Case Study: Drilling Performance in a Thin Oil Rim Carbonate Pinnacle: Central Luconia, Sarawak
Authors: Kok Keng Hung , Ajang Michael , Michael Jeremiah , Michael Runtuwene , Jong Chien Hock , Maruthanuthoo Kumaren , Chung Yvonne , Kaur Harwinder , Wong Thomas , Biusing Victor , Mohamed Sabri 
Published: November 2020

Drilling into carbonate reservoirs presents challenges involving damaging vibrations to drilling assembly components, encountering total drilling fluid losses, formation damage, potential loss of directional control if drilling into karst structures, and formation evaluation, particularly with high reservoir uncertainty. These challenges are exacerbated by the need to drill a long, relatively smooth 6,000-ft horizontal ERD well for autonomous inflow control device (AICD) smart well completion at 45 ft TVDss below the GOC. This paper describes collaborative efforts amongst different facets of drilling, fluids, and drill bits with the operator drilling team. Planning, risk assessment, directional drilling, and formation evaluation solutions are discussed for total drilling performance from spud to well TD for three wells in Phase 1 of this campaign.

The elongated narrow nature of the carbonate pinnacles and optimal oil ultimate recovery dictates horizontal production hole placement in the mid-point of the thin oil rim. Well designs were optimized with consideration for anticollision risk, optimum reservoir recovery, karst avoidance, shallow hazard, extended reach drilling the long horizontal section with minimal undulation, in-field referencing (IFR) surveys to minimize well bore uncertainty, fluid management for optimum hole cleaning, and drill bit designed for penetration rate and minimal vibration. BHAs were modeled for directional control, drilling efficiency and damaging vibrations from drill-bit and BHA components. BHA simulation incorporating downhole vibration isolation tool mitigates and decouples damaging vibrations.

The high fluctuation of 5 to 40 kpsi compressive strength from offset Carbonate sections induces vibration damage to drill-bit cutters if not designed out. Fluids design focused on drilling using a waterbased mud and readiness to combat severe or total losses. Fluids design for the horizontal section requires both an easy to clean filter-cake if switched to Pressurize Mud Cap Drilling (PMCD), and on acid-degradability of the filter-cake if drilled using conventional drilling allowing the application of an in-situ acid generator filter-cake breaker system, optimized fluid performance for hole cleaning, pressure management, and torque and drag reduction by means of nondamaging lubricants. The ultradeep-resistivity inversion successfully mapped and reduced seismic uncertainties and achieved optimal well placement within the thin oil rim.

The rigorous planning and risk mitigations paid dividends. All three wells were flawlessly drilled with one run BHA in each hole sections, no karst losses necessitating PMCD, and delivery within a 10-ft TVDss window for the 6,000-ft horizontal sections.

Drilling rate of penetration was 50% faster compared to offset benchmarking, setting new best in class performance for the Sarawak carbonate field. Particle size distribution in the drill- in fluid was kept within a narrow range to maximize bridging and reduce fines invasion. The combination of a nondamaging fluid and in situ acid precursor rendered predicted hydrocarbon production above expected productivity of the well.