Flow Behavior of Daqing Waxy Crude Oil under Simulated Pipelining Conditions

Abstract

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 °C, which is 3 °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.

Modeling the rheological behavior of waxy crude oils as a function of flow and temperature history

“…In this description, the yield stress can vary from a low value, corresponding to a fully destructured fluid, to a high value, for a fully structured material. In that case, the parameter determination is done by rheometry, exploring different shear values and structure parameter evolutions [see Cawkwell and Charles (1989), H enaut and Brucy (2001), and Wang and Huang (2014)], sometimes with a new version of the model involving more parameters [Ding et al (2006)]. However, in the literature, one can hardly find a discussion of the quality of this model to describe the effective rheological behavior of such materials obtained from careful and systematic rheometrical data.…”

Non-Isothermal Models to Determine the Rheological Constants of Different Kuwaiti Crude Oils

“…Research Online First Article 2 production, and storage applications. For instance, rheological data are required to accurately simulate and design many equipment used in petroleum refineries (Ding et al, 2006). In particular, to design pipelines, set the pump capacity, and establish production facilities, the shear stress is a key characteristic that must be predicted accurately.…”

“…When paraffin waxes, solid paraffin, or complex aromatic molecules exist in crude oil, changes in the temperature affect the viscosity, and non-Newtonian characteristics appear. All crude oils contain a certain amount of paraffin waxes, which begin to undergo deposition at low temperatures, thereby exhibiting non-Newtonian behavior (Ding et al, 2006, Ronningsen 1992. Therefore, prediction models for any type of crude oil must take into account the temperature variation.…”

The Thixotropy of Daqing Waxy Crude

“…Thixotropy is one of the most important non-Newtonian behaviors of waxy crude, and it is an indispensable property to both pipeline hydraulic calculation and evaluation on restartability of a shutdown pipeline (Ding et al, 2006;Guo et al, 2013). Thixotropy can be defined as decease in viscosity under stress, followed by gradual recovery when the stress is removed, and the effect is time dependent (Cheng, 1987).…”