Evaluation of the evolution characteristics of continental shale microstructure under CO₂ interaction based on multi-technique characterization

To investigate the evolution patterns of microstructure in continental shale during CO₂ flooding and geological sequestration, shale samples from the Chang 7 member of the Ordos Basin were selected for CO₂-rock and CO₂-water-rock interaction experiments under simulated reservoir conditions (6 MPa, 47 °C). A comprehensive analytical approach integrating X-ray diffraction (XRD), in-situ scanning electron microscopy (in-situ SEM), nuclear magnetic resonance (NMR), and micro-computed tomography (CT) was employed to systematically characterize the evolution of pore structure, mineral composition, and petrophysical properties.The results indicated that in both reaction systems, soluble minerals gradually dissolved, clay minerals became relatively enriched, while quartz and feldspar remained dominant, collectively undergoing a "dissolution-precipitation-re-dissolution" evolution process. Compared to the initial state, porosity and permeability increased significantly. Under CO₂-rock conditions, porosity and permeability increased by 18.1% and 79.1%, respectively; under CO₂-water-rock conditions, larger increases of 24.4% and 114.3% were observed, demonstrating that the latter enhances pore structure modification. NMR analysis revealed that under CO₂-rock conditions, pore evolution was predominantly limited to small pores, with no notable change in macropores. In contrast, under CO₂-water-rock conditions, both micropores and macropores underwent significant evolution, with secondary mineral dissolution at later stages resulting in further enhancement of the T₂ spectral signal.In-situ SEM observations showed that under CO₂-rock conditions, the grain surfaces of the shale samples became looser, with secondary pores forming locally. Under CO₂-water-rock conditions, pronounced dissolution occurred at macropore margins, and the pore network progressively developed. CT analysis further revealed that under CO₂-water-rock conditions, the number of pore throats and total pore volume increased by 165.6% and 398.2%, respectively, far higher than the increases of 40.3% and 125% observed under CO₂-rock conditions. This study confirms that CO₂ injection substantially alters the microstructure and pore connectivity of shale, and that the presence of water can significantly enhance CO₂-induced alteration of the shale pore network. These findings provide theoretical support for both CO₂ enhanced oil recovery and the safety assessment of geological sequestration.