Rheological study of the behavior of water-in-oil emulsions of heavy oils

Abstract

Abstract Unconventional oils (heavy and extra-heavy oils) represent approximately 55% of the oil reserves of the world. These oils are characterized for their high viscosity and gravity due to the high amount of resins and asphaltenes, natural emulsifiers, which combined with the presence of water and shear, in the production and transport of the oil in pipelines, may form the emulsions, usually water-in-oil (W/O). This study evaluated the rheological behavior of W/O emulsions of five heavy oils from the sedimentary basin of the Brazilian coast, with API gravity between 10.8 and 19.0. Factors related to the stability (temperature, amount of emulsified water, salt concentration) and rheological behavior of the W/O emulsions (dynamic viscosity, stress and shear rate) were evaluated and related to the oil chemical composition in terms of saturates, aromatics, resins and asphaltenes (SARA). The emulsions were prepared with deionized water, formation water (55\u202fg\u202fL−1 of NaCl) and water saturated with sodium chloride (270\u202fg\u202fL−1 of NaCl) under mechanical stirring at 5000\u202frpm. Rheological assays of the emulsions were performed at 30–80\u202f°C, and the obtained data were treated using the Ostwald-de Waele equation. The results of the rheological evaluation showed that in stable emulsions there was a 1212% increase in the dynamic viscosity as a consequence of the amount of aqueous phase, while temperature minimized this effect, at 60\u202f°C it was observed a reduction of more than 80% in the viscosity. In these emulsions the flow rate was positive, increasing as a function of temperature, and the inverse effect was observed when correlated to the amount of water. The unstable emulsions presented lower results for the dynamic viscosity if compared with the results of the dehydrated oils, caused by the presence of non-emulsified water. In general, the elevation of salt concentration and temperature aided in the stabilization of unstable emulsions. Oils with a total acid number minor 1.3 mgKOH·g−1 and asphaltene/resin, aromatic/saturated and asphaltene/aromatic ratios higher 0.2 formed unstable emulsions, resulting in the appearance of non-emulsified water during the rheological tests.