Impact of Alkali-Induced Pore Structure Alteration on Sweep Efficiency in Weak-Alkali ASP Flooding Systems

Weak-alkali alkaline-surfactant-polymer (ASP) flooding demonstrates significant advantages for enhancing oil recovery due to its ability to improve sweep efficiency, improve displacement efficiency, and reduce surfactant consumption. However, the alkali in weak-alkali ASP flooding can induce problems such as scaling, deposition, and emulsification, which cause damage to the reservoir and consequently affect its flow capacity. To investigate the impact of alkali on reservoir physical properties and pore structure, this study examines the scaling mechanism through laboratory experiments, and employs scanning electron microscopy, high-pressure mercury injection porosimetry, and oil displacement experiments to study the influence of alkali in the ASP system on the microscopic pore structure of the reservoir. The results indicate that the presence of alkali readily leads to the formation of calcium carbonate and magnesium carbonate, causing scaling issues. The dissolution effect of alkali on the rock is significant, leading to changes in the rock surface and pore structure where dissolution enhances the permeability of large pore channels, while smaller pores become surrounded by accumulated debrislike particles, reducing their flow capacity. After alkali soaking, the water-phase permeability of the core decreases, the average pore radius and median pore radius generally decrease, the sorting coefficient increases, and reservoir heterogeneity intensifies.