Page 185 - Fundamentals of Reservoir Engineering

DARCY'S LAW AND APPLICATIONS 123

where the subscript "d" refers to the displacing fluid, which need not necessarily be
water. To improve the displacement efficiency, M should be reduced to a value of unity
or less which will have the effect of converting the displacement from the type shown in
fig. 4.10(b), to the ideal type shown in fig. 4.10(a); this is referred to as "mobility
control". The methods by which M can be reduced are.

Polymer flooding (increase, µ d)

Polymers, such as polysaccharide, are dissolved in the injection water, this raises its
viscosity, thus reducing the mobility of the water. Polymer flooding will not only
accelerate the oil recovery but can also increase it, in comparison to a normal water
drive, because the by-passing of oil is greatly reduced.

Thermal methods (decrease, µ o/µ d)

For very viscous crudes the ratio of µ o / µ d can be of the order of thousands (which
means that M has the same order of magnitude) and therefore, water drive cannot be
considered as a feasible project (refer Chapter 10, exercise 10.1). In such cases the
viscosity ratio can be drastically reduced by increasing the temperature, as shown in
fig. 4.6(a). This is achieved by one of the following methods:

- hot water injection
- steam injection
- in-situ combustion.

Although mobility control is the primary aim in applying thermal methods, there are
other factors involved than merely the reduction of, µ o / µ d (where in this case, µ d is the
viscosity of the hot water or steam and differs from µ w at normal reservoir temperature).
In many cases distillation of the crude occurs, the lighter fractions of the oil being
vapourised and providing a miscible flood in advance of the thermal front. Expansion of
the oil on heating will also add to the recovery. Thermal methods can therefore be
considered as basically secondary recovery processes with some tertiary side effects,
such as the crude distillation, which tends to reduce the residual oil saturation.

Tertiary flooding

Tertiary flooding aims at recovering the oil remaining in the reservoir after a
conventional secondary recovery project, such as a water drive. Oil and water are
immiscible (do not mix) and as a result there is a finite surface tension at the interface
between the fluids. This, in turn, leads to the trapping of oil droplets within each
separate pore which is the normal state after a waterflood.

From a strictly mechanical point of view, the methods commonly employed in tertiary
flooding can be appreciated by considering fig. 4.11, which shows an enlargement of
an oil relative permeability curve (solid line) for water-oil displacement, in the vicinity of
the residual oil saturation point. After a water drive k ro is zero when S o = S or, point A,
and the oil will not flow.

Two possibilities for improving the situation are indicated which amount to altering the
oil relative permeability characteristics. The first of these is to displace the oil with a