Pressure-induced structuring and yield stress of waxy crude oils

Abstract

The rheological behavior of waxy crude oils at low temperatures is highly dependent on pressure, temperature, and composition. This study investigates the effects of pressure, pressurization conditions, and aging on the flow behavior and yield stress of three waxy crude oils with different wax contents and WAT values. Differential scanning calorimetry and high-pressure rheometry were performed under controlled thermal and pressure conditions up to 300 bar. Viscosity followed an exponential pressure dependence consistent with Barus law, with piezoviscous coefficients of 18–20 GPa⁻¹ at 60 ◦C and 26–32 GPa⁻¹ at 4 ◦C. Pressure increased the WAT by approximately 0.25 ◦C MPa⁻¹ and promoted earlier gelation. When applied at the onset of cooling, pressure significantly enhanced gel strength, whereas late pressurization reduced yield stress and produced a more gradual yielding process. After 1 h of aging, yield stress increased by approximately 0.3% per bar. The pressurizing medium also affected the gel structure. Using crude oil as the pressure-transmitting fluid produced the highest gel strength, while nitrogen caused minor reductions and helium induced a more pronounced weakening effect. The extended Sisko-Barus model accurately described the flow behavior. These findings demonstrate that pressure history and aging significantly modify gel structure and yield behavior, with important implications for flow assurance in subsea pipeline operations.