Experimental Investigation on the Pore-Scale Mechanism of Improved Sweep Efficiency by Low-Salinity Water Flooding Using a Reservoir-on-a-Chip.

Low-salinity water flooding, known as an environmentally friendly and efficient oil recovery technology, has attracted the attention of several researchers all over the world. However, its field application is suffering restrictions because of the ambiguous mechanisms of the oil recovery by controlling the salinity. In this study, a water flooding microfluidic experiment was conducted to investigate the pore-scale mechanism of enhanced sweep efficiency by low-salinity water flooding. This experiment used a reservoir-on-a-chip that preserved the real rock properties and morphological features. Crude oil-water-rock contact angle experiments by altering water salinity were conducted to investigate the mechanism of the improvement of sweep efficiency by low-salinity water flooding. The experiment results show that unlike high-salinity water flooding, low-salinity water flooding improves its sweep efficiency from wettability alteration. Specifically, in the microfluidic model, it clearly shows that the pore-scale sweep efficiency is improved by reducing the salinity of injected water. Low-salinity water can invade the pores that cannot be reached by high-salinity water and displace the remaining oil after high-salinity water flooding. In the altering water salinity contact angle experiments, the contact angles decrease from 91.05° (neutral-wet) to 64.41° (water-wet) as the water salinity decreases from 46.58 to 2.31 g/L. The wettability of the rock surface changes from oil or neutral-wet to water-wet and induces the imbibition process, during which the hydrophilic pores absorb the low-salinity water into the smaller pores where the high-salinity water cannot invade. This investigation provides a further in situ and pore-scale evidence of improved sweep efficiency and wettability alteration by low-salinity water flooding and a possible reference to solve the difficulty in upscaling fluid flow behavior from microfluidics to reservoir rocks.