Non-Newtonian steady shear flow characteristics of waxy crude oil

Abstract

The experimental research on the non-Newtonian flow characteristic of a waxy crude oil was conducted through a rotational parallel-plates rheometer system. The test temperature is about 6.5 ℃ higher than its gel point. The shear stress and viscosity of the waxy crude oil show sophisticate non-Newtonian characteristics in the shear rate of 10 −4 −10 2 s −1 , in which the shear stress can be divided into three parts qualitatively, i.e. stress-up region, leveling-off region, and stress-up region. This indicates that there is a yielding process in shearing for the waxy crude oil at the experimental temperature, which is similar to the yield phenomenon in thixotropy-loop test discussed by CHANG and BOGER. Furthermore, the steady shear experiment after the preshear process shows that the stress leveling-off region at low shear rate disappears for the waxy crude oil and the stress curve becomes a monotonic climbing one, which demonstrates that the internal structure property presenting through yielding stress at low shear rate can be changed by shearing. The experimental results also show that the internal structure of waxy crude oil presenting at low shear rate has no influence on the shear viscosity obtained at the shear rate higher than 0.1 s −1 . The generalized Newtonian model is adopted to describe the shear-thinning viscosity property of the waxy crude oil at high shear rate.

A new semi-empirical spectrum model for complex steady shear viscosity of complex fluids

“…(4) denoted by the bold dashed line agrees well with the experimental viscosities of the oil in the whole experimental region of the shear rates from about 10 −4 to 10 2 s −1 . The previous fitting [4] by other three generalized Newtonian models, i.e. power-law model, Carreau model, and Cross model, are also shown in Fig.…”

“…As we mentioned that the power-law model can only be applied to a limited range of shear rate and the Carreau and Cross models are inflexible when describing the whole range of viscosity of complex fluids such as waxy crude oil [4]. In polymer rheology, the descriptions of the steady viscosity for most polymer solutions or melts by using viscoelastic models [1][2][3]5] such as the KBKZ model, PTT model and Giesekus model can also be produced almost identically by using the purely viscous Carreau or Cross model.…”

“…The experimental temperature is about 14.5 • C, which is 6.5 • C higher than the gel point of the oil. The details of the experiments could be found in [4]. The experimental viscosities are shown in Fig.…”

“…2. In the previous paper [4], we regarded the nearly constant shear stress region between "a" and "b" in Fig. 2 at low shear rates of about 10 −3 − 2 × 10 −2 s −1 to be yielding behavior as termed by Chang and Boger [6].…”

“…The Cross model is a similar modification to the simple power law. In a recent experiment [4], the steady shear viscosity of a waxy crude oil near its gel point was measured and the data of the whole range of experimental shear rates from about 10 −4 to 10 2 s −1 show sophisticated behavior as presented in Fig. 1.…”