Phase behavior tests were carried out on a Nisku crude oil (a candidate reservoir for nitrogen flooding) with nitrogen and nitrogen mixed with carbon dioxide or propane. These measurements were carried out in a batchwise multiple contact mode to simulate what occurs when gas displaces oil in a porous medium. None of the mixtures achieved miscibility with the oil in four contacts at pressures up to 35 MPa. Nitrogen and methane were exchanged between the gas and oil phases in an almost ratio, while gas enrichment with C2–5 hydrocarbons proceeded at a much slower pace.
The experimental data were matched using the Peng-Robinson equation of state. Good agreement between experimental and calculated results was obtained in most cases. This applies to the equilibrium phases composition, as well as to the liquid phases densities and gas/oil ratios. The largest deviations were in the amount of heavy ends found in the vapour phase.
Nitrogen may be applied for enhanced oil recovery (EOR) in several modes. For example:
Cycling condensate reservoirs
Attic oil production
Producing gas-cap gas
Chase gas for hydrocarbon or carbon dioxide slugs
Miscible displacement of oil
These applications were discussed by Clancy et al. 1 and reviewed in detail by Sayegh et al.2 Recently Leonard3 summarized EOR activity worldwide. From this article it was noted that there are seven nitrogen injection projects in the U. S.A., four of which have already showed profit, while it is too early to tell for the remaining three. In addition, there are three flue gas injection projects. In Canada, two ongoing floods utilize nitrogen. The first is in a Pembina Nisku reef, wherein the injection gas consists of a mixture rich in nitrogen along with carbon dioxide and light hydrocarbons. The second is the Willisden Green flood, where nitrogen is being used as a chase gas for a hydrocarbon slug.
The advantages of using nitrogen for EOR include:
Widespread availability from air liquifaction
In addition, the price of nitrogen appears to be competitive with that of dry hydrocarbons, although it is difficult to give more precise estimates because of the many variables involved in costing. Wilson4 noted that nitrogen has some disadvantages in its use for enhanced oil recovery, which include:
The cost of separating nitrogen from the produced hydrocarbon gases. This is offset of the produced gases are recycled into the reservoir.
If air separation units are used to produce nitrogen, then a byproduct stream rich in oxygen is produced. This stream has to be well separated from any hydrocarbon streams in the vicinity.
In summary, the use of nitrogen in enhanced oil recovery has many advantages that can cutweight its disadvantages. This is a new topic, and there are many gaps in the information available for its proper application. It is the purpose of this work to try and fill in some of these gaps.