Page 167 - Handbook Of Multiphase Flow Assurance

Paraffin wax 163

Role of composition
The presence of light hydrocarbons and aromatic compounds helps solubilize the heavy
n-paraffin molecules in crude oil. The WAT and wax deposition both decrease with an in-
crease in the content of hydrocarbons lighter than C 17 and aromatic components in oil.
The composition of crude oil can be analyzed for the purposes of wax prediction very
roughly with a thermogravimetric analyzer, roughly with boiling point fractionation, de-
tailed with gas chromatograph and very detailed with a high temperature gas chromatograph
(HTGC). In a HTGC, a small amount of oil or wax deposit is dissolved under a vent hood in
a volatile solvent such as toluene (less toxic) or carbon disulfide (very toxic). The dissolved
sample is then injected in a temperature-controlled vaporizer and flown through an adsorp-
tion coil in carrier gas to the flame ionization detector where all molecules are dissociated into
ions to analyze mass of the components in the stream. Time of elution is calibrated to normal
paraffins of known molecular weight, and composition of the sample is determined based on
calibration. The heaviest commercially available calibration sample is C 60 . Analysis of elution
times for heavier components are based on logarithmic extrapolation.
Oil WAT increases with an increase of heaviest components content.
Majority of petroleum analyses for wax is done with GC or HTGC. In some cases labora-
tory data are supplied as true boiling point distribution. In order to use the oil composition in
a predictive tool one needs to convert the boiling point temperature of a fraction to molecular
weight of a fraction. The method proposed for doing this was presented in Chapter 3.

Miscellaneous factors

Oil-based drilling muds may be used in deepwater wellwork. Oil-based drilling muds
contain n-paraffins such as C 14 , C 16 and C 18 . Oil samples contaminated with large amounts of
drilling mud may be inappropriate for PVT analysis of oil thermodynamic properties such as
bubble point, density or gas oil ratio. However, it may still be possible to use such samples for
paraffin analysis with decontamination. The amount of components is expected to decrease
logarithmically with molecular weight. Components of oil-based drilling mud will be seen in
HTGC data as outlier peaks, and it is possible to subtract the extra peaks from the composi-
tion to make it usable for paraffin deposition analysis.

Tubular plugging
• Effect of PVT conditions
It was demonstrated (Mazepa, 1965) than wax can deposit from both saturated oil (multi-
phase flow of oil and gas) and undersaturated oil (single phase flow). As fluids flow up the
wellbore, pressure decreases and the wax appearance temperature also decreases. However,
ambient temperature also becomes lower. The colder ambient conditions cause wax to precip-
itate. In some cases vacuum insulated tubing (VIT) or downhole electric heating cables were
used to maintain heat in the flowing crude to avoid wax deposition.
• Role of composition
The presence of water helps retain heat in flowing produced fluid and reduces the amount
of wax deposition. Water is also denser than oil and creates additional shear on the wax de-
posit, partly removing it.