Stress field respond to massive injection of cold water into a geothermal reservoir study by geomechanical simulation

Abstract:

In this paper, we study the evolution and distribution of the stress tensor within the northwest part of The Geysers geothermal field during 9 years of injection (from 2003 to 2012). Based on a refined 3D structural model, developed by Calpine Corporation, where the horizon surfaces are mapped, we use the GMS™ GUI to construct a realistic three-dimensional geologic model of the Northwest Geysers geothermal field. This model includes a low permeability graywacke layer that forms the caprock for the reservoir, an isothermal steam zone (the Normal Temperature Reservoir) within metagraywacke, a hornfels zone (the High Temperature Reservoir), and a felsite layer that is assumed to extend downward to the magmatic heat source. This model is mapped into a rectangular grid for use with the TOUGH-FLAC numerical simulator. Then, we reproduce the injection history of seven active wells between 2003 and 2012. Finally, our results are compared with previous works where the stress tensor was studied from the inversion of focal plane mechanism in the same area and during the same period. As in these publications we find that: (1) changes in the orientation of principal horizontal stress are very small after one decade of injection, and (2) at injection depth significant rotations of the initially vertically oriented maximum compressive principal stress occur in response to changes in the fluid injection rates. As observed in the field, we found that σ₁ tilted towards the σ₂ direction by approximately 15° when injection rates were at their peak level. Such a rotation consequently results in a local change in the state stress from a normal stress regime (S_{v >} S_{Hmax> >} S_hmin) to a strike slip regime (S_Hmax> S_{v > >} S_hmin) above and below the injection zone. Our results show that thermal processes are the principal cause for the stress tensor rotation.