The CO2 EOR (Enhanced Oil Recovery) has been conducted especially in North America since 1970s. Recently, the CO2 EOR also draws attention as a method to enhance oil production, reducing the emission of greenhouse gas. This study proposes to inject microbubble CO2, instead of continuous super-critical phase of CO2, into mature reservoirs to increase oil recovery.
A micro-bubble is defined as a bubble with a diameter of 10 to 100 μm. Due to its small diameter, the buoyant force acting on micro-bubbles is small and the rate of the dissolution into oil/water is faster than the continuous phase of CO2 because of the large specific surface area. Previous studies confirmed the difference in flooding behavior between normal (continuous super-critical phase) CO2 injection and micro-bubble CO2 injection under the secondary recovery mode by the comparative core flooding experiments.
In this study, comparative core flooding tests under the tertiary recovery mode flooding (Cases No.1 and No.2) were conducted to investigate the effects of micro-bubble CO2 injection on oil recovery. In Case No.1, normal CO2 was injected to Berea sandstone core sample after water flooding. In Case No.2, micro-bubbled CO2 was generated with a special filter at the inlet of core sample, which was injected into the same core sample used in Case No.1. The results of these experiments showed that the recovery factor in Case No.2 was greater than that in Case No.1 by 13%. Difference in the sweep efficiency by CO2 between these two cases was also visualized by X-ray CT scanner. The CT number and 3D snapshots indicated that CO2 overrode in Case No.1, while the gravity segregation was remarkably suppressed in Case No.2.