Summary
High-pressure air injection (HPAI) is an improved-oil-recovery (IOR) process
in which compressed air is injected into a deep light-oil reservoir with the
expectation that the oxygen in the injected air will react with a fraction of
the reservoir oil at an elevated temperature to produce carbon dioxide. The
resulting flue-gas mixture provides the main mobilizing force to the oil
downstream of the reaction region, sweeping it to production wells. The
combustion zone itself may provide a critical part of the sweep mechanism.
In 1994, Fassihi et al. proposed a method for estimating recovery factors of
light-oil air-injection projects on the basis of the performance of two
successful HPAI projects. Their suggested method relies on the extrapolation of
the field gas/oil ratio (GOR) up to an economic limit. In other words, it
treats HPAI as an immiscible gasflood and neglects any potential oil that could
be recovered by the combustion front. The truth is that, although early
production during an HPAI process is caused mostly by repressurization and
gasflood effects, once a pore volume of air has been injected, the combustion
front becomes the main driving mechanism. Moreover, one of the unique features
of air injection is the self-correcting nature of the combustion zone, which
promotes good volumetric sweep of the reservoir.
This paper presents laboratory and field evidence of the presence of a
thermal front during HPAI operations and evidence of its beneficial impact on
oil recovery. An analysis of the three HPAI projects in Buffalo field, which
are the oldest HPAI projects currently in operation, shows that only a small
fraction of the reservoir has been burned and, if time allows and the projects
are managed appropriately, burning of more reservoir volumes could result in
much higher oil recoveries than those predicted by the gasflood approach.
Introduction
HPAI is an emerging technology for the recovery of light oils that has
proved to be a valuable IOR process, especially in deep thin low-permeability
reservoirs (Erickson et al. 1994; Kumar and Fassihi 1995; Kumar et al. 2007a,
2007b; Fassihi et al. 1996, 1997).
The first extended field test of HPAI began in 1963 on the Sloss field in
Nebraska (Parrish et al. 1974a, 1974b), where Amoco’s Combination of Forward
Combustion and Waterflooding (COFCAW) process was applied as a
tertiary-recovery process to a deep (6,200 ft), thin (11 ft), light-oil
(38.8°API), watered-out reservoir. This COFCAW pilot recovered 83,992 bbl of
oil, which is equivalent to 43% of the oil remaining in the five-spot pattern
after waterflood. In 1967, the pilot was expanded from an 80- to a 960-acre
project and recovered 527,000 bbl of incremental oil. However, it proved to be
uneconomical, with crude-oil prices at less than USD 3/bbl.
The second application of HPAI was the West Heidelberg pressure-maintenance
project (Huffman et al. 1983) in the US state of Mississippi, which started in
1971 as a secondary-recovery project in the deep (11,400 ft) Cotton Valley
sands. Even though oil prices were less than USD 4/bbl during the early period
of the air-injection operations, payout of the project occurred at
approximately 2.5 years, and the project continued to be a successful
air-injection project. One interesting aspect of this project was the
simulation work presented by Kumar (1991), which showed that, although the
early production was mainly because of pressure maintenance, more than half of
the cumulative oil production was mainly a result of thermal effects.
An important milestone in the advance of HPAI was the implementation of
commercial secondary HPAI projects in the North and South Dakota portions of
the Williston basin, which started in 1979 and continues to be a technical and
economic success (Erickson et al. 1994; Kumar and Fassihi 1995; Kumar et al.
2007a, 2007b; Fassihi et al. 1996, 1997).
The estimation of ultimate recovery in HPAI projects is subject to a high
level of uncertainty and requires history matching. Nevertheless, in 1994,
Kumar and Fassihi (1995) proposed a method for estimating recovery factors of
light-oil air-injection projects on the basis of the performance of two HPAI
projects. Their suggested method relies on the extrapolation of the field GOR
up to an economic limit. In other words, it considers HPAI as an immiscible
gasflood.
This paper intends to challenge that "gasflood" approach with a
"combustion" approach, on the basis of laboratory results and field
data gathered mostly from the Buffalo field, which comprises the three oldest
HPAI projects currently in operation.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
1 August 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
27 February 2008
- Manuscript approved:
6 March 2008
- Version of record:
29 December 2008
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