The Impact of Gas-Assisted Gravity Drainage on Operating Pressure in a Miscible CO2 Flood

摘要

We investigated the combined contributions of gravity drainage and miscibility as recovery mechanisms during CO2 flooding. The effects of gravity stable and unstable CO2 fronts under immiscible, near miscible and miscible displacements of crude oil by CO2 are presented. We contrast our results in porous media, with slim tube experiments, core floods, and bead packed tubes. Standard slim-tube, vertically and horizontally oriented bead packed tubes, as well as vertical and horizontal reservoir core flood experiments, were used to investigate the role of the gravitational forces in improving oil recovery under different conditions regarding the crude oil - CO2 miscibility. Three crude oils with different minimum miscibility pressure (MMP) values were used in this study. Our results show the gravity drainage mechanism has a much greater significance than previously thought when compared to the effects of phase behavior or the miscibility alone. Not surprisingly, vertically stable, downward displacement resulted in better performance compared to horizontal displacement in all cores and bead packed tubes in our experiments. Recovery is only slightly higher in the gravity stable floods when miscibility is achieved. However, in immiscible and near miscible displacements, recovery is significantly higher in the gravity stable floods, reaching up to 90% RF at 250 psi below the MMP value, compared to only 33% in horizontal floods. Our results suggest that achieving miscibility is not necessary to obtain high recovery efficiency during a gravity-stable displacement. Breakthrough is reached faster in horizontal floods as a consequence of fingering and gravity override. This work challenges the paradigm that miscibility is required to achieve high recovery factors during CO2 flooding, and highlights the overlooked role of gravity drainage as a displacement mechanism. This finding has an essential impact on field operations as it allows for lower operating pressures in CO2 flooding processes under stable gravity displacement that will result in positive impact on economics. The relevance of our results is exacerbated by the current low crude oil price environment.