This document provides information on removing gas hydrate blockages in pipelines safely and efficiently. It discusses various removal methods such as depressurization, heating, inhibitor injection, and changing the gas composition. Depressurization alone may not be effective as it can lower the temperature and reduce dissociation. Heating must start from the ends to avoid sudden pressure increases. Inhibitor injection can shift the hydrate boundary but dilution and temperature drops must be considered. Real case studies demonstrate combined methods like heating tools and inhibitors can more quickly remove blockages.

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World Class Training Solutions
Nightmare of Hydrate Blockages:
How to Remove them Safely & Efficiently
Professor Bahman Tohidi
1 September 2020
World Class Training Solutions
www.petro-teach.com

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• Expert on gas hydrates, flow assurance, PVT,
phase behavior and properties of reservoir fluids and
H2S/CO2-rich systems, production technology and EOR.
• He leads Hydrate, Flow Assurance and Phase Equilibria
Research Group at Institute of Petroleum Engineering,
Heriot-Watt University.
• He is the Director of International Centre for Gas Hydrate
Research and the Centre for Flow Assurance Research (C-
FAR) at Institute of GeoEnergy
• His is a consultant to major oil and service companies.
• Managing Director of “HYDRAFACT LIMITED” a Heriot-
Watt spin-out Company.
• Recipient of “Life Time Achievement” from the 9th
International Conference on gas hydrate, Denver, USA
• Winner of the Queen’s Anniversary Awards in 2015
• He has more than 450 publication, several book chapters
and 13 patents
• His research group work was recognized as one of the top
10 UK examples of the role of Chemical Engineering in
Modern World by the IChemE in 2016.
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Hydrate Structure and Thermodynamics
• The necessary conditions:
• Presence of water or ice
• Suitably sized gas/liquid molecules
(such as C1, C2, C3, C4, CO2, N2, H2S,
etc.)
• Suitable temperature and pressure
conditions
• Temperature and pressure conditions
is a function of gas/liquid and water
compositions.
Hydrate phase boundary
P
T
Hydrates
No Hydrates
Kihara potential for attraction
between molecules

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Gas Hydrate Structures
Gas molecule
(e.g. methane)
Water molecule
‘cage’
Methane,
ethane, carbon
dioxide….
Propane, iso-
butane, natural
gas….
Methane +
neohexane,
methane +
cycloheptane….
512
435663
51262
51264
Structure I
Structure II
Structure H
3
16
2 1
8
6
51268
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Blockage Removal
• Gas hydrate blockage in the pipeline could have some
differences with in-situ hydrates.
• They could be porous and permeable (in particular if they
are formed suddenly) unlike in-situ hydrates.
• During their formation some free water could have been
trapped between hydrate crystals.
• If flow is blocked (or for long pipelines), plug temperature
decreases to ambient temperature.
Micromodel picture
== 100 micron

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Blockage Removal
• Gas hydrate blockage in the pipeline has some differences with
in-situ hydrates.
• They are porous and permeable unlike in-situ hydrates
• They transfer pressure but limited in the transfer of flow
• During their formation some free water have been trapped
between hydrate crystals (blue sections in the figures below)
• With time, plug porosity is reduced and plug hardness is
increased (white sections in the figures below)
t=31 hrs, P=54.9 bar, T=3.4 ºC t=94 hrs, P=53.0 bar, T=3.2 ºCt=142 hrs, P=52.1 bar, T=3.2 ºC
Hydrate Blockage Removal
• It should be noted that hydrate blockages are case
specific and each case should be investigated on its
own merits.
• General guidelines
• Depressurisation (one sided or two sided)
• Heating
• Injection of thermodynamic inhibitors
• Changing the gas composition
• Combinations of the above

