Quantification of Displacement Mechanisms in Multicomponent Gasfloods
- R.T. Johns (U. of Texas at Austin) | H. Yuan (U. of Texas at Austin) | B. Dindoruk (Shell Intl. E&P)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- September 2004
- Document Type
- Journal Paper
- 314 - 321
- 2004. Society of Petroleum Engineers
- 5.5 Reservoir Simulation, 4.1.5 Processing Equipment, 5.4.2 Gas Injection Methods, 4.1.2 Separation and Treating, 5.6.4 Drillstem/Well Testing, 5.3.2 Multiphase Flow, 5.2.1 Phase Behavior and PVT Measurements, 5.2 Fluid Characterization, 5.2.2 Fluid Modeling, Equations of State, 4.6 Natural Gas, 5.3.1 Flow in Porous Media
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Local displacement efficiency in gasfloods depends strongly on the minimum miscibility pressure (MMP) or minimum miscibility enrichment (MME). The values for these design parameters depend in turn on the displacement mechanisms: vaporizing, condensing, or a combination of the two known as a condensing/vaporizing (CV) drive. Characterization of the displacement mechanism, however, is currently limited to these broad categories, with little reference to the degree to which a CV displacement is condensing or vaporizing. This "discrete" classification approach can result in significant confusion in the interpretation and comparison of various miscible gasfloods. The focus of this paper, therefore, is to present a method to quantify in a continuous way the fraction of a multicomponent gasflood that is vaporizing or condensing as the pressure or gas enrichment is increased.
The approach relies on finding key tie lines for a dispersion-free 1D displacement using method of characteristic theory (MOC). We quantify the displacement mechanism for any number of oil or gas components by calculating the displacement path lengths along ruled surfaces bounded by these key tie lines. We show how to determine the displacement mechanism along each of these ruled surfaces by the calculation and comparison of the key tie-line lengths. Several multicomponent fluid characterizations are considered, including a 12-component enriched-gasflood and a 13-component CO2 flood.
The results show that as the pressure or enrichment is increased, condensation occurs at the expense of vaporization. We also show by numerical simulations that the sensitivity of the local displacement efficiency to dispersion depends on the condensing fraction of the displacement. We show that the trends in displacement efficiency sensitivity to dispersion oppose previously published results, which showed that vaporizing displacements are more sensitive to dispersion than condensing ones. The differing trends are likely the result of improper and discrete determination of the displacement mechanism.
Pseudoternary diagrams have traditionally been used to explain the behavior of multicontact miscible (MCM) gas-drive processes.1 Both qualitative mixing cell arguments and more rigorous mathematical approaches show that a ternary displacement can be MCM only if either the oil composition (vaporizing gas drive) or the injection gas composition (condensing gas drive) lies outside the region of tie-line extensions on a ternary phase diagram.2,3 For ternary systems, the MMP is the pressure at which the oil lies on a critical tie-line extension, whereas the MME is found when the gas composition lies on a critical tie-line extension. Thus, a ternary displacement can be condensing or vaporizing, but not both, and the compositions, pressure, and temperature determine which key tie line (gas or oil) controls the development of miscibility.
Zick4 and Stalkup5 found that multicomponent oil displacements could be both vaporizing and condensing drives. They showed that MMPs and MMEs estimated by ternary methods could be significantly different from those observed for combined CV drives.
Johns et al.6 mathematically proved the existence of the combined CV mechanism for four-component systems and gave a method to calculate the dependence of the MMP or MME on the displacement mechanism. In particular, they showed that a key tie line, called the crossover tie line, controls the development of miscibility in CV drives. The MME occurs when the extension of the crossover tie line intersects the extensions of both oil and gas tie lines as enrichment is increased.
Johns and Orr7 showed that the displacement path for multicomponent dispersion-free flow is controlled by n-1 key tie lines, which include the oil tie line, gas tie line, and nc-3 crossover tie lines. They extended the simple tie-line geometric construction to show that successive key tie lines must intersect and that any one of those key tie lines could control the onset of miscibility. MCM flow is obtained when any one of the key tie lines intersects the critical locus as pressure (MMP) or enrichment (MME) is increased. Johns and Orr also showed that the displacement is purely vaporizing when the oil tie line becomes a critical tie line first as pressure is increased. Otherwise, miscibility is controlled by one of the crossover tie lines, and the displacement mechanism is CV.
More recent papers have used the geometric construction of Johns and Orr7 to calculate MMPs and MMEs for displacements of oils by multicomponent gases. Jessen et al.8 significantly increased the calculation speed of MMP estimation by inclusion of fugacity equations in the Newton-Raphson iterations. Yuan and Johns9 presented a simplified and robust analytical method for both MMP and MME calculation.
Wang and Peck10 suggested that the displacement mechanism could be quantified by determining the number of the key tie line that controls miscibility. They also showed a relationship between the number of the critical tie line and the effect of dispersion on local displacement efficiency. For example, a displacement where miscibility is controlled by the most upstream crossover tie line is likely more vaporizing than a displacement that is controlled by the most downstream crossover tie line. Johns et al.,11 however, demonstrated for a simple four-component model that the critical tie-line number is only a qualitative and discrete indicator of the displacement mechanism for CV drives. Others have also examined the sensitivity of local displacement efficiency to dispersion for a variety of systems and displacement mechanisms.11-14
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