Preparation and characterization of microcapsule acids via interfacial polymerization based on mini-emulsion templates

Deep carbonate reservoirs are rich in oil and gas reserves, but high-temperature and deep-burial conditions may cause conventional retarded acid to fail, thereby reducing acid stimulation efficiency. To address these challenges, microcapsule acids with high thermal stability, strong acid resistance, and delayed release capability were prepared. Methyl methacrylate and N-methylolacrylamide copolymer were selected as the shell material, and microcapsule acids capable of withstanding temperatures up to 180°C were synthesized via the interfacial polymerization method. Optimized parameters for mini-emulsion preparation and the interfacial polymerization process improved the uniformity of microcapsule particle size. The particle size and its distribution, micro-morphology, and shell material properties of the microcapsule acids were characterized by laser particle size analysis, infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry, and thermogravimetric analysis. The experimental results showed that the average particle size of the microcapsule acids prepared by the interfacial polymerization method was 321.7 μm, with most particles distributed between 200 μm and 400 μm, exhibiting uniformity and good dispersibility. Thermal analysis indicated that the shell material maintained its structural integrity under high-temperature conditions of 180°C. Suspension tests demonstrated that the microcapsule acids could achieve long-distance transport in clear water and be evenly distributed in the formation. Acid release experiments confirmed that these microcapsule acids could achieve delayed release. These results further indicate that the microcapsule acids have broad application potential in acid fracturing of deep carbonate reservoirs.