Stress-strain Law Analysis Of Cement Sheath For Gas-storage Wells Under Cyclic Injection And Production

Under the condition of cyclic injection and production,the cement ring of gas storage Wells is subjected to alternating stress,which causes fatigue failure of the cement ring of gas storage Wells,leading to poor sealing performance of gas storage Wells.In order to ensure the safe operation of gas storage,it is of great significance to study the stress-strain law of cement sheath under the condition of circulating injection and production in gas storage wells.The mechanical parameters such as elastic modulus,Poisson’s ratio and compressive strength of set cement are measured by uniaxial and triaxial stress tests for set cement.The cyclic loading and unloading experiments of set cement under different stress levels,different loading rates and different confining pressures were designed to analyze the stress-strain law of set cement under different conditions and establish the constitutive equation of set cement under cyclic loading.ABAQUS software is used to simulate the stress and strain of cement stone and casing-cement ring-formation combination,and the stress-strain law is further analyzed.The mechanical deformation of cement under triaxial stress is obviously different from that under uniaxial stress,which is mainly manifested as the increase of cement strength and plastic deformation.Toughening materials such as latex can improve the mechanical deformation capacity of cement,but the mechanical deformation characteristics and stress-strain curve shape of cement are not changed.The stress-strain curve of set cement under cyclic loading has obvious hysteresis characteristic and forms the stress-strain hysteresis curve.The first cycle curve forms the largest hysteretic loop and produces the largest plastic strain.With the increase of the number of cycles,the hysteretic loops become smaller and maintain a stable state.The evolution law of stress-strain curve of cement under cyclic loading is related to stress level,loading rate and confining pressure.With the increase of stress level and loading rate,the evolution of stress-strain curve of cement stone is promoted,and the hysteresis curve develops towards the ultimate strain of cement stone,which eventually leads to the failure of cement stone.The confining pressure has little effect on the stress-strain curve of cement stone.Under alternating stress,even if the stress level is lower than the ultimate compressive strength of cement,the fatigue failure of cement will still occur.The strain at the lowest point of the unloading curve was defined as the cumulative plastic strain,which was linearly related to the number of cycles and was determined by the elastic modulus,stress level,loading rate and confining pressure of the set cement.