Theory of Multicontact Miscible Displacement with Nitrogen
- Birol Dindoruk | Franklin M. Orr Jr. | Russell T. Johns
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- September 1997
- Document Type
- Journal Paper
- 268 - 279
- 1997. Society of Petroleum Engineers
- 5.6.4 Drillstem/Well Testing, 4.1.4 Gas Processing, 5.3.2 Multiphase Flow, 5.4.2 Gas Injection Methods, 4.1.2 Separation and Treating, 5.2.1 Phase Behavior and PVT Measurements, 5.2 Reservoir Fluid Dynamics, 4.6 Natural Gas, 4.1.5 Processing Equipment, 5.2.2 Fluid Modeling, Equations of State
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Solutions obtained by the method of characteristics (MOC) for displacement of mixtures of methane, butane and decane by nitrogen or nitrogen/methane mixtures are used to explain conflicting experimental observations concerning the sensitivity of minimum miscibility pressures to changes in the compositions of an initial oil or the injection gas. The solutions presented show why some investigators have reported weak dependence of the minimum miscibility pressure (MMP) on methane concentrations while others have reported significant sensitivity. The analysis of displacement composition routes indicates that either observation can be correct for some ranges of initial and injection fluid compositions, and shows that the sensitivity behavior depends on the relative positions in composition space of three key tie lines (the initial, injection, and crossover tie lines) with respect to the critical locus.
The use of nitrogen (N2) as a multicontact miscible (MCM) displacement fluid has been studied by a variety of investigators,1-7 who have considered the effects of temperature. reservoir fluid composition, and injection gas composition on minimum miscibility pressure (MMP). There is, however, considerable disagreement among those investigators concerning even the qualitative effects of changes in the composition of the injected fluid and reservoir fluid. For example, Koch and Hutchinson1 reported MMP's for N2 and lean gas mixtures that showed that the MMP declined as the concentration of lean gas in the injection fluid increased, although the declines were small. McNeese,8 on the other hand, argued that the MMP for a vaporizing gas drive is independent of N2 content of the gas, and therefore that the results of Koch and Hutchinson were incorrect.
Firoozabadi and Aziz3 reviewed experimental work for slim-tube displacements of oils and developed a correlation for N2 and lean gas MMP's. That correlation did not include an explicit dependence of MMP on composition of the injection gas, and any dependence on methane (CH4) concentration in the reservoir fluid was represented indirectly through the concentration of intermediate hydrocarbons (C2-C5). Hudgins et al.5 reported experimental data that showed significant decreases in N2 MMP as the amount of dissolved gas was increased for two crude oils. Hudgins et al.5 offered a correlation for N2 MMP that contained an explicit dependence of the MMP on CH4 content in the oil. Glaso2 proposed a correlation for N2 that ignored CH4 content of the oil and suggested that N2 MMP may be lower than that for lean gas since injected N2 is represented by an "equivalent" CH4/ethane (C2) mixture. In a subsequent paper, however, Glaso6 reported experimental data for both crude oils and synthetic oils (methane (CH4)/propane (C3)/decane (C10) mixtures) that demonstrated significant reductions in N2 MMP as CH4 content of the reservoir fluid increased, and he offered a revised correlation for N2 MMP that included an explicit dependence on CH4 content of the oil.
Stalkup4 noted the limitations of pseudoternary diagrams for N2 systems. Similarly, Nouar et al.9 noted the need for separate treatment of N2 and CH4 in phase relationships. On the other hand, they stated that MMP's of N2 and CH4 displacements coincide once the CH4 in the oil exceeds a particular value.
Boersma and Hagoort7 reported experimental data for N2 displacement of three-component oils containing CH4, butane (C4) and tetradecane (C14). They analyzed their experimental results in terms of the behavior of critical tie lines represented on a quaternary phase diagram and concluded that N2 and CH4 MMP's were the same for their oil as long as the oil contained sufficient CH4.
Yurkiw and Flock10 demonstrated the limitations of existing N2-MMP correlations. In the systems they reviewed, injection gas impurities did not change MMP's drastically. Yurkiw and Flock concluded that additional study of the effects of variation of injection gas composition is needed.
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