Experimental study of EOR mechanisms of non-chemical CO2 microbubbles and their impact on pore structures

Abstract

Non-chemical CO₂ microbubbles as a mobility control technology in enhanced oil recovery (EOR) and carbon sequestration are becoming attractive. In this study, the EOR mechanisms of non-chemical CO₂ microbubble (MB) in low permeability reservoirs are experimentally investigated by the nuclear magnetic resonance (NMR) technology. This study reveals, for the first time, the EOR mechanisms of MB in a heterogeneous reservoir and its effect on pore structure. First, mobility reduction factors of MB with various gas–liquid ratios were determined, with MB at a gas–liquid ratio of 1 exhibiting the best performance under experimental conditions. Second, the coreflood experiments with NMR scanning were performed to reveal the EOR mechanisms of MB. It was observed that MB achieved an incremental oil recovery of 13.49% and 22.80% in the core sample with a permeability of 9.51 × 10⁻³ and 2.23 × 10⁻³ μm², respectively. Benefiting from MB's conformance control, the total oil recovery was increased from 38.34% to 54.57% of original oil in place by MB in parallel core flood experiments. Third, the NMR tests demonstrated that MB significantly reduced residual oil in core samples, especially in small pore areas, which highlights the improvement of sweep efficiency by MB. Lastly, the effect of MB on pore structure was studied. The NMR tests indicated a significant increase in pore space after 1 pore volume of MB flooding. Minerals in the core sample were dissolved, leading to an increase in permeability and porosity of the core sample by 17.01% and 0.31%, respectively. Overall, the results of this study provide valuable insights into the EOR mechanisms of MB at the pore scale and offer implications for EOR and carbon sequestration in low-permeability reservoirs.