Phase Equilibrium Considerations in Using Nitrogen for Improved Recovery From Retrograde Condensate Reservoirs
Authors: Moses Phillip L. , Wilson Keith 
Published: December 2007

Summary
This paper describes laboratory studies performed to evaluate the effectiveness of nitrogen in maintaining reservoir pressure and displacing condensate from a retrograde gas-condensate reservoir. The studies indicate that nitrogen will displace condensate reservoir fluid from a packed column under miscible displacement conditions and is an attractive substitute for lean natural gas in maintaining reservoir pressure in gas-condensate reservoirs. Theoretical considerations indicate that little mixing of condensate reservoir fluid and nitrogen will occur in the reservoir.

Introduction
Lean natural gas bas been used successfully for many years as the injection fluid for condensate reservoir cycling. The limited availability and increasing value of natural gas has made its use for conventional cycling economically unattractive for most condensate reservoirs. Therefore, the use of less expensive substitutes, such as pure nitrogen, has been suggested.1 Questions have been raised, however, concerning the effect of the phase equilibrium properties of nitrogen on condensate liquid dropout in the reservoir. Computer calculations indicated that mixing of lean natural gas with condensate reservoir fluid raised the dewpoint somewhat and caused some retrograde condensation. The same source showed a much more significant effect when nitrogen was mixed with condensate fluid. To answer some of these questions a retrograde condensate reservoir fluid was chosen for laboratory experiments. The properties of the reservoir fluid are shown in Table 1.
Laboratory measurements on this reservoir fluid indicated a dewpoint of 3,428 psig at the reservoir temperature of 200°F. Depletion studies indicate that normal pressure depletion would result in recovery of 22.3070 of the stock-tank liquid and 81.5% of the primary- and second-stage separator gases at an abandonment pressure of 700 psig. Retrograde loss would reach a maximum of 20% of the hydrocarbon pore volume at 2,300 psig as shown in Fig. 1.

Laboratory Studies
A volume of the reservoir fluid was charged to a windowed equilibrium cell, and measured volumes of lean gas were injected. The composition of the lean gas is shown in Table 2. The dewpoint pressure was observed, and the initial portion of the retrograde curve was measured for each mixture. The injection of lean gas into the reservoir fluid caused the dewpoint of the mixture to increase above the original dewpoint of the reservoir fluid. Each addition of injection gas caused the dewpoint to increase further. A total of 2,467 scf of lean gas per barrel of original reservoir fluid at dewpoint conditions was added. The dewpoint increased from the original dewpoint of 3,428 to 4,880 psig after the final addition. The dewpoint and retrograde behavior is summarized in Table 3 and Fig. 2.
A similar experiment was performed using pure nitrogen as the injection gas. The dewpoint and a portion of the retrograde curve again were measured for each mixture of nitrogen and reservoir fluid (see Table 4 and Fig. 3). The dewpoint pressure elevation with nitrogen mixtures was much greater than with lean gas mixtures (see Fig. 4). A total of 940 scf of nitrogen was injected per barrel of original reservoir fluid. The dewpoint of this mixture was 7,100 psig.