Summary
Foams can improve oil recovery by reducing gas mobility and the effects of
reservoir heterogeneity. Numerous studies report that foam flow in porous media
comprises two regimes. In the “high-quality regime,” pressure gradient is
nearly independent of gas superficial velocity. In the “low-quality regime,”
pressure gradient is nearly independent of liquid superficial velocity.
Previous published data from CO2 foam studies lie either in the high- or
low-quality regime, but no single study shows both regimes. Delineating the two
foam-flow regimes is essential to modeling and predicting the behavior of CO2
foam in petroleum applications.
Experiments were performed with a sandpack and fired Berea and Boise
sandstone cores at a backpressure of 1500 or 2000 psig, above and below the
critical temperature of CO2. The data from the sandpack, Berea, and Boise
sandstone experiments at room temperature do not show the two conventional
foam-flow regimes. Instead, these experiments find a third regime evidently
related to the low-quality regime. This same behavior was observed in the
sandpack above the critical temperature of CO2. In this new regime, pressure
gradient decreases with increasing liquid superficial velocity at constant gas
superficial velocity. The Boise sandstone experiment above the critical
temperature of CO2 did find the two conventional foam-flow regimes, however. No
single experimental factor appears to explain the difference in results.
Earlier theoretical work of Hirasaki and Lawson and de Vries and Wit can
partially explain the flow regime seen in our study. A model combining a
bundle-of-tubes approach with the effective-viscosity function of Hirasaki and
Lawson predicts the behavior in this new regime.
Introduction
In producing the oil from a reservoir, on average approximately two-thirds
of oil originally in place is left in the reservoir at the end of
waterflooding. The goal of enhanced oil recovery (EOR) is to increase the
fraction of oil recovered from a reservoir. Injecting steam, carbon dioxide
(CO2), and field gas have been the most productive EOR methods. CO2 is injected
into oil reservoirs because CO2 dissolves into oil easily, reduces oil
viscosity, and can extract the light components of crude oil at sufficiently
high pressure, and CO2 can become miscible with oil at lower pressure than
other gases. The CO2 process can be highly effective within rock strata where
it contacts oil.
However, actual oil recovery with CO2 in the field is much lower, because of
poor sweep efficiency: the gas contacts and sweeps only a small portion of oil
in the reservoir. Poor sweep efficiency is caused by the low viscosity and
density of CO2 and by reservoir heterogeneity. These effects cause early gas
breakthrough and low sweep efficiency. Foam can improve the sweep efficiency of
the injected gas by reducing gas mobility and the effects of reservoir
heterogeneity. Foams are also used in matrix acid well-stimulation treatments
and environmental remediation.
Field trials of CO2 foam showed some success. A number of foam EOR field
trials with other miscible or near-miscible gases have been carried out as
well.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
12 January 2004
- Revised manuscript received:
3 May 2005
- Manuscript approved:
13 July 2005
- Version of record:
15 December 2005
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