Concern about global warming is generating interest in reducing the emissions of greenhouse gases such as CO2. One way of reducing CO2 emissions is to replace conventional (hydrocarbon fuels) energy sources for heating buildings by geothermal energy. Recently it was suggested to co-inject carbon dioxide with cold water for simultaneous geothermal energy production and subsurface carbon dioxide storage. Our data correspond to a geothermal energy project proposed for heating the buildings of the Technical University of Delft. After injection of the water/CO2 mixture a complex interaction between physical transport and the phase redistribution of the components, i.e., water and CO2, occurs. This redistribution is usually described in terms of local thermodynamic equilibrium. There are no published complete analytical solutions for 1-D problems involving complex thermodynamics that include CO2 and heat effects in the flow. We take into account the heat effects related to the cold fluid injection and related to the dissolution of CO2.

We give an analytical solution for the model equations for the temperature and for the flow of CO2, vapor and water after combined injection of a cold carbon dioxide-water mixture in a geothermal reservoir. Due to high pressures and temperatures, CO2 is in a supercritical state and it is necessary to determine the phase equilibrium for non-ideal gases. We used a modification of the Peng-Robinson equation of state and an activity coefficient based mixing rule for the thermodynamic calculations. A volume shift procedure is applied to obtain an accurate liquid density. The structure of the solution depends strongly on the injection and initial reservoir conditions. The application of the work is in the effective recovery of heat from geothermal reservoirs with simultaneous CO2 storage. Moreover, the theory provides fundamental understanding of non-isothermal flow of mixtures undergoing mass transfer between phases. The advantage of the analytical model is that it provides a simple methodology to screen injection conditions for optimal geothermal recovery or maximal storage of carbon dioxide.

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