Study On Safety Of Downhole Worn Casing Situated In Compressible Viscoelastic Rock

Intermediate casings play important roles in preventing from the collapse of wellbore and isolating the formation with abnormally high pressure in the drilling process.The rotating drill pipe contacts with the inner surface of intermediate casings in the subsequent drilling process that will lead to the wear of intermediate casings and impair the load carrying capacity and safety of casings.In particular,the worn casing is more likely to loss the stability in the viscoelastic surrounding rock under the action of the non-uniform in-situ stress.According to the result of casing wear experiment,the paper analyzes influence factors of casing wear and wear mechanisms of casing samples surface,develops a model to calculate the wear of casing.A new geological mechanical model is developed to analyze the loadings of casing situated in compressiable viscoelastic rock.The paper presents new methods to calculate the residue strength and failure criterions of stability of worn casing.The paper developes a new strain testing device of worn casing under the non-uniform loading,measures the hoop strain of the drill pipe embedded and non-concentric worn casing samples,verifies the simplified model of worn casing.The main conclusions are as follows.(1)Experimental study on casing wear:Influences of the casing steel grade,the contact force,the barite granularity,the mud type,the amount of sands and the density of mud on the wear of casing are overall analysed.The paper determines the wear mechanism of casing samples in the different test condition,adopts the energy efficiency model to calculate the remaining wall of casing.The wear coefficient of casing is inversely proportional to steel grades and hardness of casing samples,is proportional to contact force,the amount of sands in the mud and the density of mud.Under the same conditions,the wear coefficient of N80casing is 2.4 times larger than that of VM140.Under the condition of mud density 2.0 g/cm~3,the wear coefficient of VM140 casing is 2.3 times larger than that under the condition of mud density 1.4 g/cm~3.Scanning electron microscopy of the worn surface of casing samples show that the wear mechanism of casing surface is mainly adhesive wear under the all working condition.It is feasible that the wear efficiency models based on adhesive wear mechanism is used to calculate the wear of casing.A new impact-sliding-abrasive wear model is developed in the paper.According to the wear and friction coefficients of downhole casing which are measured in the paper and drilling parameters which are offered by the oil field,the wear depth of casing in a well is calculated.The remaining wall of casing is a fundamental data which is used to analyze the safety performance of downhole casing.(2)Analysis of casing loadings in the compressible viscoelastic rock:Considering the compressibility of viscoelastic surrounding rock and cement sheath,a new geological mechanics model that consists of casing,cement sheath and surrounding rock is developed.The paper calculates loadings of casing situated in the compressive visoelastic rock by mean of the elastic-viscoelastic corresponding principle,analyzes the influence of geometrical and physical parameters in the model on loading of casing.In the compressible viscoelastic surrounding rock and cement,the stable value of casing loadings is about 11%larger than that in the incompressible viscoelastic surrounding rock and cement sheath.Regardless of the compressibility of viscoelastic surrounding rock and cement sheath will underestimate loadings acting on the external surface of casing.It is very dangerous that the underestimated loadings are used to evaluate the failure of downhole casing.(3)Analysis of the residual strength of worn casing under the action of internal and external pressure:Simplifying the drill pipe embedded wear model into the non-concentric wear model,selecting suitable stress functions under orthogonal curvilinear coordinates,applying the superposition principle,the paper determines the stress distributions of worn casing under the action of the internal and external pressure,proposes new methods to calculate the residue strength of worn casing.Theoretical solutions of the hoop stress of non-concentric wear casing are almost the same as numerical solutions of that.The relative errors of burst and collapsing strength of worn casing between the theoretical and experimental values are less than 9%and 7.5%,respectively.It is feasible that developed formulas in this paper are used to calculate the residual strength of worn casing.According to the formula in this paper,residual strengths of worn casing in a well,displacement fluid density,wellhead pressure and allowed liquid level depth are calculated that ensure testing for oil,completion and safety of downhole worn casing.(4)Analysis of the stability of worn casing:The paper builds a new mechanics model to analyze the stability of worn casing with the initial ovality under the action of the nonuniform loadings,establishs a nonlinear differential equation of radial deformation of worn casing applied by the non-uniform loading.The small parameter perturbation method is adopted to linearize and solve the nonlinear differential equation by mean of two small parameters which are the the non-uniformity of loadings and thickness,respectively.The paper builds the formula to calculate the radial deformation and the relative strain of worn casing,establishes a stability criterion of worn casing under the action of the non-uniform loading by comparing with the critical relative strain of unworn casing under the action of the uniform loading for the first time.Buckling critical relative strain of worn casing is inversely proportional to the diameter of casing,is proportional to the thickness,have nothing to do with the casing ovality and the non-uniformity of loadings.Bucking relative strains of worn casing are linear relationship with the initial ovality and wear depth of casing,are parabolic relationship with the load non-uniformity.(5)Experimental study on the hoop strain of worn casing:The paper develops a new device to measure the hoop strain of worn casing,process the unworn,non-concentric and drill pipe embedded worn casing samples,measures the hoop strain of all kinds of casing samples,analyzes the analytical and numerical solutions of the hoop strain of casing samples.The errors of hoop strain between the analytical,numerical and experimental value of unworn casing are all less than 8%.This shows that the experimental data of strain of casing are reliable,numerical analysis model and the hoop strain calculation formula of casing are correct.The error is less than 5.3%and 1.8%between the experimental and numerical solution to the hoop strain of the outer surface of the two kinds of worn casing,respectively.When casing wear depth is small,the error of hoop strain between the non-concentric,90 mm and 130 mm drill pipe embedded worn casing is less than 8%and 5%,respectively.When analyzing the residual strength of worn casing,it is feasible that the drill pipe embedded wear casing model is simplified into the non-concentric wear one.