Optimization of Multistring Casing Design with Wellhead Growth

Abstract

Abstract The casing program of a well represents a significant portion of the total well cost. An approach is required to reduce the tubular cost without sacrificing the safety and integrity of the well. This paper presents a study on the optimization of multi-string casing design with complex load conditions, annuli fluid expansion and wellhead growth. Casings for different drilling phases are selected in such away that minimum cost casing design is achieved. The optimization is carried out with the objective function of minimum cost and with the multiple constraints of strength of the casing, drift diameter, casing section length, number of sections, wellhead growth and number of available pipes in the inventory. Interval branch and bound optimization technique is used to find global minima of the cost function. This optimization not only results in calculating minimum casing cost design efficiently but also helps to take reliable decisions especially for offshore wells. The heuristic technique used to search the conformational minimum cost design space with the branch and bound approach gives added structural insights of casing design. Furthermore, this addresses the adverse impact of the annulus fluid expansion and wellhead growth on the efficiency of optimization. This also presents an approach whereby the initial single string negative impact design used to find a solution that maximizes well integrity are eliminated. The optimization method proved to be cost effective wherever the annuli fluid expansion-wellhead growth is a concern. Such effectiveness will depend on the complexity of the annuli fluid expansion-wellhead growth problem. For wells where the annuli fluid expansion is not a problem may result in a higher cost casing design than expected compared to the wells where the wellhead growth and annuli fluid expansion are critical. The cost optimization will depend on the amount of wellhead movement allowed. A new concept of Wellhead Growth Index (WHI) is introduced. WHI encapsulating the annuli fluid expansion and wellhead growth provides a simple practical way to view the severity of the wellhead movement during the course of drilling and as well as the production phase of the well. The practical usefulness of the theory, backed by the fundamental and optimal analysis, is demonstrated with two numerical examples.