A review of shale wettability characterization using spontaneous imbibition experiments

Wettability is an important reservoir property that can affect the distribution and migration of geofluids within shale reservoirs, which significantly influences the occurrence state of methane in shale pore systems and the selection of fracturing fluids during shale gas exploitation. The wettability characteristics of organic matter (OM)-rich shale are much more complicated than those of conventional reservoir rocks with a relatively homogeneous composition. In addition, the low porosity and extremely low permeability, as well as the strong heterogeneity of shale, complicate the evaluation of shale wettability. As a result, traditional wettability tests (including contact angle measurements and oil/water displacement experiments) are inapplicable for shale samples. Consequently, spontaneous imbibition experiments become a practical and effective way to establish the wettability characteristics of different shales. This paper reviews shale wettability characterization using spontaneous imbibition experiments and discusses the close relationship between shale wettability and pore structure. As reported in the literature, most shale samples can imbibe both water and oil, which indicates the coexistence of water-wet and oil-wet connected pore spaces and the mixed-wet property of shales. However, different shales show a range of oil and water imbibition behaviors, indicating the wettability differences amongst them. This review reveals that the development degrees of water- and oil-wet pores within shale determine its wettability as well as its imbibition behaviors. And four simplified pore network models are proposed for more water-wet, more oil-wet, mixed-wet and intermediate-wet shale samples respectively. This work also shows the pronounced effects of thermal maturity on shale pore structure development and shale wettability characteristics. Consequently, a simplified and empirical wettability evolution model with four stages only considering the effect of thermal maturity is established in this study.