Spontaneous imbibition has been considered as one of the important mechanisms for unconventional reservoir Improved Oil Recovery (IOR). Capillary pressure and osmotic pressure are the driving forces during the imbibition process and are associated with different rock components. Ions and surfactant in water can modify the surface properties of minerals and organic component, thus wettability moves towards more water-wet to enhance oil recovery. Depending on the membrane efficiency and in-situ water salinity, osmosis can contribute to the imbibition as well. In this paper, a multi-component matrix imbibition model is coupled with wettability alteration by low water salinity/surfactant.
The simulation results are consistent with the experimental observations reported in the paper and literature. Some insights on the ion/surfactant and rock interactions are drawn and water imbibition volume is controlled by rock components (clay, organic matter and other minerals) and different concentrations of salt ions and surfactant. Depending on the ion strength of ion species and surfactant, the concentration of water-wet site on the rock surface increases, thus the contact angle is changed to more water-wet. In addition, interfacial tension can be lowered by surfactant. The combination effects determine capillary pressure imbibition. The concentration of charged ions and surfactant molecules affect the osmosis imbibition. These processes for different rock components associated with pore size distributions are incorporated into the imbibition model. The results showed that there is an optimum water salinity for maximum imbibition. Explained by adsorption/desorption theory, anionic surfactant has stronger wettability alteration ability than nonionic surfactant. High TOC rock is prone to bigger contact angle change. To validate the model, spontaneous imbibition tests were performed on cleaned and dried Three Forks core samples for fluids with varying water salinities or surfactant. The imbibition curves were matched very well using the developed model and proved that the interplay of capillarity and osmosis controls the imbibition rate and amount following the adsorption dominated imbibition.
