Liquid Spontaneous Imbibition and Its Time-Resolved Nuclear Magnetic Resonance within Differently Matured Shale Indications for Shale Pore Structure and Wettability

Organic matter (OM) content and its thermal maturation affect the wettability, pore structure, and imbibition behavior of shale, which highly affect its use as a reservoir for oil, carbon dioxide, or hydrogen underground. Core samples from the Cretaceous Qingshankou Formation with natural maturity from immature to the oil window were selected. Using different fluids (oil and water) and temperature–pressure [room (15 °C and 0 MPa) and elevated (45 °C and 20 MPa)] conditions, the imbibition process was studied with nuclear magnetic resonance (NMR) to determine the time-resolved fluid imbibition process. The results show that the rate of water imbibition is significantly higher than that of oil for all samples, and even OM-hosted pores are mainly found at oil-window mature samples. More rapid imbibition for both fluids occurred under high temperature–pressure conditions. The oil imbibition rate of oil-window mature shale is higher than that of immature shale, while the spontaneous water absorption rate is the opposite, which indicates that in contrast to immature shale samples, residual oil will occupy the space in pores or be absorbed onto pore surfaces, making the pores more lipophilic in the oil-window mature shale. Microfractures being produced during water imbibition increase the relaxation time T2 of the fluid, indicating a pore structure change. Collectively, this work provides a guideline to the influence study of maturity and pore structure on the shale imbibition test with NMR.