Mechanism And Theoretic Models Of Casing Failure And Numerical Calculation With Them

Failure prosess of casing well is complicate. Technique and practice used in drilling, production and development program all affect casing failure, which involves some subjects, such as rock mechanics, percolation flow mechanics, pipe string mechanics. All these decide the factors that affect casing failure are variety and complexity. In recent years, many scholars have studied the causes and mechanism of casing failure. Some mechanisms of casing failure are recognized, but there are not effective measures to prevent or retard casing failure, and methods to calculate stress aced on casing by formation in view of specific situations. Calculating and controlling stress acted on well casing is the key to take reasonable measures to stop or slow down casing failure, which are to calculate stress on casing in a given development condition, to determine casing strength with the stress and to promote casing string design meeting requirements of petroleum engineering.In this paper, information of 3800 wells with casing failure in Daqing and Jilin Oilfields and the factors affecting the well failure are analyzed statistically. The main characters and the trends of casing failure are grasped, and some conclusions are gotten: most locations of casing failure is in mud shale rock; and water encroachment in mud shale is a precondition of casing failure; the increase of well number with casing failure in oil reservoir and forms of casing failure and the well distribution bring to light the mechanism of casing failure in oil reservoir, which is bedding slip between shale layers in oil reservoir; the mechanism of casing failure in overlying formations is slip between layers caused by deformation in the direction of perpendicular to the layers; water injection pressure is direct dynamic force to drive formation moving that casing failure results from, and formation dip speeds up casing failure.The tests of mud shale properties by water encroachment, and the way and velocity water advanced in mud shale are taken. The result of the tests indicates that water advanced along bending interfaces and fractures vertical to the beddings. The velocity and distance water advances in mud shale is the function of and direct proportion to water injection pressure. Cohesion force and internal friction angle of mud shale are linearly decreased and shearing strength is'decreased rapidly with hydrated level of the shale. Alternations of mud shale properties are affected by clay content in the shale.According to result of statistical analysis and the tests, casing failure causes and mechanism are determined, which are casing damage by creep deformation of shale in deep formation, by shear slip in overlying formations, and by local slip in oil reservoir. Based on the mechanism of casing failure, the model of creep deformation, the model of combination finite element for bed interface problem, and the model of fluid-solid coupling, are developed. The model of combination finite element solves the contact problem of non-smooth interface of large-scale water encroachment area in mud shale. From the result of numerical simulation with the model, bedding glide does not occur in injection interval, but at hydrated weak plane in overlying formation above the injection interval. The coupling model reveals the mechanism of casing failure of water injection wells, which is pressure inequilibrium in local area results in casing damage. Percolation flow pressure field in oil reservoir is lead into the model, and coupled with reservoir. Sensitivity analysis of the factors affecting casing failure and numerical calculation are carried out with the model. 87.38% of Calculated casing failure wells conforms to the reality of casing failure in the simulated area.Based on the collapsing tests of casing, mechanical standard of casing yield failure and design method of casing equivalent strength are given. Effect of casing perforation and cement sheath on casing strength are tested and analyzed with mechanical test and theory, and their influence coeffi...