Miscibility Variation in Compositionally Grading Reservoirs
- Lars Høier (Statoil) | Curtis H. Whitson (NTNU)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- February 2001
- Document Type
- Journal Paper
- 36 - 43
- 2001. Society of Petroleum Engineers
- 5.2.2 Fluid Modeling, Equations of State, 5.4.9 Miscible Methods, 4.1.5 Processing Equipment, 5.4.2 Gas Injection Methods, 5.3.2 Multiphase Flow, 5.7.2 Recovery Factors, 5.5 Reservoir Simulation, 5.8.8 Gas-condensate reservoirs, 5.2 Reservoir Fluid Dynamics, 5.2.1 Phase Behavior and PVT Measurements, 5.4.3 Gas Cycling, 4.6 Natural Gas, 4.1.2 Separation and Treating, 4.3.4 Scale
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Minimum miscibility conditions of pressure and enrichment (MMP/MME) have been computed with an equation of state (EOS) for several reservoir-fluid systems exhibiting compositional gradients with depth owing to gravity/chemical equilibrium. MMP/MME conditions are calculated with a multicell algorithm developed by Aaron Zick, where the condensing/vaporizing (C/V) mechanism of developed miscibility is used as the true measure of minimum miscibility conditions when it exists. The Zick algorithm is verified by detailed one-dimensional (1D) slimtube simulations with elimination of numerical dispersion. The miscibility conditions based on the traditional vaporizing-gas-drive (VGD) mechanism are also given for the sake of comparison, where it is typically found that this mechanism overpredicts conditions of miscibility.
Significant variations in MMP and MME with depth exist for reservoirs with typical compositional gradients, particularly for near-critical oil reservoirs and gas-condensate reservoirs where the C/V mechanism exists. An important practical implication of these results is that miscible displacement in gas-condensate reservoirs can be achieved far below the initial dewpoint pressure. The requirement is that the injection gas (slug) be enriched somewhat beyond a typical separator gas composition and that the C/V miscibility mechanism exist. This behavior results in many more gas-condensate reservoirs being viable candidates for miscible gas cycling than previously assumed, and at cycling conditions with lower cost requirements (i.e., lower pressures) and greater operational flexibility (e.g., cycling only during summer months).
Considerable work on miscible gas injection in oil and, to a lesser extent, gas-condensate reservoirs can be found in the literature.1,2 The phenomena of compositional variation with depth owing to gravity and thermal effects has also been studied in detail the past 20 years.3,4 However, almost nothing in the literature can be found on the variation of miscibility conditions with depth in reservoirs with compositional gradients.
It is difficult to picture the variation of MMP with depth for a reservoir with varying composition and temperature. This study shows that a simple variation does not exist, but that certain features of MMP variation are characteristic for most reservoirs.
For example, the simplest variation in MMP with depth is for a lean injection gas like nitrogen, where minimum miscibility conditions are developed by a purely VGD mechanism. Here the MMP is always greater than or equal to the saturation pressure. In the oil zone, MMP may be (and usually is) greater than the bubblepoint pressure, while in the gas zone the MMP is always equal to the dewpoint.
The MMP variation with depth can be considerably more complicated when the injection gas contains sufficient quantities of light-intermediate components (C2 through C5) or CO2. Here, developed miscibility is usually by the condensing/vaporizing mechanism, but it may be purely vaporizing in some depth intervals of the reservoir. When the C/V mechanism exists, MMP may be (and often is) less than the saturation pressure, even for gas-condensate systems.
This study quantifies the variation of MMP with depth for several reservoir-fluid systems, and we try to understand the reasons for seemingly complicated MMP variation. Perhaps the most important result of our study has been to show that miscible gas injection in gas-condensate reservoirs can exist far below the dewpoint. Economic application of enriched gas injection in partially depleted gas-condensate reservoirs may be achieved by slug injection, similar to miscible slug-injection projects in oil reservoirs.5
Calculating Minimum Miscibility Pressure
Miscibility between a reservoir fluid and an injection gas usually develops through a dynamic process of mixing, with component exchange controlled by phase equilibria (K-values) and local compositional variation along the path of displacement. The exact process of mixing is not really important to the development of miscibility - i.e., the relative mobilities (permeabilities) of flowing phases are unimportant. However, to obtain the correct MMP it is important to follow a physically realistic path of developed miscibility and not assume a priori how the path to miscibility occurs.
The ability of an EOS to predict minimum miscibility conditions and compositional grading is very dependent on the accurate representation of complex phase behavior and, in particular, accurate K-value predictions.4,6,7
Before 1986, it was assumed that developed miscibility followed one of two paths: Forward contact, or VGD, where the injection gas becomes enriched in C2+ by multiple contacts with original oil and, at the gas front, eventually develops miscibility with the original oil; or backward contact, or condensing gas drive (CGD), where the injection gas continuously enriches the reservoir oil in C2-C5 at the point of injection until the injection gas and enriched reservoir oil become miscible.
Either process can be modeled with a single-cell calculation algorithm,8,9 where the critical tie-line is located by appropriate multiple contacts of injection gas and reservoir oil. For gas condensates, the vaporizing mechanism has always been assumed to exist in miscible gas-cycling projects and the VGD MMP is readily shown to equal the original dewpoint pressure.
For reservoir oils, it is usually assumed that the VGD mechanism exists for lean injection gases, while the CGD has been assumed to describe miscible displacement for enriched gas injection. Using a single-cell calculation algorithm, the calculated VGD MMP is almost always lower than or equal to the CGD MMP, unless the gas is highly enriched.
Zick6 showed that a mixed mechanism involving both vaporization and condensation describes the actual development of minimum miscibility conditions for many systems. He showed that the location of miscibility (i.e., near-100% recovery efficiency) was not at the displacement front (VGD) or the point of injection (CGD), but in between. He also showed that the true minimum conditions of miscibility could be significantly lower than predicted by the VGD and CGD mechanisms. These findings have been verified by numerous publications during the past 10 years.7,10-12
Based on Zick's findings and his description of the mixed C/V mechanism, it is clear that the true MMP (or MME) can be calculated only if the path of developed miscibility is modeled properly. Several authors have suggested methods to calculate the C/V MMP.
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