Application of miscibility calculations to gas floods
Abstract
Local displacement efficiency from gas injection is highly dependent 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. Analytical methods, which are
inexpensive and quick to use, have been developed to estimate MMP’s for
complex fluid characterizations. This thesis presents a simplified and robust
method for MMP or MME calculation and quantification of the displacement
mechanism. The calculations are also applied to develop new correlations for
CO2 floods.
The approach relies on finding key crossover tie lines for a dispersion-free
displacement using method of characteristic theory (MOC). The new method,
however, differs from published methods by significantly reducing the number of
equations and unknown parameters, and by providing a fast and robust method
that can avoid trivial and false solutions. We demonstrate the improvements by
calculation of the MMP and MME for a variety of gas/oil systems and also give
new analytical solutions for constant K-value systems that give insight into the
nature of the false solutions.
A method also based on MOC theory is presented to quantify the fraction
of a multicomponent gas flood that is vaporizing or condensing as the pressure or
gas enrichment is increased. 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. Several multicomponent
fluid characterizations are considered. The results show that as the pressure or
enrichment is increased condensation occurs at the expense of vaporization. We
also show by numerical simulation that the sensitivity of the local displacement
efficiency to dispersion depends on the condensing fraction of the displacement.
The analytical method is also applied to the displacement of
multicomponent oil by CO2. Example calculations were performed for a variety
of reservoir fluids. New correlations are also generated for more accurate MMP
prediction for CO2 floods. In addition, a new lumping scheme for psuedoization
is proposed and applied for CO2 floods so that compositional reservoir simulation
can be used in field scale where the effect of dispersion is significant.
Department
Description
text