Improving Hole Quality and Casing-Running Performance in Riserless Top Holes: Deepwater Angola

Abstract

Summary The operator and coventurers are developing several Angola Block 15 fields in water depths of up to 1400 m. To date, 83 development wells and 33 exploration/appraisal wells have been drilled in the block. Most have experienced hole quality issues in the riserless interval that are manifested as tight hole while tripping the bottomhole assembly (BHA) and as abnormal/excessive drag while running casing. In a few cases, casing could not be run to total depth (TD), and in one unfortunate case, the casing buckled in open water. There has been uncertainty as to the root cause of the hole-quality issues that have been attributable to various factors since drilling was initiated. A review of available literature revealed very little published information on riserless drilling and casing-running practices. The operator undertook a study of its riserless drilling and casing-running practices to look for trends that may suggest certain sources as the root cause and to allow a re-examination of long-accepted practices. The study, combined with some new perspectives on directional-drilling mechanics, identified micro-tortuosity as the root cause of casing-running difficulties. Micro-tortuosity has been known to exist in deeper, more consolidated sediments. The study revealed that microtortuosity can also exist in large diameter holes drilled in soft sediments. Changes to drilling practices have resulted in improved riserless hole quality and smoother casing-running operations.