Page 72 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Huff-n-puff gas injection in oil reservoirs 59

2.15.5 CO 2 injection in an unfractured vertical well in the
Middle Bakken
In shale and tight reservoirs, fractured wells are commonly used. However,
fractured wells may induce small fractures to form a stimulated reservoir vol-
ume, which may complicate the study of CO 2 diffusion in the matrix. The
heterogeneity of formation may leave a long horizontal well unideal well
type to study EOR mechanisms. To avoid these complexities, the Energy
and Environmental Research Center (EERC) and XTO Energy conducted
a pilot CO 2 injection in an unfractured vertical well in one virgin Middle
Bakken area (Sorensen et al., 2018). The well name is Knutson-Were 34-
3WIW, North Dakota state well ID number 11413. First, a small scale pre-
test was carried out on April 3, 2017. 16 tons of CO 2 was injected enough to
infill the tubing and build pressure on the perforations. When the pressure
was built to 9113 psi that was higher than the reservoir pressure of 8668
psi, the upper packed that isolated the injection zone failed. CO 2 did not
enter the formation. After the packer was repaired, the well was swabbed
and 62 barrels of fluid was produced. After swabbing, the bottom-hole pres-
sure (BHP) was about 7500 psi. Then a main test was initialed at 7 pm MDT
(Mountain Daylight Time) on June 24, 2017 and concluded at 5 am June
28, 2017. Total 98.9 tons of CO 2 were injected for about 3.2 days. On
June 27, the well was shut in for a about 5 hours to run a pressure falloff test.
After the injection was ended on June 28, the well was shut in for soak-
ing until it was opened on July 7 (about 9 days of shut in). At this time, the
BHP was 8740 psi close to the early reservoir pressure. After opening the
well, gas flowed 8.5 hours, essentially CO 2 from the tubing with some traces
of hydrocarbons in the last 2 hours. The BHP dropped to 100 psi. Those
data indicated that a significant amount of injected CO 2 was voided from
the reservoir. Because the well could not sustain flow, the well was put
back to shut in for another 6 days until July 13. The total soak time was
about 13.6 days. The BHP was built up to 3116 psi, which might result
from the reservoir oil migration to the near wellbore zone. After the well
was open and produced a mix of CO 2 and hydrocarbon gas for 10.5 hours,
some oil started to flow to the surface at a rate of about one eighth of a barrel
per minute. By that time, the BHP was decreased to 1890 psi that was below
the saturation pressure. Within 45 minutes of flow, 9 barrels of oil was pro-
duced. Analysis of produced oil compositions shows that the oil composition
produced after CO 2 injection was lighter than that before.