Technical Weaknesses in Hydraulic Fracture Modeling and Execution
complicated by weaknesses in other relevant disciplines

A potentially helpful resource: http://geology.com/dictionary

GeoScience World: Wonderful research resource! http://geoscienceworld.org/

This is a great resource for finding papers relative to hydrology:
http://www3.interscience.wiley.com/cgi-bin/home?CRETRY=1&SRETRY=0

NOTE: Generally, some of these articles are pretty technical, so my commentary in red is meant to act as a kind of quick summary highlighting the most relevant aspects.

The drilling debate from a "conditions-based" perspective

In March, 2011 I put together a paper which summarizes a collection of facts cobbled from almost a decade of research into this issue. Fragments of this information have ranged far and wide from every source who could and would knowledgably discuss any component of any issue related to drilling. It represents papers and scientific theory from the 1920's forward... from professionals in all walks including regulators, industry and private sector consultants. It began to coalesce into a framework nearly a decade ago, and I've just been adding to it, looking for a cohesive picture in the absence of disclosure.

I initially put this paper together for a reporter, then shared it with lawmakers. A number of experts began commenting on its usefulness and requesting copies for colleagues across a variety of disciplines, which prompted me to include it here. It basically says the same thing as I said on the "fracpage", phrased, perhaps, in a more technical and probably quotable manner. Here is the whole she-bang as I've provided to others.

Please feel free to share with interested others. Please share only in its entirety (preserving context) and with credit, so the person using the info can gather additional perspective if desired - and yes, I'll even discuss the matter with industry. I want this industry to evolve toward more innovative and effectual engineering and best practices, not merely regard reality as contentious and worthy of defense.

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The drilling debate from a "conditions-based" perspective

Here is my personal perspective of what is occurring during drilling / hydraulic fracturing operations. I am not a scientist. This explanation is simply what I’ve been able to independently conclude based on research into the fields of engineering, chemistry, fluid mechanics, micro-biology, hydrology and geology; my own surface observations; knowledge of coincident drilling activity and access to correlative but limited water quality and other data… all of which is relative to the seep which emerged on and around our property as long as eight years ago, continues, but remains largely un-investigated and  insufficiently explained by regulators or industry. This theory remains to be corroborated by targeted study, but the EPA has excluded West Divide Creek as an area of interest.

– Lisa Bracken  03-09-11

Summary explanation of drilling/fracturing implications into hydro-geology.

The matrix of faults and fissures pre-existing underground together with their hydrologically and gas-charged zones actually respirate, oxygenating and hydrating the earth to depth in certain areas and enriching it with microbial activity deep within the earth’s crust – much like our own skin does through its pores and capillaries.

Shale layers can thin in certain areas, making them vulnerable to penetration. Areas, like outcrops, may be possessed of a high number of vertical fractures and fissures, depending upon how they were formed. They are also likely to possess horizontal faults and fissures – all of which can tilt in planes. Further, they may possess great pressure at depth from groundwater hydrology pressing into buried geologic up-thrusts. Further, gas, located even within a mile of the surface could share similar high-pressure characteristics.

Within this geologic structure reside streams and pockets of underground water or hydrology. This water is often in some kind of motion, flowing laterally, pushed vertically or drawn deeper by gravitational forces – all at different rates depending on the geology through which it flows, which may be of differing porosity and conductive capacity. Underground water sources are fed from surface waters such as streams, snowpack and rainfall. They also express to the surface in the way of springs.

All of this amounts to pre-existing hydro-geology, which can vary according to primarily forces which long ago acted to shape the geology together with the source materials comprising it.

Gas (like oil), generated millennia ago from earth forces and earth sources like the burial and gradual decomposition of organic matter, also resides in the hydro-geologic structure, but are often sealed in place unless disturbed by earth movement. This makes fossil fuels a desirable resource target – because, held in place, they can now be reliably tapped and produced.

What I believe this represents is a house of cards, neatly and carefully stacked – vulnerable to nature’s periodic upset, but also gradual attenuation of introduced shallow gasses and hydrology into the surrounding environment at a rate that is moderate and generally absorbable without notice or significant negative impact.

Introduce drilling and fracturing: While drilling, groundwater can be encountered together with pockets of gas contributing to “kicks”, or events which produce unexpected pressure nearer the surface… containment of which, early well bore infrastructure may be insufficient. These up-ward pressure encounters can generate earth tremors at depth which can destabilize the geologic formation at depths relative to not only the pressure encountered, but how far it can reverberate through what might be an already fractured shallower geology.

Additionally, shallower hydraulic fractures may be more prone to horizontal cracking; but at deeper depths and under greater earth pressures (overburden of rock), cracks are more prone to fracturing vertically.

All of this pressure-related and faulting activity can both seal and open existing faults, as well as intercept them with newly generated faults. This can degrade the shallower geologies and contribute to the re-structuring of shallow groundwater sources of aquifers, springs and streams.

Now you have a compromised shallow hydro-geologic layer, possibly extending to depth.

As a process of drilling, the well bore is sealed with cement in strategic locations in order to isolate water sources from target gas zones. However, kicks can degrade cement. And if not properly monitored (as occurred in the 2004 West Divide Creek Seep) the cement can slough into faults, cavities, or perhaps groundwater pockets. This enables connectivity between the wellbore and the hydro-geology. Cementing can also experience “channeling” or the migration of gas through wet cement causing a weaker matrix which may or may not act as gas conduits and may weaken sealing effects.

Even if properly cemented in certain zones, the economics of drilling leave other areas along the well bore unsealed and, therefore, exposed and vulnerable to shallow gas trapped along existing fissures which may have been intercepted during drilling. This “nuisance” gas will now travel up the well bore annulus and build pressure within the well bore at the bradenhead. That pressure can cause otherwise upwardly migrating gas to seek exit back through its original fissure as well as through other, previously vacated fissures. It could also counter natural hydraulic water pressures and utilize water-bearing coal seams and underground spring conduits as pathways into water sources – at the surface, but also potentially at depth.

Should cementing prove initially adequate, fracturing (perforating the casing into producing zones and hydraulically cracking the surrounding geology) can destabilize the house of cards at depth causing slippage through the vertical and even horizontal layers of the hydro-geologic zone. Fracturing can occur over hundreds of yards horizontally and/or vertically, creating a spider web of multi-dimensional fissuring. Liquid explosives extend fractures while acidification helps dissolve some of this structure and widen pathways. Naturally occurring fines, rubble, or injected proppants help to hold the geology open and allow gas to migrate through stimulated pathways. Draw-down of gas relieves pressure in the shattered geology and causes further cave-ins and slippage – which can also destabilize cementing.

Now, you have unpredictably destabilized – severely compromised hydro-geology in place from the depth of the target formation to the surface. Multiple fracs literally pulverize, over time, what remains. Other wells drilled nearby (sometimes within ten acres – downhole) can intersect and exponentially degraded conditions.

Under these conditions, shale integrity can only do so much to act as a natural barrier. Where it thins in the geology, and where pre-existing pressure/formation conditions together with drilling/fracturing activity serve to compound risks, conditions can become compromised quickly. This enhances the earth’s vertigenic capacity to conduct gas, fluids – any material from depth to surface, as well as from surface to depth.

Once the hydrology is altered, and oxygen reaches certain areas which were previously anaerobic, microbe populations either shift or activate. Microbes then begin consuming the new food sources, introducing their own methane and mixes of CO2 as by-products of their metabolic digestion of the methane and other hydrocarbons.

Introduced fracturing chemicals such as acids, surfactants, biocides, descalers, etc. can generate physical and chemical reactions, complicating chemical signatures in observable impacts. They can also liberate other compounds, such as arsenic, into the water and soils on their way to depth or during the migration of fluids and gasses back to the surface.

Should all of this degradation at depth result in a detectable surface seep, re-cementing may attempt to correct the problem. This may aid the problem if the source of gas is strictly confined to the wellbore and involves a single, small, sealable fissure. Even if there are many fissures and sufficient cement, re-cementing may in fact significantly seal some of the fault and fissure matrix. But, depending upon the accuracy of seismic mapping; the accuracy of projections and variables involved in frac-modeling; the extent of un-intended fracturing and the amount of circulated cement; some intersected fissures may never seal. They may be too complex; run too deep; or, be too vast in size.

In reality, the fault and fissure structure may comprise a matrix of interconnected faults and fissures extending far beyond the wellbore – perhaps, as in the case of West Divide Creek - nearly a mile away, in which case the problem cannot be corrected – at least with current technology.

Even if the situation is temporarily and fully constrained, re-cementing is never truly sound, and will remain vulnerable – just like natural geology to stimulation and degradation. If zone isolation is as sound as industry asserts, and weak cementing, alone, were found responsible for seeps into the environment, re-cementing should be capable of eliminating seeps. When cementing fails, it is blamed for seeps. When re-cementing fails then one must examine the role of hydraulic fracturing.

A mile is, in reality, a very shallow depth geologically – particularly where outcrops bring deep geology to the surface. If you walk a mile from your home and visualize that distance triangulated underground, beneath your home, you will realize it simply isn’t that great of a distance. My 80 year-old uncle used to walk two miles every day - with a pacemaker. It sounds like a far distance, but it isn’t.

Industry has attempted to rely on two arguments to hide the truth – which they, themselves, know – about hydraulic fracturing:

1)       Blame cementing. (This is a very vulnerable and narrowly applicable argument.)

2)       Blame naturally occurring gas. (Here is an area worthy of great debate.)

Certainly, there is naturally occurring gas underground. It exists from the surface to great depth, but the distinction between biogenic (surface decomposition of organic matter) and thermogenic (production) is (as is now known) mainly a matter of time; depth and temperature, pressure; exposure to oxygen; and microbial activity. Deeper gas is older, hotter and under greater pressure. Surface gas is younger, cooler, more expanded and subject to great microbial feasts supercharged by oxygenation. Near-surface gas is mainly a mixture of air and metabolized methane. Production gas or "natural gas" includes other heavier hydrocarbon compounds such as propane, butane and pentane and may even contain compounds such as benzene, ethyl benzene and toluene.

