The Bantayan field, located on the northeast flank of the South Sumatra basin, was delineated by surface geological studies in the late 1920s as a narrow northwest trending anticlinal structure bisected by numerous northeast trending transbasinal normal faults. In 1932, a discovery well encountered productive upper Talang Akar sandstones which yielded 800 BOPD of paraffinic based crude with a pore point of 115 degrees F. Until year of 1982, only 12 delineation wells were drilled, from which 12 potentially productive sandstone units were obtained within a structural area of 27 sq km. Further evaluation from other new 8-well, it was found that this reservoir system is considered a complex system.

In the Bentayan Field, the hydrocarbon oil is deposited in the Talang Akar depositional complex, with the best overall reservoir quality which is relatively thick and well-connected, fine-to coarse-grained, and cross-bedded fluvial sandstones3).

Oil properties of this reservoir currently have oil gravity of 17° API and viscosity of 82.2 cp1) at the reservoir temperature. Based on the above oil properties, Bentayan field is considered as a heavy oil reservoir, and so far only 14% of Bentayan oil reserve has been produced. Since 80 years of the discovery, reservoir pressure of Bentayan field has declined from 1696 psi to about 1035 psi, and oil production has also declined significantly. In order improve the oil production, water injection has been implementing to several wells throughout the field, however the oil production is still declining.

To reduce viscosity of the oil in the reservoir, application of steam huff n puff could not work very well for improving oil production in this field. This could be caused by high reservoir pressure and also by depth of the reservoir. Since this field is considered as a mature field where all production wells have been producing with high water cut, all conventional efforts to improve oil production would be difficult to success. Implementing chemical EOR, in this case, is the best way to increase oil production. However, the selected chemical must be able to lower the oil viscosity, lower IFT and alter the oil wettability. In this case, a special chemical treatment (SEMAR) had been implemented in this field using huff n puff technique. Since, the reservoir is under strong water drive, the injection of SEMAR can also improve oil production of other surrounding wells nearby. These phenomena are very interesting to be evaluated for further steps in fullfield chemical EOR implementation.

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