Although more emphasis is placed on the interaction between CO2 and oil during a CO2 injection process, the interaction of CO2 with the reservoir brine, too, could be significant in terms of its impact. This study presents a modeling approach to evaluate the possible effects of brine salinity on CO2 injection in the context of both CO2 injection and low-salinity waterflood.
In the first phase of the study, the relevant correlations widely reported in the literature were applied for investigation of brine salinity effects on CO2-brine system properties for carbonate reservoir conditions at 248°F (120°C) and 3000psi (20.68MPa). The properties investigated include CO2 solubility in brine, IFT between CO2 and CO2-saturated brine, and density and viscosity of CO2-saturated brine. Some relevant experimental data were also incorporated in the validation of these correlations. In the second phase, a simple pore-scale model was developed to evaluate the brine salinity effect on water-isolated oil recovery by CO2 diffusion through water barrier. Furthermore, compositional reservoir simulation using a hypothetical geological model and a real PVT model was conducted to study the brine salinity impact on oil recovery of carbonated-water injection (CWI). The following findings were noted:
Under our reservoir conditions, lowering the brine salinity could significantly enhance CO2 dissolution in brine, consequently leading to great variation of the system properties, which may make a big difference on CO2 injection performance.
The CO2 diffusion-controlled modeling showed that reduction of the salinity in the water, which acted as a barrier blocking the direct contact between CO2 and oil. This could greatly promote the rate and amount of CO2 mass transfer through water barrier to the bypassed oil.
Oil recovery results from simulation studies illustrated that increasing the CO2 solubility by reducing injected brine salinity during carbonated water injection could remarkably ameliorate its performance.