We explore the complex interaction of coupled thermal, hydraulic, mechanical and chemical (THMC) processes that influence the evolution of EGS reservoirs in general, and in particular with reference to strong, low-permeability reservoirs with or without relic fracturing. We define and describe dominant behaviors that evolve with the evolution of the reservoir: from short-term stimulation through mid-term production and culminating in long-term decline. These include short-term response where effective stresses and thermal quenching dominate the behavior of the reservoir and are influenced by the local structure in the rock at the scale of a few meters and in particular the form and orientation of pre-existing fractures. Typical behaviors include the reduction of local mean stresses and the development of shear fracturing principally on pre-existing fractures but also the creation of fresh fractures and new reactive and heat-transfer surface area. Continuum models are useful analogs to represent the principal features of this intermediate-term production response. Reaction fronts may propagate through the reservoir and impact the evolution of permeability in surprising ways when considered together with the influence of the effective and thermal stress state. Finally, the long-term decline of the reservoir may be observed as flow-rates may build and the potential for the development of fluid and thermal short-circuiting as pathways grow. Finally, we apply a model with static-dynamic frictional strength-drop to evaluate the rate and severity of triggered seismicity. The changing stress state is calculated from the pore pressure, thermal drawdown and chemical effects in a coupled THMC model with dual porosity. In the rate-state friction model, we vary crack length from 1m to 1000m and examine the intrinsic scaling of energy release. Energy release increases with the cube of crack length, the square of stress drop and linearly with rock mass stiffness. Seismic activity is concentrated around the near-wellbore injection region. It is earliest for closely spaced fractures in reservoir rocks where the thermal drawdown of stress is largest at early times but results in numerous low-magnitude events. For closely spaced fractures (~0.1 m) near-injection failure develops in the short term (<1 month) and for more widely-spaced fractures (~10 m) it is delayed (>7 years) and pushed further out into the reservoir. Changes in energy release generate moment magnitudes which vary from -2 to 2 for small to large fractures. These observations are used to define the evolution of spatial seismicity within the reservoir and its migration with production, dependent on the mobilization of relic fractures. To reduce the energy release of single large pre-existing fractures we explore the role of thermally-induced micro-fractures as a mechanism to reduce stored strain energy. By allowing the development of micro-fractures in the system, more accumulated energy and deformation is released aseismically, thereby reducing the number of large events. These models are used to define the evolution of seismicity with the progress of stimulation and then production within the reservoir.
There is empirical evidence that within EGS reservoirs, injection/production-induced poroelastic stresses may promote frictional sliding on pre-existing faults with large seismic events tending to take place on developed or active faults (Majer et al., 2007; Segall, 1989).