INTRODUCTION
When a miscible fluid displaces another in a porous medium, the
displacing fluid tends to mix with the displaced fluid. The result is that a
mixing or transition zone develops at the front in which the
concentration of the injected fluid decreases from one to zero.
Experiment shows that the mixing zone grows as the displacement
progresses. This mixing and spreading of the injected fluid is known as
dispersion.
Bear (1972) describes dispersion as the "macroscopic outcome of
actual movement of individual tracer particles through pores...".
Essentially, dispersion is the mixing caused by single-phase fluid
movement through a porous medium. What is "mixed" is usually called
a tracer, but can be thought of as a concentration of any chemical
component within a given phase that is transported through the system.
Dispersion has practical consequences in contaminant transport in
aquifers and in improved oil recovery from petroleum reservoirs. If a
miscible contaminant is accidentally introduced into an aquifer at a site,
dispersion will cause the contaminant to spread to a larger area as it is
being transported by groundwater flow. Even though the concentration
of the contaminant is reduced by dispersion, a much larger area of the
aquifer will become contaminated as a result of dispersion than the
original spill area. Thus, a much larger area than the original spill will
need to be cleaned up by any contaminant remediation measure.
Miscible displacement is the most efficient improved oil recovery
method. Because there is no capillary force to trap the displaced oil, it is
theoretically possible to recover 100% of the oil by miscible displacement.
However, because the injected solvent is usually more expensive than the
oil that is to be displaced, it is usually injected in small quantities as
slugs and chased by a less expensive fluid such as water or gas.
Dispersion will dilute and reduce the effectiveness of the miscible slug as
it is propagated through the reservoir. In this case, dispersion is
detrimental to the recovery process. On the other hand, dispersion
causes a solvent to mix, spread and contact the displaced fluid even after
it had been originally bypassed by the injected solvent. In this case,
dispersion improves the displacement efficiency.
Other industrial processes that involve dispersion include (1) use
of tracers such as dyes, electrolytes and radioactive isotopes to
characterize reservoir and aquifer properties, (2) development of a
transition zone between salt water and fresh water in coastal aquifers, (3)
radioactive and reclaimed sewage waste disposals into aquifers, (4) use of
reactors packed with granular material in the chemical industry, and (5)
movement of fertilizers in the soil and the leaching of salts from the soil
in agriculture.