Surfactant selection is important for oil recovery in a hydraulically fractured reservoir. Two primary mechanisms, ion-pair coupling (cleaning) between surfactant and hydrocarbon and surfactant adsorption onto the rock surface (coating), were previously suggested to explain how surfactants can alter rock wettability, thus improving oil production. Because of the electrostatic interaction, acidic compounds in the oil tend to be adsorbed onto rock surface that is positively charged; whereas basic compounds are preferentially attracted to rock surface that is negatively charged. It has been discussed in previous literature that for wettability alteration for conventional formation rocks, the cleaning mechanism could be more efficient by inducing ion pairs between surfactants and oil compounds that have opposite charges, rather than the coating mechanism that mainly relies on how well surfactant covers the rock surface.
In this research, 90 shale oils from various liquids shale plays, such as the Eagle Ford and the Wolfcamp, were tested for total acid number (TAN) and total base number (TBN). Cationic and anionic surfactants with low interfacial surface tension, along with Berea sandstone and Indiana limestone, were used to investigate the extent that TAN and TBN can be used as criteria to select surfactants. Oil recovery, imbibition, interfacial surface tension, and emulsion tendency were conducted to examine whether the cleaning mechanism holds true for shale oil saturated formation rocks. The results demonstrated that for carbonates with shale oil having a higher TAN, a cationic surfactant provided the potential to sweep more oil than an anionic surfactant. On the other hand, for sandstone with shale oil having a higher TBN, an anionic surfactant performed better than a cationic surfactant. Those observations appear to be consistent with the proposed cleaning mechanism and resonate with production data for thousands of wells from some major liquids-rich shale plays.