259
10
Wax (Paraffin Wax) Control
10.1 INTRODUCTION
Wax deposition can be both a downhole and topside problem, blocking the ow of hydrocarbons
uids as they are cooled.
1–3,157
The term “parafn” is used in the United States to describe oileld
wax, although “parafn wax” is a better terminology. Waxes are solids made up of long-chain
(>C18), normal or branched alkane compounds that are naturally present in crude oils and some
condensates.
4
Some cyclic alkanes and aromatic hydrocarbons may also be present. It has been
established conclusively that normal alkanes (n-parafn waxes) are predominantly responsible for
pipeline wax deposition. Waxes in crudes are usually harder to control than those in condensates as
they are of longer-chain alkanes. When the molecular size is 16–25 carbon atoms, soft mushy waxes
are observed. Hard crystalline waxes have 25–50 or more carbons in the chain. The melting point of
the parafn waxes increases as the size of the molecule increases. Generally, the higher the melting
point, the more difcult it is to keep the parafn wax from forming deposits.
In the reservoir, at high temperature and pressures, any waxes within the oil will be in solu-
tion. As the crude oil temperature drops, wax will begin to precipitate from the crude oil, usually
as needles and plates. In addition, as the pressure drops during production, loss of low molecular
weight hydrocarbons (light ends) to the gas phase reduces the solubility of the waxes in the oil. The
wax appearance temperature (WAT) or cloud point is the temperature at which the rst wax crystals
begin to precipitate from the crude oil. This is a very important measurement. The WAT can be as
high as 50°C (122°F) for some oils and depends on the pressure, oil composition (in particular, the
concentration of light ends), and bubble point. Wax precipitation/deposition is normally a problem
at a higher temperature than gas hydrate formation.
5
The oil that reaches the sales tank will often
contain parafnic solids. This oil, because of the loss of light ends and the lower-temperature envi-
ronment, has lost much of its ability to hold the waxes in solution. Typical problems caused by wax
deposition include
Reduction or plugging of pipework, blocking ow—this can be downhole if the well tem-
perature is low, or topside in cold climate or subsea transportation.
Increased uid viscosity leading to increased pumping pressure.
Restartability issues caused by wax gel strength.
Reduced operating efciency and process upsets with interruptions to production or
shutdowns.
Costly and technically challenging removal, especially in deepwater pipelines.
Safety hazard due to deposits interfering with the operation of valves and instruments.
Disposal problems associated with accumulated wax.
Figure 10.1 illustrates a typical phase diagram for wax precipitation. Position A is the reservoir
pressure with the oil undersaturated. As the uids are produced, the pressure falls and the light ends
expand in greater proportion to the dissolved waxes, increasing their solubility and yielding a lower
WAT. At the bubble point (B), the volume ratio of light ends to heavy ends is at its greatest so the
WAT is at its lowest. Reducing the pressure still further (C) results in liberation of dissolved gases
and light ends to the gas phase. This reduces the solubility of the wax, and so, the WAT increases.
This effect can change the WAT by as much as 15°C (27°F) compared with stock tank oil at 1 bar.

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