A Sensitivity Analysis for Effective Use of CO₂ for Heat Mining in Geothermal Reservoirs

Carbon dioxide (CO₂) emitted from various sources, mainly fossil fuel power plants, is considered responsible of the global warming effect. Many processes and techniques are still under research for CO₂ capture and sequestration. On the other hand, it is proposed that the geothermal heat be mined from geothermal reservoirs using captured CO₂. In this sense, some theoretical studies show feasibility of using supercritical carbon dioxide (sCO₂) as a heat mining media in such geothermal reservoirs. In this work, it is carried out a set of numerical simulations to determine the most effective distance between injection and production wells for extracting geothermal energy utilizing sCO₂ (Water is used for comparison). In the study, the permeability is considered in the range of 0.5 mD to 3.5 mD, with the aim of determining also the critical point in which sCO₂ works better than water (H₂O) as a working fluid. The remaining properties such as volume, density and other thermal properties remain fixed. Afterwards, it is constructed a numerical model which is implemented in TOUGH2 and PETRASIM 5 software to simulate the cases established. In the model, it is considered a simplified control volume, i.e. only one well for injection and one for production, assuming a constant flow rate at the inlet and at the outlet, meaning that sequestration is not taken into account. A length of 300 meter is defined for reservoir thickness, considering also a pressure and temperature of 100 bar and 200 °C, respectively. The energy mined is estimated for a period of twenty-five years. As typically, the sensitivity analysis is performed by varying only one property and keeping the remaining properties constant, isolating in this way the effect of such variable. Results show that for small permeabilities H₂O works better than sCO₂, but it is possible to assure that for permeabilities greater than 1 mD, sCO₂ presents more advantages as extracting heat media instead of water. Both, H₂O and sCO₂ show a linear behavior. A deep analysis is necessary to carry out, because results shows that sCO₂ works better in an intermediate zone (greater than 200 meter length, but smaller than 800 meter length). An unusual behavior is presented when the distances between the wells are varied; water shows a linear behavior increasing monotonically, while sCO₂ shows a nonlinear behavior for some distances sCO₂ works better. As expected, the more the distance, the greater the amount of the energy mined due to the volume related with each one of the distances.