The Casing Collapse Mechanism in Salt Formations in Deepwater Fields in Brazil

Salt formations are commonly encountered during oil and gas drilling. Due to the rheological properties of salt rocks, casing collapse accidents happen frequently. Under action of nonuniform geostress, casing failure has become an important influencing factor that restricts the exploration and development benefits of deep and ultra-deep wells. Casing stress in a period after well cementation in the salt formation of Brazilian deepwater fields was analyzed. Results show that the nonuniform geostress is instantaneously applied on the cement sheath when well cementation is completed in the salt formation and then transferred to the casing. Stress on the inner wall grows with an increase in the angle with the direction of the maximum horizontal principal stress and it reaches the maximum in the direction of the minimum horizontal principal stress. As the creep continues, stress on the inner wall of casings gradually enlarges and the circumferential stress in inner rings tends to be uniform, which means that creep of the formation weakens the nonuniformity of casing deformation. The collapse pressure on the outer wall of casings tends to increase at first and then decrease as the angle with the direction of maximum horizontal principal stress enlarges. Stress on the inner wall of casings in the salt formation reduces with the increasing thickness of casings selected, and the casing strength is improved as the wall thickness increases. The research results provide certain theoretical guidance for the strength check of casings in well cementation engineering in salt formations.