Study On The Interaction Mechanism Of Solid-Liqiud-Gas In Coal Reservoir

Based on the geology situation of solid-liquid-gas coupling system in coal reservoir under stratigraphic condition, the isothermal adsorption/desorption water system model (IS-100) and electrohydraulic servo rock system (MTS815) were used to carry out the injection experiments under high pressure and isothermal adsorption experiments using coal samples which are from Sinkiang, Feicheng, Ordos and Qinshui basin. The results suggested that the methane adsorbability of coal matrix is remarkably influenced by liquid water exist in coals. According to the quantitative analyses of Van der waals force, gas interaction force and superficial potential force of coal matrix, the physical chemistry model of solid-liquid-gas interaction in coal reservoir which revealed the physical chemistry mechanisms of coal adsorption under different experimental conditions, was established from the physic and chemistry point of view. Adsorption diffusion dynamic characteristics of methane adsorption were indicated under different pressure and water content by calculating the adsorption diffusion coefficient and analyzing the adsorption diffusion dynamic. Adaptability conditions of Langmuir and Dubinin-Radushkevich model were distinguished based on experimental fitting data and theory analyzing. Therefore, the improvement of experiment method and investigation of solid-liquid-gas interaction mechanism of coal reservoir will influence the understandings of gas storage and production, and these are significant to coalbed methane exploration and development.The essence of methane adsorption by coal is physical adsorption (mainly the Van der waals force) in the solid-liquid-gas coupling system, coal adsorption capacity is decided by the intermolecular force between coal and methane molecule which is influenced by different phase water. Gaseous water in moisture-equilibrated coal sample is easier to be adsorbed on the coal surface than that of methane because of the hydrogen bond, which makes adsorption potential of coal surface lower than that of dry coal sample. As for water injection sample, liquid water enters fissures and pores, making the coal surface wet and forming monomolecular water layer under high pressure. Then methane is absorbed on the coal surface because of the Van der waals force from wet coal matrix and water film. This Van der waals force between wet coal matrix and methane of water injection sample is lower than that between dried coal matrix and methane of dried sample but it is higher than the force of moisture-equilibrated sample, and so does superficial potential of water injection sample. Coal wettability, which is related with pore structure, determines the adsorption capacity of samples with regard to different coal rank. In adsorption experiment, adsorption diffusion rate of coal dominates the speed of methane adsorption process. Adsorption diffusion rate of big grain size sample is higher than small one which has bigger total surface area and the more complicated diffusion path. The diffusion rate of high porosity sample is higher than that of low porosity sample in different coal rank. Consequently, pore structure is the reason that mainly influences the adsorption diffusion coefficient of coal. Gaseous water in moisture-equilibrated sample increases the medium viscosity of gas moving and decreases coal adsorption capacity; therefore adsorption diffusion coefficient of mosiure-equilibrate sample is lower than that of dried coal sample. Liquid water in water injection sample wets coal surface and forms water film, and the medium viscosity of water injection sample is close to that of moisture-equilibrated sample. However, the interaction force of wetted coal matrix and water film of water injection sample is higher than the force of moisture-equilibrated sample, which leads to the adsorption diffusion rate of water injection sample higher than that of moisture-equilibrate sample but lower than diffusion rate of dried coal sample.Like all models, Langmuir and Dubinin-Radushkevich have their own hypothesis conditions which make them can be applied in different reservoir pressure ranges and different type coal samples. For low rank moisture-equilibrated and water injection sample, the adaptability of Dubinin-Radushkevich model is greater than that of Langmuir model because Dubinin-Radushkevich model can be applied in wider reservoir pressure range. However, for middle or high rank moisture-equilibrated and water injection sample, the adaptability of Langmuir moedel is greater than that of Dubinin-Radushkevich model for the fitting degree of Langmuir moedel is higher than that of Dubinin-Radushkevich model.