Summary
In miscible flooding, injection of solvent is often combined with water to
reduce the mobility contrast between injected and displaced fluids and control
the degree of fingering. Using traditional fractional-flow theory, Stalkup
estimated the optimum water-solvent ratio (or WAG ratio) when viscous fingering
effects are ignored, by imposing that the solvent and water fronts travel at
the same speed. Here we study how the displacement efficiency and the mobility
ratio across the solvent front vary with the WAG ratio when fingering is
included in the analysis. We do so by computing analytical solutions to a 1D
model of two-phase, three-component, first-contact miscible flow that includes
the macroscopic effects of viscous fingering. The macroscopic model, originally
proposed by Blunt and Christie (1993, 1994), employs an extension of the Koval
fingering model to multiphase flows. The premise is that the only parameter of
the model—the effective mobility ratio—must be calibrated dynamically until
self-consistency is achieved between the input value and the mobility contrast
across the solvent front. This model has been extensively validated by means of
high-resolution simulations that capture the details of viscous fingering and
carefully-designed laboratory experiments.
The results of this paper suggest that, while the prediction of the optimum
WAG ratio does not change dramatically by incorporating the effects of viscous
fingering, it is beneficial to inject more solvent than estimated by Stalkup’s
method. We show that, in this case, both the pore volumes injected (PVI) for
complete oil recovery and the degree of fingering are minimized.
Introduction
Solvent flooding is a commonly used technology for enhanced oil recovery in
hydrocarbon reservoirs, which aims at developing miscibility, thereby
mobilizing the residual oil and enhancing the mobility of the hydrocarbon phase
(Stalkup 1983; Lake 1989). Despite its high local displacement efficiency, the
overall effectiveness of solvent injection may be compromised by viscous
fingering, channeling, and gravity override, all of which contribute negatively
to sweep efficiency (Christie and Bond 1987; Christie 1989; Christie et al.
1993; Chang et al. 1994; Tchelepi and Orr 1994). In this paper, we focus on the
effect of viscous fingering; that is, the instability that occurs when a
low-viscosity fluid (solvent) is injected into a formation filled with more
viscous fluids (water and oil).
Mobility control of the injected solvent can be achieved by simultaneous
coinjection of water—typically in alternating water and solvent slugs (WAG)
(Caudle and Dyes 1958). In this way, the mobility contrast between the injected
and displaced fluids is reduced, thereby limiting the degree of
fingering.
There is an optimum ratio of water to solvent that maximizes recovery—in the
sense of minimizing the number of pore volumes injected—while providing
effective mobility control. For linear floods in homogeneous media, and without
consideration of viscous fingering effects, a graphical construction of the
optimum WAG ratio was given by Stalkup (1983) for both secondary floods
(water/solvent injection into a medium filled with mobile oil and immobile
water) and tertiary floods (water-solvent injection into a medium filled with
mobile water and immobile oil). The design condition imposed in Stalkup’s
method is that the velocity of the water and solvent fronts be the same. Walsh
and Lake (1989) performed an interesting analysis of the WAG ratio (the ratio
of injected water to solvent) on the displacement efficiency for secondary and
tertiary floods, using fractional-flow theory. They did not include the effects
of viscous fingering, but they estimated the mobility contrast across the
solvent front as a measure of the severity of fingering.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
23 February 2006
- Meeting paper published:
22 April 2006
- Revised manuscript received:
29 January 2007
- Manuscript approved:
1 February 2007
- Version of record:
20 December 2007
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