Wetting alteration and oil redistribution within the pores of Berea sandstone during low salinity flooding

Abstract:

Low salinity water flooding increases oil recovery in many but not all sandstones. Observations in the laboratory over scales ranging from the micrometers of individual pores to the centimeters of rock core samples show that lowering the salinity of injected water can alter the wetting state of the rock, making it more water-wet. However, there remains a poor understanding of how this wetting 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, X-ray micro-CT scanning is used to image unsteady state experiments of tertiary low salinity water flooding in two Berea sandstone cores. The wetting state of one of the cores is altered with exposure to crude oil. We characterise the wetting state of samples using imagery of fluid-solid fractional wetting and pore occupancy analysis. In the unaltered rock, after high salinity water flooding, oil was trapped at a relatively high saturation of 0.74, uniformly distributed throughout the sample. This did not change significantly after subsequent flooding with low salinity brine. Similarly, the fractional mineral area covered by oil, and the size distribution of oil filled pores remain constant throughout flooding in the unaltered rock. In contrast, after high salinity flooding of the altered sample, oil was trapped heterogeneously, at relatively low saturation of 0.62. Subsequent flooding with low salinity brine resulted in an oil production of three percentage points. In the altered sample, we observe a shift in the solid surface area covered by the oil after low salinity flooding towards 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. In the altered rock, water preferentially displaces oil in small pores in response to the low salinity effect. While the shift in the wetting state resulted in only three percentage points of additional oil production, 22% of oil was redistributed within the rock. We find that the success of low salinity water flooding depends on the combined ability of the low salinity flood to alter the wetting state from more oil-wetting to more water-wetting and the pore structure to facilitate the production of the newly mobilised oil.