Gas Content Prediction Model Of Continental Shale Considering Multi-factor Coupling

Shale gas is the frontier of oil and gas exploration,and its resource distribution significantly affects the world energy landscape.The indirect method is often used for the evaluation of shale gas resources,that means the gas-bearing characteristics of shale are clarified through indoor adsorption-desorption experiments,but the experiments are mostly carried out in a dry environment,lacking deep understanding of the gas-water distribution characteristics of shale reservoirs.As a result,the measured value of shale gas content is generally high,making it difficult to objectively evaluate the exploitation potential of shale gas reservoirs.Based on the current situation of insufficient understanding of methane adsorption law of water-bearing shale,and on the basis of experimental tests,through the mineral composition analysis,reservoir space type division,pore structure and pore size distribution characteristics description of the Chang 7 Member of the Yanchang Formation in the Ordos Basin.The basic characteristics of gas reservoirs such as shale compressibility,hydrocarbon generation capacity,and shale gas enrichment and preservation conditions were analyzed in depth.Simultaneously,the reservoir environment was simulated to carry out methane isotherm adsorption-desorption experiments with shale particles of different sizes under different water conditions,to study the adsorption characteristics of methane gas in shale pore-fracture system and the main controlling factors affecting the adsorption capacity of the adsorbents.Taking theoretical analysis as the center of gravity,starting from the theory of hydrocarbon generation in source rocks and the characteristics of natural gas evolution,combined with reservoir formation geology,rock mineralogy,kerogen geochemistry,adsorption,interface chemistry,etc.,the primary sedimentary water environment of shale reservoirs,the evolution characteristics of gas-water distribution and the influence of primary water on the adsorption capacity of shale gas were studied in depth.Based on the coagulation membrane theory(DLVO theory),the formation process and microscopic mechanism of the hydration membrane on the adsorbent surface are explored.At the same time,the characteristics of the thickness and moisture content of the adsorbed hydration membrane in the pores are described on the micro scale.Taking the model construction as the goal,based on the Langmuir adsorption type of organic matter and dry clay gas-solid two-phase interface,and the Gibbs adsorption type of wet clay gas-liquid two-phase interface,the temperature field,pressure field and other factors considering water sensitivity were established.The coupled shale gas-liquid-solid three-phase interface adsorption model,combined with the calculation formula of free gas content considering the volume of the adsorbed phase,the shale gas prediction equation under different original water saturation was deduced,fitted and corrected with the experimentally measured adsorption isotherm,and the shale core site in Jiaoshiba area of Fuling was used.The desorption data validated the rationality of the model,which provided a new method for evaluating the static resource of methane under different water cuts,which laied a theoretical and technical foundation for the evaluation of shale gas reservoir development potential and the formulation of development plans.