Surface Casing Collapse Load Design Optimization Challenges for Highly Depleted Reservoir

In the current climate, cost reduction has been a major enabler for project sanction. With significant drilling activity planned in relatively benign environments offshore Malaysia, there were potential casing design optimisations, and hence cost reduction opportunities, in analyzing the relevant well planning data of such projects, with a view to further optimising the standardised 13-3/8" surface casing specifications that have historically been used in similar areas. Additionally, the feasibility of using 13-3/8" casing as slim conductor on platform development wells was investigated. Well planning data for a variety of projects planned for between 2022 and 2024, which planned to use the same 13-3/8" surface casing specification, were collated and boundary conditions were established for all of the key parameters. This standardised casing string was first assessed for suitability, and areas for potential optimisation were identified. Simplified casing load analyses optimisation were performed on this optimised 13-3/8" casing specification for typical collapse loads as laid out in the company's casing design standards, to assess its suitability. Generalized fatigue life analyses were also performed on the slim 13-3/8" conductor to try to prove viability of the concept. It was shown that collapse design is the controlling factor for the cost reduction initiatives through 13-3/8" casing specification optimization, especially in an environment of severely depleted reservoir (down to 3 ppg) in the reservoir section underneath the surface casing. This is exaggerated with conservative load assumptions that is used in the industry/company standards i.e., running mud weight, 50% to 100% evacuation, extreme low pore pressure assumption as low case prognosis etc. Combination of conservative collapse prohibited the casing specification to be optimized lower. This paper addresses the 13-3/8" casing optimization through fit for purpose collapse design approach. It was shown that the reduction of the casing specification for 13-3/8" surface casing in the majority of the projects studied was justified and acceptable based on the minimum design requirements for collapse loads. Additionally, cost savings of up to approximately 7% were observed for this reduction in 13-3/8" surface casing specification depending on connection qualification and casing manufacturer.