Improving Hole Quality and Casing-Running Performance in Riserless Topholes—Deepwater Angola

Abstract

Abstract The operator and co-venturers are developing several Angola Block 15 fields in water depths of up to 1400m. 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 manifests themselves as tight hole while tripping the BHA and abnormal/excessive drag 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 which 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 in order 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, has identified potential root causes not previously considered. Changes to drilling practices have resulted in improved riserless hole quality and smoother casing running operations. Introduction The operator and co-venturers were awarded exploration rights to Angola Block 15 in 1994. Water depths in Block 15 range from 200m in the east to over 1500m in the western portions of the Block. Several commercial accumulations of hydrocarbons were discovered and appraised during multiple successful exploration programs. The first field to be developed was the Xikomba field in 2003, followed by Kizomba A multi-field development in 2004 and Kizomba B multi-field development in 2005. Future developments are planned including a Kizomba C multi-field development and a Kizomba Satellites development for outlying fields. Angola Block 15 lies in the Lower Congo Basin as shown in Fig. 1. Shallow sediments in Block 15 are composed of soft unconsolidated Quaternary hemi-pelagic clay and silt sequences at the seafloor yielding to consolidated inter-bedded Plio-Pleistocene silt, clay/claystone and shale sequences which are highly fractured by structural adjustments during sediment consolidation and deeper salt movement/ tectonics. Depending upon location, inter-bedded sands and silty sands are possible in the sequence and often contain biodegraded hydrocarbons. Pore pressure in the shallow sediments is predominately equivalent to a seawater gradient. Shallow sand strata can have slightly elevated pore pressures when they contain gas with sufficient column to offset hydrostatic. A typical drilling hazard analysis for the shallow geologic horizons is shown in Fig. 2. Riserless Drilling Practices Wellbore construction in Angola Block 15 begins with installation of structural casing at the seafloor. Structural casing (also named conductor casing, surface casing, or surface conductor by some operators) is thick-walled pipe which provides structure and support for the subsequent casing strings. The subsea wells drilled by floating drilling rigs use 36-in diameter structural casing whereas the tension leg platform (TLP) wells employ 30-in casing. In subsea wells, structural casing must also support the subsea christmas tree and blowout preventer (BOP) stack. Structural casing is jetted in place using practices described by Akers 1. An inner string and bent housing motor mounted in the shoe assist the jetting operation by removing the majority of the sediment beneath the casing as it is washed down.