The 6-1/2 Inch Monobore Expandable System for the North Sea Application

Abstract

Abstract The use of monobore expandable systems has been theorized in previous works (Dean 2003, Dupal 2002) with various proposed embodiments. Commercially available sizes are limited to 8-1/2 and 12-1/4 wherein the size designation refers to the drift diameter which the monobore system may pass through and the post-expanded drift diameter of the monobore liner. In particular, the 12-1/4 in. size has a run history which includes a recent successful commercial installation (Baker 2021). An application within the North Sea where significant pressure gradients are typically encountered during drilling of a 6-1/2 in. section below the 7-3/4 in. base casing was identified as a candidate to leverage existing monobore expandable technology in a 6-1/2 in. size. Monobore expandable technology is the preferred method to manage challenges presented by steep pressure gradients or fluid loss zones while drilling due to the robustness of a solid steel barrier with external isolation seals, which may be cemented in place without a reduction in wellbore diameter. Beginning with the 12-1/4 in. platform and scaling down to a 6-1/2 in. platform created challenges regarding geometry, critical load ratings, and achieving post-expanded system ratings. A feasibility study was performed which relied heavily on finite element analysis to determine the possibility of the system scale down from 12-1/4 in. to 6-1/2 in. The analysis proved the design of a monobore expandable system in the 6-1/2 in. size based on the requirements for the North Sea application. It also proved that the design would require a non-standard pipe size. The pipe and expansion system were tested in a series of surface level qualification tests and at a test rig within a downhole environment. The results of testing validated the finite element analysis approach to the feasibility study and proved system readiness for field qualification trials.