Geothermal energy production can be limited by insufficient and/or inadequate working fluid and by pressure depletion, whereas geologic CO2 storage (GCS) can be limited by overpressure, which drives the risks of CO2 leakage and induced seismicity. We investigate how synergistic integration of these complementary systems may be of mutual benefit, enhancing the viability of each system. We analyze two approaches for utilizing CO2 to enhance geothermal energy production in saline aquifers, located in a stack of permeable sedimentary layers, separated by impermeable layers. The first approach emphasizes utilizing CO2 as an efficient working fluid because of its advantageous thermo-physical properties—namely the low viscosity and thermosyphon effect of CO2—which reduce the parasitic power consumption of the working-fluid recirculation system. This approach was first suggested for enhanced geothermal systems (EGS) in crystalline rock, and then later extended to sedimentary formations, in a concept called CO2 plume geothermal (CPG). Regardless of geologic setting, this approach uses small injector/producer spacing to promote early CO2 breakthrough and recirculation, thereby maximizing the heat-extraction benefit per tonne of delivered CO2. The second approach emphasizes utilizing supercritical CO2 injection as a means of providing pressure support for geothermal production wells, rather than utilizing CO2 as a working fluid. The second approach uses large injector/producer spacing to delay CO2 breakthrough at the production wells, thereby avoiding operational issues of co-producing CO2 with formation brine. We conduct reservoir analyses of both approaches. For the first approach, we address operational issues associated with the co-production of brine, and suggest potential brine-management strategies. Note that early studies of the use of CO2 as a working fluid have not addressed brine production, or its operational implications. For the second approach, we consider a range of pressure-management strategies to address concerns about the risks driven by overpressure. We present results of a generic well pattern consisting of four concentric zones of wells in the primary storage aquifer: (1) inner producers, (2) CO2 injectors, (3) brine injectors, and (4) outer brine producers; and two zones (3 and 4) in the overlying saline aquifer. This well pattern manipulates the CO2 plume by creating a hydraulic ridge and cap to suppress CO2 migration and leakage, and creates an outer hydraulic trough to nullify the hydraulic ridge, thereby isolating the geothermal-GCS operation from neighboring subsurface activities. In addition to reducing the risks associated with overpressure, this generic well pattern has the potential to yield a high heat-extraction benefit per tonne of delivered CO2, which improves the viability of CO2-enabled geothermal energy production. This work was performed under the auspices of the U.S. Department of Energy by LLNL under contract DE-AC52-07NA27344.

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