Rheology

 Dynamic Viscosity Dynamic Viscosity vs. Temperature curves for assessing the flow behaviour of a waxy [Non-Newtonian] fluid are produced at a range of shear rates corresponding to typical production flowrates during normal steady-state pipeline flowing conditions. As such, each curve is produced at a single constant shear rate [simulating a constant flow rate] while the test fluid is cooled. KAT utilise a series of Brookfield Digital Portable Viscometers fitted with cup and bob UL sensors. Viscosities were then determined at several shear rates ranging from 1 to 200s-1 as the samples are cooled. Additionally, KAT can employ its Brookfield Rheometers with a range of interchangeable cup and bob sensor systems, to further examine the shear thinning behaviour [flow curves] at specific conditions up to a maximum shear of 1000s-1. Pour Point The pour point of a crude is the lowest temperature at which it will flow under static conditions. It points to problems that could occur given the sometimes-limited available pumping pressure to maintain flow in a pipeline, either during normal operations or following a prolonged shut down. KAT offers manual pour point determinations following the ASTM D97/D5853 [IP15/IP441] standard methodology, plus low volume [≤ 20mls] variations developed in-house for when available sample volumes may be limited. Stabilised crude samples (i.e., degassed or stock tank) are usually considered adequate for most export and production systems. However, in the field, dissolved gases, pressure, and flow regime will influence the rheological characteristics of a fluid, including its pour point. KAT has developed the bespoke test equipment and methodology for studying the pour point of pressurised fluids up to a maximum working pressure of 5,800psi [400barg]. Pipeline Restart Following a prolonged shutdown, the fluids may cool sufficiently to start to solidify [gel]. Once this occurs a finite pressure [yield stress] may be required to re-instigate flow. The strength of the gelled fluid and hence pressure required to restart a line will be dependent on several factors including the fluid’s shear / thermal history, the cooldown rate, and the shut-in temperature and duration. KAT has two 50-foot-long model pipeline loops of 1/2” and 1/4” internal diameters capable of operating at modest pressures and temperatures between -10 and +60°C [14 and 140°F]. Along the length of each flow line are seven ports through which fluid temperatures and pressures can be accurately monitored. These unique multi-port model pipelines allow the development of a gelled oil during cool down and shut-in to be monitored. This data can then be used to assess the condition of the gelled oil prior to restarting the line and the subsequent development of fluid flow within the pipeline, even before flow is recorded at the outlet.

 

Once flow has been observed the extreme viscosities may prevent acceleration so that the column can only be slowly displaced. However, for many fluids, even with slight shear, the degradation of gel structure can be high leading to significant lowering of viscosity and thus rapid acceleration and displacement of the cold fluid.

 

KAT can determine flow development following the restart in both the model pipeline test and by using controlled stress rheometry.

 

As with the pour points, above, restart determinations made with stabilised crude samples are usually considered sufficient. However, KAT has developed the bespoke test equipment and methodology for studying the restart of pressurised fluids up to a maximum working pressure of 5,800psi [400barg].