Effect of asphaltenes on thermally- and shear-driven regimes of wax deposition

Waxes and asphaltenes co-exist in crude oils, yet few studies consider the effects of one component on another. The influence of asphaltenes on wax deposition and its properties is studied using a bespoke cold rotating finger which allows measurements to be made in both static and dynamic (sheared) conditions. For static deposition, adding asphaltenes reduces the rate at which the wax deposit is formed, as well as the overall thickness of the deposit. This change is attributed to changes in the fluid properties, with asphaltenes lowering the wax appearance temperature, meaning a greater degree of subcooling is needed to achieve the same amount of precipitated wax. For dynamic measurements, both the rate of deposition and deposit thickness are unaffected by asphaltenes, demonstrating that hydrodynamics strongly govern the behavior. However, the average molecular weight and the wax content of the deposit increases significantly with asphaltenes present. With the deposit thickness being similar with and without asphaltenes, it suggests that the yield strengths of the two deposits are also similar, since thickness in the hydrodynamic regime is affected by the fluid shear stress. The changes in composition and content of the wax deposit will increase yield strength, yet this is balanced by a reduced wax gel yield strength in the presence of asphaltenes. These two factors effectively cancel one another to result in the same deposit thickness when sheared. The study provides new insights into the mechanisms by which asphaltenes modify wax deposition and deposit properties. • Static condition, wax deposition inhibited due to asphaltenes lowering the WAT. • Dynamic condition, wax deposition unchanged by the presence of asphaltenes. • Asphaltenes increase wax content and shift composition to higher number alkanes. • Changing composition compensated by reduced wax yield strength, deposit thickness unchanged.