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 2,000s-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.
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 also determine the yield stress and the subsequent flow development following the restart by controlled stress rheometry.
KAT Restart R&D
In-house research to investigate the causes of predictive error between pipeline restarts in the lab and field has resulted in two key findings. These significant insights are detailed in the following publications:
- Phillips, D. A., Forsdyke, I. N., McCracken, I. R., Ravenscroft, P. D., Novel approaches to waxy crude restart: Part 1: Thermal shrinkage of waxy crude oil and the impact for pipeline restart. J. Pet. Sci. Eng. 2009; 77: 237-253
- Phillips, D. A., Forsdyke, I. N., McCracken, I. R., Ravenscroft, P. D., Novel approaches to waxy crude restart: Part 2: An investigation of flow events following shut down. J. Pet. Sci. Eng. 2009; 77: 286-304
High-Pressure Rheology
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. KAT has developed the bespoke test equipment and methodology for studying the pour point and pipeline restart of pressurised fluids up to a maximum working pressure of 5,800psi [400barg].
Fluid Analysis
KAT offers a suite of analyses to characterise crude oil and gas condensate samples and help assess the potential impact on production operations.SARA [latroscan]Separates the test fluid into four solubility classes: Saturate (Paraffin), Aromatic, Resin, and Asphaltenes using the Iatroscan…
Asphaltenes
Asphaltenes are a natural constituent of many crude oils and may be precipitated in production systems when the crude’s natural solvency for them is reduced. Several factors including, pressure, temperature, and composition can change the stability of these high molecular…
Separation
Foaming When produced and transported from the reservoir to processing facilities, fluids experience a drop in pressure. This will release dissolved gases that can cause the fluids to foam. Evolved gas is removed in a separator, but foaming can lead…
Gas Hydrates
Gas hydrates are crystalline solids with cage-like structures [clathrates] in which a hydrocarbon molecule is enclosed in a lattice of water molecules. Although they have the appearance of ice or snow, gas hydrates crucially form at pressures and temperatures above…
Oilfield Scale
Oilfield scale is the term used to describe deposits of insoluble inorganic minerals such as calcium carbonate, barium sulphate, and metal sulphides. In general, scale deposits occur when waters with different ion contents are mixed although pressure and pH can…
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…
Wax Appearance
Waxes are generally defined as paraffinic material with carbon numbers greater than nC17. Waxes are present in oil as a distribution of molecular weights and thus exhibit a range of solubilities, precipitating over a range of conditions. Precipitation is temperature…
Wax Deposition
The build-up of solid waxy layers onto cooled surfaces such as pipe walls is generally considered to be a temperature-dominated phenomenon. Several theories have been proposed to describe the effect and several commercial semi-empirical models have been developed to predict…
Arn Acids
Napthenate Solids Naphthenate solids are naturally occurring oilfield fluid scales formed from reactions between a specific group of high molecular weight cyclic naphthenic ARN acids, also known as Tetra Protic Acids or Tetra-Acids, with dissolved divalent cations [such as Ca,…
T-SEP®
Compared to the relatively high concentrations of nC10 – 20 in crude oils and gas condensates [analysed as unadulterated “Whole” sample] the concentrations of >nC30 can be relatively low and either close to or below the limit of detection /…