Study on homogeneous and heterogeneous microemulsion acid plugging removal systems and mechanisms in low-permeability sandstone reservoirs

Low-permeability sandstone reservoirs are more susceptible to form blockage during the oil development process due to their smaller pore throat. Microemulsion acids （MEAs） with nano-level droplets and great deformability have better injection ability and adaptability for low-permeability reservoirs. However， research is still lacking on the relationship between the phase state of MEAs and the plugging removal effect. Therefore， in view of the conditions of a certain low-permeability sandstone reservoir in the Bohai Sea， the pseudo-ternary phase diagram method was adopted to prepare homogeneous and heterogeneous MEAs composed of the same chemical agent but with different phase states， and its plugging removal and corrosion effects were evaluated through laboratory experiments and field applications. In addition， the influence mechanism of the MEA phase on the effect was analyzed through chemical property evaluation， microscopic morphology， and adsorption experiments. The results show that by adjusting the volume of water and oil phases， MEA can be smoothly transformed from the double continuous type to the oil-in-water （O/W） type and the oil-in-water （W/O） type， thereby constructing a homogeneous and heterogeneous MEA system. The MEA system has a good dissolution effect on the mixed blockage. Among them， the plugging removal effect of the O/W type MEA system is 10% higher than that of the traditional acid solution， and it can increase the daily oil production by 53.44% in the field experiment. However， the W/O type MEA system shows lower corrosiveness， with a corrosion rate of only 47.78% of that of pure acid. The results of the mechanism study show that the strong plugging removal property of the O/W MEA system stems from its strong oil-carrying capacity， which can more efficiently strip and carry the crude oil around the blockage， thereby enabling the acid solution to contact the solid blockage effectively. In the W/O type MEA system， the encapsulation property of oil relative to acid solution and the synergistic effect produced by the mixed adsorption of emulsifier and light oil on the surface of the steel sheet make it have low corrosiveness.