Storage Capacity and Microdistribution of Pore Water in Gas-Producing Shales: A Collaborative Evaluation by Centrifugation and Nuclear Magnetic Resonance

Both adsorbed and free water occur in shale matrix pores and intensely impact methane accumulation and transport in a shale gas reservoir. As a result of multiple water coexistence in complicated nanoporous shale, there still is a large challenge for the determination of the storage capacities and microdistributions of adsorbed and free water. In this study, storage characteristics of adsorbed and free water in deep gas-producing shales from the southern Sichuan Basin of China were investigated using a collaborative evaluation of centrifugation and nuclear magnetic resonance (NMR) under atmospheric pressure and 30 °C temperature conditions. Results show the following: (1) The amounts of adsorbed and free water in water-saturated shales are 1.7967–16.0964 mg/g (mean of 9.3875 mg/g) and 6.4267–19.8020 mg/g (mean of 11.0377 mg/g), respectively. The adsorbed water accounts for 15.83–66.10 wt % (mean of 45.14 wt %) of total water. Adsorbed water mainly exists in pores developed in solid bitumen and clay mineral. More free water is stored in the pores of solid bitumen than that in clay mineral-related pores. (2) The adsorbed water primarily distributes in pores of <20 nm; in contrast, the free water is mainly stored in >2 nm pores. The microdistributions of adsorbed and free water were controlled by the pore size distribution in shales. The mass ratio of adsorbed water decreases with increasing the pore size. (3) The presence of adsorbed water implies that some of water stored in matrix pores cannot flow. The free water is a potential source of the flowback fluid of the shale gas well. This study not only provides an evaluation theory and method but also contributes to insight into the gas–water storage in shale pores.