Karst reservoirs in the Chumphon Basin of the Gulf of Thailand have produced oil at well rates exceeding 10 000 BBL/d. Meteorically karstified buried hills were recognized as a potential exploration play. The Nang Nuan discovery well appeared to confirm such a play, and the concept prevailed despite the accumulation of contrary and unusual data. By the time a subsequent well had produced nearly 4 × 10 6 BBL oil, there was a desire to better understand the prospectivity of the concession. The accumulated data indicate that the highs are probably syn-rift horsts and inversion features. Karst reservoirs occur in Ratburi carbonates, and Mesozoic and Tertiary clastics, apparently unrelated to subaerial exposure. The karstification appears to be primarily of deep-burial origin, as indicated by the nature of the karst, substantial pore volumes that are difficult to account for, and temperature and flow anomalies consistent with active geothermal circulation. There are granites and hot springs in the vicinity, and abundant CO 2 in this and neighbouring basins. Such deep-burial karst reservoirs have different implications for reserves estimation, prospect ranking and well completions.

Thai Shell operates three onshore oil fields producing from the Yom/Pradu Tao Formations in the Phitsanulok Basin in Thailand: Sirikit D block, Thap Raet and Sirikit West. The Yom/Pradu Tao Formations originate from a low energy fluvial depositional environment with a low net-to-gross ratio of 20–40%. After a comprehensive evaluation of EOR methods, waterflooding was found to be the most attractive development concept to achieve a significant increase in recovery factor in the Yom/Pradu Tao reservoirs. However, waterflooding in a complex low net-to-gross fluvial environment is not straightforward. Thai Shell has followed a step-by-step approach in the implementation of waterflood starting with the Sirikit-D block in 2000, which is geologically least complex, followed by Thap Raet in 2001. Sirikit D block and Thap Raet are both showing a positive response to waterflooding, substantially increasing production and reserves. Sirikit West, the largest and geologically most complex of the three fields, has started water injection in May 2003. Prior to the implementation of waterflood in Sirikit West, the field had been subjected to extensive studies using multiple static and dynamic model realizations. This paper will provide an overview of the experiences and performance of waterflooding to date in the Sirikit-D and Thap Raet fields. In addition, it will explain the modeling approach and waterflooding plans in the Sirikit West field. Introduction The Sirikit D block, Thap Raet and Sirikit West fields are located in the S1 concession within the Phitsanulok Basin some 400km north of Bangkok (figure 1). Oil production from the S1 concession started in 1980 from the lacustrine Lan Krabu Formation in the Sirikit Main field. Production from the Yom/Pradu Tao Formation started in 1997 and current production amounts to some 8500 bbl/d. The Sirikit D block is located west of the Sirikit Main field (figure 2) and consists of a north-south trending fault block with dip closure to the east, north and south and fault closure to the west. The Sirikit D block has an areal extent of 3500m × 250m. Thap Raet is situated north of the Sirikit Main field (figure 2) and consists of four fault blocks, with most production coming from the two fault blocks in the south-west with an areal extent of 2km × 2km. Sirikit West is situated north-west of the Sirikit Main field (figure 2) with an areal extent of 6km × 6km. The field sits at the triangular intersection of two regional strike-slip fault zones and consists of a series of down-thrown blocks created by west-hading faults. A more detailed structural map for Sirikit West is provided in figure 3. Thai Shell Exploration and Production Co. Ltd. operates the S1 concession in partnership with PTT Exploration and Production Co., Ltd.

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