Page 174 - Enhanced Oil Recovery in Shale and Tight Reservoirs

158 Enhanced Oil Recovery in Shale and Tight Reservoirs

vessel for a set puff time. At the end of each cycle, the core was pull out of
the vessel, the liquid on the core surface was wiped, and the core was
weighted with the weight recorded as W exp . That ended one cycle of
huff-n-puff water injection. Then the core was placed back in the vessel
for the next cycle.
Oil recovery factor (RF) during a water huff-n-puff test can be deter-
mined in the following. Based on the material balance, the weight of the
core that is saturated with oil and water at the end of each cycle (W exp ) equals
the weight of the original oil saturated core (W sat ) minus the weight of oil
produced (V p $RF$r o ) plus the weight of water imbibed (V p $RF$r w ):

W exp ¼ W sat RF$V p $r þ RF$V p $r w (7.2)
o
The pore volume in the above equation V p can be calculated by
W sat W dry
V p ¼ (7.3)
r o
Then the oil recovery factor which is oil produced divided by the pore
volume can be derived from the above equations:
ðW exp W sat Þ$r o
RF ¼ (7.4)
ðW sat W dry Þ$ðr r Þ
o
w
During the huff period, water may finger into the oil phase; water may
also imbibe into the core through the countercurrent flow with oil; water
preferentially invades in large pores and then imbibes into small pores to
displace oil. During soaking the period, water may continue fingering
into the oil phase in the early period as the pressure outside the core may
still be higher than that inside; of course, water imbibes into the core. During
the puff period, oil comes out of the core owing to the pressure difference
between the inside core and the annulus and possibly by water imbibition.
Probably, an important mechanism is that the invaded water and imbibed
water increase reservoir pressure and local pressure so that the drive energy
is boosted. From the imbibition point of view, water-wet formation is
preferred. This conclusion is supported by experimental data by Huang
and Xiang (2004).
Fig. 7.4 shows the effect of soaking time. As water is not as compressible
as gas, when water was injected, the pressure in the vessel quickly reached
the set pressure of 1000 psi. The huff time was short, and a relatively longer
soaking period was needed. In the figure, the soaking time actually included
the huff time. The puff time was 3 h. On increasing the huff and soaking