Initially, oil production in the Cuu Long Prospect field within Cuu Long Basin, comes from the primary fractured basement reservoir target and secondary targets in the overlaying Oligocene and Lower Miocene thick clastic reservoirs. Currently the field production is rapidly declining over the years. To reverse the trend, minor reservoir pay zones in the Lower Miocene and Upper Oligocene have been identified as new oil prospects to be appraised and developed. This paper presents three case studies of drilling horizontal appraisal wells with fit-for-purpose geosteering methods to address the challenges in different oil prospects while reducing the number of pilot holes. The lessons learnt and best practices from the earlier drilled well such as well design, technology selection, real-time geosteering data monitoring and communication process were applied in the subsequent wells to improve the result.

Geostopping technique during landing with resistivity-at-the-bit measurement was used to land the well precisely at the top reservoir with minimum sand penetration and avoid pilot hole. Proactive well placement method using deep directional measurements and boundary mapping capability was applied to optimize the lateral length in the sand. An integrated project team involving geosteering specialists, sub-surface team, drilling engineers, and well-site personnel were brought together to ensure efficient decision making process. As a result, production rate from each of the three wells was more than 50% above the anticipated rate with zero water cut and the number of pilot holes was reduced to one out of potentially three. After these well placement successes, more horizontal wells targeting the undeveloped oil prospects are being planned for future development in the field.

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