Research On Mechanism And Prevention Of Casing Damage In Heavy Oil Thermal Recovery Wells

During the oilfield development, casing damage will cause tremendous economic loss. Through analyzing main casing damage forms and positions in heavy oil thermal recovery wells, combined with field severely sanding characteristics, using theoretical analysis and numerical calculation methods, mechanism and prevention of casing damage in heavy oil thermal recovery wells were researched under different thermal recovery conditions.The stress of casing and cement sheath which is subject to non-uniform geologic stress on the outer surface and hydrostatic pressure on the inner surface were analyzed by studying the stratum-cement sheath-casing combination. Based on the elastic theory, stress components of casing during production were deduced. The influence of elastic modulus and wall thickness of cement sheath and wall thickness of casing on the stress of casing and cement sheath was studied. Taking mudstone creep formation as an example, the reason of casing damage due to creep was analyzed.Based on the theory of M.M.PROMOJIYFAKONOV, cavity configuration was analyzed by considering the interaction of casing and formation. The calculation formula of the axial force acting on the casing at the cavity was deduced. The relation between casing destabilization damage and sand production volume was established. The influence of different steel grades and wall thickness of casing on sand production volume was analyzed while casing was destabilized.Assuming the constraint of cement sheath and wall rock to casing to be continuously distributed radial spring constraint along casing axis, stress formulas of casing caused by stratum subsidence were deduced through an approximate 3D analytic method. Considering the geologic stress, the stress caused by stratum subsidence and thermal stress, a method was established to analyze the influence of stratum subsidence on casing in conventional wells and thermal recovery wells.Based on the elastic theory, the triaxial stress formulas of casing under thermal stress and non-uniform geologic stress were derived. According to the principle that the casing's Mises stress should be under the minimal yield strength, the triaxial prestress of casing was deduced. A method was presented to design and check casing strength in heavy oil thermal recovery wells. Reliability and validity of analysis theories and application methods established in this dissertation were verified through the field application.