Creep Model Of CO₂ Geological Storage Caprock Based On Fractal Derivative And Engineering Application

Since the 14th Five-Year Plan,the goals of carbon peaking and carbon neutrality have been elevated to a national strategy.Under the present economic model,the capture,use and storage of CO₂ is the most important part of the strategy to achieve carbon peaking and carbon neutrality.The captured CO₂ is transported to the specially constructed brine layer by using the supercritical nature of CO₂,which is the most direct and effective CO₂ storage way.To ensure the long-term effectiveness of storage,it is essential to study the caprock creep characteristics of the CO₂ geological storage area.In this paper,the creep characteristics of CO₂ geological storage caprock under different stress conditions is investigated experimentally,a nonlinear viscoelastic-plastic damage constitutive model is theoretically studied based on fractal derivative and damage mechanics theory,the application research of the existing geological storage project is realized by the secondary development of the constitutive model.The main conclusions are as follows:（1）The creep properties of salt rock under different loading conditions were investigated by uniaxial and triaxial salt rock creep tests.The results show that the complete salt rock creep consists of three stages:attenuation,steady and accelerated creep.The accelerated creep is related to deviatoric stress,and at low deviatoric stresses,the salt rock only shows attenuation and steady creep,while at high deviatoric stresses,the salt rock shows a complete three stages.Triaxial creep damage is mainly in the form of shear damage due to the confinement of the confining pressure.The confining pressure has a weakening effect on creep deformation.（2）Based on the fractal derivative theory and damage mechanics theory,a fractal viscoelastic-plastic damage constitutive model is established and extended in three dimensions.This model has a concise form,a small number of parameters,high computational efficiency,and is capable of effectively characterizing the deformation process during the entire creep stage of rock.The accuracy and applicability of the model are verified by comparing with the creep test data of salt rock and mudstone.Different model parameters have different effects on the creep curve,and different forms of creep deformation can be described by varying the values of the parameters.（3）The finite difference form of the fractal viscoelastic-plastic damage constitutive model is derived,and the secondary development is carried out based on FLAC3D software.The accuracy of procedure is verified by simple examples,which shows that the numerical solution is in general agreement with the analytical solution.Meanwhile,a comparison between the results of the test and the numerical simulation is carried out with the help of the triaxial creep test of the mudstone,which further verified the correctness of the program development.（4）A part of the Permian Shiqianfeng Formation in the Ordos Basin is selected as the geological environment for engineering application.The pore pressure of the reservoir during the injection process and the long-term stability of the caprock after injection are analysed,taking into account the creep characteristic of the caprock.The simulation results show that a small amount of CO₂ migrates upwards and invades the cover layer while CO₂ moves along the reservoir.With the increase of the total amount of CO₂injected,the invasion of CO₂ into the cover layer becomes more and more obvious.The intrusion of CO₂ will cause uplift and deformation of the overlying rock layer in the injection zone,and the deformation will increase the closer to the injection zone.Cap rocks occur creep deformation over the time frame studied and are in a stable creep phase.In addition,the deformation value of the rock mass is small,and the upper overburden can maintain good stability for a long time.