12B. Hydrates
Transcript of 12B. Hydrates
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TAMU - Pemex
Offshore DrillingOffshore Drilling
Lesson 12B
Hydrates
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Lesson 21 - Hydrates What are they?
Why are they important?
Where are they found?
Conditions for existence
Drilling-related problems
Remedies -
Procedures
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Hydrates - What are they?
Gas Hydrates are solids formed
from hydrocarbon gas and liquid
water They resemble wet snow and can
exist at temperatures above the
freezing point of water
They belong to a form of
complexes known as clathrates
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The Burning
Snowball
Methane hydrate
supporting its
own combustion
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Clathrates - What are they?
Clathrates are substances having a lattice-
like structure or appearance in which
molecules of one substance are completely
enclosed within the crystal structure of
another
Hydrates consist of host molecules (water)
forming a lattice structure acting like a
cage, to entrap guest molecules (gas)
LATIN: clathratus means to encage
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Types of Hydrates
The following gases when combined with
water under the right conditions are known
to produce hydrates:
Natural gas molecules ranging from
methane to isobutane Hydrogen sulfide
Carbon dioxide
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Hydrates - Why are they
important?
A very large potential source
of natural gas
A hindrance to the natural gas industry
Often cause plugging of lines and
equipment (like an ice plug)
In drilling, under well control situations,
hydrates may plug lines and chokes
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A Natural Gas Resource?
Conditions for hydrate formation are
satisfied in more than 90% of the ocean
floors, but hydrates will only be present if
there is a source of natural gas and a
structure suitable for gas accumulation
It has been estimated that total worldwide
hydrate resources are as much as 1016 m3,or twice as large the combined fossil fuel
resource.
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A Natural Gas Resource?
Possibly as much as 98% of the hydrate
resource is below the worlds oceans
The remaining 2% that is found on land,
below permafrost, is estimated to be twice
the size of the conventional natural gas
resourse
Natural gas has been produced from
hydrates for decades in Russia
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A Natural Gas Resource?
It is estimated that gas contained in
naturally occurring gas hydrates mayexceed 16 trillion tons of oil equivalent
One cubic foot of hydrate can hold 170
standard cubic feet of gas
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A Problem in the Natural
Gas Industry?
In the 1930s it was discovered that
natural gas hydrates were blocking gas
transmission lines, frequently at
temperatures well above the freezing
point of water
This discovery led to the regulation of the
water content in natural gas pipelines
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A Problem in the Natural
Gas Industry?
It has since been determined that gas
hydrates may exist at temperatures ashigh as 20-30
oC.
As the pressure increases, hydrates can
exist at higher temperatures
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A Problem in Drilling?
Where hydrates are present in-situ in
petroleum reservoirs, they can cause
blowouts if drilled into inadvertently Extreme conditions of temperature and
pressure mean that hydrates may form
during the drilling process if fluids
containing water come into contact with
the reservoir fluids
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A Problem in Drilling?
Formation of solid hydrates can plug up
subsea risers, choke and kill lines, and
BO
Ps Conditions during well shut-in are
particularly favorable for hydrate formation
if high pressures are combined with falling
temperatures and there is sufficient time
for equilibrium to be reached
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A Problem in Drilling?
Water depths near the West Shetlands
and Hebrides rapidly reach 1,000 meters
or more, with seabed temperatures down
to -2o
C In the deepwater regions of the Gulf of
Mexico the seabed temperature is
typically around 4o
C or even lower Such extreme conditions present risks of
hydrate formation
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Where are Hydrates found?
