Flow behavior of Daqing waxy crude oil under simulated pipelining conditions

Daqing oilfield is the largest oilfield in China. This crude oil is a typical waxy crude oil, with a wax content of 26% and a gel point of 32 C. Flow behaviors of waxy crude oils at temperatures near the gel point/pour point are vital for both pipeline hydraulic calculation and evaluation on restartability of a shutdown pipeline. In this study, experimental simulation was conducted by using a stirred vessel with the energy dissipation of viscous flow as the shear simulation parameter. Crude oil specimens were taken from the stirred vessel at different temperatures during flow simulation, and the DSC (differential scanning calorimetry) curves and rheological properties, such as viscosity, gel point, yield stress, and thixotropy, were measured. The viscosity under simulated pipelining conditions was found to be much less than that measured under quiescent cooling conditions, and the gel point decreased with decreasing temperature of sampling, i.e., the end temperature of the dynamic cooling process. The DSC curves of crude oil specimen captured at different sampling temperatures and from different positions such as the bulk of crude oil and the wall of the stirred vessel showed no difference, which indicated the depletion of wax due to wax deposition during the shear simulation was not responsible for viscosity and gel point reduction in the dynamic cooling measurement. Therefore, this reduction is attributed to shear disturbance on the developing wax crystal structure, called the shear history effect. At sampling temperatures above 35 degrees C, which is 3 degrees C above the gel point measured under quiescent cooling conditions, both the yield stresses and the thixotropic parameters showed little dependence on the shear history. However, at lower sampling temperatures, remarkable shear history dependence was found. Empirical correlations were developed between the yield stress and the sampling temperature as well as the measurement temperature and between the thixotropic parameters and the sampling temperature. These correlations provide not only quantitative information on the shear history effect of waxy crude but also accurate rheological properties for assessment of the operation state and restartability of pipelines transporting this crude oil.