Page 164 - Enhanced Oil Recovery in Shale and Tight Reservoirs

148 Enhanced Oil Recovery in Shale and Tight Reservoirs

in the reservoir was 6.6 cP. There were one injector and five producers. The
benefit was observed at the producers by August 2004. By the end of 2006,
total 0.33 PV CO 2 was injected. The injector was not fractured. The CO 2
injectivity was about 6.3 times water injectivity. The pilot performance was
positive. However, uneven CO 2 sweep to the producers were observed.
This pilot was expanded to 14 injectors and 26 producers. CO 2 injection
was started in November 2007 before oil production in April 2009. In the
beginning, CO 2 injection rate was 22 tons/day, oil rate 0.2 tons/day. After
the producers responded to CO 2 injection, oil rate increased to 0.6 tons/
day. By 2014, water-alternate-gas injection was applied. By May
2015,204,000 tons of CO 2 had been injected, and 9000 tons of oil had
been produced. The ratio of oil produced to CO 2 injected was 0.044 tons/
ton which is equivalent to 53.75 MSCF/bbl oil if it is assumed that the oil
density is 0.85 and 1 ton of CO 2 is approximately 17.5 MSCF. Such CO 2
injection performance was not good. The main reason was that the net-to-
gross ratio was low (less than 0.5), and the formation was very heterogeneous.

6.4.4 CO 2 injection in Yu-Shu-Lin Field, Daqing (Wang, 2015)
The permeability in the Fuyang reservoir, Well-101 block, Yu-Shu-Lin
field was 0.96 mD in the injection area. The reservoir oil viscosity was
3.6 cP. Before CO 2 injection, the field was under waterflooding. The injec-
tion pattern was 5-spot with well spacings of 300 and 500 m. There were
total seven injectors and 17 producers. Two injectors started work in
December 2007, and five injectors in July 2008. By September
2013,110,600 tons of CO 2 had been injected, and 55,300 tons of oil had
been produced. The ratio of oil produced to CO 2 injected was 0.5 (tons
of oil to tons of CO 2 ) which is equivalent to 4.73 MSCF/bbl. Several points
are worth noting. CO 2 injection was half years ahead of production. CO 2
injectivity was more than four times water injectivity, making the pressure
maintenance by CO 2 injection easier than water injection. The wells
were not fractured. The estimated oil recovery was 21% compared with
12% from a waterflooding well pattern in the same field. This pilot area
was expanded to 70 injectors and 140 producers.

6.4.5 Summary of gas flooding performance
The above discussed gas flooding projects may be briefly summarized in
Table 6.3. These observations may be made.