Thermal drawdown and late‐stage seismic‐slip fault reactivation in enhanced geothermal reservoirs

Abstract

Late-stage seismic slip in geothermal reservoirs has been shown as a potential mechanism for inducing seismic events of magnitudes to~2.6 as late as two decades into production. We investigate the propagation of fluid pressures and thermal stresses in a prototypical geothermal reservoir containing a centrally located critically stressed fault from a doublet injector and withdrawal well to define the likelihood, timing, and magnitude of events triggered by both fluid pressures and thermal stresses. We define two bounding modes of fluid production from the reservoir. For injection at a given temperature, these bounding modes relate to either low-or high-relative flow rates. At low relative dimensionless flow rates the pressure pulse travels slowly, the pressure-driven changes in effective stress are muted, but thermal drawdown propagates through the reservoir as a distinct front. This results in the lowest likelihood of pressure-triggered events but the largest likelihood of late-stage thermally triggered events. Conversely, at high relative non-dimensional flow rates the propagating pressure pulse is larger and migrates more quickly through the reservoir but the thermal drawdown is uniform across the reservoir and without the presence of a distinct thermal front, and less capable of triggering late-stage seismicity. We evaluate the uniformity of thermal drawdown as a function of a dimensionless flow rate Q D that scales with fracture spacing s (m), injection rate q (kg/s), and the distance between the injector and the target point. This parameter enables the reservoir characteristics to be connected with the thermal drawdown response around the fault and from that the corresponding magnitude and timing of seismicity to be determined. These results illustrate that the dimensionless temperature gradient adjacent to the fault dT D /dx D is exclusively controlled by the factor Q D . More significantly, this temperature gradient correlates directly with both the likelihood and severity of triggered events, enabling the direct scaling of likely magnitudes and timing to be determined a priori and directly related to the characteristics of the reservoir. This dimensionless scaling facilitates design for an optimum Q D value to yield both significant heat recovery and longevity of geothermal reservoirs while minimizing associated induced seismicity.

Geomechanics of subsurface water withdrawal and injection

“…A hot-fractured-rock project was launched at Cooper Basin, South Australia, in 2002 to exploit the Habanero granite reservoir at a depth of 4000-4500 m. Various stimulation experiments have been conducted which triggered earthquakes with moment magnitude between 1.7 and 3.1 with hypocentral distances between 2.4 and 7.8 km and depth between 3900 and 4500 m [Baisch et al, 2006]. In these cases, thermal drawdown of the rock superpose to the pressure change due to fluid injection causing significant changes in the effective stress regime, which in turn can increase the likelihood of fault reactivation and consequently, induced seismicity [Gan and Elsworth, 2014].…”

“…A reconnaissance study aiming to identify major geological discontinuities is of paramount importance, along with a modeling tool capable of predicting fault/thrust activation resulting from the removal or injection of fluid Gan and Elsworth, 2014;Jha and Juanes, 2014;Teatini et al, 2014]. With the aid of an ad hoc model we can estimate the sliding of the fault/thrust, and hence predict the seismic moment.…”

Thermo‐Hydro‐Mechanical Model and Caprock Deformation Explain the Onset of an Ongoing Seismo‐Volcanic Unrest

“…However, there is no satisfactory numerical model that fully explains the influence of fluid injections on seismic slip at this volcano. We address this gap by building on recent studies that modeled microseismicity in geothermal reservoirs (Gan & Elsworth, 2014a, 2014b; Gan & Lei, 2020) and explained the high‐magnitude seismicity induced, for example, at Geysers geothermal field (Kwiatek et al., 2015).…”

The risks of long-term re-injection in supercritical geothermal systems

“…In this study, we focus exclusively on the prolonged fluid re-injection and assume a successful stimulation of the wells’ surroundings already took place. Although the connection between cooling and reservoir stability is known and numerical studies applied to traditional geothermal systems have highlighted the influence of thermal gradients 45 , fault and rock permeability 46 , stress regime 30 , competition between thermal and hydraulic processes 47 and tensile fracturing effects on permeability 48 , the issue has never been addressed in ESGS systems. Specifically, cooling in ESGS could take on a whole new dimension, as the potential temperature drop and associated thermal stress variation can be very high.…”