ABSTRACT
The effects of supercritical CO2 on the reservoir property changes of the Chang 7 Member tight sandstones of the Yanchang Formation in the Ordos Basin, China, was investigated using CO2-brine flow-through experiments and in conjunction with a detailed characterization of the rocks and fluids before and after the experiments using X-ray diffraction mineral analysis, 3D Computed Tomography (CT) scanning, high-resolution scanning electron microscopy (SEM), and ICP-AES. An in situ 3D porosity-mineral comparison model has been established in the paper. Minerals such as potassium (K) feldspar, albite and calcite are variably dissolved. The dissolution, migration and re-precipitation of clay minerals are common, which is characterized by the dissolution of chlorite and the re-precipitation of kaolinite, calcium montmorillonite, and sodium montmorillonite. The original chlorite-kaolinite framework was destroyed. It is the precipitation and re-migration of clay particles (e.g., kaolinites and smectites) released by the dissolution, which have been transported in the fluid flow path and accumulated at pore throats, that result in the changes of the pore system and physical properties. The trend of physical property evolution varies among the studied samples, and the porosity is reduced by 4.5%~12.8% after the experiment except an increase of 13.7% in one sample. The permeability reduction is 33.3%~81.2%, which is positively related to the clay mineral content. The influence of the type and content of clay minerals on the porosity is in conclusive and needs to be further studied.
