Numerical Investigation on Effect of Dual Casings Design on Casing Collapse Prevention, Case Study: One of Iranian Southwest Oilfields

ABSTRACT: This study investigates the effectiveness of a dual-casing design in reducing casing collapse during CO2 injection in a southwestern Iranian oilfield, where over 100 of 450 wells have experienced severe collapse within the high-pressure Gachsaran Formation, particularly across marly and salt layers. A representative well was analyzed, featuring a dual-cased (9x7-inch) section in the upper Gachsaran interval using 9%-inch V-150 BTS and 7-inch P-110 BTS casings, and a single 7-inch P-110 BTS liner in the lower section, where multiple collapses were reported. A comprehensive 3D geomechanical model was developed from surface to underburden, including the Gachsaran and Asmari formations. One-way hydromechanical coupling was used to simulate stress, strain, and displacement changes due to reservoir pressure variations. These were applied as boundary conditions to a near-wellbore 3D finite element mesh incorporating actual casing and cement properties. Material behavior was modeled using Mohr-Coulomb for anhydrite, Cam-Clay for marl, and creep for salt.The results confirmed the integrity of dual-cased sections, where inner casing stress was about 50% lower than in the outer casing, indicating strong support. Conversely, single-cased sections showed stress beyond yield strength and large deformations. The study recommends extending dual casing (V-150 and P-110) into deeper intervals to enhance well integrity in similar geological conditions.