Deformation And Fractures Evolution Characteristics Of Coal And Caprock During CO₂Sequestration

At present, the "greenhouse effect" has caused a rise in the average globaltemperature, which obviously led to the frequent natural disasters and great casualtiesand economic losses. An effective way of reducing CO2, the main cause of"greenhouse effect", is by storing CO2under the earth surface and hence controllingits greenhouse effect. For geological storage of CO2injection into the coal seam, thelong-term safely storage of CO2requires carefully consideration. This paper focuseson the deformation and fracture evolution characteristics of coal and caprock duringCO2injection into coal seam. A comprehensive study has been conducted usingexperimental, numerical, theoretical and analytical approaches and main researchcontents and conclusions drawn from the study can be listed as follows:(1) An experiment of deformation and failure characteristics of CO2-saturatedcoal is conducted to identify effect of CO2on the mechanical properties of coal. Intriaxial experiment, strength, peak strength and residual strength of CO2-saturatedcoal samples are gradually reducing with the increase of injection pressure. And, thepressure effect on peak strength is much more significant than on residual strength.Also, due to stress drop coefficient gradually decreases with the increasing injectingpressure, the destruction form of coal samples are transformed from shear failure tosplitting failure. In conventional triaxial creep, the predetermined axial stress is muchless than the long-term strength of coal samples, steady creep stage are only shown;the predetermined axial stress is less than the long-term strength of coal samples, coalsamples show the deceleration creep and steady creep stages.(2) Pore pressure distribution function during CO2injection into coal seam isdeduced, which take the boundary effect into consideration when CO2permeate to theboundary. Based on three-layer laminated beam model and Winkler assumption aboutfoundation, the relationship between deformation and normal stress of the caprockand CO2injection time is obtained. The normal stress reaches its maximum in theinitial stage of CO2injection, and then decreases. To modify Winkler assumptionapplying double parameters model, the results show that the coal seam withconsidering shear stress of foundation has little impact on caprock normal stress. Tomodify three-layer laminated beam using Winkler assumption with consideration ofoverburden and underlying rock’s restriction on caprock and base, the results showcaprock normal stress obviously reduces.On this basis, considering normal stress distribution of caprock on initial stage under different CO2injection pressure, the bestCO2injection pressure is obtained.(3) Similarity model on deformation of overburden rock during CO2injection isdesigned and the relationship between the model and the prototype is analyzed.During water injection, the rock over the palace, where water in water-stop strippermeates, will induce uplift deformation. With experiment goes on, the deformationcontinues to increase. Due to the influence of the adhesion between the waterinjection pipe and rock, the accumulative deformation in12cm is larger than in2cmfar away from the wellbore.(4) A study of the influence of CO2injection on sealing environment is conductedto obtain evolution of relative mechanical properties of sealing environment and thedamage degree of caprock. The improvement of CO2injection pressure willincreasingly affects relative mechanical properties of sealing environment and thedamage degree of caprock. A new injection method of improving gas injectionpressure by step is presented. Through this method, no damage would occur incaprock when injection interval is30days. A study of the influence of CO2injectionon sealing environment with imperfect caprock is conducted to obtain relativemechanical properties of sealing environment with two kinds of imperfection and thedamage degree of caprock.