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Haft Kel field
The Haft Kel field is located in Iran. Its Asmari reservoir structure is a strongly folded anticline that is 20 miles long by 1.5 to 3 miles wide with an oil column thickness of approximately 2,000 ft. The most probable original oil in place (OOIP) was slightly > 7 × 109 stock tank barrels (STB) with about 200 million STB in the fissures; numerical model history matching resulted in a value of 6.9 × 109 STB. The matrix block size determined from cores and flowmeter surveys varied from 8 to 14 ft. The numerical simulation model considered matrix permeabilities from 0.05 to 0.8 md. The overall horizontal and vertical permeabilities are approximately equal. There was an initial gas cap on the oil column. The oil gravity is approximately 37°API. The interfacial tension (IFT) at the bubblepoint pressure (1,412 psi and 116°F) is approximately 9 dynes/cm.
The field was discovered and put on production in 1928. It was produced on primary production from then until 1976 with a plateau rate of 200,000 barrels of oil per day (BOPD) for several early years. In 1976, gas injection began at a rate of 400 MMcf/D using gas from the nearby NIS gas dome. In 2003, the field was producing at approximately 35,000 BOPD.
Complex matrix-block/fracture-system reservoir
Saidi describes the many oil recovery mechanisms at work in this oil-wet reservoir as gravity drainage at constant IFT and reservoir pressure; oil swelling in the present gas-invaded zone because of the increase in reservoir pressure; oil swelling in the present oil zone through thermal convection/diffusion process; oil imbibition within the oil column; oil gravity drainage from the partially saturated blocks within the gas-invaded zone; and oil gravity drainage from the fully oil-saturated block in the oil zone and the blocks between that and the present gas-oil contact (GOC).
The flow behavior developed from the history match is that the oil-drainage performance follows that of stacks of discontinuous blocks, supporting practically no vertical capillary continuity between the matrix blocks (see Fig. 1).
Fig. 1 – Schematic of oil saturation profiles (dark shading) from stacks of matrix blocks: (a) without capillary continuity and (b) with capillary continuity bounded by vertical and horizontal fractures.
Immiscible gas injection
By going to immiscible gas injection, oil recovery is increased by about 500 × 106 bbl by returning to the original reservoir pressure and could be increased by another 100 × 106 bbl if the reservoir pressure is increased an additional 100 psi because of the reduction in gas/oil IFT with increasing reservoir pressure.
Overall, the application of immiscible gas injection to the Haft Kel field has been considered a success. The estimated displacement efficiency by water was 17%, whereas that estimated for immiscible gas displacement was 32%.
- Saidi, A.M. 1996. Twenty Years of Gas Injection History into Well-Fractured Haft Kel Field (Iran). Presented at the International Petroleum Conference and Exhibition of Mexico, Villahermosa, Mexico, 5–7 March. SPE 35309. http://dx.doi.org/10.2118/35309-MS
Noteworthy papers in OnePetro
Alavian, S.A. and Whitson, C.H. 2010. CO2 EOR Potential in Naturally Fractured Haft Kel Field, Iran. SPE Reservoir Evaluation & Engineering. 13 (4):172 http://dx.doi.org/10.2118/139528-PA
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