Effect of Reservoir-Caprock Interface Dip and Circulation of Produced Fluid on CO2-Based Geothermal Heat Extraction from Saline Aquifers

Abstract:

CO₂-Plume Geothermal (CPG) energy utilization involves injection of CO₂ as a working fluid to extract heat from naturally high permeability geologic units. The injected CO₂ forms a large subsurface CO₂ plume that absorbs heat from the geothermal reservoir and eventually buoyantly rises to the surface. The CO₂ plume can be “tapped” for thermal and/or electric power production in a geothermal power system. In actual systems, the CO₂ plume would likely be skewed opposite any likely dip direction of the reservoir-caprock interface. Here, we numerically analyze the characteristics of CO₂ plume formation and geothermal heat extraction from geothermal reservoirs with dip. We find that the heat extraction rate and the total amount of heat extracted over time is the same for symmetric and skewed CO₂ plume systems when the circular, horizontal production well is arranged according to the CO₂ plume distribution around the injection well. We also conduct simulations of CO₂ plume formation within a pre-existing groundwater flow field and find that groundwater flow is not capable of skewing the CO₂ plume. Furthermore, we investigate the effects of reinjecting small amounts of brine that are produced with the CO₂. Brine has a smaller mobility than supercritical CO₂ at a given temperature and thus accumulates near the injection well. Such brine accumulation reduces the relative permeability for the CO₂ phase, which in turn increases the pore-fluid pressure around the injection well. For this reason, and as injection of two fluid phases is problematic, we recommend removal of any brine from the produced fluid before the cooled CO₂ is reinjected into the reservoir. Separated brine may be reinjected into the formation away from the CO₂ plume, providing an additional means of controlling and directing the CO₂ plume pressure field and flow direction and avoiding the need to treat and dispose of the CO₂ near the land surface. In summary, we show that the geothermal heat extraction characteristics from saline aquifers is minimally affected by details of whether the CO₂ is symmetrically distributed around the injection well or skewed, as long as production well geometries take this into account and that reinjection of produced fluid without removal of any brine is not recommended to avoid pressurization of the reservoir.