Development of a Screening System to Identify Deepwater Wells at Risk for Annular Pressure Build-up

Abstract

Abstract Annular Pressure Build-up (APB) in wellbore annuli is a problem in most deepwater wells. If APB induced loads compromise string integrity, the selection and design of a suitable APB mitigation strategy must be pursued. The calculations required for such analyses can be tedious, since a number of cases to identify key variables and "what-if" scenarios must be evaluated. Though analytical techniques required to evaluate APB and mitigation strategies are well understood, there is a need to perform these calculations quickly and reliably. This paper discusses the development of a procedure and computational tool to determine deepwater wells at risk for APB. Though the tool structure is simple enough for spreadsheet implementation, it is technically and numerically rigorous. The role of the tool is illustrated with a case study where one or more mitigation strategies are necessary for well integrity. Details required to build such a tool, i.e. assemble the matrix of APB equations, develop quantitative models of mitigation strategies, and, most importantly, create a mathematical representation of a wellbore for use in the APB algorithm, are presented in the appendices. Finally, for the first time, the role of formation elasticity (i.e. the annular boundary conditions) on APB is examined by using equations that treat the formation as an elastic foundation. Introduction Investigations that followed the well failure on the BP Marlin project identified annular pressure build-up (APB) as a potential cause1,2,3. Subsequent operations such as Pompano have shown that APB can threaten well integrity during both drilling and production operations4. Though the impact of APB on casing loads and wellbore integrity has been recognized for more than a decade5, recent well design and construction challenges posed by deepwater developments have led to focused examination of APB and its effects on wellbore integrity. These efforts have identified a set of deepwater wells as APB candidates, in which the effects of APB are considered explicitly in the design basis. Analysis of APB and its impact, and selection of APB mitigation strategies have been identified as key technologies for the successful design and installation of these deepwater wells6. Though APB models are conceptually simple, arriving at an acceptable answer (or range of answers) is riddled with subtle details and requires experience and engineering judgment. At an algorithm level, these details include the choice of a suitable PVT model of the annular fluid(s), the selection of appropriate thermal simulators, the use of the proper boundary conditions in assessing annular flexibilities, and an efficient numerical method to solve the APB equations. Once the details are identified, software implementation should not pose serious computational difficulties. However, it is somewhat hindered by the difficulties of creating a mathematical representation of a wellbore. This step requires translation of the well geometry into a mathematical representation of annular geometries, locations and boundaries, as well as how annuli are connected. Though this problem is similar to the problem of generating the grids in FEA software, and could potentially be adapted, the challenge here was to identify a method that could be implemented in a spreadsheet. This paper discusses the development of a screening procedure and a computational tool to identify APB candidates. Since the conceptual basis of APB is fairly well understood, the next section begins with a review of the more subtle details required for APB design and analyses, and the quantification of mitigation strategies. This review is accompanied by a geometrical interpretation of the APB problem, an approach that greatly simplifies and aids computation. Appendix A explicitly lists the APB annular flexibility matrix equations and examines the role of formation elasticity on APB. By treating the formation as an elastic foundation, the differences caused by neglect of formation elasticity for water based and synthetic muds are presented. These concepts are illustrated with a case study that examines a typical deepwater APB candidate. The paper concludes with a procedure (Appendix B) to create a mathematical representation of a wellbore. The representation treats the plan and elevation views of the wellbore as a set of binary matrices.