Property Evaluation Of Gas Adsorption&Desorption And Gas Flow Experiments In Shales

Compared to conventional gas reservoir,the porosity and permeability of shale gas reservoir is extremely low,which means the hydraulic fracturing is an essential step for the industrial producting.Nevertheless,the production rate of shale gas declines very fast and therefore the study on gas flow mechanism in shale gas reservoirs is the basis for reasonable and effective development of shale gas reservoirs.Experimental research and analysis on the physical properties,adsorption/desorption characteristics and gas flow mechanism were carried out in this study.According to the research of physical properties,the shale samples in this experiment are mainly composed of clastic minerals,clay minerals and authigenic brittle minerals;Organic carbon content?TOC?is relatively high in these shale samples;isolated nano scale pores can be observed in the SEM images of core samples and the tested porosity and permeability of them are extremely low.The isothermal adsorption/desorption experiments have shown that the adsorption capacity increases with the increase of equilibrium pressure and decreases with the increase of temperature and moisture;the adsorption capacity of different gases in turns is CO₂>CH₄>N₂;the hysteretic characteristics of desorption is oberverd through the misalignment between adsorption isotherm and desorption isotherm.To study the gas flow mechanism of the shale gas,a set of experimental installation was designed and established.Experimental results show that the tested gas permeability exponentially increases with the increase of average pore pressure and decreases in the form of fractional function with the increase of confinement pressure;and the tested permeability in the process of increasing confinement pressure is higher than that in the process of decreasing confinement pressure.In this study,Langmuir,Freundilich,L-F,Toth,BET,D-R and D-A models were used to fit the isothermal adsorption experimental data.The results have shown that all these models have good performance in fitting the absolute adsorption capacity,howerver,the fitting results are greatly affected by the values of adsorption phase density used for fitting.For the models based on the adsorption potential theory,the fitting results are also greatly affected by the values of pseudo-saturation vapor pressure.To solve the above problems,an improved four-parameter D-A adsorption model(?1₀??8??_?6?was established.The adsorption data in this study's experiments and the adsorption data of North American shale gas were fitted by this new model.Fitting results suggest that this new model has satisfied fitting effect and the fitting goodness are all above 0.99.