Process Variations and Optimization
Steamflood Followed by Waterflood
As steamflood matures oil production rate declines, and the steam-oil ratio (SOR) eventually becomes uneconomically high. The high SOR generally indicates that a large amount of heat is retained in the reservoir and that some of this heat is cycled through the reservoir without affecting oil recovery.
The recent trend has been to convert a maturing steamflood to waterflood (Ault et al. 1985; Hong, 1987), which redistributes the heat in the reservoir, producing additional oil from zones that have been bypassed by the injected steam.
Water-Alternating-Steam injection has been used to delay or eliminate premature steam breakthrough and to improve sweep and recovery efficiencies.
The Russians used this method successfully in their heavy oil fields to improve oil production by 25%-30% annually from 1981-1984 (Hong, 1999). In several California applications water-alternating-steam eliminated steam breakthrough and improved recovery efficiencies.
Steam Injection Followed by Air Injection
Air injection as a follow-up process has the potential to recover significant additional oil after steam injection. BP Canada’s Wolf Lake “Pressure-Up Blowdown Combustion” process (Hallam and Donnelly, 1988) tested wet oxygen combustion after CSS in the Cold Lake bitumen reservoir. Total oil recovery factor was approximately double that of CSS alone and the steam-oil ratio was reduced from 6.1 m3/m3 to an equivalent of 2.3 m3/m3.
Hybrid Steam Processes
The use of co-injectants with steam, such natural gas, carbon dioxide, flue gas, solvents, etc. has received attention in the laboratory and several field trials. Depending on the choice of co-injectant, oil recovery may be improved through one or more of the following: enhanced viscosity reduction, a lowering of residual oil saturation, improved gas drive, reduced heat losses. Note that solvent-based processes alone for heavy oil recovery are generally slower than thermal processes.
These co-injectants have the potential to reduce the steam requirement, and increase oil production rates and recovery factor. Lower steam requirements (or steam-oil ratios) translates into lower capital investment, CO2 emission, and water and natural gas usage per barrel of oil recovered.
Expanding Solvent SAGD (ES-SAGD) is a variation of SAGD that involves injecting a combination of a low concentration light hydrocarbon and steam. This process takes advantage of the benefits from the heat provided by steam injection and the viscosity reduction through dilution offered by the solvent (Orr, 2009). This process has already been tested in the field, resulting in significant improvements in oil rate and steam-oil ratio. Long-term solvent returns on average are approximately 70%.
Imperial Oil’s LASER process employs steam-solvent cyclic steam stimulation (Leaute and Carey, 2005), using 6% volume fraction of C5+ condensate. Data from a field test at Cold Lake shows an incremental oil recovery of 10 m3 over regular CSS for every 1 m3 of unrecovered solvent.
Zhu et al. (2001) reported on a field test of simultaneous flue gas injection with steam in the Liaohe oil field in China. Following injection of the mixture the well was shut-in for 4 days to allow the gases to fully diffuse and penetrate the reservoir, and then turned around. The results indicate the following improvements over steam injection alone:
- Higher downhole steam quality during injection
- An overall reduction in steam-oil ratio of 30%.
Fracturing with Steam
Finally, there is substantial field data to show that cyclic steam stimulation can be used to commercially exploit bitumen reservoirs with no or very little injectivity to steam below fracture pressure. Provided there sufficient cap rock, no overlying gas cap or underlying aquifer, steam may be injected above reservoir fracture pressure to mobilize the bitumen around the well.
This process has other desirable features. In fresh-water sensitive formations, the fracture-heated pathways will extend some distance from the well, whereas otherwise swelling clays would inhibit well injectivity and productivity. The fracture-heated pathways may also make accessible geologically areally isolated pay volumes. In vertically heterogeneous reservoirs, vertically oriented fractures may provide more uniform heating through the pay.