While all fossil fuels are believed to be the result of the decomposition of organic matter, there is a school of scientific thought that suggests they are a microbially generated process even within deep geologic structures. If this is the case, natural gas is not as finite as was once believed decades ago, and, more than oil and certainly coal, it can be easily manufactured. This notion explains the commercial viability and appeal of bio-fuels.

In examining natural gas buried underground… from bottom geology to top there is a broad range of deviation in gas characteristics relative to what process it has been exposed to and for how long. Currently, these characteristics are differentiated primarily by microbial and compound signatures. But, as you can probably guess, oxygenation, hydration, and intersections of even natural fissures at just a mile depth can introduce a comfortably wide margin of analytical interpretation, particularly due to the presence of microbes in deep geology – sustained by differing tolerances for oxygen deprivation or saturation, water as well as hydrocarbon and other food sources.

In fact, in order to stimulate methane recovery, natural gas as a food source as well as other food sources may be injected together with genetically altered, highly efficient microbes and a beneficial blend of oxygen and water directly into, say, coal seams. This isn’t science fiction. It is actually occurring and substantiates the notion that optimal blends of biota, food sources and supporting conditions can be manipulated. If these variables can be manipulated, they can be measured, which bodes well for scientific inquiry when it comes to identifying environmental influences and sourcing gas-related contamination. This also enables better targeting of remediation efforts.

So, microbial colonies (and their potential to alter gas signatures) naturally exist deep underground. Because microbes may naturally find themselves subject to very favorable conditions of food sources, oxygen and water, their digestive activity can accelerate the conversion of produced (but migrating) natural gas into metabolized methane which appears more newly formed and of a purely biologic nature (just as discussed in the induced coal seam scenario above).

Whether naturally occurring; the result of drilling/fracturing mishap; or, induced, the mixing of microbes with production gas and the conversion of methane and other hydrocarbons offers a margin of interpretation which can be used as either a tool of confusion or a tool of clarity… dependent, in part, upon the agenda of the person presenting findings.

These inherent complexities, then, compel science to better determine source gas in instances where production gas has infiltrated aquifers, domestic water wells or the surface environment.

Forget cementing. Cementing is a minor argument, easily measured and quantified. Therein lies the big question… biogenic or thermogenic… pre-existing or introduced…?

In the natural environment, surface observations could contribute to a better analysis of gas sources. Some of these impacts include swaths of dead vegetation; flammable gases in creek waters; a proliferation of sulfate or iron-reducing bacteria; dead wildlife; and, hydrocarbon odors. In homes, it equates to flaming faucets.

Failures to correlate environmental impacts with engineering records and pre-existing hydro-geology simply results in improperly segregated observation and conclusion. This leads to drilling protocol advanced on false assumptions, which has not only been the very sad state of affairs on West Divide Creek for eight years, but literally supports industry’s on-going efforts to bury the truth of hydraulic fracturing and drilling in general.

For me, the answer is one of aggregate probability, supported by the coincident appearance of seeps with drilling activity. I personally knew the two were related due to their concurrent appearances in West Divide Creek where there had previously been only a normal wetland environment. Since I know I am not lying, the quest for me then became one of “how”, which adds a phenomenal amount of certainty to specific postulations. I can say with confidence the two are related, and the reality of pre-existing and introduced conditions have provided the other clues to the puzzle. Now, all that is needed is confirmation, which directed study can provide.

Again, our region was rejected as a part of the EPA’s hydraulic fracturing study.

Doubtless, this issue is a very complicated one, and because I know, in context, key points can be difficult to isolate, following is a point-by-point extrapolation of industry’s major arguments including counter-argument, based on what I believe to be true.

Extrapolation of relevant points contained in comments above…

1)       Underground geology is not perfectly defined – as industry asserts when convenient to do so.

2)       The shales themselves are being fraced - so they are not an effective barrier as industry asserts.

3)       Cementing is only a part of the problem – not the whole problem as industry asserts.

4)       Re-cementing is not sound and cannot ensure adequate seal – as industry asserts.

5)       Fracing is dependent upon computer modeling which is, itself, dependent upon accurate hydrologic / geologic data. This makes fracture patterns unpredictable – not predictable as industry asserts.

6)       Aquifers are at risk due to interconnected underground faults – not isolated as industry asserts.

7)       Biogenic gas is essentially the same as thermogenic gas… differentiated according to its conditions and subject to a range of interpretation – not nearly as ‘black and white’ as industry asserts.

8)       Recovering gas; multiple fracs; and chemical injections destabilize the geology even more – which are phenomena industry doesn’t even discuss.

Additional Note: Industry frequently states that no cases of groundwater contamination have ever been found in association with hydraulic fracturing.

Where exemptions exist, there is no concern on the part of industry to safeguard human health or the environment, freeing operations to occur within a strictly profiteering framework of least expensive, aggressive and rapid development. Where exemptions exist there is also no accountability nor perceived and compelling regulatory need to investigate. Where there is no need to investigate, there can be no discovery and quantification of impact. Where there is no discovery or quantification, there can be no accountability. This set of circumstances creates a catch-22 cycle of perpetual and undiscoverable devastation.

While the oil and gas industry claims there have been no cases bearing direct evidence of groundwater contamination from hydraulic fracturing operations, rather, a carefully implemented and broad-brush federal approach has been instituted in order to totally insulate the industry from accountability in the way of exemption – or as like to think of it, structured ignorance.

In fact, the West Divide Creek seep of 2004 released an estimated 115 million cubic feet of natural gas (as well as other associated hydrocarbons) into the aquatic and terrestrial environment approximately a mile from the offending well. The seep occurred during the hydraulic fracturing of that well, which had lost cement into, presumably, an underground fault prior to hydraulic fracturing. The cementing was never put back before hydraulic fracturing occurred.

After the seep was discovered, the well was re-cemented, but, as one may expect, it failed to correct the problem. Almost eight years later, the seep still spews benzene into the shallow groundwater of West Divide Creek. While data shows the benzene trending downward then stabilizing, collected samples are quite diluted due to on-going air-sparging efforts (forcing air into the groundwater) even occurring directly into the ground water monitoring well itself. Even with skewed data the benzene appears to have stabilized. It has not disappeared.

In lobby documents the industry distributed to U.S. Congressional lawmakers in 2009, they refute high-profile cases of groundwater contamination; but the West Divide Creek seep case (very high-profile) was conspicuously absent. Industry will not say it wasn’t hydraulic fracturing that fouled ground water in that case, because it is perhaps the only case in existence which industry cannot disprove. Hearing documents from the Colorado Oil and Gas Conservation Commission have implicated hydraulic fracturing in the occurrence of that seep.

Because re-cementing has not corrected the seep, hydraulic fracturing becomes the primary mechanical suspect. Thorough study of this particular situation has the potential of either confirming or absolving the groundwater risks of hydraulic fracturing. To date neither the Colorado Oil and Gas Conservation Commission nor the EPA has demonstrated a desire to investigate the on-going cause of the seep, beyond early modeling assumptions – now shown to be false.

In 2008, another seep emerged approximately a mile from the seep that emerged in 2004. At the 2008 seep site, black ooze bubbled to the surface, which the COGCC found to be diesel–range organics – at nearly 10%. That finding was disregarded by the COGCC, and despite my urging, they refused to test for BTEX at a time when that seep was still very active. To date, neither the Colorado Oil and Gas Conservation Commission nor the EPA has demonstrated a desire to fully investigate the on-going cause of that seep. A small bit of evidence emerged after waiting two and half years for a gas sample to be collected near this site. That sample recently revealed the presence of significant methane as well as pentanes, butanes, propane and ethane (the latter being signature components of production gas). The COGCC and EnCana have deemed the presence of this gas “biogenic” and presumably pre-existing.

Hydraulic fracturing is used to stimulate all wells in this area. There are now approximately 60 natural gas wells within a mile of my home and the seeps in West Divide Creek. An additional 20 will be developed. The COGCC has recently permitted EnCana to drill into the 2004 seep geology in order to develop more wells in that zone. We anticipate that action will completely devastate what remains of this wetlands area.

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Additional resources

Following is a paper I put together to help folks understand – at least as well as I think I do – the issues surrounding hydraulic fracturing. Please note the attendant link on this page which helps support some of my thinking with scientific perspective.

www.journeyoftheforsaken.com/fracpage.htm

http://www.journeyoftheforsaken.com/fracmodeling.htm  (scientific perspectives)

Following are ten pages from this website showing impacts from the 2008 seep. There is a photo on one of these pages of a black seep oozing. Diesel-range organics were found in this black substance, but we were told by EnCana they never used diesel to frac. Now, we learn they have used it, even during the time this seep emerged during drilling operations.

www.journeyoftheforsaken.com/dividecreekseep2008.htm

On the following page, if you scroll down to “video clips”, you can find links to a number of short “You Tube” videos from the 2004 seep as well as from the 2008 seep.

www.journeyoftheforsaken.com/sitemap.htm

[Here, specifically, is the video of the black (diesel) seep actually bubbling out of the ground.]

http://www.youtube.com/watch?v=dIrJMLkx56U

Notes:  

Okay, let's get into the perspectives of qualified experts relative to specific drilling / fracing issues...

The Procedure

4. Warpinski, N.R. and Teufel, L.W., Influence of Geologic Discontinuities on Hydraulic Fracture Propagation,” Journal of Petroleum Technology, pp. 209-219, February 1987.

6. Warpinski, N.R., “Hydraulic Fracture Diagnostics,” Journal of Petroleum Technology, pp. 907-910, October 1996.

Chris A. Wright and Leen Weijers

Pinnacle Technologies, San Francisco, California, U.S.
Corresponding author: chris.wright@pinntech.com

(Article preview) http://tle.geoscienceworld.org/cgi/content/short/20/10/1185

[Comment: This excellent (pay to read) article describes the dynamics of hydraulic re-fracturing and the resulting re-orientation of its associated faulting. It also discusses microseismic consequences and influencing factors on the accuracy of modeling and its longevity through changes in the formation brought about by the interplay of gas drawdown and responsive geologic characteristics complicated by re-fracturing stages.]

