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 dominated although pressure, through compositional change, may have a significant effect.
Wax Appearance Temperature [WAT]
The Wax Appearance Temperature [WAT] is the temperature at which the highest melting point waxes first precipitate out of solution on cooling from reservoir conditions and as such is one of the most important factors effecting system design and production operations.
How it works
KAT determine the WAT of a stock tank oil using crossed polar microscopy [CPM]. The methodology involves sealing a sub-sample within 100μm capillary tubes and observing the development of wax crystals through crossed polar filters at a magnification of x 100 as the capillaries are cooled from 80 to 0°C at 6°C / hour [0.1°C / min].
The WAT is defined as the appearance of the first crystal(s) during the cooling and typically, the higher value from two cooling cycles is reported. Normal experimental variation is within ±1°C.
Wax Dissolution Temperature [WDT]
The Wax Dissolution Temperature [WDT] is defined as the temperature at which the last crystals are observed to dissolve in the oil on heating. This is not to be confused with bulk wax or deposit melting temperatures. In this [WDT] case the individual wax crystals are dissolving within the bulk oil.
High-Pressure CPM Technique
KAT have also developed the bespoke test equipment and methodology to measure the WAT/WDT by CPM at pressures up to 6,525 psi [450barg] and study the impact of compositional change, due to pressure, on wax solubility.
How it works
The methodology is essentially the same as for the atmospheric test, described above, except a high-pressure windowed cell with an optical path length comparable to that of the microcapillaries is used to contain the test sample.
Further details of the equipment, the methodology and its use by BP to assist pipeline design can be found in:
- Ravenscroft, P. D., McCracken, I. R., Chilcott, N. P., Forsdyke, I. N. ‘Live’ Wax Appearance Measurements - Using ‘Real Data' to Strip out Conservatism in Pipeline Design. Paper presented at: 19th Oilfield Chemistry Symposium, March 2008 (Geilo, Norway).
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 /…