Abstract

In mid 2007, two deepwater exploration wells made the first significant oil discovery offshore Ghana, West Africa (Jubilee Field). Drilling of the first two wells and subsequent appraisal wells indicated hard and abrasive formations, notably in the 12-1/4" reservoir sections, where up to four bits were needed using a combination of polycrystalline diamond compact (PDC) and tungsten carbide insert (TCI) bits. Therefore amongst the many project challenges there were significant drilling challenges to overcome. The primary goal was to drill the 12-1/4" section in one run, with the benefits of substantially reducing drilling time and costs where full-spread operating rates are in excess of $1MMUSD/day.

To address these challenges, the drilling team adopted a holistic planning approach prior to drilling the first development well. This used, in conjunction with a detailed analysis of the offset performance, a 4D finite element model that accurately predicts the performance of bit and bottom hole assembly (BHA). This approach allowed a fundamental understanding of the proposed system and its behaviour. The goal of drilling the section in one run was achieved far sooner than would have been expected using "trial and error" methodology, but with high vibration levels. Ongoing optimisation of the dynamic model with the actual down-hole data justified a BHA change to reduce vibration levels. Subsequently, the changes made to the BHA successfully reduced vibration levels, more than doubled rate of penetration (ROP) and facilitated good quality log while drilling (LWD) data.

With the high cost base of deepwater drilling activity, optimisation solutions that can be applied before and during the drilling process will make significant contributions to the overall project economics. Efforts were therefore applied to making the best pre-well bit and BHA selection, then analysing and understanding initial results, with an emphasis on maximising any learning and incorporating recommendations into the subsequent well. On the Jubilee Project, on-going optimisation can now focus on maximising overall drilling performance.

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