Enhanced Heavy Oil Recovery by Liquid CO2 Injection under Different Injection Strategies

Abstract

Abstract There are vast resources of heavy oil located in reservoirs in which the reservoir temperature is lower than or in the vicinity of the critical temperature of CO2 (31.1 °C). A good example of that are the huge resources of heavy oil in the Alaska North Slope. In these reservoirs, CO2 would be in liquid state and would exhibit higher viscosity and solvent power compared to gaseous or supercritical CO2. The objective of this study is to experimentally assess the potential of liquid CO2 injection for enhancing recovery from these reservoirs. A positive outcome could bring significant economic and environmental benefit by improving oil recovery and at the same time managing produced CO2 from neighboring reservoirs. A comprehensive series of coreflood experiments were performed using a heavy crude oil under the conditions of a heavy oil reservoir in the North Slope, Alaska. Different injection scenarios were experimentally simulated including water flood, CO2 flood, as well as combined water and CO2 injection in the form of WAG and SWAG injection. In order to investigate the impact of an initial depletion stage on the performance of CO2 injection, the impact of the amount of dissolved hydrocarbon gas in crude oil was also investigated and the results of the different injection scenarios were compared with each other and with the base case (waterflood). The coreflood results show a very good performance for liquid CO2 injection in the heavy oil system tested in this work. Dissolution of CO2 in heavy oil and the consequent significant reduction in the oil viscosity leads to a prolonged period of oil recovery during CO2 injection. This is different from waterflood whereby oil recovery almost ceases as soon as water breaks through. Combining CO2 and water and injecting CO2SWAG (simultaneous injection of water and CO2) increased oil recovery more than water or CO2 injection alone. CO2WAG injection (alternating injection of water and CO2) brought about an ultimate oil recovery of 74 % IOIP. This was a staggering 30% higher than the ultimate oil recovery by waterflood under the same conditions.