Study On The Adsorption Characteristics Of CH₄ And CO₂ On Shale

CO₂ injection for enhanced shale gas recovery(CO₂-ESGR)is a highly efficient and economical technology that can increase CH₄ recovery and at the same time store CO₂,resulting in a win-win situation in terms of economic and environmental benefits.Therefore,in CO₂-ESGR technology,it is important to understand the adsorption characteristics and factors affecting CH₄ and CO₂ on shale,to understand the competitive adsorption relationship between CH₄ and CO₂,and to establish the prediction model of adsorption amount and rate.In this paper,the adsorption characteristics of shale samples taken from Huadian,Jilin,were comprehensively studied using the volumetric method with a high-pressure adsorber experimental system.The elemental composition of the shale samples,the specific surface area and pore size of the samples were analyzed.Then,the adsorption properties of the shale samples on CO₂ at pressures of 1.5,2,4,and 5 MPa and temperatures of 20,40,and 80?,and the adsorption properties of the shale samples on CH₄ at pressures of 1,2,4,5,9,and 12 MPa and temperatures of 20,40,and 80? were investigated.The results show that the dynamic adsorption process of CH₄ and CO₂ on shale samples can be split into two parts.The first part is the rapid adsorption phase at the beginning and the second part is the later adsorption equilibrium phase.During the whole process,the excess gas adsorption and the adsorption rate are positively correlated with pressure and negatively correlated with temperature.And the study found that when the pressure is below 5 MPa,the adsorption capacity of CH₄ is lower than the adsorption capacity of CO₂,and the shale will adsorb CO₂ first,which is favorable to improve the recovery rate.The optimization of model parameters and prediction of adsorption were carried out using the Langmuir model,and the ARD of CO₂ and CH₄ were 4.29-5.25%and 2.41-3.36%,respectively.The nL of CO₂ was larger than CH₄ and PL was smaller than CH₄ at the same temperature,indicating that the adsorption of CH₄ gas on shale was smaller than that of CO₂.nL of the same gas decreased gradually with increasing temperature and PL increased with increasing temperature.Meanwhile,the adsorption rate constants k1 of the two gases were calculated by the calculation model,and the results showed that the adsorption rate constants k1 of the two gases on the shale were positively correlated with the temperature and pressure,indicating that the time taken to reach the equilibrium state was shorter when the pressure was higher and the temperature was higher from the whole adsorption process.This is due to the fact that the whole adsorption process consists of both diffusion and adsorption of gas in the shale matrix,while increasing pressure and temperature can significantly increase the diffusion rate of gas and therefore speed up the time taken to reach equilibrium.