The influence of capillary pressure on the phase equilibrium of the CO2–water system: Application to carbon sequestration combined with geothermal energy

Abstract

We quantify the capillary-pressure effect on the phase equilibrium of the CO2–water system and subsequently on the CO2 storage capacity and heat-energy recovery for CO2–water injection into geothermal reservoirs. Our interest is in the capillary-pressure range between 0 and 100bars for temperatures between 293 and 372K and wetting-phase pressures between 25 and 255bars. For this purpose, we have implemented capillary pressure in the PRSV equation of state.The results show that capillary pressure promotes interfacial evaporation. Capillary pressure reduces the CO2 solubility in water and the aqueous-phase density up to 64% and 1.3%, respectively, whereas it increases the water solubility in the CO2-rich phase and the CO2-rich-phase density up to 1172% (1.0+11.7=12.7 times) and 13%, respectively. If the CO2-rich-phase properties were calculated as a function of the wetting-phase pressure, capillary pressure would shift the CO2 liquid–vapor transition and consequently the upper critical point of the CO2–water system to a lower pressure. Therefore, the CO2-rich-phase properties must be calculated using the non-wetting-phase pressure to avoid this shift.For mixed CO2–water injection into a geothermal reservoir, the influence of capillary pressure on the phase equilibrium reduces both the heat recovery up to 37% and the CO2-storage capacity up to 37%. We construct a plot of the recuperated heat energy versus the maximally stored CO2 for a variety of conditions; we compare the results including and excluding the effect of capillary pressure in the phase-equilibrium calculations. We also provide a cursory evaluation of the energy and economics of mixed CO2–water injection into a geothermal reservoir.