Geomechanical simulation of the stress tensor rotation caused by injection of cold water in a deep geothermal reservoir

Abstract

We present a three‐dimensional thermohydromechanical numerical study of the evolution and distribution of the stress tensor within the northwest part of The Geysers geothermal reservoir (in California), including a detailed study of the region around one injection well from 2003 to 2012. Initially, after imposing a normal faulting stress regime, we calculated local changes in the stress regime around injection wells. Our results were compared with previously published studies in which the stress state was inferred from inverting the focal plane mechanism of seismic events. Our main finding is that changes in stress tensor orientation are caused by injection‐induced progressive cooling of the reservoir, as well as by the seasonal variations in injection rate. Because of the gravity flow and cooling around a liquid zone formed by the injection, the vertical stress reduction is larger and propagates far below the injection well. At the same time, the horizontal stress increases, mostly because of stress redistribution below and above the cooling area. These two phenomena cause the rotation of the stress tensor and the appearance of a strike‐slip regime above, inside, and below the cooling area. The cooling and the associated rotation of the stress regime can play a significant role in the observed long‐term deepening of the microseismicity below active injection wells.

Insights Into Complex Subdecimeter Fracturing Processes Occurring During a Water Injection Experiment at Depth in Äspö Hard Rock Laboratory, Sweden

“…However, during injection and after shut-in faulting regimes alternate between strike-slip and thrusting due to the close similarity of intermediate and minimum stress magnitudes. At reservoir scale, such changes between faulting regimes due to fluid injection have been previously observed (Dreger et al, 2017;Martínez-Garzón et al, 2013;Schoenball et al, 2014) and were also suggested by modeling (Jeanne et al, 2015;Ziegler et al, 2017).…”

Estimation of the differential stress from the stress rotation angle in low permeable rock

“…The time‐dependent rotations of the principal stress axes correlate well with the variations in the monthly injection rates into the reservoir from both nearby wells (Figure a) [ Martínez‐Garzón et al , , ]. The relationship between fluid injection and stress rotation has recently been observed in another part of The Geysers field [ Dreger et al , ] and was also confirmed by geomechanical‐numerical modeling calibrated on The Geysers scenario [ Jeanne et al , ].…”

“…More recently, systematic temporal stress rotations have also been observed in reservoirs in relation to massive fluid injection [ Martínez‐Garzón et al , , ; Schoenball et al , ]. These stress rotations have been shown to correlate with fluid injection rates in accordance with pore pressure changes (Figure ) and reduced in situ temperatures by the cold fluid [ Jeanne et al , ; Yoon et al , ]. In addition to these local perturbations, stress rotations depend mainly on the initial background stress [ Sonder , ; Zoback , ].…”

Pore pressure behavior at the shut‐in phase and causality of large induced seismicity at Basel, Switzerland

“…In addition to an increase in pore pressure, there are several other phenomena that can cause shear slip on an existing fracture, such as local stress change due to shear slip of preceding events (so-called Coulomb stress change) [Mukuhira et al, 2012;Catalli et al, 2013], stress change caused by a thermal effect [Ghassemi and Zhou, 2011;Jeanne et al, 2015;Kwiatek et al, 2015], and change in the friction coefficient by chemical reaction on the fracture surface [Taron et al, 2009]. However, the contribution of these phenomena to triggering microseismicity was not considered here.…”