Conventional drilling techniques were utilized by the operator in Gabon to drill surface-hole through fractured carbonates prone to severe losses up until 2004. At that time, the operator launched a series of cost-cutting initiatives, one of which was the employment of Casing Drilling technologies.

Casing/Liner drilling is the process of drilling or reaming down a casing/liner interval utilizing a Casing Drill Bit. The casing/liner replaces the drill-pipe and is rotated at surface by the rig's top-drive. This system enables problematic zones to be drilled and sealed-off in one run reducing potential non-productive time (NPT) associated with hole problems, eliminating tripping-time for conventional drill-pipe and bottom hole assemblies (BHAs) and improved rig-floor safety by eliminating the risk associated with pipe handling.

The bit company recommended a polycrystalline diamond compact (PDC) – drillable Casing Drill Bit that is screwed onto the bottom of the casing. This simple, yet robust tool would ensure the greatest chance of success. Three Casing Dill Bit sizes have been run successfully to date on six wells; 12-1/4" (modified to 12-3/4") and 16".

Early runs raised some concern as to whether the casing connections could withstand the torque levels required to drill with casing. The first three runs used a 12-1/4" Casing Drill Bit run on 10-3/4" casing. Total mud losses and high torque levels were experienced during these runs. The Casing Drill Bit size was subsequently increased to 12-3/4" thereby increasing the annular clearance, reducing borehole friction and potential losses. The modification proved successful with the fourth well drilled without losses and torque levels reduced by half compared to the first three runs.

The fifth and sixth runs employed a 16" PDC Casing Drill Bit conveyed on 13-3/8" casing. On the fifth well the bit was pulled-out prematurely due to formation problems. The bit came out in excellent condition with no wear and only one missing PDC cutter. The same bit was re-run on the sixth well where it drilled to section total depth (TD) with full returns throughout the run.

The paper will describe the PDC Casing Drill Bit system, its performance in Gabon plus the systems positive economic impact and safety benefits.

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