Understanding the behavioral trends of the effect of water salinity and sand size on oil recovery in sandstone reservoirs

Abstract

Enhanced oil recovery techniques are deployed to subsequently improve oil production having already employed primary or secondary recovery techniques or both methods simultaneously. Literature have shown that salinity impacts on oil recovery due to wettability alteration, but grain size which is closely related to the mechanical behavior and petrographical properties is also one of the main parameters controlling the phenomenon. There is need to investigate the effect of these two parameters in oil recovery. This study considered the effects of water salinity in the range of 0 to 20,000 ppm and sand grains of 45 to 300 µm on oil recovery. Flooding experiments and statistical analyses of the designed experiment was used to determine the residual oil saturation. The properties of sand samples such as bulk and pore volumes, porosity as well as, wet and dry weight for the various groups were also analyzed. Gauss-Newton algorithm with Levenberg-Marquardt modifications were the numerical scheme executed in MATLAB to formulate residual oil saturation model. The optimum percentage recovery for the core flooding experiment in each groups are 82.4%, 81.1%, 79.4%, 81.1% and 78.8% for A4, B4, C4, D4 and E5 respectively. Based on the experimental results, the 300 µm sand grains and salt concentration of 15,000 ppm should be used for operations in this reservoirs because, those conditions guarantee oil recovery as high as 82%. High oil recoveries correspond to low residual oil saturation and vice-versa. The recovered residual oil percentage had no direct correlations with the net dry and wet weights of the samples. The 45-micron grains had the least pore volume and it was observed that the percentage recoveries had no direct correlations with the core sample weights. During validation of the formulated residual oil saturation model, the model gave a good prediction of the experimental data within the 95% confidence interval, and from the statistical analyses, residual oil saturation decreased with decrease in water salinity. Thus, the larger the size of a grain, the lower its superficial area exposed to the wetting and non-wetting fluids which subsequently impacts on oil recovery.