Studying low-salinity waterflooding recovery effects in sandstone reservoirs

Abstract

Numerous core-flooding experiments have shown that low-salinity water flooding (LSWF) could improve oil recovery in sandstone reservoirs. However, LSWF recovery effects remain highly contentious primarily because of the absence of crucial boundary conditions (boundary conditions are defined throughout the paper as the initial and final, contact angle and interfacial tension values). The objective of this paper is to conduct a parametric study using statistical analysis and simulation to measure the sensitivities of LSWF recovery effects in sandstone reservoirs. The summary of 411 core-flooding experiments discussed in this paper highlights the extent and consistency in reporting boundary conditions, which has two implications for statistical analysis: (1) the statistical correlations of the residual oil saturation to chlorite (0.7891) are strong, whereas the statistical correlations of the residual oil saturation to kaolinite (0.4399) contents, as well as to the wettability index (0.3890), are comparably lower, the majority of dataset entries are missing, and no prediction model can be generated; (2) if a prediction model is generated without clay content values and a wettability index, even though LSWF effects emphasizes wettability modification by virtue of oil aging time and the strong influence of brine cation and divalent ion concentrations on Sor, the prediction model’s regression curve and confidence level are poor. Reservoir simulations conducted to examine LSWF recovery sensitivities conclude that LSWF recovery effects are governed based on the initial and final wetting states. In all wetting states except for weak water-wet conditions, the increase in oil relative permeability is the underlining recovery effect. In weak water-wet conditions, LSWF incremental recovery is driven by low capillary pressures. In weak oil-wet conditions, the secondary LSWF recovery effect is the change of the non-wetting phase to oil. In all wetting states, an appreciable decrease in interfacial tension (IFT) is realized at the breakthrough recovery. The decrease in IFT is the primary recovery effect in strong water-wet conditions.