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The process
works best for fields where the temperature is high
enough for injected air to spontaneously oxidize the
oil. The injected air is converted to flue gas,
with approximately 13 percent carbon dioxide, in a
low-temperature reaction zone near the injector. A
flue gas displacement is generally immiscible, and
swelling of originally under-saturated oil is the most
important recovery mechanism. Thus, as Figure 1
shows, Light-Oil Air Injection might not recover as much
oil as a miscible rich gas or carbon dioxide injection.
However, it could make more money because its largest
cost is gas compression, i.e., the injected gas is free.
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Figure
1 – Predicted Recovery of 32 API
Oil During 20 Years of
Primary Production
In
this example, a 320-acre, 20-md, 1/4 five-spot model at 220° F with a
625 psi bubble point and 205 GOR oil, 70% more oil is recovered with
rich gas than with air, but an additional 0.4 pore volumes of NGL needed
to be injected as part of the rich gas, i.e, the oil in the model was
replaced by expensive NGL.
Obviously, the economics of air, rich gas, and carbon dioxide injection
depend on the availability and value of the injected fluid at the field.
Enough data has been published on the Medicine Pole Hills Unit in Bowman
County ND to calculate its profitability. The production from this
38 API oil is reported in Figure 2.
Figure 2 – NGL and Oil
Production at the Medicine Pole Hills Unit
Air injection began in 1988. The
response was very fast. An NGL plant and two new injectors
were added in 1991 and 1992. The maximum incremental
production was 800 BPD, yielding an Air Oil ratio of 8 MCF/STB.
Power for gas compression costs $0.20 per MCF. Total
investment, excluding infill primary wells, was $12.7 MM. So,
the earning power of the investment is 17% without any EOR tax
credits.
The
most important concern after potential profit is “Will the oil
spontaneously oxidize?” This is determined from an
Accelerating Rate Calorimeter (ARC) test conducted at the
University of Calgary. In this test, a sample is heated in a
sealed bomb with air until rapid self-heating begins.
Most
light oils oxidize like the oil in Figure 3.
Figure 3 – Spontaneous
Oxidation of a 32 API Gulf Coast Crude Oil
Figure 3 shows that a mixture of oil, rock and water begins to oxidize
at 110 oC and heats at up to 1,000 °C/min - reaching a 300 °C
quickly. This represents burning of the volatile oil components.
As the sample is heated further, the asphaltenes and heavy ends begin to
decompose and oxidize above 400 °C causing the temperature to rise
again. For most light oils, there are not enough heavy ends to
sustain the high temperature reaction causing the temperature in the
reaction zone to stay below 350 °C.
The
temperature that can be reached is determined by the heat generation
rate, i.e. amount of fuel and the oxygen flux, minus heat losses to the
reservoir and surroundings. Thus, higher porosity, thick reservoirs and
high injection rates are favorable for the light oil air injection
process. In addition, a higher initial reservoir temperature
promotes spontaneous ignition. Trends caused by these variables
are illustrated with the simulation results in Figure 4.
Figure 4 – Effect of
Reservoir Porosity, Thickness and Temperature on Oil Production
The
plot shows that production increases almost linearly with thickness,
implying that override of gas decreases with increasing ratio of well
spacing to thickness in the reservoir. Reduced porosity has a
large negative effect since the heat produced by combustion is retained
by more reservoir rock. This reduces the maximum temperature and thermal
expansion of gas and vaporization of water. Initial reservoir
temperature lower than 220°F has the largest effect since oil is more
viscous and the oxidation reactions cannot start as easily.
While deeper, hotter, thicker fields are excellent candidates for
Light_Oil Air Injection, Figure 4 also shows that owners of thin, lower
porosity, cooler fields can benefit from this inexpensive technology.
A
more complete summary of this study is available in SPE 59331 which is
available from the Society of Petroleum
Engineers, or from MK Tech Solutions.
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