Excerpts from this excellent article follow:

"There are many additional sources of stress perturbations that may complicate fracture reorientation:

Induced stress changes are not purely elastic, because large drawdowns in reservoir pressure often lead to formation failure, particularly in softer rocks;

Reservoir depletion can be highly asymmetric due to local reservoir faults, boundaries, or heterogeneities; and due to the depletion effects of offset wells throughout the field;

Placement of proppant or other solids in the initial fracture alters the state of stress around the fracture;

The elevated fracturing fluid pressure destabilizes planes of weakness during a treatment creating copious irreversible "microseisms.""

....

"Unfortunately, there is still not enough data available from a wide enough variety of environments to generalize on how common refracture reorientation is likely to be. While models are available to estimate the likelihood and degree of fracture reorientation, key input parameters are often uncertain, most notably the magnitude of deviatoric stress."

....

"Hydraulic fracture reorientation is not a rare anomaly. The implications of this phenomenon on hydrocarbon production can be considerable. But it remains both little understood and little appreciated."

Citations and Further Reading from Article:

"Diagnostic techniques to understand hydraulic fracturing: What? Why? And how?" by Cipolla and Wright (SPE 59735, presented at the 2000 SPE/CERI Gas Technology Symposium, Calgary).

"Refracturing: observations and theories" by Elbel and Mack (SPE 25464, presented at the 1993 SPE Production Operations Symposium, Oklahoma City).

"Parameters affecting azimuth and length of a secondary fracture during a refracture treatment" by Siebrits et al. (SPE 48928, Proceedings of the 1998 SPE Annual Technical Conference and Exhibition).

"Refracture reorientation enhances gas production in Barnett Shale tight gas wells" by Siebrits et al. (SPE 63030, Proceedings of the 2000 SPE Annual Technical Conference and Exhibition).
 

The Geology

Simultaneous inversion for Q and source parameters of microearthquakes accompanying hydraulic fracturing in granitic rock

Bulletin of the Seismological Society of America; April 1991; v. 81; no. 2; p. 553-575
© 1991 Seismological Society of America

Michael Fehler and W. Scott Phillips

Los Alamos National Laboratory, Mail Stop D443
Earth and Environmental Sciences Division, Los Alamos, New Mexico 87545

(Article Preview) http://bssa.geoscienceworld.org/cgi/content/abstract/81/2/553

[Comment: While this paper reflects upon hydraulic fracturing's consequence on granitic rock, the assumptions and consequences extend to sandstone formations also.]

Excerpted from the (very technical, but good) Abstract:

"The low stress drops are interpreted to result from underestimation of the actual stress drops because of a nonuniform distribution of stress drop and slip along the fault planes. Spatially varying stress drops and slips result from the strong rock heterogeneity due to the injection of fluid into the rock. Stress drops were found to be larger near the edges of the seismic zone, in regions that had not been seismically active during previous injections."

....

"The constant of proportionality between cumulative number of events and seismic moment is higher than that found for tectonic regions. The slope is so high that the seismic energy release is dominated by the large number of small events. In the absence of information about the number of events smaller than we studied, we cannot estimate the total seismic energy released by the hydraulic injection.^"
 

The Pressure

Flow focusing in overpressured sandstones: Theory, observations, and applications

Peter B. Flemings^*, Beth B. Stump^*,**, Thomas Finkbeiner^***,**** and Mark Zoback^***

^* Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802
^** Now at ChevronTexaco Production Co., 935 Gravier Street, New Orleans, Lousiana 70112
^*** Department of Geophysics, Stanford University, Stanford, California 94305-2215
^**** Now at Geomechanics International, Technologie Zentrum Mainz, IBM-Gelände, Gebäude 51, Hechtsheimer Str. 2, 55131 Mainz, Germany

flemings@geosc.psu.edu

(Article Review) http://www.ajsonline.org/cgi/content/abstract/302/10/827

[Comment: this is an excellent article that discusses in great detailed depth - while providing citations and equations for greater predictability - the interplay of unique sets of pressures in mudstone/sandstone formations and their tendencies in hydrocarbon stimulation/migration. It also suggests the vulnerability of these types of formations to underestimated and unpredictable geologic characteristics.]

Excerpted from the abstract :

"In one severely overpressured reservoir, bounding mudstones are less compacted at the reservoir crest than at the reservoir base, and we interpret that flow is focused along the reservoir and expelled at the crest. In the second reservoir, mudstone is compacted around the base of the sandstone, and we interpret pore fluids were drawn into the sandstone.

Dipping sandstone bodies encased in overpressured mudstone regulate hydrocarbon migration, affect borehole stability, and impact slope stability.^"

Excerpts From the pay-to-view article:

"We used simple hydrodynamic models and observations of compaction to illuminate the spatial distribution of pressure and flow within a dipping sandstone surrounded by overpressured mudstone. One fundamental result is that pressures within sandstones follow the hydrostatic gradient, and pressures in bounding mudstones follow steeper, often lithostatic, gradients. A consequence of this behavior is that sandstone pressure converges on the absolute stresses at the crests of structures. This pressure convergence controls hydrocarbon migration, affects well bore stability, and contributes to submarine landslides."

....

"We use the steady flow model to predict hydrocarbon entrapment in an overpressured synclinal reservoir where the crest of one limb is 1500 meters shallower than the other (fig. 16).At the crest of the shallow structure, P_ss is 3 MPa less than S_hmin; as a result, only a 364 meter gas column is trapped before additional hydrocarbons leak vertically (fig. 16). On the deeper limb, P_ss is 11.8 MPa less than S_hmin and 1506 meters of gas could be trapped. However, because the lower limb only has 1500 meters of relief, the hydrocarbons fill the deeper limb and then migrate from the base of the sand (the synclinal spill point) to the higher structure (fig. 16). Thus, flow focusing within overpressured sandstones controls the amount of hydrocarbons that can be trapped and the migration pathway.^"

^....

"Others have described the consequences of pore pressures that converge on the least principal stress at the crest of structures. Watts (1987) suggested that hydrocarbons could not be trapped at elevated fluid pressures because hydraulic fracturing resulted. In the North Sea, Gaarenstroom and others (1993) documented small hydrocarbon columns where pore pressures converge on S_hmin. Darby and others (1996, Darby and others, 1998) and Illife and others (1999) described how permeable systems with relief create leak-points at the crests of structures. Cosgrove (2001) and Boehm and Moore (2002) documented the presence of sedimentary dikes recording hydraulic fracturing where overpressured and unconsolidated sands have been injected into more cohesive overlying mudstones at the crests of structures.

^....

"If the borehole is open to the formation in this zone, the drilling fluids will fracture and enter the formation instead of returning to the drill rig. For this reason, casing is periodically set to protect the shallow borehole from the pressures necessary to drill the deeper horizons."

.....

"Fluid pressures in the crest of overpressured reservoirs converge on the least principal stress. In this situation, permeability will self-generate by hydraulic fracturing or other mechanisms. An understanding of this behavior can be used to predict the migration and trapping of hydrocarbons, drill stable boreholes, and estimate the stability of the continental slope."

Citations and Further Reading from this Article:

Bethke C. M., 1985, A Numerical Model of Compaction-Driven Groundwater Flow and Heat Transfer and Its Application to the Paleohydrology of Intracratonic Sedimentary Basins:Journal of Geophysical Research, v. 90, p. 6817–6828.

Boatman W. A., 1967, Measuring and using shale density to aid in drilling wells in high-pressure areas:Journal of Petroleum Technology, v. 19, p. 1423–1429.

Bruce B., Bowers, G., and Borel, R., 2001, Well planning for shallow water flows and overpressures-The Kestrel Well, Offshore Technology Conference: OTC 13104, 10 p.

Byrd T. M., Schneider, J. M., Reynolds, D. J., Alberty, M. W., and Hafle, M. E., 1996, Identification of "flowing water sand" drilling hazards in the deepwater Gulf of Mexico:Dallas, Proceedings of the 28th annual Offshore Technology Conference, p. 137–146.

Cosgrove J. W., 2001, Hydraulic fracturing during the formation and deformation of a basin:A factor in the dewatering of low-permeability sediments: American Association of Petroleum Geologists Bulletin, v. 85, p. 737–748.

England W. A., Mackenzie, A. S., Mann, D. M., and Quigley, T. M., 1987, The movement and entrapment of petroleum fluids in the subsurface:Journal of the Geological Society of London, v. 144, p. 327–347.

Green D. H., and Wang, H. F., 1986, Fluid pressure response to undrained compression in saturated sedimentary rock:Geophysics, v. 51, p. 948–956.

Jaeger J. C., and Cook, N. G. W., 1979, Fundamentals of rock mechanics: Science Paperbacks v. 18: New York, Chapman and Hall, 593 p

Karig D. E., and Hou, G., 1992, High-stress consolidation experiments and their geologic implications:Journal of Geophysical Research, v. 97, p. 289–300.

Mascle A., and Moore, J. C., 1990, ODP Leg 110; tectonic and hydrologic synthesis, in Winkler, W. R., editor, Proceedings of the Ocean Drilling Program, Scientific Results: College Station, TX, Texas A & M University, Ocean Drilling Program, p. 409–422.

–––– 1994, How permeable are clays and shales?:Water Resources Research, v. 30, p. 145–150.

–––– 1995, Abnormal pressures as hydrodynamic phenomena:American Journal of Science, v. 295, p. 742–786.

Rubey W. W., and Hubbert, M. K., 1959, Overthrust belt in geosynclinal area of western Wyoming in light of fluid-pressure hypothesis, [Part] 2 of Role of fluid pressure in mechanics of overthrust faulting:GSA Bulletin, v. 70, p. 167–205.

