LAUSR

Abstract Due to the favorable physical properties of micro bubbles (MBs), MB CO2 injection into saline aquifers was proposed as a novel, economic, and leak-free underground injection method in the purpose of sequestration. In this study, supercritical CO2 injection into a Berea sandstone sample at 0.05 mL/min under reservoir conditions was investigated using X-ray CT. Experiments with micro bubble (MB) and normal bubble (NB) CO2 injection were conducted. The dependence of CO2 migration on pore space shows macroscopic bypassing via preferential flow paths, as the distribution and saturation of CO2 were largely influenced by the heterogeneous pore structure. Most CO2 migrated forwards via three flow paths that formed along the bedding layers with high porosity. The CO2 distribution was discontinuous, and isolated CO2 clusters appeared due to the heterogeneity of the pore structure and tortuosity of the flow paths. MB CO2 migrated into the areas adjacent to major flow paths, forming irregular channels, while NB CO2 migrated through smooth channels. MB CO2 breakthrough occurred at 0.18 pore volume (PV), which was larger than NB CO2. A long breakthrough time reflects high pore space utilization. A significant differences of MB and NB CO2 in the existing duration of flow paths in the whole displacement period was observed, which reflected higher stability of MB CO2 in the sample. Additionally, little MB CO2 was observed near the inlet, suggesting that MBs accelerated dissolution in the early stage of injection (0.02 PV). Smaller saturation and larger volume differences were observed for MB CO2 compared to NB CO2 in the early stage because of dissolution. The enhanced dissolution efficiency of MB CO2 reached 18.7% at 0.02 PV. Moreover, pore space utilization improved because MB CO2 permeated into regions of low porosity and low flow ability. The improvement in the utilization efficiency reached 4.9% in low-porosity regions. And the storage efficiency was increased up to 23.6%.The results of this study suggest that MB CO2 injection has higher dissolution and improved pore space utilization, making it an efficient method of CO2 sequestration in low-porosity regions.