Mechanical Calculation And Judgement Of Casing Failure

Casing failure is a problem to be solved in the world. A lot of scholars researched on the problem of casing failure in many fields at home and abroad,but they can't resolve it fundamentally.Through analyzing the mechanism of casing failure,we know the essence of casing failure is the calculation and decision of casing failure mechanics.Since the calculation and decision of casing failure mechanics are not only related with the distribution of original earth stress,formation body and the fluid-solid coupling action of fluid vadose field,but also related with the loading calculation of casing-cement mantle's external stress because of reservoir stress redistributed and the decision of casing failure's yield strength under the loading. The article analyses the present situation of casing damaged wells,casing failure reasons and mechanism,the types and affecting factors of casing damaged wells comprehensively.It highlights the stress field of the area around the casing damaged wells,and establishes earth stress and fluid pressure'mathematical model on the base of fluid-solid coupling theory.It establishes mechanical model that reflects casing damaged wells under the complex geologic condition, and calculates used of FEM analysed by nonlinear mechanics.It establishes the calculated model of casing-cement mantle's stress under the external stress,analyses the affecting factors of casing strength,and establishes the mathematical model gived casing's extra stress and the calculated model of the mechanical decision of casing failure's yield strength.The article expounds the mechanism of casing failure reasonably, offers theoretical criterion for designing casing and lays the compact foundation for the establishment of casing failure's criterion.Analyzing the geologic and engineering factors that affect casing failure, we know that mudstone's hygroscopic afterflow,rock's bloating,rock's sand production,high pressure water injection,fault activity and formation activity are main geological factors of casing failure;well network deployment,cementing-completion operation and casing strength are main engineering factors.The calculated models of the erect and horizontal crustal stress and the mathematical model of the stress distribution in the area of wellbore are established.Two and three-dimensional fluid-solid coupling mathematics-mechanics models in the area of casing damaged wells are established. When fluid-formation pressure changed computed by FEM,the changement of strain and stress of initial body is distinguished,and smoothes out along with time-lapse,that means the original phase is main phase in which casing damaged wells buildup.The changement of strain and stress of reservoir and casing is calculated by three-dimensional linear elastic and elasticoplastic fluid-solid coupling model.The results agree well with the actual situation.A model of casing-cement sheath stress is established by perforating, the distribution rule of casing-cement sheath stress and the affection of perforation form on stress are analysed in such case.Sand inflowing in reservoirs helps formatting local empty in the sand around casing and forms additional stress.A model of casing-cement sheath stress is established under sand inflowing in reservoirs, using the stress model the cause of casing failure in such case is analyse.In the process of mudstone creeping, mudstone bits error and slips, the constitutive model of mudstone creeping is established,the cause that mudstone creeping make casing broken is analysed.Casing mechanic constitutive model of mudstone formation conditions is established. The influence of cement sheath on casing is analysed, FEM calculation results show that, the elastic modulus of cement cylindrical decreases,the stress of casing increases at the same condition.The mechanical estimation models of casing collapse resistance is established, we consider that after the casing set down, before cemented, the casing hanging in a drilling fluid will be stretched axially and compressed radially because of the axial stress caused by its gravity and buoyancy and the radial and circumferential stress caused by its inside and outside force. Based on Yitaihe-jianceng collapse resistance criterion we fetch the new casing yield mechanical criterion.