Coreflooding experiments were conducted on Athabasca oil sands in cylindrical cores 31 cm long by 9 cm diameter at 3.6 MPa (500 psi) production pressure and production temperatures below 150 °C. The objective of the experiments was to investigate different injection strategies for using carbon dioxide and sodium hydroxide as dual additives in hot water for in-situ recovery of bitumen. The additives were introduced at a later stage of the flood following the injection of a neutral pH hot water to establish hot communications between the injection and production wells. Injection strategies included commingled C02-NaOH injection, sequential injection of NaOH and CO2 and CO2-NaOH inject ion with variable CO2 injection rates to maintain the pressure drop across the core.
The results indicate that adding a small amount of NaOH in a hot water-CO2 mixture improved bitumen recovery over that from hot water-CO2 or hot water NaOH injection. Mixing CO2 and NaOH was made at concentrations such that the resulting mixture consisted of a mild alkaline solution and free CO2 beyond the solubility limit in hot water. The alkaline solution will help in lowering the interfacial tension and CO2 has the beneficial effects of lowering bitumen viscosity and resulting in bitumen swelling. In one experiment a stable low viscosity oil-in-water (O/W) emulsion was produced while hot water plus NaOH was injected. When CO2 was then added to the injection fluid, the pH 01 the fluid was reduced and resulted in a higher oil cut and recovery of bitumen in the form of water-jn-oil (W/O) emulsion. There is a strong evidence that the produced W/O emulsion was moving through the core as an O/W emulsion i.e. oil droplets carried by an external water phase and apparently this emulsion is unstable and when produced the bitumen droplets quickly coalesce to form the observed produced W/O emulsion plus a relatively large amount of water.
Maintaining the pressure drop across the core by gradually increasing the CO2 injection rate as the flood proceeded proved to be the most promising strategy for bitumen recovery at production temperatures as low as 100 °C.
The present investigation demonstrates that it is possible to both mobilize and produce bitumen at low temperatures and it is believed "that it may be possible to recover bitumen from reservoirs with as little as 40 m of overburden. This is much less than the 200 m of overburden generally required for steam injection processes.
One of the mechanisms by which a hot water-caustic process results in improvement of oil recovery is the in-situ emulsification of the oil and its entrainment into a continuous flowing alkaline water phase. This formation of oil-in-water (O/W) emulsion is influenced of the lowering of interfacial tensions, particularly for oils which contain enough acidic components to form, in-situ, natural surfactants within the caustic.
In a previous investigation, Nasr. et. al. 1 examined the potential of using hot water and caustic or carbon dioxide at 150 ° C production temperature or lower for the recovery of bitumen from Athabasca oil sands.