Experimental Study On Swelling Characteristics Of CO₂ Adsorption And Storage In Different Coal Rank

In recent years,with the rapid economic development,the CO₂ content in the atmosphere has increased dramatically,leading to global warming,which has brought a great negative impact on human production and living and sustainable economic development.Therefore,take some technical measures to reduce the CO₂ content in the air has become urgent.Due to the characteristics of large-scale storage and technical operation,CO₂ geological storage technology has received extensive attention from the international community.The deep coal seam has high temperature and high pressure,CO₂ is injected into the coal seam,it is easy to reach supercritical state.The density of supercritical CO₂ is close to that of liquid CO₂,but the viscosity is similar to that of gaseous CO₂,and its diffusion coefficient is nearly 100 times larger than that of liquid CO₂.Therefore,supercritical CO₂ has strong solubility,permeability and transferability,and can extract some of the organics in the coal,increase the fracture opening and gas migration channels in the coal,change the permeability and mechanical properties of the coal,and the adsorption capacity of different coal rank samples in supercritical CO₂ and the expansion deformation have an important impact on the deep coal seam geological storage.Therefore,this research is conducted on the changes of coal surface cracks?the permeability and mechanical properties of coal after supercritical CO₂ soaking for different times and the adsorption characteristics and the expansion deformation of different coal rank samples in gas and supercritical CO₂.The results are shown as below:?1?After soaking in supercritical CO2 for 24 hours,the crack length on the surface of the coal body becomes significantly longer,the original micro-cracks expand,and the crack space expands.Some of the original cracks penetrate directly,some new cracks appear,and the crack width is also obvious.Broadening,in the selected points,the maximum increase in the length of cracks reached 113.6%,the smallest increase reached 50.5%,the maximum increase in crack width was 199.8%,and the smallest increase was also 21.2%.However,as the soaking time increases,the surface crack morphology of the coal changes tiny.?2?Under the same temperature and pressure conditions,the permeability of coals soaked in supercritical CO₂ for different periods of time decreases with the increase of the effective stress,and decreases with a negative exponent.Under the same effective stress conditions,the longer the time for the coal soaked in supercritical CO₂,the higher the permeability of the coal body,and the permeability increased from 0.013mD⁰.048mD before soaking to0.304mD⁰.424mD,with increased by 8 to 22 times.?3?The uniaxial compressive strength of coal after supercritical CO₂soaking is obviously reduced,and with the increase of soaking time,the uniaxial compressive strength of coal continues to decrease,soaking in supercritical CO₂for 5 days,10 days,15 days,20 Days later,the uniaxial compressive strength reduces from 10.53 MPa before soaking to 5.24 MPa,4.49 MPa,3.88 MPa,and3.51 MPa,respectively,with a decrease of 50.24%,57.36%,63.15%,and66.67%.?4?The elastic modulus of the coal body decreases as the soaking time of supercritical CO₂ increases.After soaking in supercritical CO₂ for 5 days,10days,15 days and 20 days,the elastic modulus of the coal decreases from 1.3GPa before soaking to 0.65 GPa,0.61 GPa,0.57 GPa,and 0.48 GPa,with a reduction of 50%,53.08%,56.15%,63.07%?5?The adsorption of CO₂ in coal is closely related to the coal rank.Under the same adsorption pressure,the CO₂ adsorption amount increases with the increase of coal rank.When CO₂ is in the gas state,with the increase of adsorption pressure,the excess adsorption of coal increases gradually,and CO₂reaches the supercritical state when the pressure increases to 8MPa,the excess adsorption reaches the maximum value,and then the excess adsorption gradually decreases with the increase of the equilibrium pressure.?6?The strain produced by the adsorption of CO₂ of different coal ranks first increases with the increase of CO₂ pressure and then gradually becomes stable.After the CO₂ reaches the supercritical state,the strain of each coal rank reaches its maximum value.The volumetric strain can be described by the DR model with the introduction of CO₂ density;the expansion deformation after adsorption of CO₂ in each coal rank has obvious anisotropy,the strain perpendicular to the bedding direction is significantly larger than the strain parallel to the bedding direction.?7?When the CO₂ pressure is low,the volumetric strain after adsorption of CO₂ by each coal rank increases linearly with the absolute adsorption amount.When the CO₂ reaches the supercritical state,with the increase of the absolute adsorption amount,the volumetric strain of the coal body gradually tends to stable and no longer increase,and the volume strain produced by adsorbing the same amount of CO₂ decreases with the increase of coal rank.