Molecular and Pore Structures of Coal on CO₂ Hydrate Formation: Insights from the Adsorption-Hydrate Hybrid Process

Although CO2 hydrate formation technology in porous media is regarded as an effective means to address carbon emissions, the effects of the physicochemical properties of porous media on the growth characteristics of hydrates remain to be studied. In this work, the influence mechanism of the molecular/pore structures of three different ranks of coals (Ro, max = 0.99% for XZ-02, 1.39% for YT-09, and 2.29% for ZC-15) on CO2 hydrate formation was studied at 40, 70, and 100% water saturation rates via the excess gas method. The results show that the adsorption and hydrophobicity controlled by the molecular structure are beneficial for the synthesis of CO2 hydrates. A greater amount of CO2 adsorbed on the coal surface increased the gas pore pressure, shortened the induction time, and promoted hydrate formation. Moreover, a strongly hydrophobic surface is conducive to the nucleation of CO2 hydrates. CO2 hydrates are synthesized mainly in macropores (>50 nm). The macropores of YT-09 are mainly 400–10,000 nm in size, which is much larger than the critical pore size (radius of 58.68 nm) of the capillary effect, avoiding the influence of the nanopore constraint effect and promoting the synthesis of hydrates. XZ-02 and ZC-15 contain smaller macropore sizes and throats, greatly shortening the induction time of hydrate formation while hindering mass transfer, resulting in less hydrate synthesis. The water consumption and conversion rate decrease with increasing water saturation. In addition, water cannot be completely converted into CO2 hydrate because of the influence of mass transfer in the late stage of massive hydrate synthesis. CO2 hydrates tend to form in the cementation mode at 40 and 70% water saturation, whereas they form in the floating mode at 100% water saturation. Coal with low apparent density and wide macropores is more suitable as a porous medium for solidifying and storing CO2 in the form of a hydrate. This work provides theoretical guidance for CO2 capture and storage in coal measure gas hydrate reservoirs.