Page 231 - Hydrocarbon Exploration and Production Second Edition

218 Fluid Displacement in the Reservoir

oil producers

oil by-passed oil

water water injectors
rock
grain
macroscopic
sweep
residual oil

by-passed oil

original OWC

microscopic
displacement
Figure 9.13 Oil remaining in the reservoir at abandonment.

layers, and if the wells are abandoned due to high water cut arising from water
breakthrough in the permeable layers, then oil will remain in the yet unswept parts
of the less permeable layers.
The macroscopic sweep efficiency is the fraction of the total reservoir which is swept
by water (or by gas in the case of gas cap drive). This will depend on the reservoir
quality and continuity, and the rate at which the displacement takes place. At higher
rates, displacement will take place even more preferentially in the high permeability
layers, and the macroscopic displacement efficiency will be reduced.
This is why an offtake limit on the plateau production rate is often imposed, to
limit the amount of by-passed oil, and increase the macroscopic sweep efficiency.
2
On a microscopic scale (the inset represents about 1–2 mm ), even in parts of
the reservoir which have been swept by water, some oil remains as residual oil. The
surface tension at the oil–water interface is so high that as the water attempts to
displace the oil out of the pore space through the small capillaries, the continuous
phase of oil breaks up, leaving small droplets of oil (snapped off, or capillary trapped
oil) in the pore space. Typical residual oil saturation (S or ) is in the range 10–40% of the
pore space, and is higher in tighter sands, where the capillaries are smaller.
The microscopic displacement efficiency is the fraction of the oil which is recovered in
the swept part of the reservoir. If the initial oil saturation is S oi , then

S oi S or
Microscopic displacement efficiency ¼ 100ð%Þ
S oi