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Cyclic Air Injection:
Stimulation, Sand Control, Upgraded Oil
Cyclic in-situ combustion (analogous to cyclic-steam stimulation or cyclic CO2 injection) has proven to be effective in increasing oil production from heavy oil reservoirs. Additionally, this air injection application is capable of reducing sand influx in unconsolidated formations and upgrading the produced oil.
The success of the method depends on generating sufficient heat in the reservoir to increase the oil mobility and not polymerizing the oil. Reservoir energy (compressibility) is also augmented by the injected non-condensable gases.
Patos-Marinza Oilfield, Albania
Sand production and lack of oil productivity in the Driza formation of the Patos-Marinza field prompted the state-owned operator, Albpetrol, to take remedial action in the form of cyclic in-situ combustion.
The formation lies at a depth of 450 m, with oil gravity of 11°API, and viscosity of approximately 9000 cp. A total of 8 wells, which had been shut-in for years, were treated with cyclic in-situ combustion.
After cleaning the wellbores, about 200,000 sm³ air were injected in each well with rates from 1200 to 3800 sm³/d at wellhead pressures varying between 20 and 40 bar. This resulted in a burned zone of approximately 5 m radius and a stabilized reduction of sand inflow from 8 % down to less than 2 %. Technical difficulties with the downhole igniters resulted in two treatment failures. Incremental oil production in the successful treatments was as high as 650 tons cumulative, over a period of about 2 weeks, corresponding to an air/oil ratio of 300 sm3/m3.
Pleito Creek, California
This field produces from the Santa Margarita sandstone which is a faulted anticline with an average sand thickness of 130ft. The reservoir lies at a depth of 3,600-4500 ft. Porosity averages 27% and oil saturation varies from 70% at the top of the anticline to about 50% at the oil-water contact. Oil gravity and viscosity are 11°-14°API and 278 - 2,713 cp, respectively. Reservoir temperature is 105°-115° F.
In 1959 Humble Oil (Exxon) tested cyclic air injection on two flank oil producers (B-8 and B-7) for 4 months each. By 1964, the company had used air injection to thermally stimulate the nine best wells in the field, with a total of 22 stimulation cycles.
All stimulated wells suffered some liner damage that resulted from coke plugging. The tests however proved that well productivity and oil gravity could be significantly increased. The B-8 well saw a 19-fold increase in oil rate and an increase in gravity to 44° API. At the B-7 well the oil rate increased to 7.5 times the pre-stimulated rate.
Most individuals, notionally familiar with air injection EOR, balk at the idea of cyclic air injection. The concept of injecting air, intentionally igniting the oil, then putting the same well on production (“burn and turn”), seems fraught with opportunities for disaster.
The Patos-Marinza and Pleito Creek cyclic air injection tests demonstrate two important points:
- Cyclic air injection is feasible as a heavy oil production stimulation method, and
- If appropriate procedures are followed (not discussed here), it is possible to safely reverse flow in a high-temperature air injection well.
- Gjini, D., Buzi, X., Mastmann, M. and Tare, S. : “ Experience with Cyclic In Situ Combustion in Albania”, Paper 99-51, presented at the CSPG and Petroleum Society Joint Convention, Calgary, Alberta, Canada, June 14 -18, 1999.
- McGee, R., Fraim, M. and Hagen, S.: “Combined Miscible Drive Enhances Pleito Creek Oil Production”, Oil and Gas Jour., June 6, 2011.
- Mansdorfer, J., Personal Communication.