8 1 Introduction
oils, adverse mobility ratios can be improved by using steam injection or some
other thermal method to heat the oil to reduce its viscosity.
Water flooding, polymer injection, and steam injection are all examples
of methods for so-called enhanced oil recovery (EOR). Another example is
miscible and chemical injection, where one uses a solvent or surfactant that
mixes with the oil in the reservoir to make it flow more readily. The solvent
may be a gas such as carbon dioxide or nitrogen. However, the most common
approach is to inject natural gas produced from the reservoir when there is
no market that will accept the gas. Surfactants are similar to detergents used
for laundry. Alkaline or caustic solutions, for instance, can react with organic
acids occuring naturally in the reservoir to produce soap. The effect of all
these substances is that they reduce the interfacial tension between water and
oil, which enables small droplets of oil that were previously immobile to flow
(more) freely. This is the same type of process that takes place when you use
detergent to remove vaxy and greasy stains from textiles. A limiting factor
of these methods is that the chemicals are quickly adsorbed and lost into the
reservoir rock
Often, one will want to combine methods that improve the sweep efficiency
of mobile oil with methods the mobilize immobile oil. Miscible gas injection,
for instance, can be used after a waterflood to flush out residually trapped
oil and establish new pathways to the production wells. Water-alternating-
gas (WAG) is the most successful and widely used EOR method. Injecting
large volumes of gas is expensive, and by injecting alternating slugs of water,
one reduces the injected volume of gas required to maintain pressure. Sim-
ilarly, presence of mobile water reduces the tendency of the injected gas to
finger through the less mobile oil. In polymer flooding, it is common to add
surfactants to mobilize immobile oil by reducing or removing the interface
tension between oil and water, and likewise, add alkaline solutions to reduce
the adsorption of chemicals onto the rock faces.
While the mechanisms of all the above methods for enhanced oil recovery
are reasonably well studied and understood, there are other methods whose
mechanisms are much debated. This includes injection of low-salinity water,
which is not well understood even though it has proved to be highly effective in
certain cases. Another example is microbial enhanced oil recovery which relies
on microbes that digest long hydrocarbon molecules to form biosurfactants or
emit carbon dioxide that will reduce interfacial tension and mobilize immobile
oil. Microbial activity can either by achieved by injecting bacterial cultures
mixed with a food source, or by injecting nutrients that will activate microbes
that already reside in the reservoir.
Use of secondary recovery mechanisms has been highly successful. On the
Norwegian Continental Shelf, for instance, the average recovery factor is now
almost 50%, which can be attributed mainly to water flooding and miscible
gas injection. In other parts of the world, chemical methods have proved to be
very efficient for onshore reservoirs having relatively short distances between
wells. For offshore fields, however, the potential benefits of using chemical
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