Hydrodynamic Analysis of CO2 Migration in Heterogeneous Rocks: Conventional and Micro‐Bubble CO2 Flooding Experiments and Pore‐Scale Numerical Simulations

Abstract

AbstractThe hydrodynamics of CO2 flooding determine its capacity and efficiency in CO2 geological storage; however, this is not sufficiently understood. In this study, a medical X‐ray CT was used to visualize CO2 flooding in a Berea sandstone sample, and the migration pattern and distribution of injected CO2 were carefully analyzed in a series of comparison experiments. For conventional flooding, CO2 migrated mostly along the high‐porosity and large‐pore‐size layers, leaving a large area of low‐porosity and small‐pore‐size regions undisplaced. Comparatively, before CO2 broke through, CO2 fingering developed and migrated slowly for micro‐bubble flooding, showing that CO2 micro‐bubbles infiltrated into more regions than the CO2 continuous fluid. After CO2 breakthrough began, the micro‐bubble CO2 continued to enter some previously undisplaced regions; however, this was inconspicuous for conventional flooding. Finally, micro‐bubble flooding increased CO2 saturation by 6.1% over conventional flooding. Compared to conventional flooding, an essential hydrodynamic change by micro‐bubble flooding weakened the resistance of interface tension over CO2 infiltration into the small pores. Accordingly, two types of hydrodynamic models were proposed for conventional and micro‐bubble CO2 flooding, and their simulated pore‐scale CO2 migration results in a cluster of heterogeneous pores were compatible with the aforementioned core‐scale experimental observations. According to these models, both the small sizes and high injection rates of micro‐bubble CO2 were instrumental in enhancing the displacement effect. The filter for micro‐bubble CO2 injection in our experiment achieved even injection of the small CO2 micro‐bubbles at a relatively high rate, ensuring its practical potential in field applications.