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 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. Introduction Pressure maintenance is generally required for gas condensate reservoirs to prevent (or minimize) retrograde liquid loss. Pressure maintenance is also needed for volatile oil and many black oil reservoirs to improve recovery. Historically, pressure maintenance projects have utilized many different types of gas for injection. High pressure cycling projects for gas condensate reservoirs have projects for gas condensate reservoirs have generally used dry gas (lean process plant residue gas after recovery of gasoline type components), and even flue gas (88 % nitrogen) for injection. However, the current value of hydrocarbon gas essentially prohibits its use for pressure maintenance. Nitrogen can now be produced from air separation plants at considerably less cost than the value 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. General phase behavior characteristics and physical properties of reservoir fluids have been physical properties of reservoir fluids have been known for many years. Nitrogen is a component commonly found in most reservoir fluids, and methods have been presented for determining gas compressibility factors of gases containing nonhydro-carbons. However, the possibility that significant changes in phase equilibria and physical properties could occur in reservoir fluids upon properties could occur in reservoir fluids upon injection of nitrogen, has hindered its acceptance and use as an injection gas. The purpose of this study was to conduct experimental laboratory tests with several different reservoir fluids to determine the effects of contact by varying amounts of nitrogen. It was also desired to determine if an equation of state could be used to reliably predict these effects. This paper presents the results of these experimental tests. The effects of nitrogen on the phase equilibria (dew point pressure, retrograde liquid phase equilibria (dew point pressure, retrograde liquid condensation, and revaporization of liquid) of three gas condensate reservoir fluids are shown. The changes in oil formation volume factor, solution gas-oil ratio, oil viscosity, oil density, and solution gas gravity of an oil which was contacted by nitrogen are also shown. The effects of nitrogen calculated with a modified Redlich-Kwong equation of state are also compared with the experimental data.