Effect of Pore Structure on Tertiary Low-Salinity Waterflooding in Carbonates: An In-Situ Experimental Investigation

Summary Low-salinity waterflooding (LSW) is an environmentally friendly and economically feasible technology that enhances oil recovery by controlling ionic composition or brine salinity. The recovery efficiency of this technique is strongly affected by the rock pore structure that governs the flow behavior of the injected brine. However, existing experimental studies elaborating on the relationship between pore structure and LSW performance in carbonates remain scarce. To address this gap, three carbonate plugs with different pore structures were displaced sequentially with synthetic high- and low-salinity brine under the capillary-dominated flow regime. High-resolution micro-computed tomography (CT) was used to obtain 3D images of different displacement stages, visualizing the fluid distribution. After image processing and contact angle calculation, it was found that the primary mechanism for enhanced recovery was wettability alteration, transitioning from oil-wet to weakly oil-wet. Significant differences were observed among the three samples. Sample 1 showed the highest additional recovery (22.2%), followed by Sample 2 (11.2%), and the lowest was Sample 3 (4.5%). Despite Sample 1 and Sample 3 having similar and narrow pore size distributions, they exhibited different fluid behaviors during LSW: In Sample 1, oil was mainly displaced from medium-sized pores, whereas in Sample 3, small pores were the main target for brine. The large coordination number likely enhanced the relative permeability of the high-salinity brine. The low-salinity brine followed the pathway formed by the high-salinity brine, affecting the LSW performance. This work provides novel insights into how pore structure affects oil recovery by comparing the response of multiple carbonate samples to LSW.