Study On The Adsorption/desorption Law And Mechanism Of CH₄ In Deep Marine Shale Matrix In Jiaoshiba Area, Sichuan Basi

Shale gas is a crucial unconventional natural gas resource.Its exploration and production are significant for optimizing the current energy consumption pattern,mitigating energy dependence on foreign countries,and reducing environmental pollution caused by large-scale coal and other traditional fossil energy consumption.The main component of shale gas is methane（CH₄）.It mainly occurs as adsorbed state within the practical shale reservoirs.Therefore,CH₄ adsorption and desorption behavior and adsorption mechanism of deep shale matrix are of great significance to reveal shale gas accumulation and accurately estimate shale gas reserve.Hence,this study collected a total of five deep marine gas-bearing shales from the Lower Silurian Wufeng Formation-Longmaxi Formation in the Jiaoshiba block of Sichuan Basin.The properties and mechanism of methane adsorption and desorption under simulated reservoir conditions were studied.The study mainly focuses on H₂O occurrence and adsorption and desorption capacity in deep marine shales,CH₄ adsorption and desorption in deep marine shales under simulated reservoir conditions,and adsorption effect between CH₄ molecules and deep marine shale matrix based on the Polanyi adsorption potential theory.The main conclusions are summarized below:（1）By using multiple characterizations including argon ion polishing and field emission scanning electron microscopy（FE-SEM）,CO₂ molecular probe adsorption,N₂ molecular probe adsorption and desorption,and low field nuclear magnetic-T₂ spectroscopy（NMR）,the microscopic pore structure and the occurrence space and state of water in deep marine shale reservoirs were determined.Results indicate that the shale matrix contains a large amount of organic matter.The organic matter mostly distributes in the scattered and banded form,the edge of which is jagged with the authigenic minerals.Typically,the organic matter is coated with authigenic quartz,illite and further develops nanoscale pores.Besides,the shale matrix contains microfractures mainly including structural fractures,shrinkage fractures,and bedding fractures.Overall,the surface parallel to the lamination of shale matrix highly develops organic matter-hosted pores and inorganic mineral-hosted pores,while the surface perpendicular to the lamination develops more shrinkage and structure fractures.In addition,the primary moisture mainly exists in the mesopores of deep shale with diameter less than 4 nm,the main occurrence form of which is adsorption state.（2）By employing volumetric method,CH₄ adsorption and desorption isotherms of the dry and primary moisture-containing shale matrices were plotted under the simulative reservoir conditions,i.e.adsorption temperature of 353.15 K and the maximum adsorption equilibrium pressure of 30.00 MPa.Accordingly,the impact of primary moisture of shale matrix on its CH₄adsorption and desorption was discussed.Moreover,the mechanism on moisture and temperature dependence of CH₄ adsorption and desorption capacity was revealed.Results show that the Ono-Kondo lattice model can well describe the adsorption equilibrium of CH₄ on the dry and primary moisture-containing shale matrices.The primary moisture reduces the isosteric heat of adsorption of CH₄ on the shale matrix.In addition,the CH₄ adsorption capacity of shale matrix varies with its moisture content,which can be divided into the initial decreasing stage,the sharp decreasing stage,and the slowly decreasing stage.Moreover,the primary moisture shale matrix is not conducive to CH₄ desorption from the shale matrix.Particularly,the increasing moisture content raises the difficulty in desorption.Furthermore,the elevated temperature reduces the degree of CH₄ adsorption and desorption hysteresis of shale matrix.The weakening effect of temperature on hysteresis is associated with more energy provided for CH₄ desorption and the weakened shale matrix swelling at high operating temperature.（3）Based on surface coverage theory and micropore filling theory,the adsorption mechanism of CH₄ molecules in shale matrix pores was discussed.The applicability of Polanyi adsorption potential theory to CH₄-shale adsorption system was verified.Quantitively analysis indicated that the CH₄ adsorption on the deep shale matrix under simulative reservoir temperature and pressure typically follows the micropore filling mechanism.Accordingly,the Polanyi potential theory accounting for the micropore-filling mechanism is well applicable to address the adsorption and desorption equilibrium of CH₄ on the deep shale matrix.Ultimately,the adsorption and desorption characteristic curves of CH₄ on the deep shale matrix derived from the Polanyi potential theory show good coincidence for the operating temperatures of 313.15-353.15 K.Hence,the characteristic curves can serve as a useful theoretical tool to predict adsorption and desorption capacity of CH₄ on the shale matrix at given temperature and pressure.To sum up,the relevant results and conclusions derived from this study can not only help reveal enrichment mechanism of deep marine shale gas,but also favor selection and production of sweet spots of shale gas,thereby,updating theoretical system of deep marine shale gas exploration and production to a certain extent.