Drilling in carbonate reservoirs are challenging in many ways let alone coring. Recently PETRONAS Carigali Sdn. Bhd. managed to successfully core 224m of carbonate formation in a reservoir which contains 70% Carbon Dioxide (CO2) and 70 ppm Hydrogen Sulphide (H2S). This well is located in the western part of the Luconia field offshore Sarawak in Malaysia.

Core jamming is one of the biggest culprits of failure in obtaining good core recovery especially in carbonate formations. This is due to the highly vugular nature of carbonate formations which also causes losses of drilling fluid. To add to the challenge in this particular field, the carbonates here vary in their degree of porosity according to depth which means a variation of matrix compactness. This causes drilling breaks and can pose a challenge for coring if not understood properly. Core jamming in loose and interbedded carbonate reservoir pay zones has been a frequent occurrence in the wells drilled in these blocks which motivated the drilling team to optimize certain technical aspects of coring such as selectingthe most appropriate core head, bottom hole assembly (BHA), coring parameters and mud system additive chemicals to prevent jamming from happening.

The purpose of this paper is to share PETRONAS' experience of using optimized coring parameters to yield a high percentage of carbonate core recovery. The main advantage of these optimizations is to prevent core jamming problems in the carbonate reservoir in order to obtain good core recovery within the stipulated time. The paper discusses PETRONAS' experience of coring parameters optimization as well as proper coring equipment selection for successful coring in a highly sour carbonate reservoir. On the basis of offset wells and lesson learnt, these optimizations were evaluated and applied successfully to cut the 224m of core in the carbonate reservoir. Several core runs prompted realization that jamming was prone when certain parameters were not optimized hence requiring changes in parameters as well as core bits. Core bit selection and cutter size play an important role in ensuring a successful core run. The understanding of the carbonate reservoir geology is also crucial in order to apply the right parameters to cut the core successfully.

These optimizations proved its worth as it helped PETRONAS to obtain good core recovery and to prevent unnecessary trips that will result in additional time and cost to the operator. This unique PETRONAS experience will help make coring more efficient coupled with good recovery in some of the most challenging carbonate formations.

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