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- moisture occurrence in deep shales were revealed through low-pressure N adsorption and desorption, and CO adsorption measurements. The influences of pseudo- moisture on CH 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- 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 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- moisture reduces saturated adsorption capacity and isosteric adsorption heat of CH on the shales, suggesting the weakened adsorption affinity toward CH-shale system. Typically, the minor pseudo- moisture could significantly weaken CH adsorption capability of the shales with low clay mineral content through blocking pore throats of organic matter-hosted pores. However, the abundant pseudo- moisture only slightly reduces CH 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- moisture strengthens CH adsorption/desorption hysteresis on the shales associated with moisture uptake-induced clay mineral swelling, thereby raising difficulty for CH desorption from the shales.