Rupturing the ground allows releasing of accumulated stress at depths. This concept suggests that while frequency of earthquakes increases due to gas extraction overall stress in rocks decreases and eliminates danger of sudden stress release.

Segall, P., 1992, Induced stresses due to fluid extraction from axisymmetric reservoirs: pure and applied geophysics, v. 139, p. 535-560.

Dear Gunther Kletetschka, I see your point, especially in the areas close to the seismic faults.

On the other hand, this is how I see for the moment: The gas captured in the rock has an attenuation effect on the propagation of the mechanical vibrations, seismic waves. Its extraction and replacement with water for instance, will reduce the attenuation effect, adding to the deep ground ability to transmit the seismic activity with less loss of strength as we depart from the epicenter of the quake. I am not a specialist in this domain, therefor RG members contribution to this question is highly appreciated.

Thank you

Dear Adrian, I agree with your opinion about existence of attenuation effect and its possible influence because it is one of the important parameters of gas and oil reservoirs exploration but as far as I know a replacement by water or other liquid reagents is a usual practice in oil and gas extraction industry many decades as enhanced oil recovery methods when pressure of a horisont with oil or gas drops significantly. Therefore I suppose that influence of such changes on earthquakes is minimal or practically absent.

Dear Georgii, yes in did, inserting water it's a long time practice in gas and oil extraction.

Let's consider still the fact that oil and water are both liquids (no compressibility) but extraction of compressible gas and replacing with water it may make a difference. I will extend my concern in the area of unconventional gas exploitation / extraction (drilling by fracking) where water is replacing the gas.

The fracking is a controversial activity, many unknowns are floating around the subject, the financial interests are very high and many engineers are for it; no wonder somebody disliked my initial / top question :-) ))) [see above]

The following paper may be relevant to your question:

Novel Techniques for Safer Operation of Present Nuclear Power Plants After Fukushima Disaster

Parviz Parvin, Maryam Ilchi-Ghazaani, Ali Bavali, Vajiheh Daneshafrooz, Seyedeh Z. Mortazavi, Alireza Moosakhani, Akbar Nazari-Golshan, Mohammad M. Hashemi, Samaneh S. Hosseini, Behzad Mansouri and Shahryar Abachi

MENG, 2013 01/2013; 6(1):58-74.

I am not an expert in geophysical exploration techniques but I think one should identify the first order effect on seismic risk of techniques such as fracking. The main concern is about release of accumulated stress in an uncontrolled way. For example, when we extract/inject fluids in a rock volume where seismic energy has accumulated due to tectonic processes, we could modify the force balance of this system and help generate an earthquake. In this example the potential for the earthquake is already there but we are shortening the load-release cycle by decreasing friction forces. The concern is that we don't have the control to modulate this releas of seismic energy- hopefully it will be gradual but that is not guaranteed.

I like Gunther Kletetschka's linking of earthquakes to the release of stored strain energy and his pointing out that release of that energy means that strain energy accumulation at the focus of the event begins anew.

There is quite a bit of concern about hydrofracking causing seismicity. Following on Gunther's explanation strain energy builds up in the rock and does concentrate along planes of weakness, such as faults and fractures. That is completely independent of hydrofracking. When the accumulated strain energy along any fault or fracture exceeds the shear strength in the rock along that surface at that point, or in that area, the fault is activated and an earthquake results.

As was learned during liquid waste disposal many years ago in Colorado, USA, injection of the liquid waste caused earthquakes. It was then realized that the faults were being lubricated by the liquid, thereby facilitating slippage. If, as Guther implied, there were no stress build-up caused by some other phenomenon, tectonics, there would not be any slippage therefore no quakes.

To Stephen Monna I am not aware of any moderate or larger magnitude earthquakes having been triggered by fracking. To the best of my knowledge the earthquakes are generally in the range of magnitude 2 or smaller, but if anyone does know of larger ones, explicitly triggered by fracking or reservoir ponding, would you please let me know?

Hi Joe Wallach, I have found a very recent paper published on Science by Bill Ellsworth: "Injection-Induced Earthquakes" (M=>3).

http://www.sciencemag.org/content/341/6142/1225942

Stephen,

Thank you very much. I shall try to download it and look at it. It should be noted that an induced earthquake of magnitudes larger than I cited could possibly occur if, for some reason, sufficient shear strain energy were already stored in the rock, prior to the strain-liberating activity, such as hydrofracking.

Thanks again, Stephen

Joe

A couple things to add that I don't think have been covered.

Extraction induced earthquakes - The referenced paper by Segall can be used (and I think he mentions it in the paper as well) to show that under typical rock conditions, increasing pore pressure makes faults more likely to slip. But there are some conditions (high poroelastic constant, low Poisson's ratio) under which decreasing pore pressure makes faults more likely to slip. Interestingly, I work with a seismologist at UT who postulates that in the Eagle Ford oil play, there are possibly some earthquakes induced by extraction. THis kind of analysis is idealized in order to result in analytical expressions. Another factor that needs to be taken into account in real cases is the effect of finite size reservoirs and deformation induced by gradients in pressure and deformation at the periphery of the zone of extraction.

Injection-induced earthquakes - If you look at the data published by Warpinski et al. 2012 (SPE paper #151597), most hydraulic fracturing related events are magnitude 0 or less. There are a few events (very few) that have been larger. Water injection for waste disposal is the main culprit in most people's minds, and there is evidence of spatial correlation between earthquakes swarms and waste injection wells in several plays in the US. There are frequent earthquakes occurring in North Texas and Oklahoma (as large as 3.5, but mostly less then 3) that are causing quite a stir with the public currently.

Replacing extracted gas with water - Adrian mentioned this, but this does not happen in shale gas. Waterflooding (injecting water to enhance production or maintain pressure) is a typical operation done in higher permeability oil reservoirs. It has been mentioned as a possible cause of induced earthquakes in West Texas. But there is no injection of water into shale gas reservoir beyond the hydraulic fracture treatment.

Hydraulic Fracturing has been in use since 1947 or 1949 ?

It is currently in use in about 80% of wells to create micro fractures up to

100 feet from the horizontal drill hole, with the result being that more gas,

or oil is extracted. The result is far lower unit costs for oil and gas because

more product is extracted per hole drilled.

The location of the layer being fracked. might be 7,000 feet down.

The seismicity risk associated with oil and gas activities is more commonly thought to relate to waste water injection, rather than the process of fracking. One of the better studied events of induced seismicity is the Rocky Mountain Arsenal events that occurred outside of Denver, Colorado, in the 1950's and 60's. High volume waste water injection is believed to have locally elevated pore pressures, which allowed small faults to slip. A similar mechanism is believed to have triggered the recent Youngstown, Ohio, seismic events. With that said, seismicity occurs on geologic time scales. A seismically inactive area that suddenly starts suffering earthquakes coincident with oil and operations can be just that - a coincidence. Clusters of small earthquakes occur in the upper Midwest of the U.S. from time to time, well away from any oil and gas operations.

As stated above, it is the injection of waste water into deep wells which is the cause of the induced earthquakes currently being associated with hydrofracturing, not the hydrofracturing process itself. Deep well injection has long been known to have the potential to induce earthquakes if not properly sited and controlled. The USGS has just released its report on induced seismicity related to hydrofracturing. I also recommend that you consult the resources listed in the Induced Earthquake Bibliography.

http://www.inducedearthquake.com/

http://www.usgs.gov/newsroom/article.asp?ID=4202#.VT0jt5PrTif