Page 70 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Huff-n-puff gas injection in oil reservoirs 57

From January to March 2010, there was a gradual oil productivity in-
crease. The peak oil rate reached 44 bbl/d in March 2010 (a higher rate
than what was achieved during any of the 14 months immediately prior
to the injection test). However, the operator confirmed that the higher pro-
duction rate might be related to workover activities on the well as opposed
to delayed CO 2 effects. Although the Burning Tree well did not see a dra-
matic production increase, CO 2 was able to be injected (no injectivity issue)
(Sorensen and Hamling, 2016).
One reason for the poor performance in the above tests in Montana and
North Dakota could be the too short injection duration (45 and 30 days,
respectively). In the North Dakota test, CO 2 broke through an offset well
about 5000 feet away in less than 2 weeks. This could be another reason
for the poor performance. The third reason may be too low injection pres-
sure (2000e3000 psi for both the tests).

2.15.3 Huff-n-puff CO 2 injection in Parshall field
EOG conducted a CO 2 injection test in NDIC 16713 in the Parshall field in
the Mountrail County in late 2008. A horizontal well was drilled into the
Middle Bakken and was completed using a six-stage hydraulic fracture treat-
ment. An estimated 30 MMSCF of CO 2 were injected using a huff-and-puff
approach. After 11 days of injection, CO 2 broke through the offset well
NDIC 16768, located one mile west of the NDIC 16713. The oil rates
for the test well and offset wells increased after injection.
The Parshall field has a high degree of natural fracturing, and the high
mobility of CO 2 in this fractured system indicates that conformance control
is likely a major factor in designing EOR operations. Of interest is the fact
that three other offset wells located within one mile of the injector did not
see CO 2 breakthrough, suggesting that understanding the local natural frac-
ture system is key to EOR planning (Sorensen and Hamling, 2016).

2.15.4 Eagle Ford project in La Salle County, TX
Four wells are in huff-n-puff injection starting in 2015. Initially gas was
injected for 6 months to fill up the reservoir void. Then wells produced
for 2e3 months followed by 8e10 weeks. This pattern was repeated for
four cycles. After that, a shorter cycle of 4e6 weeks of injection and soak
time and 2 months of production was used. The average well oil rate
and the cumulative production within the lease are shown in Fig. 2.47.
The oil rate clearly shows the increase (about doubled) from huff-n-puff