Occurrence of moisture in deep gas-bearing shale matrix and its impacts on methane adsorption/desorption capability under favorable reservoir conditions

To comprehend impacts of moisture on exploring and producing shale gas, the rules of pseudo-in situ moisture occurrence in deep shales were revealed through low-pressure N2 adsorption and desorption, and CO2 adsorption measurements. The influences of pseudo-in situ moisture on CH4 adsorption/desorption in the shales were explored at 353.15 K and pressures up to 30 MPa by using the volumetric method. Results showed that the pseudo-in situ moisture content of the shales ranges between 0.57% and 0.94%, which positively correlates with clay mineral content but negatively correlates with organic matter and quartz. The clay minerals contribute more to moisture occurrence mainly via adsorption effect. The pores with the diameters of 1.10-4.10 nm of the shales serve as dominant space for accommodating moisture. Moreover, the pseudo-in situ moisture reduces saturated adsorption capacity and isosteric adsorption heat of CH4 on the shales, suggesting the weakened adsorption affinity toward CH4-shale system. Typically, the minor pseudo-in situ moisture could significantly weaken CH4 adsorption capability of the shales with low clay mineral content through blocking pore throats of organic matter-hosted pores. However, the abundant pseudo-in situ moisture only slightly reduces CH4 adsorption capability of the shales with high clay mineral content due to continuous distribution of organic matter-hosted pores. The aforementioned different roles are dominated by the difference in occurrence characteristics of organic matter-hosted pores and clay mineral-hosted pores between the shales with low clay mineral content and the shales with high clay mineral content. Furthermore, the pseudo-in situ moisture strengthens CH4 adsorption/desorption hysteresis on the shales associated with moisture uptake-induced clay mineral swelling, thereby raising difficulty for CH4 desorption from the shales. Moisture dependence of CH4 adsorption capability of shales relates to occurrence characteristics of organic matter-hosted pores and clay mineral-hosted pores between shales with low clay mineral content and shales with high clay mineral content.