Quantitative Characterization Of Gas Adsorption And Desorption Characteristics Of Coal Shale By Nuclear Magnetic Resonance Spectroscopy

Coal shale gas is widely distributed and reserves are abundant in China’s coal fields,which has high exploration and development value.At present,low efficiency and poor stability of gas production in coal shale production stage are the bottlenecks that restrict its industrialized mining.Essentially,it is closely related to the unclear of gas adsorption and desorption mechanism in coal shale under three-dimensional stress.Therefore,it is of great significance to carry out macro and micro quantitative research on related adsorption and desorption characteristics for safe and efficient mining of coal shale gas.Taking the coal shale in Shuangyashan Basin as research object,introducing low-field NMR spectroscopy technology,and a comprehensive experimental platform of coal shale under three-dimensional stress by using NMR spectroscopy has been built independently.Core loading and unloading experiment,free gas calibration experiment,pore size distribution experiment of coal shale and quantitative characterization experiment of adsorption and desorption characteristics of coal shale gas under three-dimensional stress were carried out in turn.The influence laws of loading and unloading process on basement signal,characteristics of free gas by using low field NMR Spectroscopy,quantitative characterization method of pore and fissure in coal shale,macro-micro adsorption and desorption characteristics and hysteresis laws of coal shale gas under three-dimensional stress were studied respectively.A new model for permeability and productivity of coal shale gas was established and the influence of quantitative characterization of adsorption and desorption characteristics on permeability and productivity was discussed.The main research conclusions are as follows:（1）The quantitative characterization of the distribution of micropores,mesopores,macropores and fractures in coal shale is realized by using the low-field NMR T₂ spectroscopy,which will be a physical property characterization method that can be widely applied to low porosity and low permeability complex porous media reservoirs.（2）The macro isothermal volume method and micro nuclear magnetic resonance T₂spectroscopy method were used to quantitatively characterize the adsorption-desorption characteristics of coal shale gas under three-dimensional stress,and it was found that the gas adsorption and desorption of coal shale gas was closely related to the action of external stress.During the adsorption and desorption process,there is a linear relationship between the micro T₂spectroscopy amplitude integral of porous medium-confined gas and the average effective stress,while the relationship between the micro T₂ spectroscopy amplitude integral of adsorbed gas and the average effective stress conformed to Langmuir Freundlich adsorption model and Dubinin astakhov desorption model respectively.The quantitative characterization models between the amount of porous medium-confined gas was linear function.The quantitative characterization models between the amount of adsorbed gas was logarithmic function.The macro-micro quantitative characterization of adsorption and desorption of coal shale gas under three-dimensional stress provides a new idea for quantitatively studying the occurrence and migration law of unconventional oil and gas reservoirs under complex stress environment.（3）Through the established T₂ spectroscopy quantitative characterization model of adsorption and desorption hysteresis effect,the micro quantitative characterization of adsorption and desorption hysteresis effect of coal shale gas under three-dimensional stress was carried out.During the desorption process,the microscopic quantitative characterization coefficient of the hysteresis effect of porous medium-confined gas was 42.13%,while the relationship between the microscopic quantitative characterization coefficient of the hysteresis effect of adsorbed gas and the average effective stress conformed to the Bi Hill model,and the hysteresis degree of adsorbed coal shale gas first increased and then decreased.The microscopic quantitative characterization of adsorption and desorption hysteresis effect of coal shale gas under three-dimensional stress provides a new experimental reference for the accurate design of unconventional oil and gas reservoir development scheme.（4）New permeability and productivity models of coal shale gas considering adsorption and desorption characteristics and introducing NMR spectroscopy T₂ parameter was established,and it was found that the influence of quantitative characterization of coal shale gas adsorption and desorption on permeability and productivity could not be ignored.This study further improved the micro adsorption and desorption mechanism of coal shale gas under complex stress state,and it provides a theoretical and experimental reference for the high and stable production of coal shale gas in the production stage.This paper has 85 figures,30 tables and 206 references...