Roles and timing of onset of various thermal-hydrologic-mechanical-chemical process couplings in EGS reservoirs

A numerical simulator is used to model the thermal-hydrologic-mechanical- chemical processes in a dual-porosity EGS reservoir. Specifically, the model is utilized to examine some of the dominant behaviors and permeability-altering mechanisms that may operate in naturally fractured media. Permeability and porosity are modified as fracture apertures dilate or contract under the influence of pressure solution, thermo-hydro-mechanical compaction/dilation, and mineral precipitation/dissolution in a deformable medium. Simulations focus on a prototypical enhanced geothermal system to examine the relative importance of hydro-mechanical vs. thermo-mechanical vs. chemical changes in fluid transmission as cold (70°C) water is injected at geochemical disequilibrium within a heated reservoir (275°C). For an injection-withdrawal doublet separated by ∼670m, the results demonstrate the strong influence of mechanical effects in the short term (several days), the influence of thermal effects in the intermediate term (<1 month at injection), and the prolonged and long-term (>1 year) influence of chemical effects, especially close to injection. In most of the reservoir, cooling enhances permeability and increases fluid circulation under pressure-drive. Thermo-mechanical driven permeability enhancement is observed in advance of the thermal sweep, counteracted by the re-precipitation of minerals previously dissolved into the cool injection water. Near injection, calcite dissolution is capable of increasing permeability by nearly an order of magnitude, while precipitation of amorphous silica onsets more slowly and can completely counteract this increase over the very long term (>10 years). For the reinjection of highly-silica-saturated water, amorphous silica is capable of drastic reduction in permeability close to the injection well. With combined action from all mechanisms, permeability change varies by two orders of magnitude between injection and withdrawal. Further results illustrate the importance of the coupling between reactive transport and geomechanics. Mineral behaviors alter fluid flow paths and, in so doing, change the characteristics of thermo-hydro-mechanical aperture changes, and vice versa. We show how each incurs changes in the system that fundamentally alter the evolutionary paths of reaction and chemical/mechanical deformation in a manner that mandates the accommodation of process couplings for the full THMC suite of interactions.