Based on Pujol and Boberg's scaling criteria, a series of experiments on steam-CO2 injection strategies was conducted in a high temperature, high pressure scaled model to evaluate oil recovery processes for bottom water reservoirs. The scaled model simulated 1/8th of a jive-spot pattern for a Cold Lake oil sand deposit of 12.8 m thickness underlaid by a 2.2 m bottom water zone. In addition to steam-CO2 conrinuous injection. CO2 soak prior to steam injection, hot water-CO2 injection and CO2 followed by steam injection (CO2-steam sequential injection) were evaluated.
The results indicate that the co-injection of a gas with steam accelerates and improves oil recovery rates, as compared to steam-only injection. during the initial Slage of the process. The CO2-steam continuous injection resulted in a better performance (final recovery and oil-to-steam ratios) than that from steam alone or steam-CO2 sequential injection. Soaking the reservoir with carbon dioxide prior to steam injection reduced steam injectivity due to blocking Of the bottom water zone with a high viscosity oil (due to asphaltene precipitation).
The design of an effective oil recovery process for bottom water reservoirs depends on oil sands to bottom water thickness ratio, permeabilities, and oil saturations as well as on the injection/production strategies used, For favourable reservoir parameters, the bottom water zone can be beneficial in the recovery of high viscosity oil since it provides initial injectivity of hot fluids into the reservoir.
Steam injection has been successful in bottom water reservoirs where the water zone volume is small relative to the oil sands volume1. Variations of the steam injection process, such as the creation of barriers at the oil-water contact2, injection of gas3,4 and polymers5 have been suggested in the literature.
To thermally reduce the viscosity of Cold Lake bitumen to 200 mPas (200 cP), it is necessary to increase the temperature to 120 °C. A reduction in bitumen viscosity could also be accomplished by contacting the biturnen with suitable gases such as CO2, methane or propane, either as alone or in combination. For example, the reduction of biturnen viscosity to 200 mPas could be achieved at 85 °C by dissolving CO2 in bitumen at 3 MPa The solubility of CO2 in reservoir fluids also results in oil swelling, vaporization of oil and interfacial tension effects. All of these are beneficial in the recovery of bitumen from oil sands.
Nasr et al4,6 have published experimental and numerical results on the use of steam, steam-N2 and steam-flue gas in the recovery of oil from Cold Lake oil reservoirs with bottom water zones, The results indicated that the processes are gravity dominated and steam injection now rate has a major impact on process performance, The sensitivity of the process to well completion interval, initial oil saturation in the bottom water zone. and bottom water-to-oil sand thickness ratio was published by Law and Nasr7. The present study provides an assessment of a series of experiments designed to evaluate CO2-steam injection strategies for improved oil recovery as compared to from steam-only.