Evaluation of Low Salinity Water Injection Improved Oil Recovery in Liquid-Rich Unconventional Reservoirs

Abstract

Since the shale matrix has a very low permeability, the flow in the matrix is often transient flow. Hence, conventional reservoir simulators often do not accurately estimate the mass exchange between matrix and fractures. To evaluate the effect of water injection on oil recovery realistically, reservoir simulators need to incorporate the mass transport in the reservoir at different scales. These issues have motivated us to evaluate the contribution of water injection on oil recovery in liquid-rich unconventional reservoirs accounting the effects of salinity, fluid type, shale swelling, and wettability alteration. In this research, a new mass transport model is used to compute the mass transfer mass exchange between the rock matrix and the fractures. A geomechanical model is also used to account for the effect of fluid–rock interaction on the permeability and porosity. The coupled model is solved for every matrix block within the reservoir-scale model to evaluate the overall effect of salt concentration, shale swelling, and wettability alteration on the mass exchange between the fractures and the shale matrix. The reservoir-scale simulation helps to determine the phase pressure, saturation, solute concentration, and liquid production. A computer code was developed to solve the transport and geomechanical equations simultaneously, and the code is validated with the benchmark problems as shown in the Appendix. The results show that osmosis significantly contributes to the recovery of oil from very low permeability shale matrix over a long time period. Shale matrix swelling also significantly reduces the permeability of the shale matrix and fractures, reducing overall oil recovery. Therefore, water injection is not recommended for formations with swelling potential. The modified-zipper pattern is recommended for enhanced oil recovery operations. The simulation results also suggested that oil recovery can be improved by increasing the density of hydraulic fractures.