Yardley G. S., and Swarbrick, R. E., 2000, Lateral transfer; a source of additional overpressure?:Marine and Petroleum Geology, v. 17, p. 523–537.

The Water

Bulletin of the Geologic Society of America / Vol. 70, pp. 167-206. 11 Figs. February 1959
By: William W. Rubev and M. King Hubbeet

http://gsabulletin.gsapubs.org/cgi/reprint/70/2/167?ijkey=90641d1d72dabbe9c24276869429fe6d0f0cb7ac

[Comment: This is an excellent article which describes the geology beneath Western Wyoming and adjacent states. It discusses the tendency for rapidly established sediments - like we find in our region of alluvial type stream-bed deposits - to maintain high water (and gas) pressures beneath their sandstone formations. It discusses the tendency for deeper layers of sand, mud and clay - which are under greater cumulative Earth pressures from their own weight - to retain tighter porosity and permeability than higher layers. Layers nearer the surface are not as compacted and encourage fluid pressures to establish equilibrium through a myriad of pathways in the formation - (a natural tendency enhanced by hydraulic fracturing, though not mentioned in this article written in 1959).]

Excerpts from the paper follow:

"For simplicity in this hypothetical example, it is further assumed that both the upper and lower elements of clay rock lie equally near a porous sandstone or an open joint or fracture in which the fluid pressure is normal or hydrostatic. Under these circumstances, the rate of escape of pore water from the two elements of clay rock will be proportional to their respective permeabilities. According to the Kozeny-Carman equation, the permeability at a depth of 2129 feet will be about 10 times as great as that at 3730 feet. According to the relationship found by Archie, the upper element of clay rock will be about 2000 times as permeable as the lower element. After a lapse of time just sufficient for the clay rock at 2129 feet to reach essential "compaction equilibrium", the clay rock at 3730 would have lost only %o or %ooo of its excess pore water (depending upon which of the two porosity-permeability relations is used for the calculation). At any moment of observation short of the final time when both the upper and lower elements of clay rock have attained their new "compaction equilibria",

the lower element will still, because of its lower permeability, be further from its equilibrium porosity and will therefore show greater excess fluid pressure than the upper one. Consequently, because the deeper lying clay rocks are in general much less permeable than the clay rocks above them, the observed fluid pressure- overburden ratio will, under the conditions here designated as a moderate rate of loading, tend to increase with depth.

Under conditions of extremely rapid loading, however, the fluid pressure-overburden ratio decreases rather than increases with depth. This situation arises when the rate of escape of pore water is negligible compared with the rate of application of new sedimentary load."

.....

"Thus under conditions of very rapid sedimentation, but short of truly instantaneous loading, some intermediate depth of maximum fluid pressure-overburden ratio would be the expectable result.

We may thus summarize these conclusions with the generalization that, other things being equal, the greater the depth of burial, the lower the porosity and hence the lower the permeability of clay rocks. Consequently when new load is added, either slowly or rapidly, to a sedimentary pile, it will take longer to establish a new "compaction equilibrium" at greater depth than near the surface. For any given rate of new loading, there is thus some depth not here specified at which the fluid pressure-overburden ratio is a maximum; this depth of maximum ratio will lie deeper the more slowly the column of older rocks is being buried by new sedimentation."

by: Tom Myers | Independent Consultant, 6320 Walnut Creek Road, Reno, NV 89523, USA

2009 Elsevier B.V. All rights reserved.

Following is the author's summary:

"Coal bed methane (CBM) development will eventually pump more than 124,000 ha-m of groundwater, or more than 40% of the recharge, from the coal seam and sandstone aquifers of the Montana portion of the Powder River Basin (PRB). This will relieve the hydrostatic pressure, by causing a drawdown in the potentiometric surface and drawing groundwater from storage and natural discharges, to release the methane gas. A numerical groundwater flow model simulated drawdown that will exceed 90 m in the middle of the CBM fields with 6-m drawdown extending up to 29 km from the fields. Simulation results indicate that river flux will decrease up to 40% and drawdown will encompass hundreds of wells and springs. Recovery requires up to 45 years for significant decreases in river flux to recover and is not complete for 200 years. CBM development impacts can be mitigated in two ways. First, reinjecting produced water into depleted coal seams would replenish the lost storage so that recovery would draw less groundwater from long distances. Second, rapid infiltration basins near potentially-affected rivers could decrease the short-term river flow depletion. Modeled artificial recharge replaced up to 4000 ha-m of deficit in the depleted coal seams and is a feasible option for mitigating some effects of CBM development. Reinjection would be more effective if the development period were lengthened."

The Chemistry

"Based on the above, the development of pH extremes appears to be a primary factor in the mobilization of naturally occurring metals by chemical oxidation, followed by the effects of direct oxidation (attacks organic matter to which metal cations may be sorbed and redox-sensitive metals such as chromium). In a field application, the most mobile metals liberated by oxidation would be the products of oxidation such as hexavalent chromium. For the most part, the mobility of any other metals liberated should be limited by pH neutralization and moderation of ionic strength. These will support both precipitation and sorption to naturally occurring aquifer materials. In some cases, the precipitation of certain metals could further enhance the precipitation of others (fresh aluminum and iron hydroxides can interact with many other metals through sorption and coprecipitation).

The Biology

Groundwater Monitoring and Remediation
by P.M. Bradley, F.H. Chapelle, and F.E. Lo¨ffler

http://www3.interscience.wiley.com/cgi-bin/fulltext/119413586/PDFSTART (HTML)
http://www3.interscience.wiley.com/cgi-bin/fulltext/119413586/PDFSTART (PDF)

[Comment: This is an excellent article which has the added benefit of demystifying the often seemingly contradictory technical references to hydrologic environments across different scientific disciplines. Further, it discusses the significance of bias between laboratory assumptions and field realities in failing to account for the myriad of possible metabolic pathways for microbes to reduce a contaminated environment relative to such influencing factors as different levels of oxygen availability. The article isolates issues around vinyl chloride and ethenes, but similarities exits in a methane/ethane hydrocarbon environment where iron and manganese and other metals are reduced by microbial action.]

Excerpts from the article follow:

"The environmental sciences combine field investigation with laboratory experimentation in order to understand complex ecological relationships. While field application is the ultimate goal, isolation of specific environmental interactions is often impractical under field conditions.

Laboratory studies allow the isolation of relevant microorganisms and specific biogeochemical interactions but also introduce experimental biases, which can complicate data interpretation and raise concerns about the environmental relevance of the results.

The ongoing debate over the role of anoxic microbial mineralization processes in the bioremediation of environmental contaminants like vinyl chloride (VC) illustrates the challenge in assessing the environmental relevance of laboratory results. The potential for reductive dechlorination of VC to ethene in highly reducing environments is well established. Field methods for quantifying ethene are available, and the degradation processes that lead to accumulation of ethene can be evaluated in situ. In contrast, reliable field methods for assessing the degradation of VC to mineralized products (e.g., CO2 and inorganic chloride) are not currently available. Instead, our understanding of anoxic microbial VC mineralization is based primarily on laboratory observations. As a result, the environmental relevance of the process remains controversial.

....

Historically, anaerobic microbiology has focused on the microorganisms and associated metabolic pathways that operate in the strict absence of oxygen. Consequently, many of these studies have been conducted under highly reducing conditions to ensure the strict absence of oxygen.

If a study site is predominated by highly reducing conditions, then laboratory experimental procedures that establish such conditions are appropriate when assessing contaminant biodegradation. However, for sites where oxygen is not detectable but extant redox conditions include the relatively oxidized, anoxic electron-accepting processes (e.g., nitrate reducing, manganese reducing, and iron reducing), laboratory experimental conditions must address the degradation potential associated with these relatively oxidized electron acceptors.

In this context, a reasonable laboratory approach would ensure that dissolved oxygen concentrations are below detection while preserving the indigenous potential for nitrate-, manganese-, and iron reduction.

The risk associated with oxygen contamination of nominally ‘‘oxygen-free’’ systems is well known and a primary focus of traditional ‘‘anaerobic’’ microbiological methods.

For many environmental contaminants, including VC, rates of biodegradation are often orders of magnitude greater in the presence of oxygen than in its absence. Moreover, oxygen-based processes are more likely to result in complete degradation to innocuous mineralization products like CO2, whereas biodegradation under highly reducing conditions may lead to accumulation of toxic intermediates. In general, the presence of oxygen under ostensibly ‘‘oxygen-free’’ conditions can lead to an overestimation of the potential for in situ bioremediation and an unacceptable underestimation of the environmental risk associated with ground water and surface water contaminants.

....

If the potential for contaminant biodegradation under the entire range of anoxic terminal electron-accepting conditions is to be investigated, then the use of highly reducing, anaerobic techniques introduces an unacceptable, but poorly recognized, experimental artifact.

Such conditions are likely to inhibit or even prohibit the contribution of a number of terminal electron-accepting processes like nitrate-, manganese-, and iron-reduction, which occur in anoxic but relatively oxidizing environments. Indeed, the predominance in anaerobic microbiology of procedures that establish highly reducing conditions introduces enrichment biases against microorganisms that prefer or require more oxidizing redox conditions and may have contributed to the relatively late isolation of Mn(IV)- and Fe(III)-reducing organisms.

Thus, when assessing the quantitative importance of anoxic microbial mineralization processes, it is critical that laboratory investigations include relatively oxidizing, anoxic redox conditions. Bradley and Chapelle originally investigated the potential for anoxic biodegradation of VC in a shallow aquifer that was characterized by visible iron staining and predominately iron-reducing redox conditions. Initial efforts to prepare nominally anoxic sediment microcosms using chemical reductants resulted in highly reducing sulfidogenic conditions, termination of iron reduction and immediate concerns that such an approach significantly altered the microbial ecology, and pathways and products of contaminant degradation that may be important under field conditions. Subsequently, microcosms were prepared by purging oxygen to below detection (oxygen analytical detection limit of 10 lM in the headspace, less than 10 lg/L dissolved oxygen) but avoiding further changes to the sediment redox environment.

....

