For the subsea wells, the only accessible annulus is the annulus between production tubing and production casing or liner. When trapped annular fluid heats up, the fluid expands and will increase the annular pressure. Should the annulus between the surface and production casing gets trapped, the pressure buildup on this annulus would put the integrity of the well in jeopardy.
In this study, the tubular design approach is used to mitigate the annular pressure buildup by utilizing a section of lower grade casing installed on the outer casing (sacrificial casing) that immediately covers the inner casing being protected and keeps its integrity intact.
In selecting the sacrificial casing, three main parameters are needed, these are; the reservoir condition, formation characteristic, and wellbore configuration. These parameters are fed to a wellbore thermal simulator/stress analyses software, in order to create the well model with related pressure and temperature conditions. Iterative annular pressure buildup (APB) analyses throughout the life cycle of the well are then conducted in order to determine the suitable properties of the sacrificial casing. If the property of the sacrificial casing satisfies all expected loading conditions and protects the inner casing string, the design is deemed acceptable and the sacrificial casing becomes a part of the final well configuration.
This case study uses a deepwater well in Indonesia in order to apply the sacrificial casing method to mitigate the uncontrolled APB between the 20″ surface casing and 13 5/8″ production casing. A 20″ 154ppf X-56 section was selected as the sacrificial casing. In order to evaluate the effectiveness of the sacrificial casing throughout the life of the well, the various well scenarios covering the drilling operations, production operations and worst case discharge (WCD) events were evaluated. The simulations were conducted using proprietary commercial software having wellbore thermal simulator/stress analyses capability. This study suggests that utilizing a sacrificial casing to mitigate uncontrolled APB is sound engineering approach to keep the well integrity intact.