Nitrogen May Be Used for Miscible Displacement in Oil Reservoirs
Authors: Crawford P.B. , Reynolds B. , Rushing M.D. , Thomassom B. 
Published: December 2007

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Introduction
Hydrocarbon miscible displacement was suggested by Wharton and Kieschnick in 1950 and later received considerable attention. The low price and large supply of natural gas favored its use for injection. Other fluids now are being investigated because natural gas supplies are limited. CO2 and soluble-oil slugs have proved favorable as miscible agents, but their use depends largely on economics and availability.
In 1958, Koch and Hutchinson reported on miscible displacement using flue gas. They presented oil-recovery data for displacing gases containing zero to 100% nitrogen. McNeese presented a study in 1963, but his work was not available to us when writing this paper.
Some recent work on nitrogen injection shows the effect of GOR on oil recovery and indicates oil recoveries greater than 90% on a 43 degrees API gravity crude oil. Other recent work describes equipment and shows the flow diagram of a gas-fired, cryogenic nitrogen plant designed for offshore platforms or remote installations.
Pure, dry, noncorrosive, cryogenic nitrogen may be obtained by liquefaction of air and separation at the surface. The cost of nitrogen is less than one-fourth that of intrastate natural gas. A cryogenic nitrogen plant may be located at the oil field. With the elevated temperatures and pressures encountered in deep oil reservoirs, injected gaseous nitrogen was believed to lead to a miscible displacement process for some crude oils. In the tests described here, commercially pure nitrogen was used as the displacing gas to explore the feasibility of miscible displacement of crude oils with nitrogen.

Oil Recovery
Fig. 1 shows the effect of pressure and temperature on oil recovery using high-pressure nitrogen injection. These data cover the pressure range from 2,500 to 5,000 psi and were obtained on a 54.4- degrees API gravity crude oil with a GOR of 700 scf/bbl. The linear pack was 40 ft long and temperatures were 72 to 250 degrees F.
At 2,500 psi and 250 degrees F, oil recovery was about 61% of the stock-tank oil originally in place (Fig. 1). At 3,000 psi, oil recovery was near 70% for temperatures of 72 to 250 degrees F. At 4,000 psi, oil recovery ranged from about 78% at 72 degrees F to about 85% at 250 degrees F. At 5,000 psi, oil recovery ranged from 85 to 92% for a temperature range of 72 to 250 degrees F.
It is entirely possible that miscibility occurs in the last few feet of the tube in Fig. 1, even though total oil recovery is only about 85%. McNeese pointed out that although recovery was only 85% in the first 123 ft of equipment, miscibility and substantially 94% oil recovery were obtained in the last 22 ft of the 145-ft pack. Some systems require a long path to achieve miscibility.
Fig. 2 is a cross-plot showing the effect of temperature on oil recovery using high-pressure nitrogen injection for this particular fluid system. Fig. 2 indicates that oil recovery was substantially independent of temperature at 3,000 psi for this system. At 4,000 and 5,000 psi, oil recovery increased with temperature (Fig. 2).
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