Induced Seismicity

Risk Prediction

Waste water injection in sedimentary sequences has been seen to cause induced seismicity in the basement rocks below. This seismicity may also pose a threat to potential CO2 sequestration operations. Switzerland, especially, has already had problems with induced seismicity. The most notable case being in Basel in 2006, when magnitude 3+ earthquakes were induced during EGS stimulation and were enough to bring the project to a halt. Clearly, the magnitude 5+ earthquakes seen during waste water injection can therefore also pose a threat. To this end, the chair “Gaz Naturel” performs quantitative seismic risk analysis accounting for the lithology differences between the sedimentary sequences where injection is occurring and the crystalline basement rock below. This involves taking into account things like the tendency for seismic/aseismic slip, varying differential stresses, and an earthquake’s frequency magnitude distribution dependence on the stress state. An example cartoon is shown below of the kind of results that might come from this type of analysis.

Production Induced Seismicity

Production-induced seismicity has been experienced by numerous oil and gas fields all over the world – USA, Canada, The Netherlands, France, Germany, Kuwait, Oman, Iraq, Russia, Uzbekistan etc (see Suckale 2009 for specific examples). As can be seen in the above image, typically normal faulting is seen on the sides of the reservoir and reverse faulting is seen above and below the reservoir. The reason for this can be explained by the poroelasticity model used by Segall 1989. Here, production causes horizontal tensile changes on the sides of the reservoir and horizontal compression changes above and below it. This fits well with the type of faulting typically seen in the field as a reduction in the horizontal total stress can cause normal faulting and an increase in the horizontal total stress can cause reverse faulting (compression is positive in this sign convention).

Here at the Chaire Gaz Naturel, we have investigated this problem. From the momentum balance equation, we can see that gradient of pore pressure is directly responsible for the stress changes experienced in and around the reservoir. This suggests that influencing this pore pressure gradient may allow us to mitigate the stress changes that occur and ultimately lower the induced seismicity rate seen in the field. 

From Darcy’s Law, we know that the permeability exhibits control over the pore pressure gradient required to produce fluid. A higher permeability will require a smaller pore pressure gradient to produce a given amount of fluid, and vice-versa. For this reason we have investigated how processes which influence permeability may influence induce seismicity. 

Hydraulically fracturing a well will result in an increased permeability near the well. Based on the above argumentation, this should reduce the pore pressure gradient required to produce fluid which should then reduce the stresses induced around the reservoir. Less stress changes induced then should result in less seismicity. This is precisely what we investigated in our recent paper, see the reference at the bottom of the page. We found that, indeed, a hydraulic fracture could reduce the induced seismicity rate under the correct circumstances, especially in reverse and strike-slip faulting stress regimes.

Another avenue we are currently investigating is the influence of permeability loss due to reservoir compaction on the induced stresses around a production reservoir. This work is based on similar concepts to that of the hydraulic fracturing work.

B. P. Fryer; G. Siddiqi; L. Laloui : Compaction-induced permeability loss’s effect on induced seismicity during reservoir depletion; Pure and Applied Geophysics. 2019. DOI : 10.1007/s00024-019-02198-0

B. P. Fryer; G. Siddiqi; L. Laloui : Reservoir stimulation’s effect on depletion-induced seismicity; Journal of Geophysical Research: Solid Earth. 2018. DOI : 10.1029/2018JB016009