The Impact of Gas Injection On the Oil Recovery of a Giant Naturally Fractured Carbonate Reservoir
Authors: Badakhshan A. , Golshan H. , Musavi-Nezhad H.R. , Sobbi F.A. 
Published: March 2010

Abstract
This paper presents the results and discusses the impact of a long-term gas cap injection project in an Iranian carbonate oil reservoir. The reservoir is naturally fractured and is located in the south west of Iran. Its estimated oil in place is 7.15 m3 × 109 (45 billion STB), making it a world giant. Laboratory and production simulation studies that were conducted prior to implementation of the gas injection project considered the uncertain effects of natural fractures on behaviour of fluids in porous media. These studies indicated that long-term reservoir pressure maintenance by gas cap injection might improve the ultimate oil recovery of the reservoir considerably. After 15 years of gas cap injection, field data combined with predicted data under natural depletion (solution gas drive, gas cap drive and gravity drainage combined) have shown that pressure maintenance would ultimately increase the recovery of oil by nearly 318 MMm3 (two billion barrels) (16% over primary recovery), but much less than predicted from the original study. A brief economic assessment also indicated that the project, besides being effective in increasing the ultimate recovery of oil in the reservoir, is commercially viable.

Introduction
Limestone and dolomite reservoirs constitute one of the largest sources of crude oil supply in the world. Approximately 65% of the present world production comes from carbonate reservoirs mostly located in the Middle East, Mexico, and Canada. The pore structure in these reservoirs is characterized by a wide variation in the shape and the distribution of pore sizes. These widely varying characteristics of carbonate formations, together with the frequent presence of fractures, have made the porous media so complex that the application of general analytical techniques, normally used to delineate its performance to understand the recovery processes and to predict the results of operating practices for enhancing the recovery, becomes extremely difficult(1,2).
Although fractured reservoirs are scattered throughout the world, one of the highest concentration of reserves of this type is in the southwest of Iran and northeast of Iraq. The oil-in-place in the Middle Eastern fractured reservoirs represents 25 – 30% of the total oil in place in that area. This percentage may also be representative on a worldwide scale(3).
Gas cap injection as a means of increasing oil recovery on a long-term basis has been applied during the last 50 years. It is stated that it improves the oil recovery through maintaining the reservoir pressure, displacing oil and/or vapourizing the intermediate fractions of the reservoir fluid(4,5,6,7).
As gas cap injection for enhancing oil recovery has been applied mainly to sandstone reservoirs, a comprehensive study to investigate its potential in fractured carbonate reservoirs was launched on a laboratory scale(8). In 1975, after this intensive research program, gas cap injection was chosen as a possible technique for improving the oil recovery in Iranian naturally fractured oil reservoirs. The mechanism involved in enhancing the gravity drainage process in the gas invaded zone consists of lowering the gas-oil contact level, maintaining the oil pressure and allowing the gas cap pressure to rise to its original level.