Immiscible CO2 injection is a potentially viable method of
enhanced oil recovery (EOR) for medium oil reservoirs in southwestern
Saskatchewan. The relatively high reservoir pressures could result in a large
extent of CO2 dissolution, significant oil viscosity reduction and
oil swelling. Laboratory corefloods were used to compare the performance of
different modes of CO2 injection into a medium-gravity oil system.
The following methods were compared: injection of a single CO2 slug
chased by water, simultaneous injection of water/CO2, and different
water-alternating-gas (WAG) cycles.
The results indicate that both a single CO2 slug and the first
WAG cycle in a series produced oil very efficiently, possibly owing to good
gas/oil contact at the relatively high residual oil saturation at this stage.
The simultaneous injection of water/CO2 was not as effective as a
single CO2 slug, possibly because the co-injected water shielded the
oil from being contacted by the gas. Among the four runs, a coreflood with four
WAG cycles recovered the most incremental oil—20.58% initial oil in place
(IOIP)—while the co-injection process produced the least (8.91% IOIP).
This experimental study suggests that the immiscible CO2 EOR
process is viable for medium oil reservoirs with relatively high pressures, and
that proper process application is important for maximizing additional oil
Miscible/immiscible CO2 flooding is an increasingly popular EOR
technique. So far, most of the CO2 miscible/immiscible processes
have been applied in light oil reservoirs, and there has been little
application in medium oil reservoirs. The in-place resources of medium oils,
defined as 20 – 30°API, in Saskatchewan are estimated at up to 1,338 ×
106 m3 (8.42 billion barrels)(1). Most of these
reservoirs are characterized by thin pay (2 to 10 metres in thickness) and
shaly sand, and they have been under production for over four to five decades.
The estimated average recovery by conventional waterflood, as listed in Table
1, is only approximately 23% IOIP because this method has generally exhibited
early water breakthrough and high water cuts owing to the unfavourably high
water-to-oil mobility ratio. By adapting miscible/immiscible CO2
flooding technologies that have matured in light oil applications, the
immiscible CO2 process is potentially viable for recovering
additional oil from these medium oil reservoirs if we can improve its
displacement and sweep efficiency(2).
© 2010. Society of Petroleum Engineers
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- Original manuscript received:
18 March 2008
- Meeting paper published:
17 June 2008
- Revised manuscript received:
17 December 2009
- Manuscript approved:
30 December 2009