As
pipeline transportation in the oil and gas industry is moving
to offshore conditions, the prevalent high-pressure and low-temperature
conditions in the subsea flow lines may lead to hydrate formation
and wax precipitation occurring simultaneously. The presence of wax
may alter the interfacial properties and particle interactions, resulting
in the change in hydrate cohesion behavior. In this study, cyclopentane
hydrate cohesive forces are measured with different wax contents using
a micromechanical force (MMF) apparatus. A custom wax sample with
the composition from C17 to C39 was mixed with cyclopentane and used
as the bulk phase. It was found that the cohesive force decreased
with increasing wax content from 0 to 5 wt % then increased with further
wax contents from 5 to 8.75 wt %. Dilution MMF measurements demonstrated
that two competitive mechanisms, the oleophilic effect and reduced
hydrate conversion rate were synergistically responsible for the observed
changes in the cohesive force. In an MMF measurement with 10 wt %
wax and 6 h annealing period, the wax was found to deposit on the
hydrate surface and effectively reduced the cohesive force, indicating
that wax crystals have a potentially inhibiting effect on hydrate
cohesion. Furthermore, the bulk phase/water interfacial tension decreased
with increasing wax contents. Finally, a possible mechanism is presented
to illustrate the effect of wax on the hydrate cohesive force, considering
the oleophilic effect, hydrate conversion, and wax deposition. This
work provides insight into the influencing mechanisms of wax on hydrate
cohesion, which can be useful for flow assurance applications where
both hydrates and waxes are present.
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