Conclusion

Laboratory and field studies have established the environmental relevance of the reductive dechlorination process for remediating chlorinated ethene-contaminated sites. This scientific understanding has enabled ‘‘intelligent design’’ engineering of subsurface environments and improved our ability to successfully remediate field sites. However, redox conditions at chlorinated ethene-contaminated sites can range from highly reducing anoxic to relatively oxidized anoxic to oxic. Consequently, researchers and practitioners should recognize the biases introduced by establishing highly reducing conditions and should continue to explore those alternative detoxification pathways that  may predominate under relatively oxidized anoxic conditions.

Ultimately, comprehensive knowledge of all metabolic pathways affecting chlorinated ethenes is desirable and will provide practitioners additional remediation options.

The diverse data sets used in this study enhance previous water-quality assessments by providing new and more complete insights into the sources and sinks of nitrate and methane in groundwater. Field measurements of dissolved oxygen in groundwater were useful indicators of the Wasatch Formation’s vulnerability to nitrate and methane contamination or enrichment. Results from this study also provide new evidence for the movement of water, ions, and gases into the shallow Wasatch Formation from sources such as the Mesaverde Group and deeper Wasatch Formation.

Combined Considerations of
Geology, Water, Chemistry and Biology

In January 2001, State oil and gas inspectors noted bubbles of natural gas in well water during a complaint investigation near Tioga Junction, Tioga County, north-central Pa. By 2004, the gas occurrence in ground water and accumulation in homes was a safety concern; inspectors were taking action to plug abandoned gas wells and collect gas samples. The origins of the natural-gas problems in ground water were investigated by the U.S. Geological Survey, in cooperation with the Pennsylvania Department of Environmental Protection, in wells throughout an area of about 50 mi2, using compositional and isotopic characteristics of methane and ethane in gas and water wells. This report presents the results for gas-well and waterwell samples collected from October 2004 to September 2005.

Ground water for rural-domestic supply and other uses near Tioga Junction is from two aquifer systems in and adjacent to the Tioga River valley. An unconsolidated aquifer of outwash sand and gravel of Quaternary age underlies the main river valley and extends into the valleys of tributaries. Fine-grained lacustrine sediments separate shallow and deep water-bearing zones of the outwash. Outwash-aquifer wells are seldom deeper than 100 ft. The river-valley sediments and uplands adjacent to the valley are underlain by a fractured-bedrock aquifer in siliciclastic rocks of Paleozoic age. Most bedrock-aquifer wells produce water from the Lock Haven Formation at depths of 250 ft or less.

A subsequent detailed gas-sampling effort focused on 39 water wells with the highest concentrations of combustible gas (12 representing the outwash aquifer and 27 from the bedrock aquifer) and 8 selected gas wells. Three wells producing native gas from the Oriskany Sandstone and five wells (two observation wells and three injection/withdrawal wells) with non-native gas from the gas-storage field were sampled twice. Chemical composition, stable carbon and hydrogen isotopes of methane (δ 13CCH4 and δ DCH4), and stable carbon isotopes of ethane (δ 13CC2H6) were analyzed. No samples could be collected to document the composition of microbial gas originating in the outwash deposits (outwash or “drift” gas) or of native natural gas originating solely in Devonian shale at depths shallower than the Oriskany Sandstone, although two of the storage-field observation wells sampled reportedly yielded some Devonian shale gas. Literature values for outwash or “drift” gas and Devonian shale gases were used to supplement the data collection.

Non-native gases from wells in the gas-storage field and native gases from wells producing from the Oriskany Sandstone were similar in chemical composition; methane (volume fraction ranging from 94.5 to 97.2 percent) and ethane (volume fraction ranging from 2.0 to 2.6 percent) were predominant. Isotopic composition data for storage-field gases (median δ 13CCH4 of about -44.1 per mil, δ DCH4 of -168 per mil, and δ 13CC2H6 of -32.7 per mil) were different than gases from the Oriskany Sandstone (median δ 13CCH4 of -34.6 per mil, δ DCH4 of -159 per mil, and δ 13CC2H6 of -40.4 per mil). Both Oriskany  Sandstone and storage-field gases were thermogenic. Compositions of gases from storage-field observation wells were intermediate to, and likely related to mixing of, native gases from the Oriskany Sandstone and non-native gases from the storage-field injection/withdrawal wells.

In water-well samples, methane and ethane were the only hydrocarbons detected at reportable concentrations. Methane concentrations as high as 44.8 mg/L (milligrams per liter) were measured and methane concentrations were greater than 25 mg/L in 38 percent of the 39 samples. The δ 13CCH4 values were measurable in 35 well waters and had a bimodal distribution with modes at -65 per mil (14 wells) and -40 per mil (21 wells). Gas in water samples from the 14 wells in the -65 per mil mode had a small measure of microbial gas (outwash or “drift” gas) in the isotopic signature as determined by carbon- 4 content of methane. The microbial gases were found chiefly in bedrock-aquifer well waters; 10 water wells representing upland and valley settings were along the northern flank of the Sabinsville Anticline.

Waters with microbial gases contained traces of ethane (volume fraction of 0.01 percent or less) that were too small for determination of δ 13CC2H6. Gases from the 21 water-well samples in the -40 per mil mode for δ 13CCH4 were thermogenic. The δ DCH4 and δ 13CC2H6 values for the 21 samples also showed thermogenic signatures. The thermogenic gases were found chiefly in a 17-well cluster on the axis of the Sabinsville Anticline at the eastern margin of the gas-storage field. This cluster corresponds with the cluster of wells with high concentrations of methane from the combustible-gases inventory. An observation well for the gas-storage field, TW805, was nearest to the cluster and three water wells in the cluster contained gases that nearly matched the stable carbon and hydrogen isotope composition in TW805. All the water wells had gas signatures indicating mixing of gases from different origins; however, the overall isotopic composition of methane and ethane showed that the gases in water wells at the eastern margin of the gas-storage field were principally thermogenic. The δ 13CCH4 and δ 13CC2H6 values of the majority of thermogenic gases from water wells either matched or were intermediate between the samples of storage-field gas from injection/withdrawal wells and the samples of storage field gas from observation wells.

Proximity to the axis of the Sabinsville Anticline and the eastern margin of the gas-storage field correspond to the presence of thermogenic gas in water wells. Of the water-well gases with a thermogenic signature, about half are from outwash aquifer wells and half from bedrock-aquifer wells. Of the bedrock- aquifer- well gases with a thermogenic signature, the majority are from wells drilled into bedrock beneath the Tioga River valley. Clay layers in the main Tioga River valley may play a role in keeping gas migration confined to the deep waterbearing zones of the outwash aquifer and the underlying bedrock aquifer.

Isotopic signatures have been used successfully in this study to help discern the origin of the gases in water wells near Tioga Junction. The thermogenic gas found in water wells does not match the composition of native gas from the Oriskany Sandstone. Mixing of Oriskany gases with storage-field gases has occurred, and there was also evidence for mixing of a microbial component of gas in some water wells. The possibility of three or more end-member compositions and many possible mixing scenarios for gases complicate the data interpretation. The lack of samples solely representing native shallow Devonian gas and the small number of storage-field gas samples places some limits on making firm conclusions about the origin of the methane in ground water. The weight of the evidence, however, points to storage-field gas as the likely origin of the natural gases found in water wells near Tioga Junction.

Introduction

In January 2001, State oil and gas inspectors noted bubbles of natural gas in well water during a complaint investigation near Tioga Junction, Tioga County, north-central Pa. A natural- gas storage field near Tioga Junction in the Oriskany Sandstone of Devonian age is a former native gas-production zone about 3,900 ft below land surface. Many abandoned native gas wells exist in the area. By 2004, natural gases, primarily methane, in ground water from household-supply wells were enough of an issue that inspectors were taking action to plug abandoned gas wells; however, plugging of wells did not solve the problem. Questions remained on the origin of the gases, and residents remained concerned. Although it is common for ground water in the region to contain salt brine and hydrogen sulfide, natural gas in ground water was rarely noted by earlier investigators. Historic assessments of ground-water resources only mentioned natural gas in water-supply wells converted from gas wells (Lohman, 1939; Taylor and others, 1983; Williams and others, 1998).

In response to the concerns of the residents, the Bureau of Oil and Gas Management (BOGM) of the Pennsylvania Department of Environmental Protection (PADEP) sampled water from selected household- supply wells in 2001 to 2004. Analytical results for water samples indicated concentrations of dissolved methane as high as 92 mg/L, which greatly exceeds the solubility of methane of about 28 mg/L in water exposed to one atmosphere pressure of pure methane. Concentrations of this magnitude result in outgassing of methane and provide the possibility of accumulation of methane and subsequent explosion, with the potential for substantial property damage and loss of life. The potential for outgassing of dissolved methane in ground water may be estimated using well-construction factors and the water level in the well. A description of the maximum theoretical concentration of methane dissolved in ground water and a discussion of the risks and the importance of venting a plumbing system if methane gas is present in well water are presented in the appendix at the back of the report.

The extent of methane occurrence in shallow ground water in the area near Tioga Junction was not well known. The problem was most commonly reported in water wells in the valley of the Tioga River near Tioga Junction and on the ridge west  of the valley (fig. 1). However, local residents noted that wells to the north near Lawrenceville, Pa. (fig. 1), also had high concentrations of natural gas.

The origin of the natural gas was unknown at the time of PADEP’s investigation. The natural gas could be thermogenic, formed by the thermal breakdown of organic material in sediments resulting from high temperatures caused by deep burial. Possible origins of thermogenic gas include the following: native thermogenic natural gas in the Oriskany Sandstone; nonnative thermogenic gas imported by pipeline and stored in one of three natural-gas storage fields near Tioga Junction (Tioga, West Tioga, and Meeker, at depths of 3,500 to 4,100 ft below land surface); or native gas from strata at depths greater or less than the Oriskany Sandstone. Alternatively, the natural gas could be formed in the shallow subsurface by microbial reduction of carbon dioxide or methyl-type microbial fermentation of organic debris. PADEP expressed concerns that the gas could be migrating from compromised gas wells (improperly plugged, leaking, or abandoned), leaking gas-storage fields, or new uncontrolled pathways opened by some event (possibly seismic) may have changed local permeability in the bedrock.

