Evolution of permeability and triggered seismicity: Fluid pressure, thermal and chemical effects in enhanced geothermal systems

We use a continuum model of reservoir evolution subject to coupled THMC processes to explore the evolution of production-induced seismicity in a prototypical EGS reservoir. The model is capable of accommodating changes in stress that result from early-time changes in effective stress, mid-time changes in thermal stresses and ultimately to incorporate long-term changes due to chemical effects. We develop a micromechanical model to represent the failure process. We apply this model to represent energy release from individual critically oriented fractures and show that the energy release rate, timing and magnitude is conditioned by fracture shear-strength stress-drop but is insensitive to rate-state response. We apply a model with simple static-dynamic frictional strength-drop to determine energy release for cracks of different size embedded in an elastic medium. The changing stress state is calculated from the pore pressure, thermal drawdown and chemical effects in a coupled THMC model with dual porosity. This model is applied to the doublet geometry representative of the Coso geothermal field. 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.