NMR investigation of boundary condition effects on spontaneous imbibitionin Longmaxi shale

Spontaneous imbibition is an important process that wetting fluid displaces the non-wetting fluid in the rock by capillary force and is responsible for low flowback efficiency (<30%) of fracturing fluid and severe water blocking effects in shale gas reservoirs. It is crucial to understand the boundary condition effects on imbibition dynamics in shale and the shale-fluid interactions as they provide insights into fracturing fluid loss which can influence gas production. In this study, we designed imbibition experiments and used nuclear magnetic resonance (NMR) to investigate spontaneous imbibition behaviors and water-shale interactions in shale samples with varied boundary conditions including all-side-open (ASO), two-side-open (TSO), one-side-open (OSO), half-side-open (HSO) and two-side-closed (TEC) to. These five boundary effects in imbibition were analyzed by dividing the imbibition stages and comparing the imbibition dynamics. Key imbibition parameters including water saturation, gas recovery factor, residual gas saturation, imbibition capacity, diffusion ability, imbibition rate, and imbibition potential under the respective boundary conditions were selected to compare the imbibition features of five boundary effects. Our results elucidate the existence of three types of water imbibition patterns including the radial counter-current imbibition as shown in TEC boundary condition, the axial co-current imbibition as shown in OSO and TSO conditions, and the compound imbibition which exhibits both radial counter-current imbibition and axial co-current patterns as in ASO and HSO. T₂ relaxation times in OSO and TSO shifted to larger relaxation times as imbibition occurred, demonstrating the induced microfractures were generated in water imbibition due to shale-water interactions. Furthermore, imbibition parallel to the bedding plane and imbibition vertical to the beddings have different water migration patterns due to bedding structures of shale. Our experiments contribute to the understanding of the mechanisms of how different boundary conditions affect imbibition dynamics and shale-water interactions in shale gas reservoirs, which is valuable to the interpretation of fracturing liquid retention processes.