A Numerical Analysis on the Effects of Using CO2 as a Driving Fluid for a Geothermal Plant

Sammanfattning: To limit the effect of global warming society needs clean energy sources and to reduce emissions of greenhouse gases into the atmosphere. By combining carbon capture and storage technologies together with geothermal power production it is possible to gain fossil free energy while potentially storing CO2 at the same time. Furthermore, it would be possible to reduce the emissions from geothermal sites with high greenhouse gas content in the groundwater by recirculating the retrieved fluid. Many uncertainties remain however about this type of adaptation. To study this, this master thesis investigated how a geothermal system would be effected by comparing a water injection with 10 % CO2 to a pure water injection. A 2D model over a 1000 m times 1000 m horizontal fracture zone with two boreholes spaced 500 m apart and a injection rate of 1 kg/s was simulated using the iTOUGH2 software with the ECO2N module, designed for numerical simulations regarding geohydrological systems. In addition to comparing the two injection scenarios stated above, the effects of changing the injection rate, CO2 concentration in the injection fluid, permeability and surrounding pressure as well as introducing salinity and having an initial CO2 saturated liquid phase were investigated as well. It was concluded that using CO2 in the injection fluid does allow for a large greenhouse gas sequestration while slightly increasing the heat production, but the system experienced a significant pressure increase while doing so. It was suggested that further investigations regarding adjustments of injection rate and concentration of CO2 was to be performed to find an optimal injection strategy, and to study the impact of different salinity concentrations in both the fracture zone and the injection fluid.