MEchanics GAthering -MEGA- Seminar: Stress preconditioning in Enhanced Geothermal Systems

Geothermal energy is a renewable source of energy that would be capable of providing baseload power to large parts of the world were it not for a few key issues. Notably, Enhanced Geothermal Systems (EGSs), which could pave the way for geothermal electricity production even in regions with moderately low geothermal gradients, still struggle with their tendency to induce damaging seismicity when stimulated. It is therefore essential that methodologies are developed that mitigate the seismic risk associated with EGS.

Unfortunately, the mechanism by which stimulation occurs in many EGS stimulations is essentially the same as that in induced earthquakes, shear failure. Recently there has been an increased focus on soft stimulation, whereby an effort is made to encourage the shear failure required for reservoir stimulation and avoid the shear failure associated with large magnitude induced seismicity; however, so far these techniques have been unable to prevent large earthquakes. In this sense, a stimulation technique which lends itself to the induction of small seismic events and not large ones would represent a major step forward for the EGS industry.

It is suggested here that it is possible to design a stimulation treatment that begins with a long period of injection-induced temperature change with the goal of reducing the differential stress in the reservoir, preconditioning the stress. During this stress-preconditioning phase, the pore pressure increase is limited such that shear failure is avoided. Then, after this period of temperature change, a short period of high rate injection occurs with the goal of increasing the pore pressure. This second phase induces shear failure on the optimally oriented faults/fractures in the reservoir. Importantly, the shear failure that occurs on faults/fractures when following this methodology occurs on shear planes that are supporting less differential stress than they would have been had the reservoir been stimulated without the first phase of temperature change. The advantage of maintaining a low differential stress comes from the connection seen between differential stress and the Gutenberg– Richter b-value. This connection implies that, by maintaining a low differential stress, a high b-value will be seen during stimulation. A high b-value results in more low magnitude seismic events and relatively few large magnitude events that might pose a nuisance to the public, or even cause damage. In this way, EGS reservoir stimulation can still be performed, inducing shear failure on pre-existing planes of weakness, with a lower risk of inducing large events. The development and potential implications of this stimulation technique will be addressed here.