Enhancing Drilling Efficiency in Limestone HPHT Sour Reservoir: Formate-Based Drill-In Fluid Application in Madura Offshore, Indonesia

The "A" Field is Husky-CNOOC Madura Limited's (HCML) first project in offshore Madura Strait, Indonesia. The reservoir in the Kujung-1 limestone reef contains gas with condensate under near high pressure and high temperature conditions, with a bottom hole pressure of 8,100 psi, bottom hole temperature of 300 °F, 5.5% CO2, and 5,000 ppm H2S. This paper presents the utilization of a formate-based drill-in fluid system with manganese tetraoxide as a weighting agent to drill a 5-7/8 in. reservoir section. The Kujung-1 reservoir section was drilled with an overbalanced mud system to meet the company's HPHT and sour well requirements. A drill-in fluid (DIF) system treated with formate and manganese tetraoxide, with a mud weight (MW) of 14.9 ppg, was chosen for drilling the 5-7/8 in. reservoir section, providing minimal damage to the reservoir. Liquid potassium formate was used as the base fluid to minimize solids content and equivalent circulating density (ECD). Manganese tetraoxide, characterized by its small particle size and spherical geometry, can be easily cleaned up and flowed back. Offset well data shows that loss circulation occurred with a 15 ppg mud MW, while wellbore stability studies predicting mud loss at 16.2 ppg. Monitoring and maintaining equivalent circulating density (ECD) during the 5-7/8 in. hole operation was essential to mitigate lost circulation and well control issues. Challenges included minimizing formation damage while achieving a 14.9 ppg MW. Various drill-in fluid systems were evaluated based on formation pressure and temperature, reservoir compatibility, economics, contractual complexity, environmental impact, and industry records. A system using potassium formate (KCOOH, specific gravity 1.57) and manganese tetraoxide (Mn3O4, specific gravity 4.8) was selected, marking the first application for the Kujung-1 Limestone Reservoir in Indonesia. Laboratory tests showed that a 15% weight acetic acid and 50% volume chelating agent solution dissolved 100% of the manganese tetraoxide filter cake at 305 °F with a 6-hours soak time. These solutions enabled successful well execution, minimizing solid particle content and controlling HPHT fluid loss below 10 ml/30 minutes to minimize fluid invasion into the reservoir. During well clean-up and two mud cake breaker simulations, all wells delivered higher absolute open flow (AOF) than expected. The novelty of this paper is comprehensive design, selection, and operational improvement of a drill-in fluid system for the HPHT and sour Kujung-1 limestone reservoir. Using a formate-based system with manganese tetraoxide, HCML successfully completed four "A" development wells, exceeding absolute open flow (AOF) expectations with minimal reservoir invasion. The research reported herein is useful and can be used for future references of other researchers/companies.