Page 14 - Handbook Of Multiphase Flow Assurance

8 1. Introduction

temperature” (WDMT). The WDMT occurs at a temperature when a wax deposit accu-
mulated over time in a field flowline melts without added oil or solvent. WDT is typically
10–20 °C higher than WAT, whereas WDMT can be 30–40 °C higher than WAT, depending
on how long the wax had to age in the field flowline and how much of the heavy fractions
it had concentrated.
Crystals of normal paraffin wax, as well as many other crystals, can rotate the plane of
light linear polarization and shine. This phenomenon is used in cross-polarized micros-
copy (CPM) to determine WAT. Networks of waxy crystals called gels, which also shine
in a cross-polarized microscope, are therefore composed of wax crystals, not of amor-
phous non-crystalline material. The term gel is used in relation to wax to signify that the
whole bulk of oil converts to a non-flowing material when its temperature drops below
pour point and a waxy gel is formed. However, the waxy gel material is not uniform and
contains both solid wax and liquid oil trapped between solids. One should recognize
when discussing incipient, or initial, wax deposition on a pipe wall that it is wax crystals
made up of concentrated normal paraffins or isomerized saturated alkanes that deposit,
not a gel which has the same composition as base oil.
A combined hydrate and wax blockage formed in the North Sea in the past in the Staffa
field led to costly remediation by depressurization to melt the hydrate. After the first block-
age got removed, the second blockage formed, which led to an abandonment of the subsea
flowline.
In another case offshore West Africa, an incompletely dissociated hydrate plug started to
move in a pipeline and acted as a scraper, compacting the existing paraffin wax deposit into
a solid blockage, which could no longer be melted simply by depressurization, and led to a
lengthy process of solvent injection past the low-permeability paraffin plug which eventually
got removed.
Scale can form as shown in Figs. 1.7 and 1.8 when reservoir water, which may exist near
the oil or gas deposit, has some minerals dissolved in it. At reservoir conditions, the forma-
tion water is usually partly saturated with salt, or in some cases may be near the equilibrium

Fig. 1.7 Scale buildup inside a heat exchanger tube (Lebedev, 2010).