Page 240 - Hydrocarbon Exploration and Production Second Edition

Reservoir Dynamic Behaviour 227

recovered oil (30%) recovery method
considered
residual oil (15%) miscible injection
surfactant flooding
thermal recovery
by-passed oil (20%) polymer flooding
infill wells
oil remaining due to lack water injection
of drive energy (20%) gas injection
unproducible oil infill
remaining (20%) (horizontal) wells

Figure 9.21 Recovering the remaining oil.

the pore throats. Very low residual oil saturations (around 5%) can be achieved.
Surfactants such as soaps and detergents are added to the injection water.
Miscible processes are aimed at recovering oil which would normally be left behind
as residual oil, by using a displacing fluid which actually mixes with the oil. Because
the miscible drive fluid is usually more mobile than oil, it tends to by-pass the oil
giving rise to a low macroscopic sweep efficiency. The method is therefore best
suited to high dip reservoirs. Typical miscible drive fluids include hydrocarbon
solvents, hydrocarbon gases, carbon dioxide and nitrogen.
When considering secondary recovery or EOR, it is important to establish
where the remaining oil lies. Figure 9.21 shows an example of where the remaining
oil may be, and the appropriate method of trying to recover it. The proportions are
only an example, but such a diagram should be constructed for a specific case study
to identify the ‘target oil’.
One category of remaining oil shown in the above diagram is unproducible oil
in thin oil rims (typically less than 40 ft thick), which cannot be produced without
coning in unwanted oil and/or gas. Horizontal wells are an ideal form of infill well
in this situation, and will be discussed in Section 10.3, Chapter 10.