Surfactant-Polymer Compatibility in Bulk and Static Conditions vs. Confined and Under Flow Conditions

Abstract

The objective of surfactant-polymer (SP) formulation design is to simultaneously achieve ultra-low interfacial tension and good mobility ratio. However, in some cases, the presence of both polymer and surfactant can cause compatibility issues leading to a cloudy or even demixing solution. Until now, those observations were made in bulk conditions but the assessment of this behavior has not been studied in confined and under flow conditions. Three SP compatibility cases were selected using the same SP formulation and playing on the brine salinity (low, medium and high). Bulk and kinetic studies based on solubility, viscosity and cryo-TEM were performed prior to monophasic injections carried out in a transparent micromodel representing a 2D rock porous medium and in a coreflood rig with a 3D outcrop rock. The observation in micromodel was performed using an optical microscope under polarized light to visualize the physical structure of the SP formulations. The pressure drop along the core was monitored during coreflood experiments to measure the mobility reduction entailed by the injected solution. In bulk conditions it is shown that increasing the solution salinity leads, after a few to several days depending on the solution's volume, to a degradation of the SP compatibility or even demixing. This behavior can be attributed to depletion effects. In this case, depletion is due to the formation of surfactant vesicles (hundreds of namometers in size) that tend to aggregate in the presence of polymer molecules. As expected, injection of the compatible SP solution (at low salinity) in the 2D porous medium micromodel and in the outcrop rock led to an easy in-depth transport, namely mobility reduction compatible with the viscosity of the solution. More interestingly, the same formulation at higher salinity exhibited a deposit of SP aggregates having a crystalline structure when injected in the micromodel. However, this formulation at high salinity did not show any issue in terms of mobility reduction when injected in the outcrop rock as the mobility reduction stabilized rapidly at a value close to the relative viscosity of the solution. These results highlight that the presence of a demixing phase does not always induce propagation issues in cores and that some cloudy SP solutions could be injected without causing any pressure increase. The objective of this study was to correlate the bulk behavior of SP formulations showing respectively good and poor compatibilities with their performance in confined and under flow conditions. It has been proved that a poor compatibility in bulk does not always induce transport issues when the solution is injected in porous medium, despite the deposit of structured aggregates in some pores.