Coupled Effects of Initial Water Saturation and Water Film on Spontaneous Imbibition of Overmature Shale: Insights from Nuclear Magnetic Resonance and Water Vapor Adsorption Analysis

Summary Fracturing fluid retention and the resulting water blocking, both major challenges in shale gas production using hydraulic fracturing methods, are significantly governed by the initial water saturation (Swi) because it controls spontaneous imbibition (SI) behavior. However, this issue remains poorly understood, especially in overmature shales. In this study, we prepared Lower Cambrian overmature shale into three shale samples with different Swi using saturated saline solutions. The water adsorption behavior and flow process of a Lower Cambrian overmature shale was investigated, respectively, using water vapor adsorption (WVA) experiments and the SI experiments under the monitoring of nuclear magnetic resonance (NMR). The results show that the SI proceeds through the rapid SI stage, transition stage, and balanced stage. A higher Swi corresponds to a weaker capillary force, with lower imbibition volume and rate. Initial water primarily resides in small pores and gradually adsorbs on larger pores to form water films as Swi increases. These thin water films are prone to be interpreted by NMR as small pores to overestimate the volume of smaller pores. During the SI process, the water film effect gradually disappears, and the true pore size is reflected. The water film effect develops obviously within a relative humidity (RH) range of 0.59–0.75, and the T2 ranges of strong water film and weak water film effects are 0.2–0.52 ms and 1.86–5.8 ms, respectively. The critical water saturation of the shale sample for effective self-sealing ranges from 31.59% to 45.94% (corresponding to an RH value of 0.59), with a sharp decrease in gas permeability. Moreover, the gas flow becomes almost blocked at water saturation of 60.17– 64.85% (corresponding to an RH value of 0.75), with the gas permeability loss rate exceeding 95%. Therefore, the low Swi layers represent preferable sweet spots for shale gas development.