CO2-Low Interfacial Tension Viscoelastic Fluid Synergistic Flooding in Tight Reservoirs

Abstract

Tight oil reservoirs have poor physical properties, insufficient formation energy, and low natural productivity. CO₂ flooding is an important technical mean that enhances the oil recovery of dense reservoirs and achieves effective CO₂ sequestration, but strong heterogeneity of the tight oil reservoir usually results in gas channeling and poor enhanced oil recovery effect. The existing methods to prevent gas channeling are mainly to use the small-molecule amine system and the polymer gel system to plug fracture and high permeability channels. The small-molecular amine system has low flash points and pollutes the environment and the polymer gel has poor injectivity and great damage to the formation, which limit their large-scale application. Therefore, a new viewpoint of CO₂-low interfacial tension viscoelastic fluid synergistic flooding for enhanced oil recovery in a tight oil reservoir was made. The performance of low interfacial tension viscoelastic fluid (GOBT) was studied. The injectivity and oil displacement effect of CO₂-GOBT synergistic flooding were evaluated, and the mechanism of CO₂-GOBT synergistic flooding was discussed. The experimental results showed that 0.4% GOBT is a low interfacial tension viscoelastic fluid, which has strong adaptability to the salinity water of tight oil reservoirs (6788-80,000 mg/L), good viscosity stability at different pHs, excellent capacity to emulsify crude oil, and the ability to improve reservoir water wettability. CO₂ alternating 0.4% GOBT flooding has good injection ability in cores (K _a = 0.249 mD), and injecting 0.4% GOBT can effectively increase the injection pressure of subsequent CO₂ flooding. CO₂ alternating 0.4% GOBT flooding can effectively improve water flooding recovery in tight sandstone reservoirs, which is better than CO₂ flooding and 0.4% GOBT flooding in both homogeneous and heterogeneous conditions. The mechanisms of CO₂ alternating 0.4% GOBT flooding to enhance the oil recovery include that GOBT and CO₂ foam block high permeability layers, shunt and sweep low permeability layers, and GOBT emulsify and wash oil. CO₂ partially dissolving in GOBT synergistically enhances the core water wettability, which improves GOBT injectability, emulsification, and stripping ability to residual oil.