Abstract
Microemulsion phase behavior is the most important phenomenon related to any type of chemical EOR using surfactants. Although the impact of solution gas and pressure on microemulsion phase behavior has been studied for decades, only a very limited amount of data are available in the literature and this is especially true at high temperature and pressure, nor is there a good theory available to guide our understanding of this behavior. A very large number of new chemical EOR projects are underway and almost all of them now involve live oil whereas in the past most of the projects did not involve live oil. The availability of sapphire view cells rated to 150 °C and 5000 psi has greatly extended our laboratory capability to do live oil studies under conditions where almost no data exist. These view cells allow us to make visual observations very similar to the critical phase behavior observations that we routinely make using glass pipettes at low pressure. For example, the classical emulsion test can be performed to qualitatively estimate the interfacial tension and viscosity. Also, it is easy to observe phase separation of the microemulsion/water/oil mixtures, the oil and water solubilization ratios and the equilibration times. This can be done using multiple cells so that salinity or other key parameters can be varied to determine optimum salinity and other key design parameters. This has allowed us to perform more tests at higher temperature and pressure with higher solution gas ratios than previous tests. The results are highly significant with very important implications for field applications to live oil reservoirs. Specifically, we have found the optimum salinity is closer to the dead oil than expected and higher than the value for surrogate oils made based on the equivalent alkane carbon number approach. The development of formulations that meets the aqueous stability criterion at optimum salinity is therefore more challenging than previously realized.
