Degradation of the mechanical properties imaged by seismic tomography during an EGS creation at The Geysers (California) and geomechanical modeling

Abstract

Using coupled thermal-hydro-mechanical (THM) modeling, we evaluated new seismic tomography results associated with stimulation injection at an EGS demonstration project at the northwest Geysers geothermal steam field, California. We studied high resolution seismic tomography images built from data recorded during three time periods: a period of two months prior to injection and during two consecutive one month periods after injection started in October 2011. Our analysis shows that seismic velocity decreases in areas of most intense induced microseismicity and this is also correlated with the spatial distribution of calculated steam pressure changes. A detailed analysis showed that shear wave velocity decreases with pressure in areas where pressure is sufficiently high to cause shear reactivation of pre-existing fractures. The analysis also indicates that cooling in a liquid zone around the injection well contributes to reduced shear wave velocity. A trend of reducing compressional wave velocity with fluid pressure was also found, but at pressures much above the pressure required for shear reactivation. We attribute the reduction in shear wave velocity to softening in the rock mass shear modulus associated with shear dislocations and associated changes in fracture surface properties. Also, as the rock mass become more fractured and more deformable this favors

The Northwest Geysers EGS Demonstration Project, California – Part 2: Modeling and interpretation

“…Moreover, as shown in Fig. 19, Jeanne et al (2015a) found a general trend of decreasing V P and V S with calculated steam pressure. V S decreases with pressure in areas where pressure is sufficiently high to cause shear failure, which in the field would correspond to areas where shear reactivation of pre-existing fractures and high microseismic density occur.…”

“…Coupled THM modeling in Jeanne et al (2015a) showed that seismic V P and V S velocities decrease in areas with the most intense microseismicity; this is also correlated with the spatial distribution of calculated steam-pressure changes. This correlation can be observed by comparing Vp and Vs distribution in Fig.…”

“…The use of seismic tomography was tested at the Northwest Geysers EGS Demonstration and was also interpreted by coupled THM modeling in Jeanne et al (2015a). High resolution seismic tomography was produced using a dense surface seismic monitoring network, including an addition of 15 temporary three-component seismic stations inside a "focused array" around the EGS area, to collect specialized data during the start-up of the stimulation (from one month before to two months after the beginning of the injection).…”

“…To this end, Lawrence Berkeley National Laboratory developed an inexpensive, automated microseismic data collection and processing system (and computational capability) necessary to record and process large numbers of microseismic recordings-and obtain tomographic images at lower cost and less time than was previously possible (Hutchings et al, 2011). The interpretive modeling of the seismic tomography data in Jeanne et al (2015a) aimed at developing the link between the seismic tomography and the coupled THM responses in the rock mass, including the impact of fluid pressure, temperature, effective stress, and shear failure.…”

“…16 and 17 present contours of the V P and V S on a vertical cross section oriented northwest-southeast for the period 1 month before injection, and then for up to 2 months after the start of the injection (Jeanne et al, 2015a). The black points in Fig.…”