Water adsorption characteristics of organic-rich Wufeng and Longmaxi Shales, Sichuan Basin (China)

Abstract

Water is ubiquitously present within organic-rich shale and can exert an important effect on the gas-in-place and gas adsorption capacity of shale reservoirs. Thus, understanding the water adsorption characteristics of gas shale reservoir is critical for optimizing shale-gas productivity. In this work, the water vapor adsorption characteristics of organic-rich Wufeng and Longmaxi Shales in the Sichuan Basin were studied over a wide range of relative humidities (RHs, 5–95%) utilizing a gravimetric method. All the water vapor adsorption isotherms are categorized as type II, suggesting similar water adsorption behaviors between samples. Being pronounced at approximately 70–80% RH, hysteresis loops from water adsorption curves are characterized by calculating areal hysteresis index (AHI) values. The Wufeng Shales have higher AHI values than those of Longmaxi Shales, which are mainly related to the variation of TOC content and pore structure characteristics. Pore size distribution obtained from water vapor adsorption show unimodal distribution of pore sizes, which are generally comparable to that derived from nitrogen physisorption. The mineral content (e.g., quartz and clay contents) and pore structure characteristics (e.g., pore type, porosity and surface area) have great effects on the water vapor uptake in the Wufeng and Longmaxi Shales. Compared to the shales with low surface area and porosity, Wufeng and Longmaxi Shale samples with higher surface area and porosity exhibit much higher water vapor uptake, suggesting that moisture sorption is mainly controlled by the amount of adsorption sites and volumes. When RH increases from low to high values, methane capacity of partially moisture-equilibrated shale could have experienced rapid decline, moderate decrease and slow decrease stages, finally reaching the minimum adsorption capacity for full moisture-equilibrated shale.