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.
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.
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.
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.
KAT have also developed a high-pressure CPM technique to measure the WAT/WDT at pressures up to 6,525 psi [450barg] and study the impact of compositional change, due to pressure, on wax solubility. 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: “Live” Wax Appearance Measurements - Using “Real Data” to Strip out Conservatism in Pipeline Design by Paul D Ravenscroft, Ian R McCracken (BP Exploration Operating Co Ltd); Ivor N Forsdyke, Neil P Chilcott (KAT Ltd) presented at the 19th Oilfield Chemistry Symposium (March 2008, Geilo, Norway).