The effect of crystal morphology of two different polymorphs of mineral calcium carbonate, CaCO3, (i.e., calcite and aragonite) and a follow-up study (based on our previous work on emulsion characterization as reported in Arshad et al., 2018 a) on the thermal stability of emulsions are presented in this work.

Brines with varying salt concentration (deionized water (DIW), synthetic seawater (0.5SSW and SSW), and formation water (FW2 and FW1)), model oils (decane (D) and 1:1 vol. ratio of hexane–hexadecane (HH)), and a sample of North Sea crude oil (NSCO) were employed. Calcite fines (size ≤ 30 μm), aragonite fines (size ≤ 5 μm), and calcite nanoparticles of three different sizes (15–40, 50, and 90 nm) were used for emulsion formation in brine–oil mixtures. CaCO3 micron-sized fines (calcite and aragonite) and calcite nanoparticles were characterized by Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM), respectively. X-ray Powder Diffraction (XRD) was performed to examine the crystal structure of calcite and aragonite fines. Branson Sonifier® SFX250 (forced emulsification) was employed for emulsion generation in brine–oil–fines/nanoparticles. An optical microscope (Axio Scope.A1) was used for characterizing the emulsion droplet size. Thermal stability of the emulsions was examined in two stages, first by keeping them at room temperature for an extended period of 12–23 months (emulsion phase readings were taken at week–1, month–5, month–12, and month–23) followed by heating them at an elevated temperature of 80 °C for a period of 40 days in a custom-made closed water bath.

The crystal morphology study showed that although the calcite fines (≤ 30 μm) were six times bigger in size in comparison to the aragonite fines (≤ 5 μm), they generated a comparatively large amount of emulsion with relatively smaller emulsion droplets in the DIW–HH mixtures. This indicates that the crystal morphology of fines was the dominating factor in emulsion formation and emulsion droplet size instead of the size of fines and the crystal morphology should be considered as an important parameter in the selection of nanoparticles for EOR applications.

Emulsion thermal stability was examined in brine–oil–calcite nanoparticles with a wide range of brine salinity (DIW, 0.5SSW, SSW, FW2, and FW1), oil type (D, HH, and NSCO), and size of calcite nanoparticles (15–40, 50, and 90 nm). All the brine–D–calcite nanoparticles systems showed excellent thermal stability both at room temperature for 12 months and at an elevated temperature of 80 °C for 40 days. Amongst all the brine–D–calcite nanoparticles cases, a maximum cumulative de-emulsification (from month–5 to heating at 80 °C for 40 days) of 6 % was observed for the 0.5SSW–D–CaCO3 system with 50 nm calcite nanoparticles. Due to the lower boiling temperature of hexane (~ 68 °C), the brine–HH–calcite nanoparticles systems were not tested at 80 °C. They were instead tested at room temperature for an extended period of up to 23 months and all the studied systems showed excellent thermal stability with no measurable change in the emulsion phase from month–5 to month–23. In case of the brine–NSCO–calcite nanoparticles systems, nanoparticles with 15–40 and 90 nm sizes generally showed excellent thermal stability at room temperature and very good thermal stability at 80 °C. Contrary to the results of nanoparticles with 15–40 and 90 nm sizes, 50 nm nanoparticles exhibited poor thermal stability for the whole rage of brine salinity tested in this work. They showed the largest reduction in the emulsion phase of 39 % (for the SSW–NSCO–CaCO3 system) when heated at 80 °C for 40 days amongst all the studied systems. Similarly, the FW2–NSCO–CaCO3 system showed the largest cumulative (from week–1 at room temperature to heating at 80 °C for 40 days) de-emulsification of 47.5 % amongst all the studied systems. It was also observed that the presence of salts in the system (SSW, FW2, and FW1) promoted thermal stability of the emulsions compared to the DIW system, both at room temperature and at 80 °C (with an exception of 50 nm nanoparticles).

Emulsion formation behavior of particles with different crystal morphology and emulsion thermal stability over an extended period and at an elevated temperature as presented in this work can be very beneficial in developing nanoparticles based EOR applications.

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