The U.S. Geological Survey (USGS), in cooperation with the PADEP, conducted a study to investigate the occurrence and origin of natural gas in ground water in the area of about 50 mi2 near Tioga Junction, Tioga County, in the spring and summer of 2005. This report is a summary of findings from the study.

This report describes an inventory of water wells completed in bedrock and in unconsolidated sediments to evaluate the extent of elevated concentrations of hydrocarbon gas in ground water and subsequent detailed isotopic analysis of samples from water and gas wells to investigate the origin of the gas in the ground water. For the inventory, combustible-gas concentrations and field water-quality characteristics, measured in 2005 in 91 wells in an area of about 50 mi2 near Tioga Junction, Pa. For the subsequent detailed analysis, analytical results from sampling 39 water wells for field water-quality characteristics, natural-gas hydrocarbons, other gases (oxygen, carbon dioxide, nitrogen, argon, hydrogen, and helium), stable carbon and hydrogen isotopes of methane, stable carbon isotopes of ethane and dissolved inorganic carbon (DIC), and carbon-14 of methane (9 samples) are presented and discussed. The ground-water gas compositions are compared to 17 gas-well samples (nearby storage gas and native deep gas) and literature values for microbial gas to characterize the origin of natural gas in ground water in the study area. This study focused on the question of gas origin in well water and did not determine a specific source of the stray gases, mechanism of migration, or evaluate the ground waters for other contaminants.

Description of Study Area

The study area in northeastern Tioga County north of Mansfield (fig. 1) includes the Tioga River valley and adjacent uplands about 4 mi east and west of the valley (fig. 2). The area extends from just north of the Tioga and Hammond Reservoirs to the general area of Lawrenceville, Pa., a few miles north of Tioga Junction. Two gas wells south and east of Mansfield also were studied (fig. 1). The Meeker, West Tioga, and Tioga gasstorage fields lie at a depth of about 3,900 ft below a ridge on the western side of the Tioga River valley near Tioga Junction and extend about 8 mi westward.

Ground water for rural-domestic supply and other uses near Tioga Junction is from two aquifer systems in and adjacent to the Tioga River valley. An unconsolidated aquifer of outwash sand and gravel of Quaternary age underlies the main river valley and extends into the valleys of tributaries. Fine-grained lacustrine sediments separate shallow and deep water-bearing zones of the outwash. Outwash-aquifer wells are seldom deeper than 100 ft. The river-valley sediments and uplands adjacent to the valley are underlain by a fractured-bedrock aquifer in siliciclastic rocks of Paleozoic age. Most bedrock-aquifer wells produce water from the Lock Haven Formation at depths of 250 ft or less. Structural Geology Eastern Tioga County straddles glaciated and non-glaciated areas of the Appalachian Plateaus Physiographic Province (Sevon, 2000). The geology of the study area is described in Fuller and Alden (1903), Cathcart and Myers (1934), Lytle (1963), Luce and Edmunds (1981), Williams and others (1998), and Harper (1999) and is excerpted here for the Silurian and younger-age rocks.

The Devonian and older rocks were structurally deformed from folding at the end of the Paleozoic Era into a series of synclines and anticlines with axes trending southwest-northeast. The Sabinsville Anticline (Fuller and Alden, 1903, p. 5) trends northeast from the southwest corner of Farmington Township to Tioga Junction and continues northeast (fig. 2). The anticline axis plunges to the southwest at 50 ft/mi or less. The dips on the north flank of the anticline are 150 to 350 ft/mi; the south flank has beds with dips ranging from 700 to 900 ft/mi. A fault block on the south flank of the anticline, bounded by two north-dipping faults with down-thrown beds to the south, is documented and mapped from gas-well drilling records (Cathcart and Myers, 1934, p. 16) at the top of the Lower Devonian Oriskany Sandstone (fig. 2). In the vicinity of Tioga Junction, the top of the Oriskany Sandstone is about 3,900 ft below land surface. The displacement of the faults is less in limestone beds above the Oriskany Sandstone than in the Oriskany Sandstone (Cathcart and Myers, 1934, p. 19). Additional faults are shown in figure 2 as mapped at the top of the Oriskany Sandstone by Lytle and presented in Luce and Edmunds (1981, p. 63). The fault traces are approximated as dashed lines in all the earlier work. Recent seismic and drilling data have been used to refine the structural geology of the Oriskany Sandstone (Beardsley and others, 1999, p. 289; NE Hub Partners, L.P., 1996, project application on file at Pennsylvania Bureau of Oil and Gas Management). The recent mapping shows the fault traces as solid lines; these imply the faults are located with more certainty than in the earlier work. The faults in the Oriskany Sandstone do not extend into near-surface rocks (Beardsley and others, 1999, p. 289).

Upper Silurian (Salina Group) salt deposits occur (Norris, 1978) at depths greater than 4,500 ft and are proposed targets for solution mining and development of salt caverns for gas storage (Susquehanna River Basin Commission, 1996; NE Hub Partners, L.P., 1996, project application on file at Pennsylvania Bureau of Oil and Gas Management). The Upper Silurian Tonoloway Limestone is part of the Salina Group (Laughrey, 1999, p 105) at a depth of about 4,200 to 4,300 ft. Above the Tonoloway Limestone are Lower Devonian limestones and shales that include the basal Devonian Keyser Formation. Drillers call the Keyser Formation the “Helderberg” (Harper, 1999, p. 113).

Devonian age Oriskany Sandstone (hereafter termed “Oriskany”) overlies the Keyser Formation. The Oriskany is equivalent to the Ridgeley Sandstone and drillers sometimes use the names interchangeably. The Oriskany is an almost pure quartzose sandstone (Harper, 1999, p. 114); however, it is described as calcareous. Above the Oriskany, the bedrock consists of shale, siltstone, and sandstone along with beds of limestone and calcareous rocks of Devonian age. Near the land surface, the rocks are part of the Upper Devonian Lock Haven Formation (Harper, 1999).

Unconsolidated sediments of glacial and postglacial origin cap the geologic section. Glacial ice entered the study area during the Pleistocene Epoch. The glaciers eroded the bedrock and deepened and widened the valleys. During glacial melting, the valleys were partly filled with sediments deposited by glacial ice,  meltwater, and proglacial lakes. The uplands were mantled with till. In the postglacial period, the streams deposited alluvium and organic-rich deposits. Glacial and postglacial valleyfill sediments consist of stratified drift, outwash, alluvium, swamp deposits, lacustrine deposits, and till (Fuller and Alden, 1903). The unconsolidated deposits are referred to in this report as outwash or alluvium. The outwash consists of poorly sorted to well-sorted sand, gravel, silt, and some clay deposited by glacial meltwater or glaciofluvial processes. Outwash, as indicated in this report, refers to the deposits of sand and gravel deposited as valley-trains in the Tioga River and tributary valleys in the study area (fig. 2). The alluvium is generally shallow sand and gravel or silt and fine sand found on floodplains and typically overlies the outwash deposits in the study area. The generalized extent of mapped unconsolidated deposits (Commonwealth of Pennsylvania, 1989; Williams and others, 1998) is shown in all the maps in this report. Till, consisting of clay, silt, some sand, and rock fragments deposited by the glacier, is on the hillsides and ridge tops of the study area but is unmapped in this report.

The hydrogeologic characteristics in the study area were described by Williams and others (1998) and are excerpted here. The generalized framework of the outwash aquifer system with major ground-water recharge and discharge areas is shown in figure 3. The outwash aquifers are stratified unconsolidated deposits of valley fill overlying lacustrine deposits or bedrock with till and bedrock as basal confining units. The lacustrine deposits, till, and bedrock typically have low primary permeability. Hydrogeologic sections (traces of sections shown on fig. 4) from Williams and others (1998) show sequences of glacial deposits that occur along the major stream valley in the study area (fig. 5). The shallow deposits of alluvium and some outwash generally are thin and are unconfined aquifers. In the Tioga River valley, lacustrine deposits may form a local confining unit, and confined aquifers of outwash and related deposits may be present beneath. Sources of natural recharge to the outwash aquifers consist of infiltration of precipitation on the valley floor, infiltration of unchanneled runoff from the uplands at the valley walls, ground-water inflow from the uplands, and infiltration from tributary streams (fig. 3). Mitchell Creek (fig. 5) has been shown by Williams and others (1998, p.16) to have losing reaches. Recharge to the confined outwash aquifers occurs mainly near the valley walls where surficial sand and gravel is in hydraulic connection with the outwash aquifer (fig. 3).

Most ground water flows from the outwash aquifers toward points of discharge, generally either the Tioga River, the lower reaches of principal tributaries to the Tioga River, or swampy areas. In the confined aquifers, ground-water flow discharges to the Tioga River and reaches near the mouths of the principal tributaries through either the confining-unit materials or through overlying unconfined aquifer materials where a confining unit is absent.

The outwash aquifer, unconfined or confined, in the glaciated valleys of Tioga and adjacent counties is characterized by a 20 gal/min median yield to wells (Williams and others, 1998, p. 26); however, yields can be larger for industrial and municipal supply wells. Domestic wells drilled into the outwash aquifers are completed as open-ended casing in a water-bearing zone.

The Lock Haven Formation is the principal bedrock aquifer and is recharged by precipitation. The bedrock is commonly mantled by till on hillsides and ridge tops.

