Page 110 - Handbook Of Multiphase Flow Assurance

106 5. Flow restrictions and blockages in operations

4 MPa. The graph illustrates that the constant overpressurization line corresponds to a lower
subcooling at higher pressures, which fits empirical observations for KHI performance. At
lower pressures, higher subcooling may be achieved for a set overpressurization before hy-
drates start to form. At higher pressures, hydrate starts to form at a lower subcooling for a set
overpressurization.
An observation was made in laboratory tests by Makogon and Sarkisyants (1966, p. 36) that
a hydrate formation condition for a multi-component gas mixture differed from the hydrate
dissociation condition. The start of hydrate formation was observed at temperatures lower than
hydrate equilibrium (dissociation) condition by 1–10 °C approximately as read from the graph.
Makogon (1974, 1981) also studied the effect of water preheating to reduce hydrogen bond
structure in water on subcooling required to start hydrate formation. Table 12 and Fig. 37
in this work show that subcooling ranged from 0.6 to 8.3 °C at 127 atm and from 3.0 to 8.2
at 75 atm for methane hydrate. For ethane hydrate, Table 13 in this [1974, 1981] work shows
that subcooling ranged from 1.5 to 8.6 °C depending on pressure. He found that preheating of
water had effect on subcooling, but preheating beyond 30–35 °C gave no extra effect.
This is relevant to flow assurance project design because some operator companies base
the hydrate risk mitigation on a fixed subcooling value. Some companies use a positive sub-
cooling and take a calculated risk by allowing the system to operate inside the hydrate stabil-
ity region by a fixed number of degrees in temperature. Some companies take a conservative
approach and use negative subcooling, designing the system operation to stay away from the
hydrate region by a fixed safety margin of a number of degrees. Overpressurization margin
could be a more appropriate measure for use in designing a production system for hydrate
risk management. Also it is not advisable to rely on positive subcooling or overpressurization
(i.e. to design a system to operate inside a hydrate region) as measured in clean laboratory
conditions because in field operations produced fluids introduce impurities and solid par-
ticles which act as crystal nucleation sites and allow hydrates to start forming more easily,
closer to the equilibrium than in a laboratory.

Chemistry

Gas hydrates are chemically neutral as no chemical reactions occur during their formation
or dissociation, only a fraction of hydrogen bonds between water molecules changes.
As hydrate may consume sour gas components such as CO 2 and H 2 S, the overall pH of the
fluid may change as a result of hydrate formation.
Crystal surface of hydrate may be both electronegative and electropositive as shown in
Fig. 5.9, depending on which level the crystal surface is cleaved at.
sII hydrate {1,1,1} dominant plane is commonly seen in real crystals.
Chemical conformations, bonding energy and active sites can be viewed on crystals with
molecular modeling as shown in Fig. 5.10. This helps compare the effectiveness of chemicals
before lab synthesis and tests.

Thermodynamic features

Thermodynamically the gas hydrates act as solids with low compressibility. A summary
overview of phase diagrams for gas hydrates for various components, including several