Study On The Distribution Characteristics Of Primary Water In Gas-bearing Shale And Its Effect On CH₄ Adsorption/desorption

To achieve sustainable development of economy,the demand for clean and efficient energy sources has been increasing in China.As an emerging unconventional natural gas,shale gas has attracted broad interest.Promoting the exploration and production of shale gas will not only help optimize China's current energy consumption structure,mitigate environmental issues caused by traditional fossil energy consumption such as coal and petroleum,but also help reduce the external dependence of conventional natural gas and protect China's energy security.Shale gas is mainly present as adsorbed state within the practical shale reservoirs.Therefore,the study on the adsorption mechanism of shale gas is of great significance to explore the accumulation of shale gas and accurately estimate the reserve of shale gas.The adsorbed shale gas is typically dominated by the shale reservoir conditions and the physicochemical properties of the shales.Among them,the primary moisture in the gas-bearing shale reservoir is a crucial factor affecting adsorption/desorption capability of shale gas.Therefore,this study collected a total of four Lower Silurian Longmaxi shales containing primary moisture from Sichuan basin.Multiple experimental methods,including TOC analysis,CO₂ adsorption and N₂ adsorption/desorption,FTIR analysis,XRD analysis,water vapor adsorption/desorption,and high-pressure CH₄ adsorption/desorption,and adsorption theory have been adopted to explore the distribution characteristics of primary moisture in gas-bearing shales and its effect on CH₄ adsorption/desorption.The main research contents include(1)the distribution characteristics of primary moisture in gas-bearing shale reservoirs;(2)the rule on primary moisture dependence of CH₄adsorption/desorption in gas-bearing shale reservoirs;(3)the mechanism on primary moisture dependence of CH₄ adsorption/desorption in shale reservoirs.The main conclusions derived from this study are as follows:(1)The primary moisture content of the four shale samples is controlled by the clay mineral content within shales.The primary moisture in shales is mainly distributed in pore space with pore diameter of 1-4 nm of shales.The water vapor adsorption isotherms of shales well follow the modified Brunauer-Emmett-Teller(BET)equation.The oxygen-containing functional groups such as carboxyl and carbonyl in shales serve as the primary adsorption sites for water molecules.The clay minerals of shales and water molecules previously adsorbed on the primary adsorption sites can serve as the secondary adsorption sites for water molecules.(2)The adsorption equilibrium of CH₄ on shales conforms to the Ono-Kondo lattice model.The fitting results confirm that the primary moisture in shales reduces the maximum adsorption capacity of CH₄ of shales.The decresing amplitude varies between 12.86%and 45.45%.Furthermore,the primary moisture in shales weakens the adsorption isosteric heat of shales for CH₄;the decresing amplitude ranges between10.64%and 14.43%.The aforementioned adsorption equilibrium and thermodynamics confirm that the primary water weakens the adsorption affinity between CH₄ and shale matrix.(3)The adsorption kinetics of CH₄ on shales follows the pseudo-second-order kinetic model.The fitting results indicate that the primary moisture reduced the adsorption and diffusion rate of CH₄ in shales.(4)The CH₄ adsorption and desorption hysteresis of shales containing the primary moisture is more pronounced than that of the dry shales,implying that the primary moisture in shales hinders CH₄ desorption from shales.(5)The competitive adsorption relationship between primary moisture and CH₄within shales lead to the negative influence of the primary moisture on CH₄ adsorption.With respect to the more noticeable adsorption and desorption hysteresis of CH₄ on shales containing the primary moisture,it is probably related to pore throat blocked by the desorbed H₂O molecules,and CH₄molecules re-adsorbed on the adsorption sites previously occupied by H₂O clusters.This work not only helps to improve the theoretical system of shale gas storage,but also promotes the exploration and production of shale gas by studying the occurrence characteristics of shale primary water and its mechanism of CH₄adsorption performance.