Page 163 - Handbook Of Multiphase Flow Assurance

Paraffin wax 159

Molecules diffuse from areas with high concentration to low concentration. Concentration
of n-paraffins can become low near a pipe wall if the wall is cold which affects solubility of
n-paraffins in oil. At low temperature, n-paraffins crystallize and precipitate out from solu-
tion as solids, which causes concentration to decrease. This in turn causes diffusion from high
concentration to low concentration areas.
Besides thermal diffusion there is also shear dispersion mechanism for deposition of par-
affins. Dispersion also depends on fluid cooling down below the paraffin crystallization tem-
perature, but does not require diffusion or temperature difference. Dispersion requires flow.
Flow causes dispersion and movement of precipitated solids from area of high content (bulk
of oil) to the periphery of flow (pipe wall or well tubing). Dispersion contributes between 1
and 10% of deposition. This means that even in a perfectly insulated pipes flowing oil with
precipitated wax crystals, there will still be adhesion and accumulation of precipitated wax
crystals on pipe walls. Regular maintenance for removal of wax deposits may be accom-
plished with scraping such pipe line.
Brownian diffusion and settling are commonly ignored in wax deposition analysis.
Brownian diffusion is eclipsed by flow turbulence. Gravity settling is only observed in static
non-flowing systems such as shut-in pipes or storage tanks, and settled wax particles are
usually easily re-dispersed when flow resumes.
Branched or isomerized paraffins also can form wax. Such wax is observed in biodegraded
oils where normal paraffins have been isomerized by bacteria. While the degree of crystal-
linity is less or absent, such wax also can deposit as solids on cold surfaces when there is a
temperature difference between bulk oil and pipe wall.

Chemistry
Wax is usually composed of n-paraffins which are inert, apolar and non-reactive at normal
production operation conditions.

Composition
Besides the solid crystalline matrix made up of normal paraffins, the wax deposits may
contain occluded oil, sand, precipitated asphaltenes, resins, corrosion products, and other
materials.
The measurements of mass fractions for asphaltenes, resins and solid hydrocarbons
was reported for samples deposited at various depths (Liushin and Iksanova, 1965) for
the Bavlinskoye, Romashkinskoye and other fields. The summary table is shown below in
Table 5.5.
Similar data were compiled for eight oil fields. Authors note that the solid hydrocarbons
extracted from oils of different fields significantly differ in fractional composition, such as in
heavy high-temperature melting solid hydrocarbons. Authors suggest that the intensity of
paraffin deposition is directly proportional to the content of high-melting temperature hy-
drocarbons in oil, all other parameters being equal. The authors also discuss the differences
in properties and crystallization habits of macrocrystalline (paraffinic) and microcrystalline
(isoparaffinic) wax. Authors note that strength of paraffinic wax deposits increases with an
increase of n-paraffin content in oil and decreases with an increase of aromatic content in oil.