Experimental Study on Enhanced Oil Recovery by Carbonated Water Flooding in Ultra-Low Permeability Sandstone Reservoirs

Abstract

Ultra-low permeability sandstone reservoirs represent a crucial replacement resource for petroleum in China. However, due to extremely low reservoir permeability and severe fluid channeling, conventional development methods yield a recovery factor of less than 30%, posing significant challenges. While CO₂ flooding is a key enhanced oil recovery (EOR) technology for such reservoirs, it faces limitations including gas channeling, high minimum miscibility pressure (MMP), and water blocking effects. Carbonated water flooding (CWF), as a novel EOR technique, had not been systematically studied for ultra-low permeability reservoirs prior to this research. This study focuses on ultra-low permeability sandstone with a permeability of less than 0.3×10⁻³ μm². Core flooding experiments were conducted to comparatively analyze the oil recovery performance of carbonated water flooding, CO₂ Water-Alternating-Gas (CO₂-WAG) injection, surfactant flooding, and a combined carbonated water-surfactant flooding. Additionally, Scanning Electron Microscopy (SEM) was employed to investigate the impact of carbonated water on reservoir minerals and pore structure. The displacement experiment results demonstrate that carbonated water flooding increased the recovery factor by 6.43% compared to water flooding, significantly outperforming CO₂-WAG (5.33%) and surfactant flooding (3.39%). The combined carbonated water-surfactant flooding exhibited the best performance, enhancing recovery by 10.58% relative to water flooding, indicating a synergistic effect. Studies on mineral and pore structure evolution reveal that carbonated water induces selective dissolution of clay minerals, carbonate minerals, and silicate minerals. Kaolinite, illite, ankerite, and quartz are prone to dissolution, facilitating pore enlargement. In contrast, chlorite shows strong resistance to dissolution. Dissolution products from calcite, mica, and feldspar are prone to secondary precipitation, leading to pore-throat blockage. The research confirms the favorable applicability and feasibility of carbonated water flooding in ultra-low permeability sandstone reservoirs, providing a novel technical pathway and theoretical foundation for enhancing oil recovery in such reservoirs.