Abstract
<jats:title>Abstract</jats:title> <jats:p>Critical conditions required for promoting oil mobilization in successful chemical enhanced oil recovery (cEOR) operations include decreased interfacial tension (IFT), surfactant stability, and microemulsion structure. These conditions in turn are dependent on surfactant chemistry and properties of the reservoir (salinity, oil, temperature). This study evaluates the link between these properties and surfactant micelle size to identify better criteria for surfactant formulation and relate these behaviors to performance.</jats:p> <jats:p>Surfactants were systematically selected based on groupings of alcohol structure (length, branching) and hydrophile nature (ethoxylation degree, ionicity) to note trends among the chemistry classes. To focus on micelle formations, Dynamic Light Scattering (DLS) was used to measure the particle sizes of surfactant micelles and microemulsion droplets. Solutions were prepared at several concentrations to determine a robust concentration region for study. This practice also revealed any dependence of micelle size and polydispersity on concentration, ensuring consistent comparison across different surfactants and formulations. IFT was measured at test temperatures by spinning drop tensiometry with representative crude oils and surrogate oils of known EACN. Evaluations accounted for the proximity to optimum salinity of the formulations in their respective conditions.</jats:p> <jats:p>For alcohol ethoxylates, micelle size was found to be more sensitive to changes in alcohol chain length than ethoxylation degree. In deionized water at 25°C, 5000 ppm solutions of C13 ethoxylates with medium, high and very high ethoxylation degrees had micelle sizes ranging from 8.1 to 9.7 nm, while a C17 with high ethoxylation degree produced 11.6 nm micelles. This upward trend was consistent up to 60°C. Interestingly, at 70°C the C13 with medium ethoxylation begins to approach its cloud point, and the average micelle size increases by approximately +1 nm. The IFT of some formulations with crude and pure component oils were measured and compared with the micelle size at the corresponding temperature and salinity. Notably, no correlation was found between small micelle size and low IFT. Winsor Type I, II and III microemulsions were prepared using sodium dihexyl sulfosuccinate (SDHS) and toluene, and the microemulsion droplet sizes in each case were compared to the micelle size of SDHS in aqueous solution. A novel screening technique was developed to qualify surfactant stability under reservoir conditions by monitoring changes in micelle size and polydispersity over time.</jats:p> <jats:p>This study provides insight into how information on surfactant micelle size can be applied in a meaningful way when formulating surfactant packages for cEOR. The results also challenge common industry assumptions that attribute superior performance of formulations to smaller micelle size. Ongoing work will test the applicability of these observations across additional conditions, chemistries, and brines.</jats:p>