Page 105 - Fundamentals of Enhanced Oil and Gas Recovery

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Enhanced Oil Recovery Using CO 2

CO 2 injectivity was 4 times greater than that of water. The estimated oil recovery
was 21% compared with 12% from a water flooding well pattern in the same field.
• Song-Fang-Dun Field, Daqing [81]
The injection was carried out in a layer with an average gas permeability of
0.79 mD. The reservoir oil had a viscosity of 6.6 cP. The project started in
March 2003 with one injector and five producing wells. The first response was
observed in August 2004. The CO 2 injectivity was estimated to be 6.3 times
higher than that of water. Then WAG process was applied to this field in 2014.
Overall, CO 2 injection performance was not good due to high heterogeneity of
the formation.

3.7 CO 2 INJECTION FOR ENHANCED GAS RECOVERY

CO 2 injection into gas reservoirs for enhanced gas recovery (EGR) is a new
topic of concern, and no field trial has been reported so far. However, recent
researches have shown that CO 2 injection would profit those countries that have very
large gas reservoirs but limited number of oil reservoirs. Gas reserves can also provide
good storage capacity for CO 2 . The main mechanisms responsible for EGR due to
CO 2 injection are: displacement, which is analogous to water flooding process in oil
reservoirs; reservoir pressure support, which prevents the reservoir from being
depleted and also inhibits from subsequent subsidence and water intrusion [13,82].
Despite the promising results of CO 2 EGR for gas reservoirs, it has never been
tested in the field. This can be attributed to two main reasons. Firstly, CO 2 costs are
high as a commodity, and thus geological storage is not accepted worldwide.
Secondly, concerns exist about the potential of CO 2 mixing with natural gas, which
could downgrade the natural gas resource.
CO 2 and CH 4 are able to be mixed at any pressure, and CO 2 CH 4 mixture has
some characteristics that promote the CO 2 EGR efficiency. In summary, these charac-
teristics are as follows [13]:
1. An achievable gravity-stable displacement due to higher density of CO 2 compared
to that of CH 4 (normally 2 4 times higher). This also could be attributed to
lower mobility of CO 2 , as it is more viscous than CH 4 at reservoir condition.
2. A delayed CO 2 breakthrough due to higher solubility of CO 2 in formation water
compared to that of methane.
3. Higher injectivity of CO 2 by reason of its nearly gas-like viscosity.
In order to optimize a CO 2 EGR project, a sensitivity analysis should be conducted
on CO 2 resources, mixing of injected gas with methane and project time. Other effec-
tive factors like injection gas rate should also be accounted. In conclusion, more
research works are needed to assess CO 2 EGR viability and effectiveness [11,83,84].