Theoretical Study Of Casing Stress And Strength Under Long-Term In-Situ Stresses

In this paper, for the problem of casing collapse on cementing status caused by long-term In-Situ Stresses, it puts forward the research method of casing stress and anti-collapse strength under long-term In-Situ Stresses. The following contents were analyzed and studied by this method: Original casing stress generated by non-load on cementing status; The stress distribution on casing and stratum around the cementing borehole under long-term In-Situ Stresses; To determine the casing stress & deformation and possible location of casing collapse & load under In-Situ Stresses on different cementing status; The theoretical methods provided a effective simple means for explaining the partial casing damage on engineering practice, establishing technology of preventing casing collapse and casing drilling program.Through analyzing the actual cementing technique, it summarized and founded three solid models and casing stress mechanical ones for casing cementing status. By analyzing the cementing technology, it put forward the concept of casing cementing circumferential stress generated by cementing hydration and solidification. This cementing circumferential and the axial stress caused by other load are fixed in the casing at the same time, and called the casing original stress in the casing by no-loading.The casing original stress had an adverse effect on casing anti-collapse. It tested the circumferential stress of cementing casing by experimentation and proved the existence of casing original stress on cementing.By applying the classical theory of elasticity of the semi-inverse method and the superposition one, it set up firstly the distribution theory of stress in stratum around the cementing borehole under long-term In-Situ Stresses, put forward the opinion that the radial, annulus and shear stress are not zero on the well wall. The paper reckoned that the stress on the wall is cycle at cementing status. Distribution theory of casing stress is obtained at cementing status under long-term In-Situ Stresses. It pointed out the collapse is partial. The theory in the paper could be applied to engineering calculations directly. It is simple and convenient.Analyzed the casing stress at general cementing status, the conclusion was obtained that more homogeneous stratum stress less easily damaged and in contrary the casing were easily damaged partially under long-term In-Situ Stresses. Firstly applied adjacent balance criterion to discuss the steady critical load for eccentric cementing status, it provided the opinion that the security working load for eccentric cementing was decided by the strength and stability. Firstly applied the thin-shell theory to analyze the casing stress and deformation on random lacuna cementing, it analyzed the casing stress and deformation under cement sheath rigidity and elastic partial annular cementing lacuna, by trial calculation with the semi-inverse method, it determined the border issues problem, solved the displacement , stress and stress distribution that consisted with the boundary conditions and differential equations, identified that casing collapse occurred mainly in the central lacuna under two types of constraints , and on the edge under rigid constraint, set up the condition of casing strength design and failure ones under this environment. It simply studied the casing stress and deformation under elastic partial square lacuna cementing, and put forward new ideas deeply to analyse casing partial collapse.The casing stress and deformation at different lacuna cementing were calculated with FEM and engineering example.The result proved that the theory and method were right. Analytical and numerical results indicated that the cement sheath can greatly enhance the casing anti-collapse at scope of stress range by combined the analytical and numerical solutions; The conclusions that casing had the possibility of collapse under long-term In-Situ Stresses at four cementing status would explain availably the engineering facts that the partial collapse is taken place on cementing casing. The engineering example and the academic result in common use proved that the analyses theory was validity and more universality for casing stress and anti-collapse strength. The theoretical calculation is simple, convenient and exact.There would be high significance in engineering for reasonable cementing technology, formulating prevention for casing collapse.