Mechanisms behind CO₂-brine-Sandy Conglomerate interactions and resulting variations in mineral components, fluid occurrence, and pores: A case study of the Mahu Sag, Junggar Basin

Tight Sandy Conglomerate reservoirs in the Mahu Sag, Junggar Basin exhibit extreme heterogeneity, posing significant challenges to the field tests and prediction of the CO₂ sequestration performance following CO₂ flooding. Understanding the mechanisms underlying CO₂-brine-Sandy Conglomerate interactions is crucial to evaluating the CO₂ sequestration performance following reservoir development. Based on CO₂-brine saturation experiments and data from integrated quantitative evaluation of minerals by scanning electron microscopy (SEM), SEM-based quantitative evaluation of minerals (QEMSCAN), micro-CT scanning, and nuclear magnetic resonance (NMR) spectroscopy, we investigate mechanisms underlying CO₂-brine-Sandy Conglomerate interactions and resulting variations in the mineral components, fluid occurrence, and pores in tight Sandy Conglomerate reservoirs of the Mahu Sag. The results indicate that CO₂-brine-Sandy Conglomerate interactions led to the dissolution of rock minerals. This mineral dissolution and subsequent migration resulted in elevated average pore-throat sizes and enhanced pore connectivity in cores. The X-ray diffraction (XRD) analysis reveals that the interactions increased albite and illite contents by 3.20 % and 2.32 %, respectively, while producing minimal impact on quartz content. SEM analysis results demonstrate that CO₂ soaking led to an increase in both the quantity and width of grain-edge fractures, as well as the formation of new lateral fractures. The carbonate and feldspar dissolution generates substantial intragranular micropores and microfractures, with these microfractures propagating along cleavages. NMR experiment results reveal that saturated water primarily occurs as clay-bound water, capillary-bound water, and movable water in the cores. CO₂ soaking contributes to the enhanced content and expanded distribution range of saturated water in the pores, with movable water in macropores changing the most significantly. The research results highlight the complexity of CO₂ interactions in Sandy Conglomerate reservoirs, which is of great significance for guiding the field test effectiveness of CO₂ storage after flooding in the Mahu Sag Sandy Conglomerate reservoirs.