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Blockage Removal Through Depressurisation
• The idea is to reduce the system pressure and
come out hydrate zone, and dissociate hydrates
• A common misconception is that
depressurisation alone can cause hydrate
dissociation
• In-fact, when the system is depressurised, some
hydrates remove heat from surrounding and
dissociate, resulting in a reduction in system
temperature.
• This reduction in the temperature will reduce or
eliminate the driving for dissociation
Blockage Removal Through Depressurisation
The objective is
to move the
system outside
hydrate stability
zone.
Constant
pressure systems
Pressure
Temperature
No Hydrates
Hydrates
Lw-H-V
Initial pressure
conditions
Final pressure
conditions
273 Ambient tempHydrate temp
Driving force for heat transfer and hydrate dissociation

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Blockage Removal Through Depressurisation
Constant volume
systems (i.e., if the
valve is closed after
initial
depressurisation)
A combination of
pressure increase and
temperature
reduction will occur
Pressure
Temperature
No Hydrates
Hydrates
Lw-H-V
Initial
conditions
Final
conditions
273
Blockage Removal Through Depressurisation
Multi-stage
pressure
reduction.
The system
temperature
could drop to
below zero and
ice could form.
Pressure
Temperature
No Hydrates
Hydrates
Lw-H-V
Initial
conditions
Final
conditions
273
Water resulting from hydrate dissociation can form ice, which works as
an insulator, reducing heat transfer and hydrate dissociation.

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Blockage Removal Through Depressurisation
• The thermal gradient (between the ambient and
hydrates) will result in heat flow through pipe-
wall.
• The second misconception is that during
depressurisation, the hydrate plug dissociate at
its end(s).
• In fact although the initial plug dissociation is at
its ends, the hydrate plug will dissociate radially
resulting in plug dislodge.
Pipeline Hydrate Plug
P1 P2
HEAT
Pipeline Hydrate Plug
HEAT
P1 P2
If P1 >> P2 ( or in case
of multiple plugs)
Blockage Removal Through Depressurisation

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Blockage Removal Through Depressurisation
• The thermal gradient will result in heat flow through pipe-
wall.
• The second misconception is that during depressurisation,
the hydrate plug dissociate at its end(s).
• In fact although the initial plug dissociation is at its ends,
the rest of hydrate plug will dissociate radially resulting in
plug dislodge.
After 1 Hour After 3 HoursAfter 2 Hours
Pictures courtesy of CSM, Prof Dendy Sloan
Blockage Removal Through Depressurisation
• Depressurisation from one end or both
ends
• Projectile (less in both end depressurisation)
• How much DP is safe?
• Ice formation, if the temperature drops to
below ice point
• The problem with ice formation
• Low heat transfer
• Forming a protective layer on hydrates
• Does not respond to pressure reduction. Ice will
dissociate on temperature increase not pressure
reduction

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A hydrate plug moves down a flowline
at very high velocites.
Where the pipe bends, the hydrate plug can rupture
the flowline through projectile impact.
Ahydrate plug moves
down a flowline at very
high velocites.
Closed Valve
Closed ValveIf the velocity is high enough, the
momentumof the plug can cause pressures
large enough to rupture the flowline.
Safety Hazards of Moving Hydrate Plugs
(From Chevron Canada Resources, 1992)
a) b)
Courtesy of CSM, Prof Dendy Sloan
Safety Models Could Prevent Accidents
Chevron
Hydrate
Accident
Pupstream Pdownstrea
m
Newton’s Laws of Motion describe the position, velocity & acceleration of the plug
DP
friction
Courtesy of CSM,
Prof Dendy Sloan

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Blockage Removal Through Heating
The objective is to
move the system
outside hydrate
stability zone.
The system could
be in Lw-H-V or H-V
equilibria. Pressure
Temperature
No Hydrates
Hydrates
Lw-H-V
Initial
conditions
Final
conditions
Blockage Removal Through Heating
It is often difficult to locate the end of a pipeline
hydrate plug to begin heating.
Heat must be supplied with caution, beginning from
the end and progressing toward the middle of the
plug.
If a hydrate plug is dissociated in the middle, the
pressure might increase suddenly, resulting in
equipment failure, blowouts, or hydrate projectiles
in pipelines.

