Effect of capillary pressure and salinity on CO2 solubility in brine aquifers

CO2 storage is a process in which CO2 is injected into depleted oil and gas reservoirs or deep aquifers to store CO2 through several mechanisms. Since supercritical CO2 phase and saline brine are immiscible at a wide range of pressure and temperature in a permeable medium, capillary pressure exists between the CO2 and saline brine. Although there have been many efforts to study solubility of CO2 in brine aquifers, the most published models neglect the capillary pressure effect during phase behavior calculations to investigate the amount of dissolution trapping. This work focuses on effects of capillary pressure and salinity on solubility of CO2 in the region, where brine saturation increases from a dried zone and/or below residual saturation toward a 100% water saturated zone at the transition zones of CO2 saturations. For this purpose, we developed a new model accounting for inequalities of aqueous and gas phase pressures to calculate the solubility of CO2 in the brine phase. Peng-Robinson equation of state and criterion for local equilibrium are applied. The capillary pressure is a function of phase saturation. We verified the proposed method against the experimental measurement. Subsequently, we performed the case studies including the capillary pressure effect on the solubility of CO2, water vaporization and K-values. The results indicate that increasing capillary pressure leads to a reduction in CO2 solubility. The importance of this effect on CO2 solubility depends on the capillary pressure curve which represents petrophysical properties. In a case considered in this paper, the decrease of CO2 solubility is 23% when the capillary pressure increases from 0.48 to 9.45 MPa. Additionally, effects on equilibrium constant (K-values) and water vaporization were also investigated in the presence of capillary pressure, which can be used to obtain accurate simulation results in commercial reservoir simulators. Furthermore, we analyzed the influence of salinity on CO2 solubility and compared the results with and without the capillary equilibrium. Comparisons between different salinities reveal that CO2 solubility decreases nearly 49% when the salinity increases from 0 to 4 mol aqueous Nacl solutions. It is the first attempt to theoretically investigate the capillary pressure effect on the subject of CO2 geological storage. (C) 2016 Elsevier Ltd. All rights reserved.