Page 33 - Primer on Enhanced Oil Recovery

24 Primer on Enhanced Oil Recovery

Figure 2.7 Combination of gravity and waterdrive in the horizontal and inclined liquid
containing strata.

named elastic-pressure mode. Elastic mode the mode of reservoir operation in
the process of reducing the pressure in the reservoir. The reservoir energy exhibits
itself in the form of an elastic expansion of the reservoir fluid and rock. Compared
with the water-pressure mode, the elastic-pressure mode of the reservoir operation
is less effective. The oil recovery coefficient in this mode of reservoir development
can be between 0.3 and 0.6.
Water drive mode (Waterdrive) is an active water drive mode. It is the most
effective mode in which oil moves in the reservoir to the production wells under
the pressure of contour (or bottom) water. In this mode it is possible to extract
35 75% of OOIP. Sometimes the extraction is even higher. In average the recovery
factor for Waterdrive is thought to be at 50%.
It is also possible that the gas above the oil in the formation, so named gas cap,
has some significant pressure. As we pump the oil from the reservoir the oil level
lowers and gas expands but still presses oil downwards towards out production well
perforation. This situation is named gas cap drive. The recovery factor in this case
is within 0.2 0.3 and is taken in average as 0.3. For most economical exploitation
it is recommended to maintain gas cap pressure by injecting natural gas. In some
cases it was possible to do natural gas injection even after significant depletion of
reservoir pressure and restore or initiate gas drive.
As we start production the pressure in the reservoir might drop. This leads to
slight oil expansion as an elastic body and this expansion for a while will keep the
pressure and oil flowing to the production well. It is also possible that during pres-
sure drop the liquid passes the bubble point pressure, e.g. some dissolved gas will
start to form bubbles. This gas separation can continue for a while and maintain
some almost stable, below the original, pressure in the formation. This is so named
solution gas drive.
The drawings on Fig. 2.8 reflect on the above diving mechanisms.
In almost all cases there is so named mixed natural drive. It means that two or
more of the above mechanism are present and are active or will activate as the pres-
sure in the reservoir change.