Study On Strength Reliability Assessment Of Well Casing In Deep Complex Formations

Due to the complexity of well structure, multiple levels of casing, downhole conditions exist more uncertainties, casing failure accidents have occurred more occasionally in deep well. At present, the wellbore casing safety and reliability analysis are mainly based on the casing strength calculation and the magnitude of the safety factor. The evaluation methods on the basis of the existing API/ISO and GB/SY standards design of casing can’t fully ensure well integrity during the process of casing serving lives under complex loadings. For wellbore system, the composite casing-cement-formation interaction mechanism of multilayer structure, deformation and stress distribution law in deep and complex formation are still needed further research. Therefore, the relationship between the wellbore casing failure mechanism and external load is a key scientific problem. The casing-cement- formation, as a complete system, The research is necessary to establish evaluation criteria and reliability evaluation method for wellbore casing failure considering conditions of stratigraphic information uncertainty, to ensure the safety and reliability during drilling operations and subsequent wellbore productions.Firstly, the paper put forward a calculation method for casing external loading in which cement unloading effects and internal pipe empty could be considered, through the establishment of casing-cement-formation mechanic models under uniform and non-uniform loading conditions. According to mechanic analysis, the main factors influencing on casing external loadings are in-situ stress and formation elasticity modulus, second factors includes pipe empty coefficient, cement elastic parameters and formation Poisson’s ratio and density of drilling fluid, etc. Examples calculation shows that the method is suitable for casing external loadings calculation in deep wells.Secondly, considering the parameters randomness of deep complex stratigraphic information, the paper put forward a probability method to describe drilling geological characteristic parameters and formation pressures. In term of treatment of original well logging data, single well and regional well profiles including rock cohesive force, internal frictional angle, axial compressional and tensile strength, elasticity modulus, Poisson’s ratio and other parameters were set up. Thereafter, vertical and horizontal distribution profiles of formation pressures and in-situ stresses were given. According to wells field data in Bock Xinchang at Sichuan oilfield, the range of in-situ stress of each strata group are gotten, the heterogeneous degree of casing external loadings in the research region is in the range of 1.14 to 1.26.At last, according to probability distributions of casing external loadings and casing pipe performance parameters, the randomness of these parameters were discussed, then probability distribution functions for casing external loadings and internal pressure were obtained. Thereafter, probability distribution functions of collapse resistance and internal pressure resistance were also established for casing strings before and after wear. On these bases, the casing safety evaluation method for calculating external strength and internal pressure strength were put forward based on the “load-strength” interference theory. Single well and regional wells as examples analysis in some fields were completed. The results reveal that safety and reliability evaluation as an auxiliary means of safety factor method could provide the basis for the selection of regional safety factors.