Surfactants have been used for decades to enhance the production of hydrocarbons from oil-bearing subterranean formations. Production improvement is tied to optimization of the interaction of surfactant within a given oil/water/rock system. Ideal surfactants will alter the wettability and water/oil interfacial tension. While the mechanism of interfacial tension reduction is well-established, the mechanism of surfactant-driven wettability alteration is still up for discussion. This study aims to give insight into the matter by investigating the surfactant adsorption and desorption process using a quartz crystal microbalance with dissipation (QCM-D) apparatus.
QCM-D is, in essence, an ultra-sensitive mass balance with nano-gram sensitivity. This technique exposes a sensor to flowing fluid at a controlled temperature and directly measures surface associations through the change in mass over time. Altering the material composition of the sensor surface, SiO2 for quartz and CaCO3 for calcite, and modifying the wettability with North American oil samples gives better representation of the surface interactions present in oil producing reservoirs. Surface activity for an anionic, cationic, non-ionic, and microemulsion surfactant were evaluated to determine both static and dynamic adsorption properties.
The surfactant systems have drastically different static and dynamic adsorption properties. The charged surfactant had no measurable interaction with the quartz surface at 1 gallon per 1000 gallons (gpt). At higher concentrations the cationic surfactant reacted more slowly than the anionic and left more residual mass on the quartz and carbonate surfaces. Non-ionic surfactants had more measurable mass even at lower concentrations and the non-microemulsion had faster adsorption kinetics and was more resistant to washing off with fresh water than the microemulsion. The impact of job design for the various surfactant interactions with the silica surface was evaluated by altering the pumping schedules for the same volume of surfactant, showing the difference in accumulated residual mass on the surface using low concentrations throughout the fluid or front loading a concentrated plug volume. For charged surfactants, front loading was the least effective method; consistent concentration throughout the pumping schedule was more effective. Fast adsorbing surfactants quickly saturated the surface at high concentrations and had more effective loadings by splitting the surfactant into two equal medium concentration plugs. Ultimately, the surfactants were evaluated for removal of oil from the quartz surface. Without surfactant, very little oil is removed from the surface and it remained oil wet and fluorescent; the addition of surfactant improved the oil recovery by vastly different mechanisms.
This study provides an understanding of surfactant adsorption processes on rock surfaces and the role of job design for mobilizing hydrocarbons. Understanding surfactant adsorption and its effect on wettability improves the current understanding on the matter.
