The objective of applying underbalanced drilling (UBD) technology to drill basement plays in Malaysia was to determine the true, undamaged production capability, and at the same time determine productivity index while drilling, in order to characterise the reservoir inflow performance versus depth. Previous exploration and appraisal wells had determined oil flow from the basement, but at lower than expected rates, with the understanding that this was due to formation damage.

The key approach was to maintain bottom-hole pressure below formation pressure while drilling. The lower boundary of the formation pressure uncertainty range was hydrostatic pressure. Therefore, the drilling system had to be sub-hydrostatic to ensure underbalanced conditions and eliminate the risk of creating formation damage. Two fluid choices were suitable from a density perspective, base oil and nitrified water. The latter was selected because it enabled simplified separation, avoiding the risk of live oil being returned to the rig pits.

The well design was conventional down to the top of the reservoir, with 9-5/8? liner set 10m inside fresh basement. A tieback liner was run to improve annular velocity and facilitate running a downhole isolation valve to provide improved well integrity and faster bit trips. At first, nitrified seawater was used. However, this was changed to nitrified drillwater to reduce corrosion and scale. Membrane nitrogen was selected on both wells to avoid logistical constraints of supplying cryogenic nitrogen. Two UBD wells were drilled; the first experienced problems with the tieback liner, which backed off while drilling. The liner was re-run and cemented in place to eliminate the risk of recurrence. This approach was repeated on the second well. Although underbalanced conditions were maintained throughout the basement, there was no influx while drilling, or during a subsequent 1000psi drawdown test that was performed with the final drilling BHA. The ROP and bit life exceeded all expectations and demonstrated one of the recognised benefits of UBD. Although oil staining was visible on sidewall cores, it was determined that there were insufficient open and connected fractures to enable the well to flow. The drawdown test was conclusive in avoiding potential additional expenditure on a well test.

These wells demonstrate that basement wells in Malaysia can be successfully drilled using UBD technology and that qualitative reservoir performance information can be obtained while drilling, allowing optimal decision making. From worldwide UBD experience, key benefits include faster ROP, longer bit life, early reservoir characterisation and enhanced productivity. The first three of these have been demonstrated on this project, while the last would still be expected if drilling into open and connected fracture zones.

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