Study on the evolution and prevention of wellbore casing failure induced by marker bed water invasion

Marker bed related casing failure under intensive water injection has become a major constraint during the late stage of compound flooding in oilfields, but the quantitative relationship among marker bed water invasion, formation weakening, and well-scale casing failure remains unclear. To address this issue, a typical well in the Sazhong development area of the Daqing Oilfield was investigated through theoretical analysis, laboratory experiments, and numerical simulation. Based on effective stress theory and a Drucker-Prager elastoplastic equivalent model, the strength degradation of the marker bed after water immersion was calibrated, and a numerical model for marker bed weakening and casing failure was established. The simulated minimum casing drift diameter differed from the field measurement by 7.2%, indicating that the model can reasonably reproduce the casing deformation observed in the representative well. The results show that marker bed strength decreases with soaking time, and abnormal water invasion is highly consistent with the intervals of casing failure. Failure is mainly concentrated at the interfaces between the marker bed and adjacent formations, where deformation incompatibility induces significant slip and subsequent casing deformation. Within the investigated parameter range, the sensitivity of the main controlling factors follows the order: injection pressure > marker bed thickness > marker bed dip angle > marker bed permeability. Injection pressure is the most direct controlling factor, whereas marker bed thickness is the dominant geological factor. Based on a minimum casing drift diameter threshold of 95 mm, zonal pressure control limits and corresponding operational recommendations are proposed for casing failure prevention.