Practical and Successful Prevention of Annular Pressure Buildup on the Marlin Project

Abstract

Abstract In late 1999, British Petroleum (BP) experienced a well failure in the Marlin development in deepwater Gulf of Mexico (GOM). Within hours of starting production, the production tieback casing collapsed, causing failure of the production tubing. Pressurization of outer annuli due to production thermal effects was identified as one of two possible root causes of the failure. The oil and gas industry has been observing, documenting, and reporting cases of annular pressure buildup (APB) in annuli for several years.1 On land, platform, and spar-type wells having access to annuli, the problem can usually be dealt with by bleeding off the annular pressure as needed. Subsea completions, however, do not allow this capability, and the technology to provide access is still being studied. Especially susceptible to APB are deepwater developments in which the differential between mudline temperatures and flowing-production temperatures can exceed 125° to 200°F. A key technique developed during the Marlin project to help prevent APB was the use of nitrified cementing spacers. These spacers provide a compressible cushion that can absorb the pressurization effects caused by thermal heating in the annulus. In addition to carrying nitrogen, the spacers can also provide adequate mud removal and remain stable for at least 72 hr. These features help prevent the migration of nitrogen into the riser prior to setting the wellhead seals. Based on a series of pressure/volume/temperature (PVT) tests and well design simulations, the minimum foam quality of nitrogen was determined. Service company and operator engineering staffs worked proactively and integrated their efforts to identify, evaluate, plan, and implement multiple options for resolving this substantial well-integrity issue. This paper reviews the problems associated with APB, details the large-scale testing that was conducted, and the resulting best practices that help prevent APB from affecting the casing design. These best practices were successfully implemented on the Marlin subsea development, and other projects are also using these techniques. Introduction Marlin History, A-2 Well Failure Summary An incident investigation team was formed to evaluate the failure of Well A-2. The team provided detailed analyses of probable failure modes and determined that there were two possible root causes of the failure:excessive annular pressure buildup, orannular hydrate disassociation. The failure analysis results were applied to the five remaining Marlin wells.2 When these original Marlin wells were batch drilled, a number of mitigation techniques presented in this paper were not implemented and were no longer available for consideration. As a result, the redesign of the Marlin completions had to focus on VIT (Vacuum Insulated Tubing) and fiber optic monitoring systems as a means of both controlling and observing thermal behavior.3 Early in the failure investigation of Marlin Well A2, a root cause analysis was performed to discern the most likely causes of tubing ovalization.4 The possible causes included the following:Excessive helical (column) buckling of the production tubingHydrate formation and dissolutionTrapped annulus pressure leading to casing collapseMiscellaneous issues such as proper tubulars and wellhead movement Marlin History, A-2 Well Failure Summary An incident investigation team was formed to evaluate the failure of Well A-2. The team provided detailed analyses of probable failure modes and determined that there were two possible root causes of the failure:excessive annular pressure buildup, orannular hydrate disassociation. The failure analysis results were applied to the five remaining Marlin wells.2 When these original Marlin wells were batch drilled, a number of mitigation techniques presented in this paper were not implemented and were no longer available for consideration. As a result, the redesign of the Marlin completions had to focus on VIT (Vacuum Insulated Tubing) and fiber optic monitoring systems as a means of both controlling and observing thermal behavior.3 Early in the failure investigation of Marlin Well A2, a root cause analysis was performed to discern the most likely causes of tubing ovalization.4 The possible causes included the following:Excessive helical (column) buckling of the production tubingHydrate formation and dissolutionTrapped annulus pressure leading to casing collapseMiscellaneous issues such as proper tubulars and wellhead movement