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 10⁻¹³ to 10⁻¹⁶ m²), 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.

We present a model coupling stress and fluid flow in a discontinuous fractured mass represented as a continuum by coupling the continuum simulator TF_FLAC^3D with cell-by-cell discontinuum laws for deformation and flow. Both equivalent medium crack stiffness and permeability tensor approaches are employed to characterize pre-existing discrete fractures. The advantage of this approach is that it allows the creation of fracture networks within the reservoir without any dependence on fracture geometry or gridding. The model is validated against thermal depletion around a single stressed fracture embedded within an infinite porous medium that cuts multiple grid blocks. Comparison of the evolution of aperture against the results from other simulators confirms the veracity of the incorporated constitutive model, accommodating stress-dependent aperture under different stress states, including normal closure, shear dilation, and for fracture walls out of contact under tensile loading. An induced thermal unloading effect is apparent under cold injection that yields a larger aperture and permeability than during conditions of isothermal injection. The model is applied to a discrete fracture network to follow the evolution of fracture permeability due to the influence of stress state (mean and deviatoric) and fracture orientation. Normal closure of the fracture system is the dominant mechanism where the mean stress is augmented at constant stress obliquity ratio of 0.65—resulting in a reduction in permeability. Conversely, for varied stress obliquity (0.65–2) shear deformation is the principal mechanism resulting in an increase in permeability. Fractures aligned sub-parallel to the major principal stress are near-critically stressed and have the greatest propensity to slip, dilate and increase permeability. Those normal to direction of the principal stress are compacted and reduce the permeability. These mechanisms increase the anisotropy of permeability in the rock mass. Furthermore, as the network becomes progressively more sparse, the loss of connectivity results in a reduction in permeability with zones of elevated pressure locked close to the injector—with the potential for elevated pressures and elevated levels of induced seismicity.

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