On Causes of Partial Cement Deficiency at Doglegs and its Impact on Casing Failure; An Integrated Modeling Approach

Directional wells with complicated trajectories are developing highly non-linear contact relationships between the casing string and borehole, which may result in poor cement displacement efficiency due to the variable annulus geometry at doglegs. Since some pockets of drilling fluids may remain after the cementing, the solidified cement sheath presents eccentricity or deficiency problems. In such cases, this may intensify the load nonuniformity around the borehole which escalates potential threats for casing failure. By incorporating methods of large deformations theory, we first simulated casing running at doglegs while considering nonlinear contacts with the borehole. Then, the resultant annulus geometry was extracted from the results through Boolean operations of the deformed and undeformed surfaces to determine the volume to be cemented. CFD models are utilized to simulate the process of displacing drilling and completion fluids in the resultant annulus space. Eventually, we estimated mechanical properties of cement based on these displacements. The effects of partial cement deficiency on casing stress distribution were studied. The complex annulus space prevents a perfect sweeping of cement due to the reverse flow and bypassing drilling mud pockets. Casing coupled with deficient cement and formation simulations indicate that perfect cement could be acted as a cushion to the casing and help reduce the load transferred from the formation in-situ stress. No matter what the cement deficiency located in, the maximum stress of casing occurs on the internal wall pointing to the void. The stress magnitudes for deficiency cases are even less than that with perfect cement everywhere but at the voids. Hence, we may take advantage of this characteristic in some specific practices which needs to reduce the stress level of casing. This paper provides a novel workflow to discuss the mechanical behavior of casing coupled with deficient cement and formation at large doglegs, while current methods only assume the homogeneous cement properties and uniform deficiency shapes.