Water Adsorption Characteristics and Mechanisms in the Organic Components of Shale Gas Reservoirs

To reveal the adsorption characteristics and mechanisms of water vapor in the organic matter (OM) of shale gas reservoirs, OM was isolated from the Longmaxi shale and used in water adsorption experiments. The primary, secondary, and entire adsorption behaviors of OM were compared with those of montmorillonite, kaolinite, illite, and intact shale to characterize its contribution to the water adsorption capacity of shale gas reservoirs. Thermodynamic parameters of Gibbs free energy change (ΔG), enthalpy change (ΔH), entropy change (ΔS), and isosteric heat (Q_st) were calculated and compared among all adsorption carriers to discuss their differences in adsorption intensity, spontaneous nature, and driving factor. Moreover, the water vapor adsorption mechanisms were identified from the perspectives of composition, chemical structure, and pore structure of the adsorbent. The results indicate that (i) the adsorption amount on montmorillonite is the largest and, thus, greater than that on OM, kaolinite, illite, and intact shale. (ii) The primary adsorption intensity of clay minerals is significantly higher than that of OM due to their strong hydrophilicity, whereas the secondary adsorption intensity of OM is slightly higher, caused by the superimposition of the adsorption potential in its extensively developed nanopores. (iii) Montmorillonite is expandable and characterized by large interlayer space, cation exchange capacity (CEC), and pore structure parameters. Although OM is composed mainly of hydrophobic aromatic structures, with low hydrophilic polarity and alkaline functional group content, the multilayer adsorption and capillary condensation in its widely developed pore structures are significant. (iv) The water adsorption capacity of OM is greater than that of kaolinite and illite, and the adsorption amount is mainly influenced by accumulation spaces and adsorption sites rather than the adsorption intensity.