Abstract
X field is a carbonate reef pinnacle gas reservoir located in the Central Luconia area offshore Sarawak, Malaysia. The reservoir is over-pressured with temperatures in excess of 170°C. The gas contains relatively high concentration of CO2 (up to 20% mole) and 140 ppm of H2S. The field was developed by five gas wells in a single drilling campaign. All development wells were planned to be completed with 7-inch production tubing to deliver a gas rate of which would deliver committed sales volumes with all wells flowing. Each well has an open-hole length of 300 - 400m inside the carbonate section and completed with pre-drilled liner (PDL) to prevent the borehole from collapsing.
From experiences in drilling other carbonate rocks in the surrounding area, severe losses are expected. A Pressurized Mud Cap Drilling system (PMCD) was employed to mitigate the risk. The system would be activated if severe losses were encountered. Upon reaching total depth (TD), the well would be secured by setting a composite bridge plug (CBP) at the production casing section. The plug would provide mechanical isolation while making up the lower completion, which comprised of PDL with fluid loss control valve (FLV). The plug would be later drilled out by sacrificial motor while the PDL is run-in-hole. This is a common procedure to deploy PDL in total losses condition. This system has been successfully implemented in the past.
However, with the combination of severe loss rate (up to 700 bph) and utilization of heavy mud weight that contains barite, the CBP failed to set. The usage of barite as weighting agent is inevitable due to heavy mud weight requirement: 12.8 – 15.0 ppg and solids free mud had been ruled out due to economics.
Without the CBP, there was no mechanical isolation. It is only possible to run a shortened PDL (3 joints) or a barefoot completion in term of operational safety. These two completion options were likely to stop the well from meeting its objective because of potential borehole collapse i.e. reduction in well productivity and no accessibility for well intervention. The subsequent problems that occurred had caused significant impact to the project cost and schedule and could potentially jeopardize its objectives.
Hence an innovative open-hole completion design was proposed. The new solution addressed the challenges by:
More accurate prediction of top of carbonate depth
Switching from open to closed system in lower completion assembly
Simplifing lower completion deployment method
Applying direct displacement for wellbore clean up
The application of this innovative open-hole completion strategy had improved safety during drilling/completion and ensured project to meet the production target and commitment as per schedule. This paper will document the initial challenges, problem identifications, application of the new solution, and its results.
