During an acid fracturing treatment, the preflush and spent acid injected by multistage alternate injection will enter the matrix; this part of the aqueous phase entering the matrix will cause serious water blocking damage during crude oil production, hinder the migration of crude oil in the matrix, and reduce the well productivity. There is a large amount of small-sized pore throats in low-permeability bioclastic limestone reservoirs. The small-sized pore throats are important channels for hydrocarbon flow and are more vulnerable to water blocking under the effect of capillary pressure. The multistage alternating injection of microemulsion acid is a key technology to unlock the commercial potential of low-permeability porous limestone reservoirs, which has the advantages of high acid conductivity, long action distance, less solid residue, and low water blocking damage. This technology has demonstrated good usability in field tests, and the relevant laboratory studies mainly focus on understanding the stimulation effect of microemulsion acid through macroscopic core experiments. But there is still no clear explanation at the microscopic level for why this system can expand its invasion range and mitigate the water blocking damage. To evaluate the effect of microemulsion acid in limestone reservoirs and provide a reference for its application, the properties and mechanism of this system are studied in this paper, while the systems commonly used in the acid pressing process (spent acid without cleanup additive, spent acid added with cleanup additive, and spent emulsified acid) are selected as the control group. According to the characteristics of multimode pore throat in limestone reservoir, a special micromodel with CaCO3 coating is developed. The microfluidic research is carried out to visually present the dynamics of multiphase flow during different spent acids invasion and flowback processes. The cleanup mechanism of microemulsion acid was revealed, while highlighting its application advantages through comparative analysis.

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