Twenty Years of Gas Injection History into Well-Fractured Haft Kel Field (Iran)

Abstract

Gas injection into Haft Kel field was started in June 1976 at a rate of 400 million scf/d, when its oil production dropped to about 14 000 stb/d with an oil column thickness of about 110 ft. The reservoir pressure was increased from about 1100 psi to about 1410 psi, at the crest. Both water-oil and gas-oil contacts were smoothly moved down shortly after the start of gas injection. The oil column thickness was gradually increased to about 350 ft with water-oil contact at about 2650 ft sub sea and has become nearly stationary after about nine years. The present field production is about 35 000 stb/D. Haft Kel field was simulated using specially developed reservoir simulation model described by Saidi(1). Its history was successfully matched using: 1- Gravity drainage and imbibition. 2- Full block to block interaction process. 3- No capillary continuity between blocks. 4-Thermal convection in fractures and diffusion between fracture oil and that in matrix blocks, during both natural depletion and repressuring. Through these processes, gas-oil ratio (GOR) could be correctly and rigorously matched during natural depletion, with a decreasing produced GOR, and that during repressuring with a higher but a constant producing GOR. The field behavior confirms oil drainage takes place under practically no capillary continuity but under full block to block interaction. In addition, the present field behavior confirms the results of an early reservoir simulation study, estimating that approximately 500 million stb of extra oil could be recovered by gas injection under the conditions: 1-Reservoir pressure reaches 1412 psi at the crest. 2- Water-oil contact reaches its initial depth of 2087 ft depth. 3-50 ft final oil column thickness. If the reservoir pressure could be further increased to 1512 psi, nearly an extra 100 million stb of oil could also be recovered. The calculated water displacement efficiency, supported by field measurements is about 17%, whereas the calculated gas displacement efficiency, at a reservoir pressure of 1512 psi at crest, is about 32%.