Page 122 - Handbook Of Multiphase Flow Assurance

118 5. Flow restrictions and blockages in operations

Upon dissociation, gas evolves into the water layer present on the hydrate crystal surface
as nano-bubbles and micro-bubbles which coalesce into larger gas phase. The importance and
pressure of nano-bubbles of dissolved gas for hydrate formation was described by Makogon
(1996). The significance of nanobubbles and microbubbles during hydrate dissociation was
reported by Uchida et al. (2016a, b, c) who developed acronym MNB or micro- and nano-
bubbles. They used a transmission electron microscope to confirm the existence of MNB after
hydrate dissociation, with most frequently observed size in 200–400 nm range, and Raman
spectrometer to estimate the pressure inside the MNB around 7 ± 5 MPa. The authors also de-
scribed the importance of MNB for the memory effect of gas hydrate recrystallization (Uchida
et al., 2016a, b, c). The industrial application of MNB for wastewater purification and physi-
ological promotion was evaluated with respect to NaCl effect (Uchida et al., 2016a, b, c). The
authors found that low concentrations of NaCl stabilize MNB for at least 1 week, but at salt
concentrations >100 mM the MNB decay faster.

Hydrate blockage remediation
The time to dissociate a hydrate blockage depends on the blockage location and on the
insulation of the pipeline containing the blockage.
As a preliminary estimate, a hydrate plug in an insulated deepwater flowline will take as
many days to fully dissociate by depressurization as is the pipe diameter in inches.
(
time dissociation ( days) ≈ Inside Diameterin ) .
This estimate is based on heat transfer, mass of hydrate and the time it takes for the en-
ergy for dissociation to transfer through the flowline insulation as the majority of deepwater
flowlines are thermally insulated. An operator should plan for at least this many days for a
hydrate removal, plus time for mobilization and demobilization of depressurization equip-
ment. A more accurate analysis for the anticipated time to dissociate a hydrate plug by de-
pressurization can be done with detailed heat transfer analysis.
Hydrate may be dissociated in many ways, but the common practice is to depressurize the
plugged flowline to a pressure below hydrate stability.
Usually the dissociation pressure at the plug location will be below 10 bar in deepwater
conditions because structure II hydrate forms most commonly in oil and gas production, and
its stability pressure is near 10 bar at the typical deepwater temperature of 4 °C.
Initial pressure communication through the plug may start at as early as one tenth of the
time of complete dissociation, provided that pressure at the hydrate remains below its stabil-
ity pressure. Pressure of the hydrostatic head of the liquid column in a deepwater riser must
be taken into account when planning a hydrate remediation project.
Thermodynamics of hydrate dissociation are described to explain this. Water molecules in
a hydrate crystal lattice are held together by hydrogen bonds, just like bricks in a building a
held together by mortar. It takes 5 kcal per mol of water to break the hydrogen bonds. Thus
the process of hydrate dissociation is endothermic or consuming energy.
Energy is required to agitate the bonded water molecules in a hydrate lattice to a point
where the hydrogen bonds begin to break. The energy in a deepwater environment comes
from the cold seawater. Deepwater flowlines are usually insulated with approximately 3 in. of
insulation layer, designed to keep fluids warm during normal produciton. However, during