An adsorption model with multiple factors for shale gas:Taking the Wufeng Formation-Longmaxi Formation shale in southeast Sichuan as an example

The adsorption of methane on shale is simultaneously affected by the physical properties of shale and external storage conditions.To establish a shale gas adsorption model that takes into account the influence of multiple factors,the Wufeng FormationLongmaxi Formation shale in southeastern Sichuan is used to carry out the experiments on the adsorption of methane on shale with different total organic contents (TOC) at different temperatures and different water contents.The Langmuir model is used to fit the adsorption data and the quantitative relationship between saturated adsorption capacity,Langmuir pressure and temperature,TOC,water content are analyzed,and finally,the multifactor adsorption model which considering the comprehensive effects of temperature,TOC,and water content is established,also the accuracy of the model is verified by comparing with the measured adsorption data.The results show that the Langmuir model can fit the adsorption data of Wufeng Formation-Longmaxi Formation shale well with high fitting accuracy,the coefficient R2 is between 0.972 8 and 0.998 2.The saturated adsorption capacity has a positive linear correlation with TOC and a negative linear correlation with temperature and water content.In contrast,Langmuir pressure has a negative linear correlation with TOC and a positive linear correlation with temperature and water content.The adsorption capacity of dry shale with TOC of 4.17% is about 39% higher than that of dry shale with TOC of 2.95% at 30 ℃.The adsorption capacity of dry shale with a TOC of 4.17% decreases by 30.6% when the temperature increases from 30 ℃ to 80 ℃.For the shale with TOC of 3.66%,when the water content increases from 0 to 4.2%,the saturated adsorption capacity of the wet sample decreases by 23.1 and 11.6% at 30 ℃ and 60 ℃,respectively.The established adsorption model which based on the Langmuir model and considering the effects of multiple factors can calculate the adsorption capacity of shale with different TOC at different temperatures and different water contents accurately.From comparing with 2 sets of experimental data,the relative error in the entire experimental pressure range is less than 5%,and the average errors are 2.48% and 3.67%,respectively.The proposed model has been verified to have good applicability to different shales by using adsorption data of different shales in other literature.