External Load Calculation And Strength Analysis Of Casing Under The Combined Effects Of Strata And Cement Mantle

Casing damage in various oil fields at home and abroad is growing which has become aworldwide problem solved. To achieve safe drilling, there must be acompleteborehole.Borehole system is consists of casing , cement mantle and strata . Through theanalysis of casing damage mechanism it can be know that the casing damage problem iscalculation and criterion of the casing mechanical damage question.Therefore, the loadcalculation and strength theories of strata- cement mantle-casing system must not be delaied.Based on previous theories this article has been mainly do the following :Theory formula of casing external pressure on the uniform and non-uniform strata stressis analyzed through which factors influencing casing external pressure were got. And then afinite element model is created under non-uniform strata stress which can calculate externalpressure of casing. This model includes the twelve factors which affect casing externalpressure. And then calculation results of the new method and theoretical formula arecompared. The error is small with a good accuracy.This new method can provide a referencefor the integrity of the wellbore and casing damage criterion.With the application of the new method of external pressure calculation ,by changing thefactors'size which affect casing external pressure, the law of cement mantle elastic modulusand Poisson's ratio impacting on casing external pressure is analysised. The laws of cementmantle'thickness, strata elastic modulus and Poisson's ratio impacting on casing externalpressure are also analysised . The degree of non-uniform strata stress is also affecting the sizeof casing external pressure .By analyzing the theoretical formulas of casing collapsing strength, factors that casinginfluencing casing collapsing strength are known. By simulating casing loading with finite element software Ansys, law of casing stress distribution are got. The maximum Von Misesstress point of casing was on inside wall, so casing collapsing strength can be analyzed bystudying the stress of casing inside wall . Changing the factors that affect casing stress, therules of strata elastic modulus and Poisson's ratio affecting casing collapsing strength areanalyzed by calculating the size of the stress inside wall. The degree of non-uniform stratastress is also affecting the size of casing collapsing strength. And then the law of cementmantle elastic modulus, Poisson's ratio and thickness affecting on the very collapsing strengthof casing are studied. Cement mantle wether or not discharging casing load were discussed.Also cement mantle missing and shape will affect the casing collapsing strength. Finally, thetheoretical formula of the article is programmed in order to facilitate the comparison of theproposed algorithm results.