Wettability Evolution of Sandpacks with Clay Content during Long-Term Waterflooding: A Pore-Scale Contact Angle Characterization

Summary Wettability of rocks may undergo a dynamic evolution during long-term waterflooding. Existing studies usually assume a constant wettability property of rock and ignore the wettability evolution during long-term waterflooding, resulting in ambiguous recognition of residual oil resources. To uncover the wettability evolution within complex pore structure, the effects of clay content on local contact angles are addressed in the sandpacks during long-term waterflooding via in-situ computed tomography (CT) imaging experiments. In this study, the wettability evolution is quantitatively characterized by mean contact angle, contact angle hysteresis, and wettability representative element volume (REV) size. Based on surface roughness and clay deposition patterns, we propose a physical model to analyze the mechanism of wettability evolution. The results indicate that the contact angle distribution within the pore structure is heterogeneous while locally continuous. Water-clay turbid treatment is effective in altering the wetting state of sandpack to less water-wet, reducing the contact angle hysteresis, and enhancing the heterogeneity of contact angle distribution. Waterflooding makes the sandpacks more water-wet and decreases the heterogeneity of contact angle distribution. Transition of surface roughness and clay deposition patterns during waterflooding accounts for the contact angle alteration and determines the overall wettability evolution.