Summary
Measurements of the interfacial tension (IFT) of mixtures of a reservoir
fluid and injection gas at various pressures have been proposed as an
experimental method for predicting the minimum miscibility pressure (MMP) in an
experiment referred to as the vanishing-IFT (VIT) technique. In this paper, we
analyze the accuracy and reliability of the VIT approach using phase
equilibrium and slimtube experimental observations and equation-of-state (EOS)
calculations of the behavior of VIT experiments for the same systems.
We consider 13 gas/oil systems for which phase equilibrium and density data
and slimtube measurements of the MMP are available. We show that tuned EOS
characterizations using 15 components to represent the gas/oil systems yield
calculations of phase compositions and densities and calculated MMPs that
reproduce the experimental observations accurately. We assume that IFTs can be
calculated with a parachor expression, and we simulate the behavior of a series
of VIT experiments with different mixture compositions in the VIT cell. We show
that compositions of mixtures created in the VIT cell are not, in general,
critical mixtures and that calculated estimates of the MMP obtained by the VIT
approach depend strongly on the composition of the mixture used in the
experiment. We show also that those MMP estimates may or may not differ
significantly from values obtained in slimtube displacements. Fortuitously
chosen mixture compositions can result in VIT-experiment estimates that agree
well with slimtube MMPs, while for other mixtures, the error of the estimates
can be quite large. In particular, we show that errors in the VIT-technique
estimate of the MMP are often large for gas/oil systems for which the
first-contact miscibility pressure (FCMP) is much larger than the slimtube
MMP.
We conclude, therefore, that the VIT experiment is not a reliable single
source of information regarding the development of multicontact miscibility in
multicomponent gas/oil displacements.
Introduction
Many oil fields are now candidates for enhanced-oil-recovery processes such
as tertiary gasfloods or miscible water-alternating-gas injection schemes. The
MMP is an important parameter in the design and implementation of these
displacement processes and, hence, it is equally important that the MMP be
determined by a method that is both reliable and accurate.
Several methods have been proposed for measurement of the MMP. The
slimtube-displacement experiment is the most commonly used approach (Yellig and
Metcalfe 1980; Holm and Josendal 1982; Orr et al. 1982). Because of the
time-consuming process of performing multiple slimtube-displacement
experiments, alternative experimental approaches have been proposed. Some
investigators have suggested use of a rising-bubble experiment, in which
observations of bubbles of injection gas rising through oil (Christiansen and
Haines 1987; Eakin and Mitch 1988; Novosad et al. 1990; Sibbald et al. 1991;
Mihcakan and Poettmann 1994), are a basis of a method for determining the MMP.
Zhou and Orr (1988) concluded that the changes in bubble behavior observed in
the rising-bubble experiment are caused primarily by changes in IFT as
components in the bubble dissolve in the oil and components in the oil transfer
to the bubble. They showed that rising-bubble experiments could be used to
measure the MMP for vaporizing gas drives, but are less accurate for condensing
gas drives, while a drop of oil falling through gas could be used to determine
the MMP for condensing gas drives. Whether either a falling-drop or a
rising-bubble experiment could be used to determine the MMP accurately in
condensing/vaporizing gas drives such as those described by Zick (1986),
Stalkup (1987), and Johns et al. (1993) has not been determined.
Rao and coworkers proposed a different use of IFT observations to determine
the MMP (Rao 1997, 1999; Rao and Lee 2002, 2003; Ayirala et al. 2003; Ayirala
and Rao 2004, 2006a, 2006b; Sequeira 2006). They measured IFTs for pendant
drops of oil suspended in a cell containing a two-phase mixture of the
injection gas and the oil. In that approach, known as the VIT experiment, the
IFT is measured at a sequence of pressures, and the MMP is taken to be the
pressure at which the IFT plotted as a function of pressure extrapolates to
zero IFT. Orr and Jessen (2007) presented an analysis of the VIT technique
based on EOS calculations for well-characterized ternary and quaternary gas/oil
systems and demonstrated that the VIT experiment may give estimates of the MMP
that differ significantly from the MMP based on critical tie-lines for
condensing, vaporizing, and condensing/vaporizing gas drives.
In this paper, we extend the analysis of Orr and Jessen (2007) and calculate
the IFT behavior that would be observed in the VIT experiment for gas
displacements of multicomponent crude-oil systems. We assess the accuracy of
MMP estimated by the VIT approach for 13 multicomponent gas/oil displacements
for which experimental phase-equilibrium and slimtube data are available, and
we demonstrate that for these multicomponent crude-oil systems, the VIT
approach can give estimates of the MMP that are close to the actual MMP or that
are significantly in error, depending on the compositions of mixtures created
in the equilibrium cell.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
2 August 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
20 February 2008
- Manuscript approved:
2 March 2008
- Version of record:
25 October 2008
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