(49c) Partial Recovery of Wax Gel Strength: Quantitative and Qualitative Analyses

Wax contents in a crude oil start to precipitate when the surrounding temperature is lower than wax appearance temperature (WAT). While the wax deposition can be initiated during the flow, wax gel formation occurs in static conditions. When the wax gel develops within a relatively short time, certain pressures are needed to overcome the yield of the gel along the pipeline for restart. It was found that paraffinic components contribute to the evolving gel strength continuously while cooling below pour point (PP). Gel strength is varied by the participating wax amount and wax composition in the gel network. It was also observed that the yield stress values increase linearly with respect to temperature decrease in a measured range. A more rapid increase of yield stress is obtained in the model oils with higher wax content. It has been reported that the gel properties are dependent upon various factors: temperature, cooling rate, cooling time, shear history, and a diverse combination of factors. This study explores gel strength and strength recovery by shear exertion near the PP followed by further cooling. Model oils were used in this study by mixing mineral oil and varying amounts of well-characterized wax. The measurements of WAT and PP were performed using ASTM methods. A controlled-stress rheometer equipped with a cone-and-plate geometry and a Peltier plate device was employed to determine the yield stress and the measurement of creep recovery. Shears were applied at the range of creep response. Various combined series of shear exertion and cooling were performed to compare the gel strength and partial gel recovery. The cooling after shear was scheduled in fixed time duration. Yield reduction was observed more in cases where the temperature gradient was larger between the temperature shear applied and the temperature after consecutive cooling. As shear is applied near the yield stress, the yield stress value after cooling was much less than the yield stress value determined without shear exertion. The result from the oil with higher wax content shows the importance of initial gel formation just below the PP.