In the acid stimulation of oil wells, slowing down the rate of the reaction between hydrochloric acid and carbonate rocks is a huge challenge, in order to increase the stimulation efficiency. In this work, acid solutions containing Nonylphenol nonionic surfactants of different degrees of ethoxylation were formulated to decrease the reaction rate of H⁺ ions with carbonate rocks. An experimental design plan was executed with a central compound design, showing that the reaction rate decreased up to 350 times (2.356 × 10⁻⁴ ± 0.133 grams of rock per second). It was possible to verify that the presence of the surfactant, in addition to reducing the surface tension and changing the wettability of the medium, also allowed an increase in the viscosity of the solution, which was the main factor for the delay of the reaction. Graphical Abstract

The application of Enhanced Oil Recovery (EOR) methods is essential to promoting increase in the oil recovery where the conventional recovery methods unsuccessful. Numerous studies have demonstrated that microemulsion and surfactant systems injection can change the properties of fluids and increase the oil recovery. In this work, flooding tests were carried out combining brine and microemulsion in different proportions and schemes of injection. Microemulsion systems constituted by: Ultramine 200, n-butanol, kerosene and water were obtained and characterized by measurements of droplet diameter, surface tension, interfacial tension (IFT), and viscosity. The surfactant concentration in the microemulsion was evaluated on the oil recovery through flooding experiments. This study also evaluated how surface and interfacial tensions influence crude oil recovery (29° API). Results demonstrate that the oil recovery increases with the increase in surfactant concentration in the microemulsion. However, that increase is significant only up to 9 wt%, reaching an additional 21% oil recovery of the original oil in place (OOIP). This research recommends chemical method to EOR tests by combined injection from microemulsion and brine. The chemical action of microemulsion system results in a miscible phase, promotes oil mobilization, improve brine flooding, and increase the oil recovery. Graphical Abstract