Hydrates are found in situ in the deep
oceans of the world, on the ocean
floor or in the sediments below the
seafloor
Hydrates are found in situ in
permafrost regions
Hydrates are also found in
extraterrestrial environments
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Hydrate-
forming
conditions
for natural
gases
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Temperatures Profile in the Gulf of Mexico
0
2,000
4,000
6,000
8,000
0,000
2,000
4,000
6,000
30 35 40 45 50 55 60 65 70 75 80
T , F
D
h, S Fl
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Temperatures Profile in the North Sea
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Pressures @ 8.6 lb/gal
2
2
2
P u p
p
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(not the GOM)
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Results of a
typical hydratethermodynamic
test
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The probability of hydrate formation increases as
you move towards the supercooled temperature
Schematic ofconstant-volume
temperature
ramping
experiment
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Hydrate-Related Drilling Problems
There are two basic types of hydrate-
related drilling problems:
Drilling through formations already
containing natural hydrates, and
Experiencing drilling conditions that maybe conducive to formation of hydrates
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Techniques for drilling through
Zones containing Hydrates Reduce the temperature of the drilling
mud
Drill at controlled drilling rates (not too
fast - to reduce heat generation rates)
Increase mud weight - if possible
Increase mud circulation rate to ensure
turbulent flow to achieve better cooling
and to remove any gas
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Techniques to avoid Hydrate
Formation while Drilling
Keeping the temperature above, or the
pressure below hydrate formation
conditions
Using chemicals to depress the hydrate
formation point, i.e., use thermodynamic
inhibitors such as methanol, glycols and
salts (methanol is very toxic)
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Techniques for avoiding Hydrate
Formation while Drilling - contdAdding chemicals that reduce the rate of
nucleation of hydrate crystals
Adding chemicals to reduce the rate ofgrowth of hydrate crystals which have
nucleated
Adding chemicals that tend to preventagglomeration of crystals, so that solid
plugs do not form (kinetic inhibitors)
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Thermodynamic Inhibitors
Basically, thermodynamic inhibitors
reduce the temperature at which
hydrates will form
The inhibitor dissolves in the water
phase, increasing the stability of the
liquid water with respect to the hydrate
An inhibitor like methanol will also enter
the gas and liquid hydrocarbons
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Thermodynamic Inhibitors - contd
Salts are the most commonly usedinhibitors: NaCl, KCl and CaCl2
Saturated NaCl (26%) provides a ~21oK
margin relative to pure water
Glycols and glycerols can also be used
Mixed inhibitors can be used and their
effect is approximately additive
20-23% NaCl polymer muds are the most
commonly used for deepwater drilling
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Kinetic Inhibitors
Kinetic inhibitors work by reducing therate of nucleation of hydrates, the growth
rate of the crystals, or the agglomeration
of the crystals They cannot prevent hydrate formation,
but they may increase the delay between
the time when a fluid enters the hydratezone and the formation of a blockage
These have not been tested in drilling
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Remedies
Depression
of hydrate-
formationtemperatures
with
methanol and
diethylene
glycol
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Remedies
Inhibition ofhydrate
formation
temperatures
caused by glycol
Glycols may
experiencesevere viscosity
increases at
cooler
temperatures
0%
20%40%
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Oil-based and synthetic-based muds also require
inhibition since they contain a water phase
Remedies
?
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Note that
below ~3,000 ft
water depth,
inhibition with
salt alone cannot guarantee
a hydrate-free
environment
Remedies
Effect of gas gravity, mud weight and
salt content on hydrate stability
Seawatertemperature
profile
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Well Control Remediation Methods
Prevent hydrocarbons from entering the
wellbore
(adequate mud weight, rapid shut-in)
If hydrocarbons enter the wellbore,prevent them from reaching the wellhead
(monitoring, bullheading)
If hydrocarbons reach the wellhead andBOP, prevent formation of hydrates
(high salinity mud; glycol mud standby)
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Well Control Remediation Methods - contd
If hydrates do form, eliminate them
(methanol on standby for pumping
down kill line, heated seawater ready
to be pumped up riser)
Methods for removing hydrate blockages:
Depressurization to dissociate the hydrate
Addition of chemical inhibitors to melt the
hydrate
External heating to dissociate the hydrate
Mechanical (drilling)
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References
Clathrate Hydrates of Natural Gases, by E.Dendy Sloan, Jr., Marcel Dekker, Inc., New
York,1998.
The Properties of Petroleum Fluids, by William
D. McCain, Jr. PennWell Books, Pennwell
Publishing Company, Tulsa, Oklahoma, 1990.
Controlling, Remediation of fluid hydrates in
deepwater drilling operations, by B.Edmonds,R.A.S. Moorwood and R. Szczepanski,
Ultradeep Engineering, March 2001.
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References - contd
IADC Deepwater Well Control Guidelines.International Association of Drilling
Contractors. Houston, Texas, 1998.
Lab work clarifies gas hydrate formation,
dissociation, by Yuri F. Makogon and Stephen
A. Holditch. Oil & Gas Journal, Feb.5, 2001.
Experiments illustrate hydrate morphology,
kinetics, by Yuri F. Makogon and Stephen A.Holditch.Oil & Gas Journal, Feb.12, 2001.
SPE, OTC...
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THE ENDTHE END