Page 186 - Fundamentals of Reservoir Engineering

DARCY'S LAW AND APPLICATIONS 124

fluid which is soluble in it, thus increasing the oil saturation above S or. This is
equivalent to moving from point A to B on the normal relative permeability curve. As a
result k ro is finite and the oil becomes mobile.

Alternatively, flooding can be carried out with a fluid which is miscible, or partially
miscible, with the oil thus eliminating surface tension, or in some way modifying the
interfacial properties, between the displacing fluid and the oil. This reduces the residual
oil saturation to a very low value, S′ in fig. 4.11, and alters the oil relative permeability
or
curve, as shown by the dashed line. In this case, when the displacing fluid contacts the
residual oil left after the waterflood, the effect is that the oil relative permeability
increases from zero to point C and again the residual oil becomes mobile.

k ro

B C

A
S o S or S’ or

Fig. 4.11 Illustrating two methods of mobilising the residual oil remaining after a
conventional waterflood

Obviously the second method appears the more favourable since it creates the
possibility of recovering practically all of the residual oil. In the first case, only part of
each swollen oil droplet is recovered. Tertiary floods generally aim at either total
miscibility or else a combination of the methods described above. The ways in which
such floods can be engineered are many and varied, some of the more popular being,

Miscible (LPG) flooding

The oil is displaced by one of the LPG (Liquid Petroleum Gas) products, ethane,
propane or butane. If the reservoir conditions are such that the LPG is in the liquid
phase then it is miscible with the oil and theoretically all the residual oil can be
recovered.
Carbon Dioxide flooding

Carbon dioxide has a critical temperature of 88°F and is therefore normally injected
into the reservoir as a gas. It is highly soluble in oil and this has two favourable effects.
In the first place the saturation of the oil droplets, containing dissolved CO 2, increases
above the residual saturation, S or, the oil permeability becomes finite and oil starts to
flow. Secondly, the viscosity of the oil is reduced resulting in better mobility control. In
addition the carbon dioxide, by extracting light hydrocarbons from the oil, displays
miscible properties.