Study On Stress Change Law Of Casing-Cement Sheath-Formation Combination In High Temperature And High Pressure Wells

In recent years,the problem of annular pressure in high temperature and high pressure oil and gas wells has become increasingly prominent,which makes the safety production and life of oil and gas wells are facing great challenges.The main reason for the formation of annulus pressure is the change of stress in casing cement annulus formation combination of oil and gas wells.The change of pressure,temperature,annulus pressure and confining pressure caused by different conditions such as pressure test,oil test,production and fracturing after cementing will result in the change of stress in casing cement annulus formation combination.Therefore,it is urgent to study the stress change law of casing cement sheath formation combination in high temperature and high pressure wells.In this paper,through the combination of laboratory experiments and numerical simulation,the stress distribution of casing cement sheath formation combination is studied.The indoor experiment is mainly based on the full-scale high-temperature and high-pressure cement ring stress transfer experimental device.According to the theory of thick wall cylinder,an experimental method of stress test of casing cement sheath formation combination under simulated downhole high temperature and pressure is proposed.The stress transfer experiment of casing cement sheath formation combination under different pressure changes in casing,different temperature changes,different annulus pressure changes and different confining pressure changes is carried out.According to ABAQUS software,the stress transfer of casing cement sheath formation combination is considered Based on the formation as elastic body and cement sheath as elastic-plastic body,the experimental model is co MPared and analyzed by using CDP,and then the stress change rule of casing cement sheath formation combination under the actual working condition is analyzed.The analysis results show that the physical model based on cohesive unit and CDP plastic damage theory is established according to the actual formation conditions and construction conditions of South China Sea,and the proposed judgment criteria are used for simulation,and it is found that there is a high degree of agreement with the experimental results.Using the equivalent plastic strain peeq,the damage coefficient sdeg,the compression failure coefficient damagec and the tensile failure coefficient damaget in the laboratory experiment and numerical simulation to study the stress distribution of the composite.The results show that: the change of the pressure in the casing has a great influence on the damage coefficient of the first and second interface of the cement sheath;the change of temperature has a great influence on the damagecoefficient of the first and second interface of the cement sheath relative to the change of the pressure in the casing The influence of the change of the interface damage coefficient caused by chemical reaction is small;the influence of the change of the annulus pressure on the interface damage coefficient of the first and the second is almost the same;the influence of the change of the confining pressure on the interface damage coefficient of the second cement ring is large.Through the experiment and numerical simulation,the pressure in the casing,the annulus and the change of the formation are summarized when the cement sheath body and the interface are damaged under different working conditions in the South China Sea.The research results provide important guidance for the optimization of parameters under different working conditions and the pressure control in the construction process.