Page 112 - Handbook Of Multiphase Flow Assurance

108 5. Flow restrictions and blockages in operations

quadruple points where ice, water, hydrate and vapor can coexist, is presented by Sloan
(1990). More rare quintuple points for hydrates also exist such as the one discovered at the
Colorado School of Mines for the coexistence of water, structure I hydrate, structure H hy-
drate, liquid hydrocarbon and vapor five phases at the same condition.
Pure gas hydrate has a relatively low electrical conductivity or high electrical resistivity
which is on the order of 20,000 Ωm (Dunbar, 2013). That property, combined with the speed
of sound different from that of surrounding rock is used to find location and saturation of gas
hydrate deposits in geophysical studies of natural deposits of gas hydrates. Typical electrical
resistivity of a gas hydrate deposit is of the order of 100 Ωm, seabed is 1 Ωm and seawater
0.36 Ωm. The compression wave speed of sound is reported at 3650 m/s, and the shear wave
speed measured simultaneously, is 1890 m/s (Waite et al., 1999).

Stability
Stability of gas hydrates varies with pressure and temperature. In general, hydrates are
more stable at low temperatures and high pressures. At lower temperatures water molecules
have less movement relative to each other. At higher pressure more guest molecules such as
methane are dissolved in water. Hydrogen bonds which hold together the water molecules in
the gas hydrate crystal are stable at lower temperature and at higher pressure.
As an example, a structure II hydrate commonly encountered in subsea production systems
typically would form at approximately 10 atm and 4 °C with fresh water without chemicals.
To make a formed gas hydrate unstable, one or more of the following is required: lower
pressure, higher temperature, fewer hydrogen bonds. Chemicals such as methanol act by
removing hydrogen bonds from the gas hydrate crystal. Methanol does that by making the
hydrogen bonds in water which makes up the gas hydrate crystal switch from hydrate to the
hydroxyl group OH in methanol where electronegative oxygen in methanol attracts elec-
tropositive hydrogens in water.
As 4 °C or 40 °F is a temperature usually found in deep water environment, production sys-
tems may require a depressurization to below 10 bar in order to dissociate any formed hydrate.
In some cases the depressurization needs to be to a lower pressure because some chemicals
such as kinetic hydrate inhibitors (KHI) stabilize hydrates (Makogon and Holditch, 2001b).

Problems related to hydrate formation
Gas hydrate formation creates mainly economic but also safety issues. In onshore produc-
tion systems, both wells and gathering lines get plugged by gas hydrates. In subsea systems,
mainly trees, flowlines and risers get plugged. A blocked well or a flowline can no longer gen-
erate revenue. However, hydrates also have plugged process equipment and flare relief lines
which led to a loss of primary containment and release of hydrocarbons. Hydrates require
four conditions to form: low temperature, high pressure, water and light hydrocarbons such
as gas or live oil. These four conditions can be met in some process operations which leads to
partial or sometimes complete hydrate blockages.
A hydrate plug occupies nearly all cross-section of the pipe. A radiographic image in
Fig. 5.11 shows a hydrate plug in a USA onshore field pipeline.
Partly dissociated gas hydrate particles can also be transported to relief lines from elsewhere
in the plugged line and restrict or block the vent relief line. In one instance (Makogon, 2017)