MODELING OF CHEMICALLY-INDUCED FRACTURE PERMEABILITY EVOLUTION AND ITS IMPACT ON GEOTHERMAL ENERGY PRODUCTION

Fluid-rock chemical interactions may potentially modify fracture permeability, and therefore influence long-term performance of enhanced geothermal system (EGS) reservoirs. Coupled fluid flow, heat transfer and reactive transport processes in fractured geothermal formations are controlled by the interplay between mineral reaction kinetics, fracture flow and matrix diffusion, leading to increase or decrease in fracture permeability, and hence flow magnitude through the fracture. Understanding and quantifying processes that affect fracture permeability evolution are critical for prediction of geothermal energy production, and may provide the key to sustainable productivity. In order to improve understanding of fluid-rock chemical interactions in fractured rocks we develop a discrete fracture model (DFM) in the GEOS framework (a LLNL-developed, massively parallel and multi-physics simulation code) to simulate fracture permeability evolution due to mineral dissolution/precipitation under geothermal reservoir conditions. In this study we apply the discrete fracture model to simulate the thermal-hydrologic-chemical (THC) behaviors of fractured geothermal reservoirs, and in particular examine effects of quartz dissolution/precipitation on overall heat recovery. Numerical results show that the strong temperature dependence of quartz reactivity and equilibrium concentration can significantly influence fracture permeability evolution. It is also found that when the injection fluid is supersaturated with respect to quartz, precipitation occurs near the injection well, resulting in a reduction in fracture aperture and energy recovery. However, if a quartz-undersaturated fluid is injected into fractured formations, dissolution dominates along fractures, and enhances fracture permeability and causes more efficient energy recovery process. Insight gained from this work will not only help develop a more accurate description of chemically-induced fracture evolution in fractured geothermal reservoirs, but also provide a useful basis for upscaling flow and reactive transport behaviors from discrete fracture to field scales. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.