Secondary permeability in the bedrock is dependent on fractures. Regional patterns of fractures or faults in the Lock Haven Formation have not been mapped; however, the southwest to northeast orientation of the channels of the Cowanesque River, Crooked Creek as it flows into Hammond Reservoir, other smaller tributary streams to the Tioga River, and the Tioga River near Mitchell Creek may be due to regional patterns of weakness or increased fracturing that allowed glacial and weathering processes to downcut preferentially in the bedrock, which led to river-channel incision and the present-day channel orientation. Reports by PADEP oil and gas inspectors of natural gas bubbling into streams at the channel bottoms (Robert Gleeson, Pennsylvania Department of Environmental Protection, oral commun., 2006) could support the concept that stream channels are zones of increased fracture density (Wyrick and Borchers, 1981) and preferential pathways for migration of natural gas from depth.

There is no evidence in the hydrogeologic literature for the Tioga  Junction area that faults at depth in the Oriskany extend to near land surface and into the Lock Haven Formation to act as fractured or preferential pathways for ground-water flow. Nevertheless, the faults are mapped for reference in figure 4.

Locally, fractures in the bedrock may be in hydraulic connection with overlying outwash aquifers. Domestic and nondomestic water-supply wells intercept water from fractures in the bedrock and are completed as open boreholes with surface casing. The median yield from domestic wells in the Lock Haven Formation is 10 gal/min (Williams and others, 1998, p. 26).

History of Gas Development and Gas Storage

The first gas well drilled into the dome in the Sabinsville Anticline in northern Tioga County near Tioga Junction produced gas from the Oriskany in 1930 (Cathcart and Myers, 1934). During the next several decades, the Oriskany gas was produced by many gas wells in the Sabinsville Anticline area.

Drilling efforts also produced dozens of dry holes, and many   drill holes were later abandoned (fig. 2). Natural gas was determined on the basis of pressure responses to reside in three independent “pools.” The pools were labeled the Boom, the Meeker, and the Elbridge (Cathcart and Myers, 1934). Southeast of Tioga Junction in a separate anticline, the Oriskany is at a depth of about 5,800 ft at the Krause gas well (fig. 1, the Krause gas well has a total depth of 5,850 ft and the Oriskany is the producing zone of the gas). Other gas-producing strata such as the Trenton and Black River Limestones of Ordovician age are much deeper than the Oriskany—approximately at a depth of 12,000 ft in Tioga County (Christopher D. Laughrey, Pennsylvania Topographic and Geologic Survey, oral commun., 2006).

In the 1960s, the native Oriskany gas in the Sabinsville Anticline near Tioga Junction was depleted, and the pools or reservoirs were converted into gas-storage fields (Lytle, 1963). These reservoirs are now called the Tioga, Meeker, and West Tioga storage fields (fig. 1), respectively, and are collectively referred to as the Tioga gas-storage field.

The gas-storage field receives gas primarily from the gas producing areas of the Gulf of Mexico region of the United States but also from other areas of the Appalachian Plateaus Physiographic Province in the United States and Canada. Gas from a pipeline is injected into the field during spring, summer, and fall for future withdrawal. During winter, gas is usually withdrawn from the Tioga storage field and transmitted to a pipeline for distribution and use. The storage fields currently (2005) are managed by two gas companies. Dominion Gas Company manages the Tioga and West Tioga fields (Thomas Rice, Dominion Gas Company, written commun., 2005). Pennsylvania Power and Light manages the Meeker field (Douglas Welsh, Pennsylvania Department of Environmental Protection, written commun., 2005).
 

As of 2005, methane gas was leaking into water wells near Tioga Junction, Tioga County, north-central Pa. State oil and gas inspectors found water from household-supply wells with concentrations of dissolved methane as high as 92 mg/L, which greatly exceeds the solubility of methane in water. Concentrations of this magnitude result in outgassing of methane and provide the possibility of accumulation of methane and subsequent explosion with the potential for substantial property damage and loss of life. The origin of the methane was investigated by the U.S. Geological Survey in cooperation with the Pennsylvania Department of Environmental Protection (PADEP) in an area of about 50 mi2, using compositional and isotopic characteristics of methane and ethane in gas and water wells. This report presents the results from October 2004 to September 2005.

Ground water for rural-domestic supply and other uses near Tioga Junction is from two aquifer systems in and adjacent to the Tioga River valley. An unconsolidated aquifer of outwash sand and gravel of Quaternary age underlies the main river valley and extends into the valleys of tributaries. Fine-grained lacustrine sediments separate shallow and deep water-bearing zones within the outwash. Outwash-aquifer wells are seldom deeper than 100 ft. The river-valley sediments and uplands adjacent to the valley are underlain by a fractured-bedrock aquifer in siliciclastic rocks of Devonian age, primarily the Lock Haven  Formation. Most bedrock-aquifer wells produce water from the Lock Haven Formation at depths of 250 ft or less.

A review of previous geologic investigations was used to establish the structural geologic framework and identify four plausible origins of the natural gas. The Sabinsville Anticline, trending southwest to northeast, is the major structural feature in the Devonian bedrock. The anticline, a structural trap for a reservoir of deep native gas in the Oriskany Sandstone (origin 1) at depths of about 3,900 ft, was explored and tapped by numerous wells from 1930-60. The gas reservoir in the vicinity of Tioga Junction, depleted of native gas, was converted to the Tioga gas-storage field for injection and withdrawal of non-native gases (origin 2). Devonian shale gas (shallow native gas) also has been reported in the area (origin 3). Gas might also originate as outwash or glacial “drift” gas generated from buried and decomposed organic material in the outwash deposits (origin 4).

During May to August 2005, a combustible-gas inventory of 91 water wells (62 in the bedrock aquifer and 27 in the outwash aquifer) yielded 49 positive readings (greater than or equal to 0.1 percent) in the headspace of water samples (36 from wells completed in bedrock and 13 from wells completed in outwash). The percentage of combustible gas as measured in the headspace of ground-water samples collected in the vicinity of Tioga Junction, when multiplied by a factor of three, gave an approximation of the concentration of methane dissolved in ground water in milligrams per liter. High combustible-gas readings corresponded with methane concentrations in water greater than 10 mg/L; the occurrence was in clusters in the central and southern parts of the study area, and the largest cluster was near the eastern boundary of the gas-storage field.Southwest- to northeast-trending bands of occurrence characterized the northern flank of the Sabinsville Anticline.

A detailed gas-sampling effort from June 2005 to August 2005 focused on 39 water wells with the highest gas concentrations representing the bedrock aquifer (27 wells) and the outwash aquifer (12 wells). Detailed gas sampling also included 8 selected gas wells that were sampled twice—three wells producing native gas from the Oriskany Sandstone and five wells (two observation wells and three injection/withdrawal wells) with non-native gas from the gas-storage field. These data were combined with data collected earlier from the same wells by PADEP. Chemical composition of gas, stable carbon and hydrogen isotopes of methane (δ 13CCH4 and δ DCH4), and stable carbon isotopes of ethane (δ 13CC2H6) were analyzed. Isotopes are reliable indicators of the origin of different gases, whether thermogenic or microbial. No samples could be collected to document the composition of “drift” gas or of native natural gas originating solely in Devonian shale at depths more shallow than the Oriskany Sandstone, although two of the storage-field observation wells sampled reportedly yield some Devonian shale gas. Literature values for outwash or “drift” and Devonian shale gases were used to supplement the data collection.

Non-native gases from wells in the gas-storage field and native gases from wells producing from the Oriskany Sandstone were similar in chemical composition; methane (volume fraction ranging from 94.5 to 97.2 percent) and ethane (volume fraction ranging from 2.0 to 2.6 percent) were predominant. Isotopic- composition data for storage-field gases (median δ 13CCH4 of about -44.1 per mil, δ DCH4 of -168 per mil, and δ 13CC2H6 of -32.7 per mil) were different than gases from the Oriskany Sandstone (median δ 13CCH4 of -34.6 per mil, δ DCH4 of -159 per mil, and δ 13CC2H6 of -40.4 per mil). The Oriskany Sandstone and the storage-field gases were thermogenic, but of different origins. The δ 13CCH4, δ DCH4, and δ 13CC2H6 differences are because the Oriskany gas is more thermally mature than the storage-field gas and is more enriched in 13C and 2H.

Compositions of gases from storage-field observation wells were intermediate to, and likely relate to mixing of, native gases from the Oriskany Sandstone and non-native gases from the storage-field injection/withdrawal wells. One depleted Oriskany gas well (TW805) that is approximately 500 ft northeast of the eastern boundary of the storage field and is used as an observation well for the storage field yielded about a 50:50 mixture of Oriskany gas and storage-field gas. This provided an initial line of evidence that there was some gas leaking and migrating from the eastern boundary of the storage field toward well TW805.

Chemical and stable-isotope characteristics of water-well samples with and without combustible gas were compared by aquifer. Bedrock-aquifer waters with gas exhibited the largest range and the largest median conductivity. The median pH of 6.8 for outwash-aquifer water without gas was the most acidic; the presence of gas seemed to be associated with a buffering effect. This relation also held for bedrock-aquifer waters with and without combustible gas, although the differences in median pH were less pronounced.

Dissolved oxygen and Eh data provided evidence of a geochemical environment in the aquifers that was not oxygenated. Waters with combustible gas had lower median Eh and dissolved oxygen compared to waters without gas, regardless of aquifer. The lack of dissolved oxygen is conducive to anaerobic microbial reduction of sulfate to hydrogen sulfide and carbon dioxide to methane. During well-water sampling, the odor of hydrogen sulfide or “sulfur smell” was common and provided evidence of sulfate reduction by bacteria under conditions of low dissolved oxygen.

Stable-isotope ratios for hydrogen and oxygen of ground water in bedrock (26 samples) and outwash (10 samples) aquifers were not different for wells with and without combustible gas and did not correlate with gas concentrations. When δ Dwater was plotted against δ 18Owater for Tioga Junction ground water and compared to typical river water in north-central Pennsylvania, the ground water composition was similar to that of nearby river waters. There is no strong evidence that the waters from one or both aquifers are isolated from recent recharge or represent Pleistocene-age water or water originating from a high-altitude or cold-environment source. The stable isotopes of water provided no additional information on gas occurrence or origin.

