Preparation and Performance Study of Microemulsion Acid for Comprehensive Plugging Removal in Carbonate Reservoir

Matrix acidification is one of the most effective stimulations to dissolve scales and remove damage in carbonate reservoirs. However, existing acid systems are difficult to dissolve organic and inorganic scales simultaneously, and complex multi-stage alternative injection often introduces new precipitation and damages the reservoir. Here, based on the retardation ability of emulsified acid and the stable structure of microemulsion, an oil-phase solvent was preferably selected, and the surfactant and cosurfactant were optimized to prepare an acid-in-oil type microemulsion acid capable of dissolving both organic and inorganic scales and high solubilizing for hydrochloric acid. Based on the rotating disc experiment, scale dissolution experiment and acid driving experiment, the acid-rock reaction kinetics, scale dissolution ability and acidizing and plugging removal performance of microemulsion acid in a carbonate reservoir were systematically studied and compared. The results show that Solvesso 150 (aromatic solvent) has the advantages of low toxicity, high flash point and high-scale dissolving ability. At NP-4: OP-10: A (cosurfactant) = 3:3:4, the microemulsion acid system has the strongest ability to solubilize hydrochloric acid and can solve the problem of low H⁺ concentration. The particle size of microemulsion acid is smaller compared to emulsified acid. At 60 °C, the mass transfer coefficient of microemulsion acid is 3.2 × 10^-8 cm²/s, which is one order of magnitude less than that of emulsified acid. Microemulsion acid shows good solubility performance in dissolving organic and inorganic scales, and the comprehensive solubility of mixed scales can reach 98.28%. The stronger scale solubilization ability and lower acid-rock reaction rate enable microemulsion acid to form a thin and straight main wormhole, thus enhancing the acidizing and plugging removal effect. This study can solve the problem of poor hydrochloric acid solubilization ability of microemulsion acid to a certain extent, which provides theoretical and data support for the research and development of microemulsion acid and the efficient plugging removal technology for carbonate reservoirs.