Seismic and aseismic deformations and impact on reservoir permeability: The case of EGS stimulation at The Geysers, California, USA

Abstract

In this paper, we use the Seismicity‐Based Reservoir Characterization approach to study the spatiotemporal dynamics of an injection‐induced microseismic cloud, monitored during the stimulation of an enhanced geothermal system, and associated with the Northwest Geysers Enhanced Geothermal System (EGS) Demonstration project (California). We identified the development of a seismically quiet domain around the injection well surrounded by a seismically active domain. Then we compare these observations with the results of 3‐D Thermo‐Hydro‐Mechanical simulations of the EGS, which accounts for changes in permeability as a function of the effective normal stress and the plastic strain. The results of our modeling show that (1) the aseismic domain is caused by both the presence of the injected cold water and by thermal processes. These thermal processes cause a cooling‐stress reduction, which prevent shear reactivation and favors fracture opening by reducing effective normal stress and locally increasing the permeability. This process is accompanied by aseismic plastic shear strain. (2) In the seismic domain, microseismicity is caused by the reactivation of the preexisting fractures, resulting from an increase in injection‐induced pore pressure. Our modeling indicates that in this domain, permeability evolves according to the effective normal stress acting on the shear zones, whereas shearing of preexisting fractures may have a low impact on permeability. We attribute this lack of permeability gain to the fact that the initial permeabilities of these preexisting fractures are already high (up to 2 orders of magnitude higher than the host rock) and may already be fully dilated by past tectonic straining.

Long-term thermal effects on injectivity evolution during CO2 storage

“…Data from several injection sites will be required to generalize the actual amount of injectivity increase induced by cooling, but this study suggests that there is potential to save a significant amount of compression energy all over the duration of injection projects. Similar observations of injectivity enhancement have been observed in fractured geothermal reservoirs as a result of strong cooling (e.g., Koh et al, 2011;Jeanne et al, 2015). Overall, our simulation results show that cooling has the potential to enhance injectivity in fractured reservoirs.…”

Seismic Response to Injection Well Stimulation in a High‐Temperature, High‐Permeability Reservoir

“…We assume L = 1 × 10 −5 K −1 (Bauer & Handin, 1983), E = 20 × 10 9 Pa (Cant et al, 2018), = 0.25, and a temperature difference of 204 • C (interpreted from pressure-temperature spinner data at feed zone depth during stimulation). However, the lack of seismicity suggests that such cooling effects may have stabilized the fracture network, consistent with preferential reduction of 1 (≈ V ) due to density-driven, downward flow of the cold injectate (as detailed by Jeanne et al, 2015). In the future, numerical modeling of the thermoporoelastic problem should be undertaken in order to characterize the effect of thermal contraction on the stability of the fracture network and to determine the utility of using hypocentral locations to estimate the extent of the permeable reservoir (e.g., Ghassemi et al, 2007;Riffault et al, 2018).…”

“…This relationship has led other groups to develop seismicity-based reservoir characterization (SBRC) to infer diffusivity from the location and occurrence time of induced seismicity (e.g., Jeanne et al, 2015;Parotidis et al, 2004;Shapiro & Dinske, 2009;Shapiro et al, 2002). The advance of the induced pressure front is controlled by the hydraulic diffusivity of the reservoir (also referred to as transmissivity, the permeability thickness product: kh), which is closely linked to the reservoir permeability.…”

“…During a stimulation operation at the naturally fractured Geysers geothermal field, Jeanne et al (2015) modeled a similar thermal front propagating >100 m in the span of 2 months. During a stimulation operation at the naturally fractured Geysers geothermal field, Jeanne et al (2015) modeled a similar thermal front propagating >100 m in the span of 2 months.…”

The seismo-hydromechanical behavior during deep geothermal reservoir stimulations: open questions tackled in a decameter-scale in situ stimulation experiment

“…Various authors also explored the vast induced seismicity dataset of > 500 000 events recorded since the 1960s at the Geysers geothermal site, where recently also an EGS demonstration stimulation has been performed (Garcia et al, 2012;Jeanne et al, 2014). The observed seismicity was partly related to injections (Jeanne et al, 2015) and thermoelastic stress changes (Rutqvist and Oldenburg, 2008). Here, local variability in the stress field (Martínez-Garzón et al, 2013) and volumetric source components (Martínez-Garzón et al, 2017) were inferred from detailed analysis of injection-induced seismicity.…”