Design of CO2-Based Geothermal Power Plant for Transient Reservoir Dry-out – or ‘Enhanced Water Recovery\'

Abstract

Carbon-dioxide-based engineered geothermal systems provide an opportunity to combine CO2 storage with geothermal power generation. They accomplish this by displacing the water initially present in the reservoir with supercritical CO2, which has been shown, for some reservoir system parameters, to also enhance overall geothermal energy extraction. However, one complexity of this concept is the transition period, during which substantial quantities of both water and carbon dioxide are simultaneously present in the reservoir, and which is poorly understood in its effect on energy extraction rates. Here we assess two aspects of the energy flows during the transition period. Firstly, we address the energy flow impact of residual reservoir water on CO2-rich production flows. We find that water content in the CO2-rich phase may enhance total energy extraction for high temperature reservoirs by a mechanism of in-reservoir evaporation combined with condensation in surface plant heat exchangers. Secondly, we consider the energy production rates as CO2 displaces water from the reservoir for a number of cases of different modes – plug flow, mixed, and combination – of reservoir flow. We find that initial displacement of water by carbon dioxide in the near-injection-well region substantially reduces the total flow resistance of the geothermal reservoir, leading to higher production flow-rates for the same in-reservoir pressure gradient. This enables much high energy extraction rates than are possible in water-based injection-driven geothermal systems. This finding is important as it implies that the largest geothermal energy extraction rates are achievable by a hybrid ‘enhanced water recovery’ design, in which injected CO2 provides a gas cap to supply the pressures necessary to sustain large production flows of brine.