Study On Shale Gas Adsorption-diffusion Behavior And Its Controlling Mechanism

Study on methane adsorption-diffusion in shale matrix is of great significance to improve the understanding of gas adsorption process in organic-rich shales.In order to fill research gaps,several methods,including experimental test,mathematical statistics,numerical simulations and correlations,were employed on shale samples from Lower Permian Shanxi and Taiyuan Formations in Southern North China Basin to reveal the gas adsorption-diffusion behavior and its control mechanism.(1)The “dynamic change” of shale pore structure and gas adsorption-diffusion process is the main reason that affects model fitting results.For unipore diffusion model,the shale pore structure and gas adsorption-diffusion process can be seen as “dynamic change”,ie,“homogeneous large pore contribution” under low pressure steps,and“large pore and small pore contribution” under medium pressue steps,and“homogeneous small pore contribution” under high pressure steps.(2)The gas effective diffusion coefficient is significantly affected by organic matter,clay minerals,brittle minerals and other factors.The organic matter has no significant effect on macropore effective diffusion coefficient,but has a negative correlation with micropore diffusion coefficient.The clay minerals have a negative correlation with macropore effective diffusion coefficient,and positive correlations with micropore effective diffusion coefficients.The brittle minerals have a positive correlation with macropore effective diffusion coefficient,and a negative correlation with micropore effective coefficient.(3)Shale pore structure controls the gas adsorption-diffusion behavior in organicrich shales.Except for part of adsorption rate curve under higher pressure step,most of adsorption rate curves are characterized by “two stage” adsorption-diffusion process:the initial rapid macropore adsorption-diffusion phase and the later slow micropore adsorption-diffusion phase.In addition,by comparing the "inflection point" and "with or without inflection points" of the adsorption rate curves under three pressure steps conditions,four different types of adsorption rate characteristic curves were established.(4)Based on the bidisperse diffusion model,a new calculation method for lost gas content was established.The bidisperse diffusion model can meet the requirements of gas diffusion simulation under various conditions.For the theoretical deficiencies of the USBM linear regression method and the Amoco curve fitting method,this study proposed and established the MCF curve fitting method based on bidisperse diffusion model,this MCF method can meet lost gas content calculation under the condition that the free gas volume accounts for more than 75%.In this study,the gas adsorption-diffusion behavior and its control mechanism were studied,and part of research findings were extended to geological applications in Souther North China Basin.Overall,as an extension of mechanism research,it will have a positive role in promoting shale gas exploration and development in China.