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to be first-contact miscible. Light and intermediate-molecular weight hydrocarbons,
such as methane, propane and butane, are first-contact miscible agents. However, the
injected gas and the oil may form two different phases, that is, they may not be first-
contact miscible, but mass transfer between the two phases and long-time contact
between the fluids may achieve miscibility. This is usually called multiple-contact
or dynamic miscibility. In the petroleum industry, the gases or fluids that achieve
either first-contact or dynamic miscibility are usually called miscible solvents.In
this chapter we restrict ourselves mostly to first-contact miscible displacements, as
modeling multiple-contact miscibility involves computation of the phase equilibria
diagrams for the mixture of the reservoir fluids and the injected gas, a subject which
is beyond the scope of our discussions.
Atypical miscible injection, or flooding, is carried out by injecting into the reservoir
a limited volume or slug of the solvent. The solvent slug may be displaced miscibly
by an appropriate drive fluid (miscible slug process), or immiscibly by, for example,
water. The latter process leaves a residual solvent saturation in the reservoir, and
causes dilution and fingering of the drive fluid in the solvent, and fingering of the
solvent in the oil, all of which reduce the overall effectiveness of the process. Such
miscible displacements have received considerable attention since the early 1950s.
Over 100 studies were undertaken in the 1950s and early 1960s to investigate the
feasibility and economics of miscible displacement processes, and in particular gas
injection, as an effective tool for enhanced oil recovery (EOR).
The early studies indicated that natural gas, flue gas, nitrogen at high pressure,
and enriched hydrocarbon gases achieve miscibility with the reservoir oil. The high
injection pressure and the specific composition that an enriched gas must have
limit the number of prospective oil reservoirs in which a miscible flood can be
used. Slug of fluids containing oil, water, surfactants, and co-solvents (e.g., alco-
hols) in various compositions – known as micellar polymer solutions – have also
been found to be efficient miscible displacement agents, although they are not used
commonly.
Perhaps the best advantage of N 2 flooding is that it can potentially be used anywhere
in the world, if it can be cheaply extracted from the air, where other injection fluids are
either not available or the cost of their delivery to the oil reservoir is prohibitive. On
the other hand, since light hydrocarbons contain considerable combustion energies,
their use as EOR agents is generally limited to remote locations, which makes the cost
of delivery of the oil to commercial markets too high. In the United States, the main
emphasis has been on miscible CO 2 flooding since it offers two main advantages
over other gases. One is that CO 2 requires a relatively low operating pressure to
attain miscibility with reservoir fluids. The second advantage is that CO 2 is relatively
inexpensive as it has no value as a fuel, and is available in large amounts from natural
deposits or as a waste product of industrial processes.
However, even if the gas that one wishes to use as a miscible displacement agent
is economically available, its use is not without problems. Gases are normally less
viscous than typical crude oil. The viscosity contrast between the oil and the injected
gas, together with the phenomenon of gravity segregation (see below), render the