Risk assessment of cement placement failure due to cement loss caused by design changes in deepwater well construction

Abstract

The Well Containment Screening Tool (WCST) has been adopted by deepwater oil and gas operators, in the aftermath of the Macondo/Deepwater Horizon tragedy, to evaluate well integrity during worst-case survival loads. It favors a more conservative deepwater casing scheme with thicker casing walls and, hence, narrower cementing annuli for the same borehole diameters. These design changes may increase the risk of cement loss during narrow-annuli cementing, possibly compromising the cement barrier. Thus, the WCST may actually increase the risk of another well control incident.To examine this possibility, we developed a model to simulate and quantify fluid and cement loss in narrow casing-formation annuli. This model analyzes the influence of design parameter changes, such as a change in casing wall thickness, on generating additional zonal isolation risk in deepwater. Our analysis suggests that a minor increase (within 0.1”) in casing thickness, through increasing outer diameter, has little influence on the risk of cement loss, as does a slight decrease in mean open-hole diameter. Larger increases in casing diameter and reductions in open-hole diameter, however, increase the risk of cement integrity failure, particularly at flow rates above 100 gpm, because of increased cement loss risk, as expected. Beyond the specific problem we analyze, the quantitative risk-analysis approach described here allows for quantification of risk associated with deepwater well design and cementing changes.