Analysis of Boundary Condition of Stress Calculation on Casing/Cement-Sheath

Reasonable calculation of casing stress is the basis for design of casing string strength and for judging whether casing failure happened.At present,the common method of casing stress calculation is to apply the non-uniform in-situ stress to the casing/cement-sheath.Then according to the calculation results,casing string stress and deformation are analyzed,but in fact,stress concentration effect caused by non-uniform in-situ stress is already released on sidewall rock before well cementation,and can not be applied to casing/cement-sheath.Usually an oval hole is formed on the effect of non-uniform in-situ stress,the sidewall rock already reaches balance under the effect of non-uniform in-situ stress and fluid pressure before cement setting,so the load caused by non-uniform in-situ stress is not taken by casing/cement-sheath,and in short period uniform stress of salt water column is taken by casing/cement-sheath.In the long period development of oilfield,due to rheological property of formation rock,some even all of vertical crustal stress will be taken forward to casing/cement-sheath,then casing/cement-sheath will take the effect of uniform crustal stress,what is the main reason for casing collapse and necking down.The circumferential stress distribution is changed by oval cement sheath which is formed by non-uniform in-situ stress.But non-uniform in-situ stress cannot be concerned as the boundary condition of stress calculation on casing/cement-sheath.