Since shale matrix has very low permeability, conventional reservoir simulators often do not accurately estimate the mass exchange between matrix and fractures. To evaluate the effect of water injection on the oil recovery, the mass transport in the reservoir at different scales should be modeled accurately. These issues have motivated us to evaluate the contribution of low salinity 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 phenomenological model was formulated to compute the mass transfer mass exchange between the rock matrix and the fractures. This mass transport model was validated using experimental data. A shale-swelling model was also derived to account for the swelling effect on the permeability and porosity by solving the coupled geomechanics and mass transport models. The coupled model was solved for every matrix blocks within the reservoir-scale model to evaluate the overall effect of salt concentration, shale swelling, and wettability alteration on the mass exchange between fractures and shale matrix. The reservoir-scale determines phase pressure, saturation, solute concentration, and liquid production.
The results show that swelling decreases matrix and fracture porosity, forcing the fluid out of the rock matrix and maintaining the pressure in fracture. However, matrix swelling reduces the permeability of the matrix and fractures, reducing oil recovery. Therefore, water injection is not recommended for formations with high swelling potential. The modified zipper pattern is recommended for enhanced oil recovery operations. The simulation results also suggested that higher density of hydraulic fractures increase oil recovery.
Recent research studies suggest the use of low salinity waterflooding, chemical flooding, CO2 injection, and gas injection for improving the recovery in shale oil reservoirs (Morsy et al. 2013, Chen 2013, Nguyen et al. 2014, Sheng 2015, He et al. 2015). However, we are currently using conventional models implemented in conventional simulators to evaluate the potential of each method. Further research on the mass transport mechanisms and modeling at various scales should be conducted to make any quantitative recommendations. Hence, modeling the mass transport in shale oil reservoirs and evaluating the overall effect of different factors on water injection enhanced oil recovery (EOR) is the focus of this study. A multi-scale reservoir model with a better evaluation of the mass exchange between the fractures and the rock matrix enables the investigation of the low salinity water injection on oil recovery in liquid-rich shale reservoirs.