Conventional methods for wettability determination of shales: A comprehensive review of challenges, lessons learned, and way forward

The wettability is a critical property that governs the in situ fluid distribution, fractional flow, and hydrocarbon trapping in the porous media. This topic is of immense interest to the petroleum industry for the exploitation of unconventional resources, which have become a significant source of oil and gas production, especially in North America, Canada, and China, as a result of the advancement of horizontal drilling and fracking technologies. The conventional techniques as the central focus for wettability determination of shales include contact angle, spontaneous imbibition, and flotation. This paper reviews the published literature and analyzes the challenges and lessons learned from each method applicable for shales. The results of various techniques are compared, and their consistency is highlighted. Comparative contact angle measurements can characterize the surficial wettability and hysteresis of dynamic processes in shales. Spontaneous imbibition is recommended as the most realistic and representative measurement method, whereas it reveals the pore connectivity of the internal pore network. However, different liquid/rock interactions, mainly clay swelling and the creation of micro-fractures, affect the results thus necessitate the application of reservoir triaxial stress conditions on shale samples. In this review, finally, some concluding remarks and recommendations are presented.