Mechanical Analysis Model And Application Of Casing-cement Sheath-formation System With Consideration Of Its Initial State

Annulus with pressure and gas channeling are a worldwide problem,which is mainly caused by wellbore integrity failure.Calculating the stresses of casing-cement sheath-formation system is a base to wellbore integrity design and verification.However,all analytical models and most finite-element models never consider the influence of its initial loaded state,including its initial stresses and initial strains.Only a few finite-element models have taken the initial stress of cement sheath into account,but neglected the initial strain of wellbore.The results and understanding are not comprehensive enough.To calculate the stresses and strains of casing-cement sheath-wellbore system accurately,to direct wellbore integrity design and verification perfectly,a new analytical model which can consider the influence of its initial loaded state is needed.On the basis of the results of previous studies,according to process of oil and gas well construction and porous medium properties of cement sheath,this paper speculates the initial stresses of casing and cement sheath and formation at the initial moment of the end of wait-on-time of cement slurry.The casing-cement sheath-formation system is divided into the initial stress state,the virtual complete unloaded state and the reload state.Based on the assumption of porous medium of the cement sheath and the formation,the geometry size of casing-cement sheath-formation system under the condition of complete unloaded state is inversed by the initial stress state with the application of thick walled cylinder theory of elastic mechanics and modified displacement continuity condition.Then,casing pressure and in-situ stress of later operations are reloaded to the system to obtain a new casing-cement sheath-formation model which can consider the influence of its initial loaded state.A living example has been finished and discussed.It is showed that whether taking the influence of its initial loaded state into account can cause significant differences to the stresses and their variation trends of casing-cement sheath-formation system.Considering the influence of its initial loaded state is quite necessary and conducive to direct wellbore integrity design and verification perfectly.The application of the system model considering the influence of the initial state on the design and verification of the wellbore integrity is discussed.In order to evaluate the mechanical integrity of the cement sheath,dimensionless tensile failure index and shear failure index and equivalent stress failure index are defined using the failure criterion of cement sheath.The casing-cement sheath-formation model established in this paper is applied to provide the reasonable range of internal pressure in casing during the later operations of a given oil and gas well and optimize the mechanical properties of cement stone according to engineering demands of the later operations.