Research On The Deformation Law And Repairing Technology Of High-Strength Casing

Underground casing has different levels and kinds of failure on the effect of service period and adverse working conditions, which impedes the normal operation of oilfield gravely. The uneven ground stress can result in the casing deformation, which is one of the most important damage types. Casing deformation law and repairing techniques under the effect of ground stress have drawn plenty of attention and emphasis. Investigating the deformation law and repairing technology is of vital significance to reduce the cost of remedial well treatment and to evaluate oil well production dramatically.In terms of the practical conditions of Puguang gas field, the casing withΦ177.8mm(7inch), SM-C110 steel grade and 12.65 mm wall thickness is taken as the research objective.The deformation law, repairing boundary and the effect of uneven ground stress on plastic repair stress of the casing is investigated.Based on the stress status of underground casing, the casing is assumed to be absolute circular and has complete contact with stratum. The nonlinear contact numerical model which includes casing, cement sheath and stratum is established. The uneven ground stress is applied on the outside of the model to study the effect of uneven degree of ground stress on casing deformation. The casing has elliptical deformation under the effect of uneven ground stress,and deformation becomes bigger with the increase of uneven degree of ground stress.The repairing technology of casing is investigated according to the deformation of casing.The pyriform shaper and ball shaper are studied respectively. As the single amount of plastic repairing cannot be too large, the shaper needs to be replaced many times in the plastic repairing process. Since the outer diameter of pipeline and packer after plastic repairing of casing is different, the inner diameter after casing repairing is different. According to thepractical requirement of gas filed, the maximum stress after single plastic repairing should be less than the tensile strength of casing and the required minimum repairing inner diameter should be achieved. When the required inner diameter after repairing is 131 mm, the minimum inner diameter is less than 120 mm after casing deformation. The repairing amount decrease with the deformation degree of casing.The extrusion of uneven ground stress on casing exists before and after casing deformation. The strength of casing and uneven ground stress need to be overcame in plastic repairing process. According to the numerical model, the effect law of uneven ground stress to repairing stress is investigated. When the minimum ground stress is determined, the plastic repairing stress enlarges with the rise of uneven degree of ground stress. When the uneven ground stress is determined, the plastic repairing stress becomes larger with the increase of frequency of repairing operation. The plastic repairing stress of ball shaper is much less than that of the pyriform shaper.