Pore-Scale X-ray Imaging of Wetting Alteration and Oil Redistribution during Low-Salinity Flooding of Berea Sandstone

Observations over a range of scales show that lowering the salinity of injected water can alter the wetting state of a rock, making it more water-wet. However, there remains a poor understanding of how this alteration affects the distribution of fluids over the pore and pore-network scale and how this wetting alteration leads to oil recovery. In this work, we observe how oil and brine redistribute in the pores of rocks in response to low-salinity flooding. We use X-ray μ-CT scanning to image tertiary low-salinity waterflooding in two Berea sandstone cores. The wetting state of one core is altered with exposure to crude oil. We characterize the wetting state of both samples using imagery of fluid–solid fractional wetting and pore occupancy analysis. In the unaltered rock, oil saturation, fractional mineral area covered by oil, and size distribution of oil-filled pores remain constant between high-salinity flooding and subsequent low-salinity floods. In contrast, in the altered sample, we observe a shift in the mineral area covered by oil after low-salinity flooding toward decreasing coverage, consistent with a change to a less oil-wetting state. This change is also reflected in the observed variation in fluid pore occupancy. The wetting alteration results in the redistribution of 22% of oil within the rock, but an additional recovery of just three percentage points. The success of low-salinity waterflooding depends on both a wetting alteration and a pore structure, which facilitates the production of mobilized oil.