Emulsion effects on the yield stress of gelled waxy crude oils

Waxy crude oils subjected to low temperature conditions in ultra-deep water environment for a long enough time start a crystallization process and exhibit a transition to a gel state. Flow assurance is greatly affected by this process. In most cases, this oil has a significant volume fraction composed of water. Hence, it is not uncommon that stable water-in-oil (w/o) emulsions are formed and they co-exist with the paraffinic structure of the waxy oil. The present work analyzes how the volume and dispersion of the droplets in the w/o emulsion influences the rheological properties of the oil when compared with its dry version. In the case of dry oils, the reference for w/o emulsions, we found that the initial temperature of the cooling process affects the yield stress of the material at the final temperature even when Ti is far above the wax appearance temperature. In addition, there is an inversion of the tendency of the growth of the yield stress when Ti is increased. For high values of the cooling rate, we found a second critical stress, besides the yield stress, where a secondary level of structure breaks down. The results concerning the emulsion effects on the yielding behavior of the tested waxy crude oils revealed that small drop emulsions can increase significantly the yield stress. While the typical increase with the volume of water cut was found for large drop emulsions (emulsions with more than 10% of water cut) an even higher level of the yield stress was found for emulsions with less than 5% of water cut. The competition between number of crystal nuclei and size of final crystals have encounter favorable conditions in the case of small drop emulsion. This result is probably related to the fact that the number of initial nucleation points and the distribution of drops in the small-drops emulsions are such that the mean distance between drops are more likely to be covered by the final typical length achieved by the crystals in the cooling process.