Multistring Casing Design for Deepwater and Ultradeep HP/HT Wells: A New Approach

Abstract

Abstract Drilling operations in deep water and ultra-deep water are increasing around the world. The development of these substantial prospects is very challenging and requires integration of knowledge with prudent designs at different stages of the well development. However the cost premium imposed by deep water and ultra deep wells are higher and many of the discoveries are marginal in terms of the commercial viability. With the increase of developing and operating costs, there is a growing need to reduce the cost of drilling and other allied operations. The casing program constitute a significant portion of the well cost and this calls for an alternate approach to the casing design criterion particularly relevant to high temperature and high pressures are encountered in ultra deep wells. Challenges associated with extreme depth, pressures and temperatures where annular fluid expansion is a problem translate to additional problems in casing design. It is therefore required to align design objectives closer to the changed requirements, which necessitates changes in traditional casing design methods. The design implemented should be without sacrificing the safety and integrity of the well. This paper presents a study on the new approach and the alterations proposed to the casing design to adapt to the new criterion for the deep water and ultra deep high-temperature high-pressure wells. This paper will examine some of these proposed changes, while asserting that it is necessary to integrate this new approach, so that cost effective solutions can be derived as a result. This new design encapsulating annular fluid expansion and pressure provides a simple, practical way to view the severity of the composite well system during the course of drilling as well as the production phase of the well. The practical usefulness of the theory, backed by the fundamental analysis is demonstrated with three numerical examples.