Experimental study on the effect of average effective stress on micro adsorption and desorption characteristics of coal shale gas

The difference of gas production law between initial stage and later stage of shale gas well is essentially attributed to the difference of occurrence, migration and production mechanisms of adsorbed gas and porous medium-confined gas in three dimensional stress states. In order to reveal the influence of average effective stress on the production law of porous medium-confined gas in the initial stage and adsorbed gas in the later stage of coal shale gas well production from a micro perspective, and to simulate the whole processes of coal shale gas adsorption and desorption in situ stress states, nuclear magnetic resonance T-2 spectrums method was applied to the whole process of experimental research on isothermal adsorption and desorption of the same coal shale sample situ stress state from Dongbaowei Mine #36 seam in Shuangyashan basin. The average effective stress was taken as the comprehensive measurement index of coal shale, and the amplitude integral of nuclear magnetic resonance T-2 spectrum was used as the characterization index of micro adsorbed gas and porous medium-confined gas. The relationships between micro adsorbed and porous medium-confined gas content and average effective stress of coal shale in adsorption and desorption were quantitatively studied. The relationships between the hysteresis coefficient of micro adsorbed and porous medium-confined gas and the average effective stress were studied quantitatively. The influence of average effective stress on the occurrence and output of micro adsorbed and porous medium-confined gas in coal shale was clarified by multi factor linear regression. The experimental results showed that, the adsorbed gas content of coal shale and average effective stress could be fitted by Dubinin-Radushkevich function model in the adsorption and they could be fitted by Weibull function model in the desorption. The porous medium-confined gas content of coal shale and average effective stress could be fitted by linear function model. The hysteresis coefficient of micro adsorbed gas and the average effective stress accorded with the logarithmic function model. However, the decrease of average effective stress had little effect on the hysteresis coefficient of micro free gas, and the hysteresis coefficient of porous medium-confined gas was approximately 41.53%. The pore pressure of reservoir and vertical stress of overlying strata were the main control stress factors of micro adsorption and desorption, respectively. The occurrence and output of free gas were mainly related to the stress of surrounding rock.