Role of H2O of Gas-Bearing Shale in Its Physicochemical Properties and CH4 Adsorption Performance Alteration Due to Microwave Irradiation

Microwave irradiation is available to produce shale gas. Practical gas-bearing shale reservoirs often contain water (H2O). Given the special physicochemical property of H2O and its occurrence rule in the gas-bearing shale matrix, the presence of H2O has potential effect on electromagnetic energy penetration and dielectric property of the shale matrix, therefore affecting application of microwave irradiation to produce the chief component of shale gas, that is, methane (CH4), from the shale reservoir. Hence, to further explore shale gas production via microwave irradiation, the impact of H2O of gas-bearing shale in its physicochemical property response to microwave irradiation was mainly investigated. The H2O dependence of CH4 adsorption capability alteration due to microwave irradiation was also addressed. The results indicate that microwave irradiation exerts minor impact on the micropore of both dry and moist shales but a noticeable impact on their mesopore. Specifically, both the mesoporous surface area and volume of dry and moist shales decrease after microwave irradiation. The presence of H2O further decreases the typical mesoporous parameters including pore surface area and pore volume of shales in general. Moreover, fractal analysis reveals that both the roughness of the pore surface and complexity of pore structure decrease for all the shales after microwave irradiation, and those of moist shale after microwave irradiation decrease even further, thus facilitating shale gas migration and diffusion within the shale matrix. As for the surface chemical property of shale, the total amount of oxygenic groups consisting of highly conjugated carbonyl, conjugated carbonyl, and carboxyl of dry shales rises after microwave irradiation, but the aromaticity drops. Also, the same changing rule in functional groups is also found for moist shale after microwave irradiation; however, the change is even more pronounced. Finally, the CH4 adsorption capacity of both dry and moist shales decreases after microwave irradiation. In comparison to dry shale, CH4 adsorption capacity of moist shale containing low total organic carbon (TOC) and H2O decreases, but that of moist shale with high TOC content and H2O content increases. These alterations in CH4 adsorption capability of various moist shales are relevant to their pore structure and functional group response to microwave irradiation. Overall, the presence of H2O is beneficial for further weakening the adsorption affinity between CH4 and the shale matrix and strengthening migration capability of CH4 under microwave irradiation, thus making microwave irradiation become a competitive technology regarding shale gas production.