DLVO-based estimates of critical water saturation of shale: A case study of the lower Silurian Longmaxi shale in Sichuan Basin

Shale gas is an important unconventional resource, and shale reservoirs typically contain both water and gas fluids. Water can occupy the shale gas storage space, reduce the flow capacity of shale gas, and even completely seal off the shale gas. When the shale develops an effective sealing capacity, the water saturation of the shale reaches a threshold value which can be measured using physical simulation experiments. However, limited research has been conducted on the quantitative calculation of critical water saturation. In order to obtain the critical water saturation of shale, this paper proposes a theoretical calculation method to estimate the critical water saturation of shale based on DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. Two shale samples from the Longmaxi Formation in the Sichuan Basin with different total organic carbon (TOC) were selected for gas adsorption experiments to characterize the pore structure of the organic matter and inorganic matter of the shale. Based on the established theoretical and geological models, the critical water film thickness and critical water saturation of pores with different pore sizes were calculated. Taking the boundary conditions into account, the critical water saturation of the two shale samples was ultimately determined. The results showed that inorganic pores occupied 81.0% of the pores of the shale with a TOC of 0.89%, and their dominant pore sizes were dominated by mesopores around 40 nm; inorganic pores occupied 48.7% of the pores of the shale with a TOC of 4.27%, and their dominant pore sizes were dominated by micropores and mesopores around 0–20 nm and 40 nm. As the pore size increased, the corresponding critical water film thickness also increased, and the critical water saturation was normally distributed in the pore size range centered at about 10 nm. The distribution of critical water saturation in inorganic pores with different pore sizes was in the range of about 63%–76%, and the critical water saturation of shale with a TOC of 0.89% and shale with a TOC of 4.27% were calculated to be 41.7% and 32.7%, respectively. The method proposed in this study accurately calculates the critical water saturation of shale and effectively distinguishes the differences critical water saturation between shales with different TOC. Further, shale gas reservoirs can be finely characterized by comparing with the original water saturation of shale layers. This study is of great scientific significance to shale gas exploration and development, and even to the field of CO₂ geological storage.