The declining oil production from fractured reservoirs after several decades of exploitation and significant amount of oil still remaining in place are of great concern to oil companies and fully justify its interest in EOR processes. Secondary recovery processes based on miscible gas can represent a very interesting solution to extend the life of conventional reservoirs and maximize the oil recovery. But for fractured reservoirs the miscible gas injection needs to be carefully studied. In this paper, we conducted a feasibility study to investigate the application of hydrocarbon gas miscible flooding in an Iranian fractured reservoir. For that purpose, a series of experiments was performed under reservoir conditions on a carbonate rock type containing fractures. The experiments were carried out on composite cores, consisting of several carefully selected reservoirs fractured core plugs of the chosen rock type. The minimum miscibility pressure (MMP) was determined. The composite core displacement tests were conducted by LPG to evaluate the displacement efficiency. In this paper, findings from a variety laboratory slim tube and composite core flooding experiments are discussed and analyzed.


Limestone and dolomite reservoirs constitute one of the largest sources of crude oil supply in the world. Approximately 65% of present world production comes from carbonate reservoirs mostly located in the Middle East, Mexico, and Canada. 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 East fractured reservoirs represents 25–30% of the total oil in place in that area. This percentage may also be representative on worldwide scale(1).

Efforts to recover more oil from an Iranian naturally fractured reservoir led to the initiation of this laboratory study. Given the characteristics of this reservoir (depth more than 3000 m, 30 API, carbonate rock, temperature 113 ° C, pressure 307 bar), the enhanced oil recovery experiments have to be conducted under very demanding conditions. Due to the fractured type of the reservoir rock, attention has centered on the liquefied petroleum gas (LPG) displacement.

Adequate prediction of the displacement efficiency of miscible displacement is fundamental to the design of the miscible flood. Some of the important input variables for a successful design are pore volumes of displacing fluid injected, type of displacing fluid, mobilities of the displacing fluid and reservoir fluids, and reservoir heterogeneity(2). The effects of these variables on the displacement efficiency are best determined from laboratory displacement experiments.

The purpose of this paper is to examine the feasibility of applying miscible gas flooding scheme for naturally fractured reservoir in an oil field in Iran.

Core Preparation and Properties

The cores used in the displacement experiments were obtained in preserved conditions from two Iranian naturally fractured reservoirs of AS1 and AS2.The stucture of reservoir AS1 is an elongated anticline that is 50 km in length and 3 km in width.

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