Assessment of Shale Wettability from Spontaneous Imbibition Experiments

Abstract

Wettability can significantly affect the distribution and migration of hydrocarbons within shale reservoirs, and influence the occurrence state of oil and gas in shale pore systems and the selection of fracturing fluids. The wettability of shale with abundant organic matter (OM) is much more complex than conventional reservoir rocks, which typically have a relatively smaller heterogeneous composition at micrometer scales. The low porosity, ultra-low permeability, and the pronounced heterogeneity of shale at micron scales complicate the determination of shale wettability. Consequently, classical wettability determination tests (e.g. contact angle measurements, oil/water displacement experiments at core scales) have a limited applicability for shale samples. Spontaneous imbibition (SI) using different kinds of liquids becomes a useful method to obtain the wettability of various shales at mm-cm scales. This chapter presents SI experiments to investigate the wettability of four different shale gas formations with modest- to over-maturation, including Mississippian-aged Barnett Formation from the United States as well as three leading Chinese shales of the Silurian Longmaxi Formation, the Triassic Yanchang Formation, and the Jurassic Ziliujing Formation. The results show that both water and oil can enter the pore spaces of most shale samples, which confirms the co-existing water- and oil-wet pores and the mixed-wet wettability of these shales. However, different shales show different imbibition behaviors and associated wettability, and our results reveal that the presence of water- and oil-wet pores and their connectivity within shale determine their wettability at mm-cm scales. Finally, four pore network models and a simplified and empirical shale wettability evolution model are established in this study.