Analysis of fluid injection‐induced fault reactivation and seismic slip in geothermal reservoirs

Abstract

We explore the issue of fault reactivation induced in enhanced geothermal systems by fluid injection. Specifically, we investigate the role of late stage activation by thermal drawdown. A Thermal-Hydrological-Mechanical simulator incorporating a ubiquitous joint constitutive model is used to systematically simulate the seismic slip of an embedded critically stressed strike-slip fault. We examine the effects of both pore pressure perturbation and thermal shrinkage stress on the magnitude of the resulting events and timing. We analyze the sensitivity of event magnitude and timing to changes in the permeability of the fault and fractured host, fracture spacing, injection temperature, and fault stress obliquity. From this we determine that (1) the fault permeability does not affect the timing of the events nor their size, since fluid transmission and cooling rate are controlled by the permeability of the host formation. (2) When the fractured medium permeability is reduced (from 10 À13 to 10 À16 m 2 ), the timing of the event is proportionately delayed (by a corresponding 3 orders of magnitude). (3) Injection temperature only influences the magnitude but not the timing of the secondary thermal event. The larger the temperature differences between that of the injected fluid and the ambient rock, the larger the magnitude of the secondary slip event. (4) For equivalent permeabilities, changing the fracture spacing (10 m-50 m-100 m) primarily influences the rate of heat energy transfer and thermal drawdown within the reservoir. Smaller spacing between fractures results in more rapid thermal recovery but does not significantly influence the timing of the secondary thermal rupture.

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

“…This present work is a continuation of prior characterizations that define the potential for late‐stage fault reactivation in geothermal reservoirs [ Gan and Elsworth , ]. This extension is to codify the likelihood, timing, and magnitude of potential events as a function of fracture geometry and fluid transmission characteristics and applied flow rates.…”

A Review of Numerical Simulation Strategies for Hydraulic Fracturing, Natural Fracture Reactivation and Induced Microseismicity Prediction

“…Different studies dealt with the effect of the geometrical and mechanical nature of the fault on the reliability of simulation results. Cappa,and Rutquist [78], Rutqvist, et al [79] and Gan and Elsworth [80], developed an iteratively coupled model where planar fractures were defined in a geomechanical grid as weakness planes but they were not set up to account for the mechanical heterogeneous nature of the fracture planes.…”

“…The iteratively-coupled scheme was also adopted for developing a coupled geothermal model where the permeability of the fault was kept constant during fault deformation, and micro-seismic events were extrapolated using results from the 2D plane strain model in FLAC2D [80]. Fig.…”

“…Fig. (10) shows the process flow-chart used by Gan and Elsworth [80] to develop the coupled TOUGHREACT-FLAC3D model. Fig.…”

Fault zone exploitation in geothermal reservoirs: Production optimization, permeability evolution and induced seismicity

“…The issue of induced seismicity in enhanced geothermal reservoirs is of great concern both to the scientific and political community (Ellsworth, 2013). Cold fluid injection in enhanced geothermal reservoirs is critical in evaluating the fault reactivation and induced seismicity (Cappa and Rutqvist, 2010;Gan and Elsworth, 2014a). Injection of large amount of water into geothermal reservoirs increases the pore pressure and the potential in causing failure on fault planes (Hubert and Rubey, 1959;Cappa and Rutqvist, 2010;Ellsworth, 2013;Levandowski et al, 2018;Scholz, 2019).…”