Effect of CO2-Brine-Rock Interactions on the Pore Structure of the Tight Sandstone during CO2 Flooding: A Case Study of Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin, China

CO2 flooding is an effective technique to enhance oil recovery from tight sandstone reservoirs. The CO2-rich fluid reacts with the in situ minerals and results in the corrosion and precipitation of minerals in the sandstone, affecting the connectivity and morphological features of pores. However, the diagenesis of the tight sandstone is complex, and the structural modification behavior and mechanism in different lithofacies of tight sandstone during CO2 flooding are still unclear. This study combined CO2 flooding, thin-section casting, scanning electron microscopy, nuclear magnetic resonance, X-ray diffraction, and fractal analysis methods to investigate the changes in the pore structure of tight sandstone after CO2 flooding. The results show that the cement of the tight sandstone is complex and diverse. The tight sandstone can be divided into two types of lithofacies: clay cementation (CL) and ferrocalcite cementation (CA). The mineralogical alterations occur differently in each lithofacies of tight sandstone. Alterations in the cement minerals affect the pores morphology of tight sandstone depending on their mineralogical structure and texture, and the CO2 flooding mainly changes the micromorphology and heterogeneity of large pores. Clay minerals dominate the cement in the CL lithofacies of tight sandstone. The dissolution mainly occurs in small pores because the precipitation of new minerals and exfoliation of skeleton particles partially block large pores and transform them into small pores. Thus, the number of small pores of CL lithofacies increases while the number of large pores decreases. Such a phenomenon hinders the increase in porosity and permeability. On the other side, in the CA lithofacies of tight sandstone, the cement is dominated by ferrocalcite, and the dissolution of ferrocalcite is the primary mechanism of mineralogical alteration. As the ferrocalcite dissolution expands, it creates new flow paths, improving the connectivity of pores. The petrophysical properties of CA lithofacies improve significantly after CO2 flooding. There is a crucial need to study the changes in diagenetic characteristics of tight sandstone due to CO2 flooding. Such type of study provides insights related to the improvement and evaluation of the development of tight sandstone reservoirs during CO2 flooding.