Experimental and Theoretical Investigations of Waxy Crude Oil in Steady and Transient Pipe Flows

Waxy crude oils are becoming increasingly important in worldwide oil production. Due to the existence of suspended wax crystals during shear flows, waxy crude oils usually exhibit large non-Newtonian shear viscosity values and thixotropic behaviors (time-dependent yield stress). Thus, accurate pipe-flow prediction models are needed in order to support pipeline oil transportation operation and design. Recent rheological studies have shown that the presheared waxy oil slurry below its wax appearance temperature exhibits a large shear stress overshoot during start-up after a short period time of static aging. To investigate the pipe flows of model waxy crude oils, a bench-scale flow loop (test section ID = 1.02 cm, length = 1.37 m) and a pilot-scale flow loop (test section ID = 2.67 cm, length = 3.81 m) were built suitable for studying the steady-state and transient flow behaviors. The temperatures and pressures across the entire test section were measured during the flow experiments. Model oils containing 7 and 10 wt % wax were studied. Steady-state pressures were obtained at a variety of flow rates. Overshoots in pressure were observed at the beginning of the start-up flow. The overshoot was attributed to the buildup of a wax gel structure during quiescent aging periods. A pipeline model that incorporates the isotropic–kinematic hardening rheology model is proposed. The parameters of this model were obtained by laboratory cone–plate rheology measurements. The cone–plate shear stress of the waxy model oil exhibits a relative minimum when plotted against the shear rate. The pressure drop profiles of steady-state and transient start-up flows were predicted by the developed pipe-flow model.