Understanding governing fluid flow mechanisms in a mature Omani oil field - A case study

The Omani field under study was acquired by Petrogas (a local Omani company) from Elf-Oman. The field extends over 63 km 2 and is a major part of the Butabul permit. Haushi, the hydrocarbon-bearing formation, consists of tight upper sandstone containing most of the remaining oil (practically unmoved), a dolomite drain from which most of the active wells are producing and a prolific sand drain that has been nearly swept. A pinchout that terminates the sand drain over the eastern part of the accumulation is a major characteristic of the field. Another feature is an active edge water aquifer attached to the sand drain and contributing to pressure support of most of the producers. Numerous field-scale reservoir simulation runs were performed to understand the intricate fluid flow mechanisms, to investigate better production options and to recommend a better management program. A total of one hundred and eighty one runs were carried out to initialize the model, history match it and perform the different sensitivities. During the preliminary history matching exercise, simulating the different profiles was a difficult task. The turning point was the superimposition of regional reservoir pressures, which indicated the existence of three distinct fluid flow regions. A western block attached to an active aquifer; a central block under the combined influence of the aquifer dynamic front and a significant crossflow from the pinching out sand drain into the relatively less permeable dolomite drain, and an eastern block with waterflooded as well as depleted wells. In the field, most of the shut-in wells (W3, W16, W19, W20 and W21) produced from the western part of the reservoir and have been watered out by the aquifer. Other wells (W4, W9, Wll and W14) are still active and produce from the central part. W1, W5, W15 and W22, located on the eastern block, are waterflooded by W3, a producer that has been converted to an injector. W8 is another eastern block well that contributes to production. Other eastern block wells like W6, W7, W10 and Wl 7 have been shut-in due to completion and mechanical problems. Thereafter, having identified the governing fluid flow mechanisms and affected areas, a successful history matching campaign was achieved in terms of estimating initial volumes in place, fingerprinting reservoir pressures and production profiles. Prediction and sensitivity runs then followed. Strategies on how to upgrade the reservoir performance were also established.

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