Summary
Miscible injection of carbon dioxide has seen a significant increase in
interest for the purpose of enhanced oil recovery (EOR) in conventional oil
reservoirs. However, naturally fractured reservoirs, which are among the
largest oil reserves in the world, are considered poor candidates for this
process because of presumed low-performance efficiency. This paper presents the
results of an experimental study that explains the effect of connate water
saturation, matrix permeability and oil viscosity on the performance of gravity
drainage from the matrix (into fracture) when it is surrounded by a
CO2-filled fracture. Experiments were performed in an experimental
model under different operating pressures to cover both immiscible and miscible
conditions. Experiments were conducted using synthetic oil (nC10)
and light crude oil in two Berea cores having large differences in
permeability. In addition, the effect of connate water saturation was studied
by performing experiments in an initially brine-saturated Berea core and
comparing the results with those obtained when the core was 100% saturated with
oil. The experimental results showed that matrix permeability had a significant
effect on the rate of gravity drainage when CO2 was injected under
immiscible conditions. When experiments were performed at immiscible
conditions, production rate by gravity drainage was nearly five times greater
in the Berea core with 1,000 md permeability compared to the core permeability
of 100 md. The production rates in the cores investigated were similar at low
pressures (below 3,400 kPa), but slightly higher for the higher-permeability
core. As system pressure was increased beyond 3,400 kPa, the production rate
from the higher-permeability core increased significantly, compared to the
lower-permeability case. Beyond miscibility conditions (~6,900 kPa), matrix
permeability was less significant, indicating the important role of capillary
pressure in the gravity drainage mechanism. However, ultimate oil recovery was
less sensitive to the matrix permeability at pressures near or above minimum
miscibility pressure. The observations were more interesting when experiments
were performed in the presence of connate water saturation. The ultimate oil
recovery from a core saturated with oil in the presence of connate water
saturation was less at immiscible conditions. However, at near-miscible and
miscible conditions, the presence of connate water was beneficial to the
gravity drainage mechanism in that it led to higher ultimate oil recovery. The
effect of oil viscosity appeared to be important during the sustained
miscibility of CO2 and hydrocarbon phases. For the crude oil
examined, the heavier components that remain in the oil phase after the
vapourizing gas drive limited the length of the oil production period when
compared with the nC10 production. Miscible CO2 injection
in fractured reservoirs is a viable option for both oil recovery and storage
purposes because as the residual oil saturation is reduced, additional pore
volume (PV) becomes available to store CO2 in its supercritical
form. However, under immiscible conditions, when CO2 is injected at
pressures below the minimum miscibility pressure (MMP) and above the
supercritical condition, it is not beneficial for improving oil recovery by
gravity drainage. This was clearly seen when gravity drainage experiments using
crude oil were performed and MMP was not achieved at the maximum possible
operating pressures. The results obtained from this study address the knowledge
gap in the best practices for utilizing CO2 for improving oil
recovery from fractured reservoir environments and demonstrate the effects of
key parameters on the gravity drainage mechanism.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
28 March 2009
- Meeting paper published:
17 June 2009
- Revised manuscript received:
25 July 2010
- Manuscript approved:
7 August 2010
- Published online:
1 November 2010
- Version of record:
1 November 2010
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