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Blockage Removal Through Heating
HEA
T
HEA
T
HEA
T
BURST
Pipeline Hydrate Plug
P
Pressure could be very high, could be as high as 800 bar
• Pipefitter attempted to remove hydrate plug by heating
exposed pipeline with torch.
• Gas pressure from dissociated mid-hydrate plug rose
rapidly, confined by plug ends.
• Pipeline exploded: In resulting fire, one man died and
four others badly injured.
Siberian Incident February 2000
Courtesy of CSM, Prof Dendy Sloan

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Blockage Removal Through Inhibitor Injection
• Inhibitor injection will shift the hydrate phase
boundary to the left, which could result in gas
hydrate dissociation.
• However, gas hydrate dissociation will produce fresh
water reducing the concentration of the inhibitor.
• Also gas hydrate dissociation will result in the release
of gas (possible pressure increase) and a reduction in
system temperature.
Blockage Removal Through Inhibitor Injection
Pressure
Temperature
No Hydrates
Hydrates
Lw-H-V
Initial
Inhibitor
Injection
Initial Hydrate
Phase Boundary
Gas released from
hydrate dissociation in
a closed system
Temperature reduction
due to hydrate
dissociation

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Blockage Removal Through Inhibitor Injection
Pressure
Temperature
No Hydrates
Hydrates
Lw-H-V
Initial
Inhibitor
Injection
Inhibitor
Dilution
Initial
Hydrate
Phase
Boundary
MEG for Melting Hydrates

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Blockage Removal Through Inhibitor Injection
• Inhibitor’s density, viscosity, and vapour pressure
play important roles in the effectiveness of inhibitor
injection.
• Temperature reduction (e.g., due to ambient
temperature and/or temperature reduction due to
hydrate dissociation) could result in significant
increase in inhibitor’s viscosity.
• High vapour pressure inhibitors (e.g., methanol) are
effective for vapour phase transport (e.g., pipelines,
or gas processing columns).
Blockage Removal Through Inhibitor Injection
• High density inhibitor are more effective in plug
removal in wellbore.
• Sometime a mixture of inhibitors are more
efficient, base on our experiments.
• Inhibitor delivery by mechanical means (coiled
tubing) could improve the rate of gas hydrate
dissociation.
• Inhibitor injection could be combined with other
techniques (heating, depressurisation, milling,
etc.).

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Changing the Gas Composition
H2S
CO2
C2
C1
C3
i-C4
N2
Hydrate Stability of Pure Compounds (HydraFLASH)
Changing the Gas Composition
H2S
CO2
C2
C1
C3
i-C4
N2
Hydrate Stability of Pure Compounds (HydraFLASH)

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Blockage Removal by Changing Gas
Composition
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Effect of Heat and Insulation
• Hydrate formation is exothermic and its
dissociation is endothermic, similar to ice formation
and melting
• However, the amount of heat requirement is much
higher than ice, due to presence of van der Waals
forces between gas and water molecules
• The amount heat requirement depends on type of
gas, cavity occupation, and hydrate structure
• Insulation; “to be or not to be”
• It will delay hydrate formation and dissociation
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Effect of Heat
• Calculate the system temperature if 10% of the
water is converted into ice at -10 °C
• Latent heat of water to ice = 6.01 kJ/mole water
• Specific heat capacity, ice: 2.108 kJ/kg.K
• Specific heat capacity, water: 4.187 kJ/kg.K
Effect of Heat
• 100 moles water
• 10 moles are converted into ice
• Heat released = (10 moles water)x(6.01 kJ/mole water)
= 60.1 kJ
• Amount of water and ice=(100 moles)x(18
g/mole)=1800 g=1.8 kg=(0.18 kg)ice+(1.62 kg)water
• Q=miCpiDT+mwCpwDT
• 60.1 kJ = (0.18 kg)x(2.108 kJ/kgK)xDT+(1.62 kg)x(4.187
kJ/kgK)xDT
• DT=60.1/(7.16)=8.39 K
• T=-10+8.39=-1.61 °C

