The phase behavior of hydrocarbon systems can be described through the use of ternary diagrams such as Fig. 11.6. Crude oil phase behavior can typically be represented reasonably well by three fractions of the crude. One fraction is methane (C1). A second fraction is a mixture of ethane through hexane (C2-C6). The third fraction is the remaining hydrocarbon species lumped together and called C7+.
Figure 11.6 Ternary diagram illustrating typical hydrocarbon phase behavior at constant temperature and pressure.
Figure 11.6 illustrates the ternary phase diagram for a typical hydrocarbon system with these three pseudocomponents at the corners of the triangle. There are one-phase and two-phase regions (enclosed within the curved line V0–C–L0) in the diagram. The one-phase region may be vapor or liquid (to the left of the dashed line through the critical point, C) or gas (to the right of the dashed line through the critical point). A gas could be mixed with either a liquid or a vapor in appropriate percentages and yield a miscible system. However, when liquid is mixed with a vapor, often the result is a composition in the two-phase region. A mixing process is represented on a ternary diagram as a straight line. For example, if compositions V and G are mixed in appropriate proportions, the resulting mixture would fall on the line VG. If compositions V and L are mixed, the resulting overall composition M would fall on the line VL but the mixture would yield two phases since the resulting mixture would fall within the two-phase region. If two phases are formed, their compositions, V1 and L1, would be given by a tie line extended through the point M to the phase envelope. The part of the phase boundary on the phase envelope from the critical point C to point V0 is the dew-point curve. The phase boundary from C to L0 is the bubble-point curve. The entire bubble point–dew point curve is referred to as the binodal curve.
The oil–LPG–dry gas system will be used to illustrate the behavior of the first-contact miscible process on a ternary diagram. Figure 11.7 is a ternary diagram with the points O, P, and V representing the oil, LPG, and dry gas, respectively. The oil and LPG are miscible in all proportions. A mixing zone at the oil-LPG interface will grow as the front advances through the reservoir. At the rear of the LPG slug, the dry gas and LPG are miscible, and a mixing zone will also be created at this interface. If the dry gas–LPG mixing zone overtakes the LPG–oil mixing zone, miscibility will be maintained, unless the contact of the two zones yields mixtures inside the two-phase region (see Fig. 11.7, line M0M1).
Figure 11.7 Ternary diagram illustrating the single-contact miscible process.
Reservoir pressures sufficient to achieve miscibility are required. This limits the application of LPG processes to reservoirs having pressures at least of the order of 1500 psia. Reservoirs with pressures less than this might be amenable to alcohol flooding, another first-contact miscible process, since alcohols tend to be soluble with both oil and water (the drive fluid in this case). The two main problems with alcohols are that they are expensive and they become diluted with connate water during a flooding process, which reduces the miscibility with the oil. Alcohols that have been considered are in the C1-C4 range.
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