Casing Design for Dual Gradient Wells

Abstract

When planning dual gradient wells, it is important to understand the details of dual gradient drilling (DGD) operations and the resulting loads exerted on the casing strings in the wellbore. Standard casing design loads for conventionally drilled wells must be modified so that they apply to dual gradient drilling, and there are additional load cases specific to DGD that must be considered. This paper outlines those factors that should be accounted for in dual gradient casing design as compared to conventional deepwater casing design, including: Internal and external pressure profiles for typical deepwater casing design load cases for drilling and production strings. Additional load cases that should be considered for dual gradient drilling, such as running/cementing casing with an air gap in the string and tension/collapse combined loading when running in the hole. Application of dual gradient pressure profiles to worst-case discharge load cases. Annular pressure buildup analysis for dual gradient wells. Negative test magnitudes and procedures. While the dual gradient casing design loads are generally less severe than the corresponding load cases considered in conventional deepwater casing design, there are instances in which this does not hold true. Additionally, the collapse loads can be much greater than conventional due to the u-tube that exists during dual gradient operations. A thorough understanding of dual gradient operations is required to conduct a proper, diligent casing design that ensures a safe and efficient well plan and execution.