ROCK-FLUID INTERACTIONS IN A TIGHT SANDSTONE RESERVOIR

In this work, we present a comprehensive and practical framework to quantify the dynamic characteristics of pore-throat structure together with wettability alterations during a waterflooding process in a tight sandstone reservoir in addition to evaluating the impact of ionic composition and salinity on its performance. Thin section analysis, XRD analysis, SEM tests, PMI and RMI experiments, contact angle measurements, zeta potential tests, and displacement experiments with and without surfactants are integrated. In a reservoir, throats with a radius greater than 0.2 mu m are found to control less than 40% of its porosity but contribute over 95% to its permeability. While pore size distribution remains relatively stable, average throat radius with the maximum frequency is decreased from 0.5 mu m to 0.3 mu m, resulting in a 13.9% decrease in average permeability. Compared to that of using pure brine, adding surfactants reduces the cutoff movable throat radius from 0.033 mu m to 0.019 mu m, leading to an incremental oil recovery of 4.26%. In a strongly water-wetting formation with its oil filling the center of a pore in the form of oil droplets, the movable water saturation in the twophase flow zone on the relative permeability curves ranges from 22.5% to 55.1%. At the isotonic point, the water saturation is 43.6%, resulting in a waterflooding oil recovery of 42.1%. Conversely, in a weakly oil-wetting formation with its oil mainly distributing in the form of an oil film, water saturation in the twophase flow zone varies from 22.3% to 48.1%. At the isotonic point, the water saturation is 38.1%, resulting in a waterflooding oil recovery of 33.2%. Experimental observations demonstrate that low salinity water (LSW) injection expands the electric double layer (EDL) on the oil/brine and rock/brine interfaces, altering the rock wettability; however, the presence of divalent ions in the LSW undermines such wettability alteration. On the other hand, permeability reduction resulting from salinity decrease can be alleviated by introducing divalent ions into the LSW, while continuous injection of monovalent brine reduces the ultimate oil recovery. Alternatively, a mixture of monovalent and divalent ions in LSW is utilized to revive permeability and maintain wettability alteration. Furthermore, an optimized divalent ion concentration is found to be conducive to reducing the cutoff throat radius and achieving a higher oil recovery in a tight sandstone reservoir.