Methane and ethane were the only hydrocarbons detected at reportable concentrations in 37 and 32, respectively, of 37 water-well samples analyzed. The range of methane as a volume fraction in headspace gases ranged from 0.0066 to 94.4 percent; ethane ranged from 0.003 to 1.61 percent. Of the 35 wells with isotopic data for methane, 10 were completed in the outwash aquifer, and 25 were completed in the bedrock aquifer. The range in δ 13CCH4 for ground water was from -35.54 per mil (well TI593, outwash aquifer) to -75.18 per mil (well TI611, bedrock aquifer). The range of δ DCH4 in ground water was from -105.9 per mil (well TI586, bedrock aquifer) to -266.4 per mil (well TI610, bedrock aquifer). There was a bimodal distribution of δ DCH4 results with modes at about -180 and -250 per mil. The - 50 per mil mode appears related to microbial gases, and the -180 per mil mode is related to thermogenic gases. The δ 13CCH4 values in well waters had a bimodal distribution with modes at -65 per mil (14 wells) and -40 per mil (21 wells). Gas in water samples from the 14 wells appears to have had a small measure of microbial gas (outwash or “drift” gas) in the isotopic signature as determined for well water samples by 14C content of methane. The amount of 14C was low, an indication that the microbial gas represented a minor component relative to the other gases in the ground water. The microbial gases were found chiefly in bedrock-aquifer well waters; 10 wells representing upland and valley settings were located along the northern flank of the Sabinsville Anticline. Data for δ 13C of dissolved inorganic carbon give support to microbial processes contributing to the methane in these bedrock- aquifer wells; however, the data provided no new insights on the origin of the carbon in the methane.

Microbial gases typically do not contain ethane. The  waters with microbial gases contained only traces of ethane (volume fraction of 0.01 percent or less). The trace ethane concentrations were too small for determination of δ 13CC2H6. For waters with ethane, the δ 13CC2H6 ranged from -28.73 to -39.03 per mil in a unimodal distribution; the median was -32.17 per mil. Waters from the 21 wells in the -40 per mil mode for δ 13CCH4 were thermogenic. The δ DCH4 and δ 13CC2H6 values for the 21 samples also were thermogenic.

The thermogenic gases were found chiefly in a 17-well cluster on the axis of the Sabinsville Anticline at the eastern margin of the gas-storage field. This cluster corresponds with the cluster of wells with high concentrations of methane from the combustible-gases inventory. An observation well for the gas-storage field, TW805, is nearest to the cluster, and three water wells in the cluster had gases that nearly match the gas composition in TW805. This is evidence that the gas mixture in observation well TW805 was also migrating to ground-water wells.

All the water wells had gas signatures indicating mixing of gases from different origins; however, the overall isotopic composition of methane and ethane showed that the gas in water wells at the eastern margin of the gas-storage field is mostly thermogenic. The δ 13CCH4 and δ 13CC2H6 values of the majority of thermogenic gases from water wells either matched or were intermediate between the samples from storage-field gas injection/withdrawal wells and the samples from storage-field gas observation wells.

Proximity to the axis of the Sabinsville Anticline and the eastern margin of the gas-storage field corresponded to the presence of thermogenic gas in water wells. It is not known why certain wells had microbial gas and others did not. Detailed drilling records were not available for many of the inventoried water wells; hence, without expensive and invasive borehole tests, a detailed understanding of the gas occurrence in the aquifers could not be developed using the limited well data. Well depth was a factor in the observed occurrence of combustible gas for the 62 bedrock wells inventoried. As well-depth range increased from less than 50 ft to 51-150 ft to greater than 151 ft, the percentage of wells with combustible gas increased. On the basis of the known geology and the aquifer framework in the Tioga River valley, the presence of clay-rich layers formed by lacustrine deposits probably plays a role in keeping gas migration confined to the deep water-bearing zones of the outwash aquifer and the underlying bedrock aquifer. Of 11 shallow wells inventoried (with depths of 50 feet or less) in the Tioga River valley, 2 had ground waters with combustible-gas percentages greater than 0.1 percent. These 11 wells likely tap the shallow water-bearing zones of the outwash aquifer. Of 18 wells with depths greater than 50 ft inventoried in the Tioga River valley, 10 had waters with combustible-gas percentages greater than 0.1 percent.

Isotopic signatures have been used successfully in this study to help discern the origin of the gases in water wells near Tioga Junction. The thermogenic gas found in water wells does not match the composition of native gas from the Oriskany Sandstone. Mixing of Oriskany gases with storage-field gases is occurring, and there is also evidence for mixing of a microbial component of gas in some water wells. The possibility of three or more end-member compositions and many possible mixing scenarios for gases complicate the data interpretation. The limited numbers of storage-field gas samples place limits on making firm conclusions about the origin of the methane in ground water. The weight of the evidence, however, points to storagefield gas as the likely origin of the thermogenic gas found in water wells near Tioga Junction. 

The following article - primarily an industry-type study of methane production from coal seams - also describes ways in which noble gases can be used to determine the relationship between hydrology and hydrocarbon production. It illuminates the numerous factors involved which, when properly considered, aid in greater predictability of hydro geologic mechanics.

by: Zhou, Zheng; Ballentine, Chris J.; Kipfer, Rolf; Schoell, Martin; Thibodeaux, Steve

http://adsabs.harvard.edu/abs/2005GeCoA..69.5413Z

From the Abstract:

"The San Juan Basin natural gas field, located in northwestern New Mexico and southwestern Colorado in the USA, is a case-type coalbed methane system. Groundwater is thought to play a key role in both biogenic methane generation and the CO2 sequestration potential of coalbed systems. We show here how noble gases can be used to construct a physical model that describes the interaction between the groundwater system and the produced gas. We collected 28 gas samples from producing wells in the artesian overpressured high production region of the basin together with 8 gas samples from the underpressured low production zone as a control. Stable isotope and major species determination clearly characterize the gas in the high production region as dominantly biogenic in origin, and the underpressured low producing region as having a significant admix of thermogenic coal gas."

This excellent article discusses ways in which pressure, hydrology and fault structures interact under arid climatic conditions - a previously unstudied area of fault physics. It sheds very important light on the risks of contamination movement associated with induced fractures underground and ways in which intersected pathways (from existing and introduced fractures) can facilitate the exchange of fluids and gases from surface to subsurface environs. Weaknesses are also revealed in certain assumptions - chalk and shale acting as impermeable barriers - for example....

Cracks In The Earth A Danger For Global Warming

http://greenprophet.com/2009/03/18/7611/cracks-earth-global-warming/

Mar 18th, 2009 by Green Prophet

“Fractures breathe, and this process has direct relevance to the question of global warming,” says Dr. Noam Weisbrod –– With the ever increasing shortage in the world’s water resources and dire warning of Israel’s own ongoing drought, research to reduce contamination of existing water resources while developing new potable sources has become a pressing concern, here, as in the rest of the world.

....

Hydrologist Dr. Noam Weisbrod’s research is providing the basic knowledge that can help in solving this crisis by understanding how pollutants reach the subsurface and how they behave underground.

Weisbrod, a member of the Zuckerberg Institute for Water Research of the Jacob Blaustein Institutes for Desert Research at Ben Gurion University’s Sede Boqer campus, is investigating the transport of various contaminants into the groundwater from land-surface, through the soil to the aquifer, and within the aquifer to vital production wells for fresh water.

.....

The local chalk formation in the Negev desert is a very different kind of surface compared to the sandy soils of the coastal aquifer. “Hydrologists once thought chalk was a natural barrier protecting the groundwater under what is now the Ramat Hovav industrial complex, located south of Beer-Sheva, but they were wrong,” he continues.

“The chalk formation may be impermeable in most areas, but it is fractured: the maze of cracks and fissures create a kind of by-pass from the surface to the water table, allowing pollutants and salts to migrate through the cracks crossing the chalk rock and reach groundwater at surprisingly high velocities,” he explains.

....

Weisbrod found the multidisciplinary challenges in this field exciting. It involved the interface between chemistry, physics, flow behavior and material properties, against the politically charged backdrop of social, commercial and governmental inputs. All these aspects came into play during his postdoctoral studies at the Department of Bioengineering at Oregon State University, where he worked at a government nuclear waste site in Washington state.

....

“Gases, particularly water vapor,” he explains, “are being released from deep underground through the earth’s fractures into the atmosphere. Until now, scientists have neglected this critical aspect of fracture physics,” he says.

Fractures have been intensively studied for their role in aquifer recharge or aquifer contamination during periods of liquid infiltration. In general, they have been considered inactive when there is no precipitation.

In arid and semi-arid environments, this is the case more than 90 percent of the time. But here’s a surprising fact: “Fractures breathe,” declares Weisbrod, “and this process has direct relevance to the question of global warming. The role of geological fissures in global water cycles has been entirely neglected in the design of modern climate models,” he notes.

Weisbrod and his group have shown that in dry arid regions, such as the Negev desert, there is a nightly escape of water vapor from fractures. They’ve proven that, in fact, water vapors are being released at a much greater rate during the night than during the day, through fractures. During the day, the air deep inside the fracture is cooler than the warm air at its opening. This is a stable condition.

At night, the air cools down and unstable conditions develop, which is when convective movement takes place: the now cooler atmospheric air invades the fracture and the relatively warmer and lighter humid inner air goes out.

“We’ve demonstrated and quantified this process both in the field and in a specially designed climate controlled laboratory,” says Weisbrod. “We believe this mechanism plays an important role in the gas exchange rate between the earth and the atmosphere and may also be important in our understanding of the energy balance across the earth’s surface.”

In addition to water vapor, movement of gases through fractures affects the exchange of many other “greenhouse” gases, including carbon dioxide and methane. “Even small changes in such gases are thought to have large repercussions on global climatic function,” he continues.

Since there are untold numbers of cracks, shafts, caves and other cavities on the earth’s surface, if climatologists were given accurate data on earth-atmosphere gas exchange rates via these cavities throughout the world,” contends Weisbrod, “they could design better models predicting climate change and CO2 concentrations. And this is a completely untouched area of geoscience.”

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