Experimental Study On Adsorption And Deformation Characteristics Of Shale Under CO₂

The use of supercritical carbon dioxide injection into shale gas reservoirs to increase shale gas recovery can not only achieve efficient shale gas extraction,but also achieve permanent storage of CO₂ in shale reservoirs.The deep shale reservoirs with low porosity and low permeability are available to store CO₂ in a geological tine scale.The CO₂ adsorption capacity and adsorption deformation effect of shale reservoirs are important factors affecting the effectiveness of CO₂ sequestration and limiting the stable of shale gas production.Therefore,the research on the adsorption and deformation properties of shale by adsorbing CO₂ has certain scientific significance.In this paper,we performed CO₂ adsorption-desorption equilibrium time effects experiments（adsorption and desorption）and CO₂ adsorption deformation experiments under different pressures on the Longmaxi Formation shales in the Fuling area.The apparent and absolute CO₂ adsorption capacity of shale and the relationship between the deformation and the adsorption capacity were studied.The main research results are as follows:（1）The experiments obtained the law of adsorption-desorption equilibrium time and the adsorption deformation characteristics of shale.The adsorption equilibrium time gradually increases with enhancing CO₂ pressure within 0⁶.27 MPa.When the pressure reaches the critical pressure of CO₂ nearby,the adsorption equilibrium time shrinking rapidly.Continuing to increase pressure,the adsorption equilibrium time increases again.The desorption equilibrium time of shale increases with CO₂ pressure.It tended to be stable after the pressure is higher than the critical pressure.The anisotropy of shale adsorption deformation was observed at 0¹6MPa,and the axial and radial strain ratios were between 1.226 and 1.086,which gradually decreased with enhancing CO₂ pressure.（2）Improved Gibbs absolute gas absorption calculation method,establish a gas absolute adsorption capacity calculation model which adapted to the"stage balance method".The model shows that:the absolute amount of CO₂ adsorbed by shale can be determined by the amount of CO₂ injected into the system,volume of CO₂ adsorption phase and adsorption volume strain of shale.Absolute adsorption of CO₂ is mainly affected by the quality of CO₂ injected into the system under low pressure.The volume of the CO₂ adsorption phase increases rapidly with the equilibrium pressure increase,and becomes the dominant factor affecting the absolute adsorption capacity.The volumetric strain factor has less influence on the absolute adsorption capacity,because the volumetric strain-induced adsorption amount accounts for the absolute CO₂absorption capacity less than 5%（3）Have a comparative analysis of the differences between CO₂ apparent adsorption capacity and absolute CO₂ adsorption capacity of shale,and discuss the relationship between shale deformation and CO₂ adsorption capacity.There is a quadratic relationship between the amount of CO₂ apparent adsorption amount and equilibrium pressure,and an exponential function between CO₂ absolute adsorption amount and equilibrium pressure.The absolute absorption of CO₂ is 1.51 to 10.30 times of apparent adsorption of CO₂ in the same pressure range.There is a quadratic function relationship between shale volumetric strain and CO₂ apparent adsorption,and a Langmuir-type relationship between the amount deformation induced by adsorption and the absolute adsorption of CO₂.According the results to this study.The results of the study on the equilibrium time for adsorption and desorption of shale under different equilibrium pressures can be used to guide the adsorption and desorption experiments conducted by the laboratory.The absolute gas adsorption model can be used to calculate the adsorption of other adsorbent gases in rocks.The calculation result of absolute CO₂ adsorption amount can provide reference for estimating the amount of CO₂ adsorbed in the process of CO₂ geological storage in shale reservoirs.The model of shale adsorption deformation and CO₂adsorption amount established in this paper can be used to predict the shale deformation by the amount of shale adsorbed on CO₂,and provide experimental basis for the realization of CO₂ enhanced shale gas recovery.