Influence of Pore Water on the Gas Storage of Organic-Rich Shale

Previous studies that use dry samples have been conducted on the nanoporosity of organic matter in shale. However, underground shales contain pore water that may greatly influence their pore structures and adsorbed gas content. To address this, a typical organic-rich, marine, gas-bearing shale in China was analyzed in this study. The water content and isothermal adsorption under different saturation conditions were determined, and the adsorbed and free gas contents were quantified. Across scales, water occurs within clays, between grains, and in fissures. In micro- and mesopores, gas is mainly adsorbed onto the surface of kerogen, because clay surfaces are water-saturated. In macropores and fractures, gas mainly exists in a free state. Adsorbed and free gas contents decrease with increasing water saturation in shale reservoirs. In high-production reservoirs, a limited amount of adsorbed gas is stored within micropores, while a relatively large amount of free gas is stored in larger pores and natural fractures. Because shale gas occurs in low concentrations, high total organic carbon may not be the only necessary factor; high porosity and many natural fractures may be more favorable for high reservoir productivity. The results provide valuable insights for understanding gas storage and finding zones of high productivity in shale reservoirs and have potential applications in engineering conditions.