(29c) Investigating the Role of Wax Gelation in Wax Deposition and Growth in Subsea Pipelines

Abstract

Wax deposition during the transportation of crude oil
in the subsea pipelines has caused significant flow assurance problems in the
petroleum industry. One
practical but costly way used to remediate wax deposition is pigging, which involves sending
inspection gauge or 'pig' along the length of the pipeline to scrape off the
wax. In order to arrive at an
optimal pigging frequency, the growth rate of wax deposition needs to be accurately determined. Wax deposition models can serve as predictive tools for
estimating the growth rate of wax deposition in subsea pipelines.

Most
of the current wax deposition models were developed based on the mechanism of
molecular diffusion, which was developed by Probjot et. al. (AIChE J. 2000, 46, 1059–1074) and was highly regarded among wax deposition studies. However, our
recent analyses have shown that another mechanism might play a role in addition to molecular
diffusion contributing to the deposit growth, which is overlooked in most
previous studies: at low oil flow rates (i.e. low shear rates), it is expected that not
only molecular deposition takes place at the oil-deposit interface, but the wax
molecules at the boundary layer might crystallize to form a cross-linked
network and help to further grow the deposit. This gelation phenomenon is
unlikely to occur at high oil flow rates because the high shear stress tends
to break up the crystal network.

The carbon number distribution of the deposit obtained from gas
chromatography is a significant indicator of the role of gelation in the wax deposit
growth. As
the solids in
the deposit consist
of mostly the heavy components, a
higher mass fraction of heavy components in the carbon distribution indicates a
larger amount of solid formed. The difference in the carbon number distribution
between the deposit and the oil (above the WAT) is that the heavy components
are enriched in the deposit due to the diffusion of wax molecules (heavy
alkanes). As the diffusion-driven
formation represents the continuous radial transport of wax molecules
depositing on the interface and diffusing into the gel, its resulting deposit
is expected to contain more solids than a gelation-driven deposit.
Consequently, the deposit formed mainly by diffusion-driven deposition is
expected to have higher mass fractions of heavy alkanes than that formed by gelation.
The overall relationship between the flow rate and carbon number distribution
in GC is shown in Table 1.

Table 1: Influence of decreasing flow rate on the deposit gas chromatography

The GC analyses of the deposits obtained from flow-loop experiments have been
carried out and the results are shown in Figure 1.

Figure
1:
GC analyses of experiments

It is clearly seen that the
carbon number distribution of the experiments can be divided by two groups of
experiments that are formed by two drastically different mechanisms. The
experiments carried out at relatively low flow rates and low wall temperatures
(Group A) show very small amounts of heavy components (low solid fraction) in
the deposit, indicating that the deposit is gelation-driven. The experiments at
relatively high flow rates and high wall temperatures (Group B) show a great
contents of heavy components (high solid fraction), which implies that the
deposit is deposition-driven.

Therefore, a fundamental investigation of the role of wax
gelation in wax deposit growth will be carried. The work will start with several
rheometric experiments to investigate the effect of shear history and cooling rate on gelation temperature.
The goal is to quantitatively describe the gelation phenomena and develop a
theoretical model for gelation kinetics. The finally purpose would be to
incorporate the gelation model into the current wax deposition models to improve the prediction quality.