The Effect of Nitrogen on the Phase Behavior and Physical Properties of Reservoir Fluids

Abstract

Abstract Nitrogen can be produced from air separation plants at considerably less cost than the value of an equal volume of natural gas. For this reason, nitrogen has been considered for pressure maintenance operations in gas condensate and oil reservoirs. However, the possibility that significant changes in phase equilibria and physical properties could occur in the reservoir fluid has hindered its acceptance as an injection gas. Experimental laboratory tests were conducted in which gas condensate and black oil reservoir fluids were contacted by nitrogen at reservoir conditions. The results showed that the dew point pressures of gas condensates would significantly increase and retrograde liquid could condense where the nitrogen mixes with the reservoir gas. Additional contact by nitrogen revaporized a significant amount of the condensed retrograde liquid. The results also showed that the oil formation volume factor and solution gas-oil ratio decreased, and the oil density and viscosity increased when a black-type oil was contacted by nitrogen. The experimental tests were simulated using a modified Redlich-Kwong equation of state. The results showed that this equation of state can be tuned to reliably predict the effects of nitrogen on phase equilibria and physical properties of reservoir fluids. This equation of state could subsequently be used in compositional reservoir simulators to predict overall effects of nitrogen injection on reservoir performance.