A series of tertiary multiple-contact miscible CO2 corefloods have been carried out to study effects of flow rate, core length, oil viscosity, wettability, WAG ratio, and initial water saturation on displacement mechanisms. The goal of these experiments was to identify important recovery mechanisms that influence performance of reservoir-condition, multiple-contact miscible floods.
In these experiments, a bank of miscibly-displaced oil is produced first, followed by oil that is initially lighter than the original oil, and becomes increasingly heavy as the flood progresses. The implication is that some oil is initially bypassed and later recovered by extraction.
The amount of bypassing is not sensitive to flow rate or core length, but increases as the solvent/oil viscosity ratio decreases. Based on these results, it is concluded that pore-level effects such as capillarity-induced bypassing and dispersion are major bypassing mechanisms. Microvisualization experiments, in which fluids are hardened in-place, provide direct confirmation that pore-level bypassing takes place.
Viscous fingering (macroscopic bypassing) may also take place, but it is not thought to be the dominant bypassing mechanism under the conditions of this study.
Wettability also has a significant effect on coreflood performance, particularly at high WAG ratios. High WAG ratios result in less oil recovery by extraction. In water-wet rock, this is a large effect, with almost no extraction at high WAG ratios. In mixed-wet rock, however, the effect of WAG ratio is much smaller, and significant quantities of oil are recovered by extraction regardless of WAG ratio. This is likely the result of wall-coating oil films in the mixed-wet rock, which improve contact between oil and solvent.