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Heat Released on Hydrate
Formation
• Calculate the heat released if in the previous
example 10% of water is converted into hydrates,
assume methane hydrates
• 100 moles water
• 10 moles are converted into hydrates
• Heat released = (10 moles water)x(9.03 kJ/mole water) = 90.3 kJ
Effect of Heat
• Ignoring the effect of encaged gas, assume molecular
mass and specific heat of hydrates are the same as ice,
calculate the final temperature
• Specific heat capacity, ice: 2.108 kJ/kg.K
• Specific heat capacity, water: 4.187 kJ/kg.K
• Amount of water and hydrates=(100 moles)x(18
g/mole)=1,800 g=1.8 kg=(0.18 kg)hyd+(1.62 kg)water
• Q=mhCphDT+mwCpwDT
• 90.3 kJ = (0.18 kg)x(2.108 kJ/kgK)xDT+(1.62 kg)x(4.187
kJ/kgK)xDT
• DT=90.3/(7.16)=12.61 K
• T=-10+12.61=+2.61 °C

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Time Required to Remove a
Blockage
37
• Amount of hydrate = Internal
volume
• Heat requirement for melting
hydrate can be calculated
• W=UA(DT)
• Knowing the overall heat transfer
coefficient (U) and DT, one can
calculate time required for melting
hydrate blockage
We have developed devices that can monitor rate of hydrate melting and when
it is completed.
Hydrate
Plug
Blockage Removal Through Depressurisation
Hydrate gun tests!!!
Plug velocity could reach 200 m/s after
travelling 7 metres!!!
If wellhead pressure is reduced to 10
bar, the movement will result in a
pressure of 400 bar at the wellhead.
If the pressure is dropped to 1 bar, the
wellhead pressure will increase to 4800
bar as a result of plug movement.
Gas
Liquid
200 bar
100 m
100 m

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Blockage Removal
• As discussed before, hydrate blockage changes
with time.
• Initial hydrate plugs are generally soft and porous,
while with time the plug become hard.
• Procedure should be in-place to tackle hydrate
plugs ASAP.
• While patience is important, combined methods
could reduce the time requirement significantly.
• Coiled tubing could be very efficient, in particular if
combined with heating (expect some cases)
Blockage Removal, Case Studies
• Offshore gas well
• Gas hydrates formed due to leakage in SSSV
• MEG injection: 30 metres melted in 30 days
• Heating tool: 20 metres melted in 2.5 days
• MEG + MeOH performance improved.
• Blockage removed after 5 months.

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Blockage Removal, Case Studies
• Offshore gas well, 70-100 m hydrate plug
• MEG + depressurisation: 1 metre/day
• MEG + shut-in: 0.15 metre/day
• Heating tool: 8 metre/day
• 2/1 MEG/MeOH solution was used for the rest of
operations
• Plug removed after 6.5 months
Blockage Removal, Case Studies
• Hydrates formed at the start of water injection in
WAG (Water Alternating Gas Injection)
• MeOH + depressurisation to remove hydrate in
water injection manifold.
• MEG + depressurisation for two weeks
• Heated TEG + milling: 5 metre/hour
• Heated TEG without milling: 0.3 m/hour
• Milling: 3 metre/hour

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Summary
• There are various options in removing hydrate
blockage
• Choosing an option depends on the system
• Heat is required for melting hydrates, so one needs
to be patient
• Plug movement is a serious problem and DP
depend on the system and location of blockage
• We can calculate time required for hydrate melting
• There are device that can monitor hydrate melting,
saving time and chemicals
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Thank you
Any question?
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