Nitrogen has advantages in some reservoirs because it is an inert gas.  These are:

• Cool reservoirs or very small patterns where oxygen breakthrough could be a problem

• Hot high pressure reservoirs where nitrogen could be miscible with the very light reservoir oil

• Where the compression cost for air is much higher than the cost of easily recycling nitrogen, or

• Where corrosion could be a problem.

     Rich-air containing 30% oxygen can be compressed with the same equipment as used for Light-Oil Air Injection and can be considered for the same reservoirs.   The following  four examples help illustrate when nitrogen, air or rich air might be preferred.

      The first example shows the effect of reservoir temperature on production from the 1/4 - 320 acre 20 foot thick, 10 md carbonate 5-spot with a 2 degree dip described in other web pages.  The predictions in Figure 1 were made using STARS.
 

Figure 1 - Effect of Reservoir Temperature on Production from 320 Acre 1/4 Five-Spot

     This model contained 32 API gravity oil with a GOR of 220 SCF/BBL at 680 psi at 220^oF.  It was depleted for twenty years, then 280 MCFPD of gas was injected into the model.   As in Light Oil Air Injection, the hot zone with rich air remains near the injector, and the bulk of the reservoir remains unheated.  Rich-air produces the most oil because 50% more carbon dioxide is generated.  When the initial temperature of the reservoir is higher the difference between recovery with nitrogen, air and rich-air decreases.  At 100 degrees Fahrenheit in this large model air and rich-air produce 22.5% and 32.5% more oil than nitrogen.  However, at 260 degrees Fahrenheit the difference has decreased to 6.7% and 14% respectively.

     The second example, Figure 2, is for recovery of a 48 API oil from the same model.  Now, recovery decreases with increasing temperature for both air and rich-air, but increases slightly for nitrogen.  Recovery of the 48 API oil is much higher than that of the 32 API oil, so air and rich air only produce 13.7% and 16% more oil at 100 degrees Fahrenheit.  However, at 260 degrees Fahrenheit, the carbon dioxide containing gases produce just 3.7% and 6% more oil respectively.

      The third example shows that the difference between nitrogen and oxygenated gases becomes proportionally smaller as the reservoir pressure increases.  The previous simulations were conducted on a model whose initial pressure was 4,000 psi and the gas injection pressure was also 4,000 psi.  Figure 3 shows that increasing reservoir and gas injection pressure raises recovery from injection of all three gases by about the same amount.   Thus, proportionally, the extra production from air and rich-air diminishes   These figures show that increasing either the pressure or temperature increases recovery from nitrogen more than from air or rich-air.  Thus, at higher pressure and temperature nitrogen might be the more desirable injectant.

      The final example is for a small, low pressure, low temperature model with 20 acre spacing.   In this low pressure example, there is no difference between the production predicted for nitrogen, air or rich-air because a limited volume of gas dissolved in the oil at 500 psia.  This example can be contrasted to Figure 1 or the light-oil air injection web page.  In these 4,000 psi examples, gases containing carbon dioxide produce the most oil.  In this case, all gases will produce the same amount of oil.

     Using Both Nitrogen and Rich-Air: The preceding examples illustrate how much oil could be produced by injecting air, nitrogen or rich-air and introduces the possibility to inject both the nitrogen and rich-air .   The next figure illustrates that using both nitrogen and rich air could be  much more profitable than injecting nitrogen alone.  The figure shows the composition and volume of gases produced from a small pressure swing adsorption nitrogen generation plant and that of a large BCF/day refrigerated gas separation plant.  Figure 5 shows first that the small plant produces nitrogen for about 60 cents per MCF, while a plant as large as that installed at Cantarell in Mexico could produce nitrogen for 40 cents per MCF.    The small plant could supply 10 or 20 injection wells.  While the large plant could supply a world class field.

     The small plant with capacity of up to 40 MCF/day will produce almost equal volumes of low and high oxygen product.  Now if the 40% oxygen stream from the small plant is diluted with air to 30% oxygen, its volume is over twice the volume of the nitrogen stream and the average price of rich-air and nitrogen is 28 cents per MCF.

      The large plant produces almost pure nitrogen for 40 cents per MCF.    The nitrogen could be used where corrosion is a major concern, such as at an offshore field like Cantarell.  However, the byproduct rich-air contains 70% oxygen and is 30% of the plants output.  If this stream were diluted with air to 30% oxygen, the average gas price from the plant would now be 12 cents per MCF.  Thus, injecting the rich-air that is produced from a nitrogen plant as well as the nitrogen could improve the economics of regional projects dramatically.

      Process Selection: A few examples have been provided illustrating where nitrogen, air or rich air could be the better gas for enhanced oil recovery. It appears that if nitrogen should be injected (for gas cap production for instance), the rich-air should also be used nearby.  Deciding whether a reservoir is a suitable candidate requires teamwork by reservoir engineers and geologists.  For more information please contact MK Tech Solutions.