Impact of variation in multicomponent diffusion coefficients and salinity in CO2-EOR: A numerical study using molecular dynamics simulation

Abstract

CO2 injection in depleted or partially depleted oil reservoirs entails a three phase flow system governed by physical processes such as molecular diffusion and solubility. Using numerical modelling, the aims of this paper are two-fold. (i) We investigate the impact of variations in the magnitude of diffusion of CO2 into oil on dissolution of CO2 in brine, and quantify the sensitivity of the simulation outputs (recovery factor and amount of CO2 stored in water and oil phases) by use of different sets of diffusion coefficients throughout the simulation based on the variations in the compositions of the fluids. (ii) We investigate whether CO2 dissolution in brine in a water-flooded system will be a competing or limiting factor for enhanced oil recovery by molecular diffusion of CO2 into oil. To this end, we use molecular dynamics (MD) simulation to determine composition-dependent diffusion coefficients for a multicomponent fluid system in a synthetic fractured reservoir that undergoes CO2 injection. In total we consider 5 components interacting in the reservoir model, namely, CO2, CH4, C4H10, C6H14 and C10H22. The fracture-matrix interaction is simplified with the dual-porosity assumption. Our results show that (i) molecular diffusion not only enhances oil recovery but also enhances CO2 dissolution in water. The enhancement, nevertheless, depends on the values of the multicomponent diffusion coefficients and may exhibit an optimal condition for dissolution due to the impact of CO2 diffusion and entrapment into matrix oil. (ii) The amount of CO2 stored in oil is strongly affected by variation in molecular diffusion coefficients (we observe up to %13 difference). (iii) The results show that there is 4% discrepancy between estimates of the recovery factor for simulation cases that are run with different values of diffusion coefficients. Therefore it is important to account for compositions-dependent diffusion coefficients in simulation of CO2-enhanced oil recovery processes.