Effect of Shear History on Restart of Gelled Waxy Crude Oil Pipeline

Effect of shear history on restart of gelled waxy crude oil pipeline

Youquan Bao, Jinjun Zhang

National Engineering Laboratory for Pipeline Safety/Beijing Key Laboratory of Urban Oil & Gas Distribution Technology, China University of Petroleum, Beijing 102249, China

Abstract: The existence of wax in waxy crude oil makes its rheological property very complex. When the temperature of waxy crude oil is high enough, wax dissolves in the oil in molecular form and the whole system behaves as Newtonian fluid. When the temperature is reduced to a certain value which is called "wax appearance temperature", the dissolved wax begins to precipitate from the oil as a result of supersaturated and the whole system is still Newtonian fluid. If the temperature is further reduced, the amount of precipitated waxy will increases gradually, and the system turns from Newtonian to non-Newtonian. Once the mass percent of the precipitated waxy increases to 1wt%~6wt%, the wax crystals in crude oil will attract and interlock with each other to form a three-dimensional sponge-like network structure where liquid oil is wrapped in, and the whole system turns from sol to gel. The gelled waxy crude oil shows complex rheological behaviors, such as viscoelasticity, yield stress, and thixotropy.

Waxy crude oil is always heated and transported by pipeline for its bad fluidity at normal temperature. In the production process, shutdown of pipeline is unavoidable. If the shutdown costs a long time, the waxy crude oil in the pipeline will gelled because of temperature reduction. How to restart the pipeline successfully is one of the great concerns of flow assurance, and it is also a research hotspot in the last few decades.

Shear history has greatly influence on rheological properties of waxy crude oil, while the rheological properties have a determinative effect on pipeline restart. In the present work, shear history was simplistically simulated by means of rheometer (HAAKE MARS III) firstly, and the main processes include: dynamically cooling (shear rate is 10 s^-1/50 s^-1/100 s^-1) the waxy crude oil sample in the rheometer from 50 °C to 34 °C at a cooling rate of 0.1 °C/min; Statically cooling the sample further to 32 °C. Then, shear rate stepwise increasing experiment was conducted to investigate the effect of shear history on the structure breakdown characteristic of waxy crude oil, and Eq. (1) was employed to describe the breakdown characteristic. Finally, numerically simulation was performed to study the effect of shear history on the restart process.

Where is shear stress;  is shear rate; is structural parameter, and; is total shear strain;, K, , n₁, n₂, a, b and m are undetermined parameters.

The shear rate stepwise increasing results under different shear history were shown in Fig. 1.It can be seen from Fig.1 that with increasing shear rate during the cooling process, the stress overshoot increases and the breakdown curve is upward. The Eq. (1) was used to quantify the curves in Fig.1 and the associated parameter values were given in Table 1.

Fig. 1 Shear rate stepwise increasing results under different shear history

Table 1 Associated parameter values of Eq. (1) under different shear history

Considering a horizontal pipe of length L=1 km and inner diameter d=0.5 m. The weakly compressible (compressibility factor ) fully-gelled waxy crude oil is assumed to have uniform density and rheological properties (as shown in Table 1) initially. A constant pressure gradient  MPa is suddenly applied at the pipe inlet to break the gel and restart the flow. The results of numerical calculation were normalized according to the following parameters:  

The inlet and outlet velocities variations with time for different cases were shown in Fig. 2. It can be seen from Fig. 2 that, for all three case, inlet velocities increase instantaneously at t^*=0 while outlet velocities increase at t^*=1 and both oscillate before reaching the steady states. In addition, the higher the shear rate during cooling process, the higher the steady inlet/outlet velocity is.

Fig.2 Time variations of inlet and outlet velocities for different cases

The time variations of pressure at Z^*=0.5was shown in Fig. 3. It can be concluded that lower shear rate during cooling process results in higher pressure-peak magnitude. However, the steady-state pressure at Z^*=0.5 is independent of the shear history.

Fig. 3 Time variations of pressure at Z^*=0.5 for different cases

Key words: waxy crude oil; shear history; restart