Wetting, Adsorption, and Cleaning Processes - Surface and Colloid Chemistry: Principles and Applications - page 132

Oil recovery from reservoirs is not a 100% process from rock since not all of the
oil in a reservoir is recovered. This means that all the oil recovered until now leaves
some 20-40% residual oil in the depleted reservoirs. This may be considered as an
advantage in the long run because, as the shortage of oil supplies approaches, one
may be forced to develop technologies to recover the residual oil.
In order to enhance this recovery, physicochemical methods have been used.
At present, approximately 100 million barrels of oil are used per day. In most oil
reservoirs, the primary recovery is based on the natural flow of oil under the gas
pressure of the reservoir. In these reservoirs, as this gas pressure drops, a water-
flooding procedure is used. The pressure needed is determined by the capillary
pressure of the reservoir and the viscosity of the oil. This procedure still results
in 30 to 50% of the original oil remaining in the formation. In some cases, such
substances as detergents or similar chemicals may be added to enhance the flow of
oil through the porous rock structure. The principle is to reduce the Laplace pres-
sure (i.e., ΔP = γ/curvature) and to reduce the contact angle as well. This process is
called tertiary oil recovery . The aim is to retrieve oil that is trapped in capillary-
like structures in the porous oil-bearing material. The addition of a surface-active
agent (SAA) reduces the oil-water interfacial tension (from ca. 30-50 mN/m to less
than 10 mN/m).
In the tertiary process, more complicated chemical additives are designed for
a particle reservoir. In all these recovery processes, the interfacial tension (IFT)
between the oil phase and the water phase is needed.
Another important factor is that, during water flooding, the water phase bypasses
the oil in the reservoir (Figure 6.1). What this implies is that, if one injects water
into the reservoir to push the oil, most of the water passes around the oil (the bypass
phenomena) and comes up without being able to push the oil.
The pressure difference to push the oil drop may be larger than that to push the
water, thus leading to the so-called bypass phenomena. In other words, as water
flooding is performed, due to bypass, there is less oil produced, while more water is
pumped back up with the oil.
The use of surfactants and other surface-active substances leads to the reduction
of γ oilwater as described in Figure 6.1. The pressure difference at the oil blob entrapped
in a reservoir and the surrounding aqueous phase will be

ΔP oilwater = 2 IFT (1/R 1 B 1/R 2 ) (6.2)

Thus, by decreasing the value of interfacial tension (with the help of SAAs; from 50
mN/m to 1 mN/m), the pressure needed for oil recovery would be decreased.
In the water-flooding process, mixed emulsifiers are used. Soluble oils are used
in various oil-well-treating processes, such as the treatment of water injection wells
to improve water injectivity and to remove water blockage in producing wells. The
same method is useful in different cleaning processes with oil wells. This is known
to be effective since water-in-oil microemulsions are found in these mixtures, and
with high viscosity. The micellar solution is composed essentially of hydrocarbon,
aqueous phase, and surfactant sufficient to impart micellar solution characteristics
to the emulsion. The hydrocarbon is crude oil or gasoline. Surfactants are alkyl aryl