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INTRODUCTION

While the petroleum demand is continuously increasing day by day, petroleum production worldwide is in a steady state. Due to various emerging technological developments, it can be expected that substantial portion of otherwise neglected oil can be recovered. The life of an oil well goes through three distinct phases (primary, secondary and tertiary recovery) where various techniques are employed to maintain crude oil production at maximum levels. Techniques employed at the third phase are commonly known as Enhanced Oil Recovery (EOR) and they can substantially improve extraction efficiency. In the recovery of oil from reservoirs, it is usually possible to recover only minor portions of the original oil by the primary recovery methods which utilize the natural forces present in the reservoir and the major parts, nearly 2.0 × 10¹² barrels of conventional oil and 5.0 × 10¹² barrels of heavy oils remain in reservoirs worldwide after conventional recovery methods have been exhausted (Thomas 2008).

Much of these oils would be recovered by various EOR methods which involve the injection of a fluid, or series of fluids, into the reservoir through an injection system. Over the years, interest in EOR has been growing due to the increase in oil reserves. Although large volumes of oil remain in the mature reservoirs, the oil production in large quantities by EOR processes will not be possible unless these processes can compete economically with the cost of oil production from conventional sources. So it is important to find economically suitable EOR methods for oil production from the reservoir. Thermal, Chemical and Gas injection are three major EOR methods developed during the last years (Moritis 2004). Natural gas (lean or rich) has been used successfully for many years as a primary choice of the operators for gas injection (miscible or immiscible). The limited availability and increasing value of natural gas has made its use for conventional cycling economically unattractive, especially in offshore environments where initial capital investment is large. Therefore, the use of less expensive substitutes, such as inert nitrogen, has been suggested (Donohoe and Buchanan 1981). Nitrogen was selected as a substitute makeup gas on the primary basis of pioneer work reported by Koch and Hutchinson on miscible displacement of reservoir oil using flue gas (Koch and Hutchinson 1958). They correctly concluded that flue gas (88% N₂) could be substituted for hydrocarbon gas without sacrificing miscibility. Moreover, nitrogen provides a higher reservoir displacement volume per standard volume of nitrogen than any other gas injectant; that is, it provides the lowest volume requirement for pressure maintenance. In addition, nitrogen is also non corrosive. Therefore, no special metallurgy is required for the injection equipment. A cryogenic (air liquefaction and distillation) process can produce 99.999% pure nitrogen. Non-cryogenic processes employ membranes or adsorbents (PSA/ VPSA) to remove the unwanted components of air. They produce nitrogen which is typically 95 to 99.5% oxygen-free (pure). Non-cryogenic plants are less energy efficient than cryogenic plants (for comparable product purity) but may cost less to build, in particular when the required production rate is relatively small. Non-cryogenic plants are relatively quick and easy to start up, which is useful when product is not needed full time. At high production rates, cryogenic processes are the most cost-effective choice. Cryogenic processes can produce very pure end products; and must be used to produce liquid nitrogen, oxygen and argon. Using either process, nitrogen can be generated at almost any location. Specific guideline on the required purity of N₂ for injection in the reservoir is unavailable in literature.

Depending upon the pressure, quantities, and location, nitrogen may cost one-quarter to one-half the price of natural gas (Clancy et al. 1980). Here, this cost range of nitrogen mentioned above is the nitrogen generation cost (Cryogenic air separation or inert gas generation) compared to the natural gas price depending on pressure, quantities, and location. Because of the increasing cost of natural gas, nitrogen injection is becoming more popular and attractive. For example, in 1983, over 500 million cubic feet per day of nitrogen was being injected into thirty oil or gas reservoirs (Clancy et al. 1985). In 1985, this number becomes 600 million cubic feet per day (Clancy et al. 1985) and by 1990 this number grew to 800 million cubic feet per day in over forty oil and gas reservoirs. Produced cryogenically from air with an established and proven technology, nitrogen can be made available for continuous trouble free injection in vast quantities at any location. Bath et al. (1980) first highlighted the scope for EOR in the North Sea. One of the more promising processes identified was nitrogen injection, which could be considered as either a secondary process to replace the water flood in certain parts of the fields or as a tertiary process to recover water flood residual oil. The drawbacks of nitrogen injection are that the produced hydrocarbons sooner or later become contaminated with nitrogen, which then require additional separation facilities. This technical problem can be solved easily with some extra cost. Then, the plants should have the additional separation facilities in order to separate the hydrocarbons contaminated with nitrogen. So, the investors have to pay extra cost for this additional separator in the plant and this cost will depend on the available technology, existing separator and plant size.

In fact, Nitrogen injection into subsurface reservoirs does not present any major problems. It is being applied successfully in an increasing number of EOR projects (Ahmed et al. 1983, Peterson 1978, Koch and Hutchinson 1958). Also the condensate gas displacement by nitrogen in the reservoir is a simple process that has proved to be very efficient (Moses and Wilson 1981). Hence the feasibility of nitrogen injection as a substitute for gas cycling is mainly an economic problem.