Coupled mechanical and chemical processes in engineered geothermal reservoirs with dynamic permeability

A model is developed to represent mechanical strain, stress-enhanced dissolution, and shear dilation as innately hysteretic and interlinked processes in rough contacting fractures. The model is incorporated into a numerical simulator designed to examine permeability change and thermal exchange in chemically active and deformable fractured reservoirs. A candidate engineered geothermal reservoir system (EGS) is targeted. The mechanistic model is able to distinguish differences between the evolution of fluid transmission characteristics of (1) small scale, closely spaced fractures, and (2) large-scale, more widely spaced fractures. Alternate realizations of fracture frequency and scale, exhibiting identical initial bulk permeability, lead to significantly different conclusions regarding permeability evolution and thermal drawdown within the reservoir. Reactivation, primarily through mechanical shear, of pervasive, large-scale fractures is shown capable of causing both hydraulic and thermal short circuiting. Small variations in fracture scale impact the balance between the efficiency of thermal transfer and the rate of fluid circulation. Stress-enhanced chemical dissolution, initially at equilibrium within the reservoir, may be reactivated as fractures are forced out of equilibrium during hydraulic fracturing. At the conditions examined (250. °C reservoir with 70. °C injection), however, shear dilation exerts dominant control over changes to permeability. Heterogeneity in permeability, generated from a normal distribution of fracture spacing, impacts thermal breakthrough times at the withdrawal well, as well as withdrawal rates. For the given conditions, spatial variability over ~1 order of magnitude leads to a reduction of ~10% in withdrawal rates compared to a spatially uniform system. Permeability is a strongly dynamic property and at geothermal conditions is influenced by the full suite of THMC interactions.