SummarySurfactant flooding is an effective enhanced oil recovery method in which the oil/water interfacial tension (IFT) is reduced to ultralow values (<0.01 mN/m). The microscopic fluid-fluid displacement has been extensively studied at high IFT (>10 mN/m). However, the microscopic displacement dynamics can be significantly different when the IFT is ultralow because the dynamic contact angle increases with the increase of the capillary number. In this study, surfactant flooding was performed and visualized in micromodels to investigate the dynamics of multiphase displacement at ultralow IFT. Although the micromodels used were strongly water-wet, the displacements of oil by surfactant solutions at ultralow IFT appeared as drainage. Furthermore, a macroscopic oil film was left behind on the surface, which indicates that a contact line instability occurred during displacements. The shape of the oil/water meniscus was determined by the balance between viscous forces and capillary forces. The meniscus can be significantly distorted by viscous forces at ultralow IFT. Therefore, the water-wet micromodel exhibits an oil-wet behavior at ultralow IFT, and the displacements of oil by surfactant solutions at ultralow IFT manifested as drainage rather than imbibition. The flow behavior is further complicated by the spontaneous formation of microemulsion during displacement. The microemulsion is mainly formed from the residual oil. The formation of a microemulsion bank made the surfactant solution discontinuous, with transport in the form of droplets in the microemulsion bank and displacement front. The novelty of this work is to reveal the effects of dynamic contact angle on the ultralow IFT displacement.

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Bottle test and tube-blocking test are two major methods to estimate the minimum inhibitor concentration (MIC) for scale inhibitor applications, such as scale squeeze and continuous injection. However, they may not adequately represent the behaviors of scales and inhibitors in porous media. In this study, coreflood, bottle test and tube-blocking test were conducted, and the measured MIC values were compared to investigate the mechanism leading to the differences. Bottle tests were conducted to determine the MIC of phosphino polycarboxylic acid (PPCA) by measuring the Ba2+ concentration in solution. Tube-blocking tests were carried out to determine the MIC of PPCA in stainless steel tubing by recording the increase of differential pressure. Finally, coreflood experiments were conducted to evaluate the performance of PPCA in porous media. Pressure changes were monitored along the core plug and were used to determine the location of BaSO4 deposition. Ba2+ concentrations in effluents were measured and compared with the bottle test results to investigate the performance of PPCA in porous media. Results show that PPCA cannot stop scale deposition in porous media at the MIC determined by bottle tests and tube-blocking tests. Both bottle tests and tube blocking tests suggest an MIC of 6 ppm for PPCA. However, coreflood results show a noticeable increase of pressure in the first section of the core at 6 ppm. Moreover, the Ba2+ concentration in the effluent with 6 ppm PPCA of coreflood is just slightly higher that measured in the coreflood with no PPCA, which means that the MIC determined by bottle test and tube- blocking test cannot mitigate scale deposition in porous media. Sandpacks with higher permeability show higher permeability reduction after the same pore volume of water injection. This study finds that the MICs determined by commonly used bottle tests and tube-blocking tests are not adequate to mitigate BaSO4 deposition in porous media. The mechanism of BaSO4 deposition and the behavior of inhibitors in porous media need to be further studied.