Page 101 - Fundamentals of Enhanced Oil and Gas Recovery

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Enhanced Oil Recovery Using CO 2

high saturation of residual oil. This means a poor volumetric efficiency, which is not
desirable [59].
In order to overcome this issue, it is recommended to lower CO 2 mobility by
decreasing its saturation. This could be done by alternatively injecting CO 2 and water
in cycles. This process is called WAG injection. A slug of CO 2 is injected and fol-
lowed by water. As water enters into the reservoir, it fills some of the pore spaces,
resulting in reduced mobility to CO 2 .
Although WAG gives promising results, there are still some problems associated
with this mode of injection. For instance, injected water may not be distributed
uniformly in the reservoir, causing reduced microscopic efficiency. Indeed, nonuni-
form distribution can occur when there is a sufficient vertical permeability. This
causes water to move downward while gas will tongue in an upward direction.
Another important problem associated with the WAG process is the so-called
problem of “water-shielding.” Injected water blocks any possible connections between
CO 2 and trapped oil in the porous media that were swept by water injection, leaving
a high residual oil saturation. Water blocking is much more significant for water-wet
rocks and it is negligible for mixed-wet rocks.
The major design issues to be considered for an optimum WAG injection process
are WAG ratio, ultimate CO 2 slug size, rock and fluid properties, reservoir characteris-
tics and heterogeneity, injection rate, and injection pattern [63,64]. Recently, for an
optimized WAG process, researchers are working on the brine composition of water
being injected alternatively with CO 2 . A new hybrid method has been proposed as
low salinity water (LSW) alternative CO 2 injection. Results show that this new
method enhances oil recovery by both LSW and CO 2 effects. Although the results of
CO 2 LSW are promising, further research is needed to assess its efficiency [65].

3.4 CO 2 INJECTION LABORATORY TESTS

As discussed earlier in previous sections, when CO 2 is injected into a reservoir,
a series of complex mechanisms will occur. These mechanisms are effective in
improving oil recovery. Therefore it is of great importance to know the performance
of each mechanism. The performance of a CO 2 flood could only be assessed by
experimental investigations in simulated condition of the reservoir. Because reservoirs
and CO 2 injection processes vary widely, it is not possible to list a set of laboratory
experiments appropriate to all situations. Laboratory experiments performed most
often fall into three general classes as follows [66]:
• Standard PVT test
PVT experiments are commonly performed for determining the relationship
between pressure volume temperature. Phase behavior and fluid properties that