Summary
The minimum miscibility pressure (MMP) is a key parameter governing the
displacement efficiency of gasfloods. There are several methods to determine
the MMP, but the most accurate methods are slim-tube experiments, analytical
methods, and numerical-simulation/cell-to-cell methods. Slim-tube experiments
are important to perform because they use actual crude oil, but they are costly
and time consuming. Analytical methods that use the method of characteristics
(MOC) are very fast and help to understand the structure of gasfloods. MOC,
however, relies on finding the unique and correct set of key tie lines in the
displacements, which can be difficult. Slim-tube simulation methods and their
simplified cell-to-cell derivatives require tedious fluid and rock inputs, and
their MMP estimates can be clouded by dispersion.
This paper presents a simple and accurate multiple-mixing-cell method for
MMP calculations that corrects for dispersion, and is faster and less
cumbersome than 1D simulation methods. Unlike previous mixing-cell methods, our
cell-to-cell mixing model uses a variable number of cells, and is independent
of gas/oil ratio, volume of the cells, excess oil volumes, and the amount of
gas injected. The new method only relies on robust P/T flash calculations using
any cubic equation-of-state (EOS). The calculations begin with only two cells
and perform additional cell-to-cell contacts between resulting
equilibrium-phase compositions based on equilibrium gas moving ahead of the
equilibrium liquid phase. We show for a variety of oil and gas compositions
that all key tie lines can be found to the desired accuracy, and that they are
nearly identical to those found using analytical MOC methods. Our approach,
however, is more accurate and robust than those from MOC because we do not make
approximations regarding shocks along nontie-line paths, and the unique set of
key tie lines converges automatically.
The MMP using our mixing-cell method can be calculated in minutes using an
Excel spreadsheet and is estimated from a novel bisection method of the minimum
tie-line lengths observed in the cells at four or five pressures. Our
multiple-mixing-cell method can calculate either the MMP or the minimum
miscibility for enrichment (MME) independent of the number of components in the
gas or oil. Our approach further supports the notion that the MMP is
independent of fractional flow because we obtain the same key tie lines
independent of how much fluid is moved from one cell to another.
© 2011. Society of Petroleum Engineers
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History
- Original manuscript received:
25 November 2009
- Meeting paper published:
22 September 2008
- Revised manuscript received:
25 January 2011
- Manuscript approved:
26 January 2011
- Published online:
25 October 2011
- Version of record:
23 December 2011
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