TAMU Pemex Offshore Drilling

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TAMU - PemexOffshore 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 may exceed 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 as high 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 BOPs
• 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 -2oC
• In the deepwater regions of the Gulf of Mexico
  the seabed temperature is typically around 4oC 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
Sea Floor
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Temperatures Profile in the North Sea
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Pressures _at_ 8.6 lb/gal
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(not the GOM)
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Results of a typical hydrate thermodynamic test
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Schematic of constant-volume temperature ramping
experiment
The probability of hydrate formation increases as
you move towards the supercooled temperature
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Hydrate-Related Drilling Problems

• Choke and Kill line plugging
• Plugging of wellbore below the BOPs
• Plug formed around the drillpipe inside casing,
  in the the BOPs or in the riser, preventing
  drillstring movement
• Plug formed in the BOPs preventing full BOP
  closure
• Plug keeping BOPs from opening

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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 may be
  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 - contd

• Adding chemicals that reduce the rate of
  nucleation of hydrate crystals
• Adding chemicals to reduce the rate of growth of
  hydrate crystals which have nucleated
• Adding chemicals that tend to prevent
  agglomeration 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 used inhibitors
  NaCl, KCl and CaCl2
• Saturated NaCl (26) provides a 21 oK 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 the rate 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 hydrate zone and the formation
  of a blockage
• These have not been tested in drilling

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Remedies
Depression of hydrate-formation temperatures with
methanol and diethylene glycol
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Remedies
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Inhibition of hydrate formation temperatures
caused by glycol
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Glycols may experience severe viscosity increases
at cooler temperatures
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Remedies
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Oil-based and synthetic-based muds also require
inhibition, since they contain a water phase
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Remedies
Note that below 3,000 ft water depth, inhibition
with salt alone can not guarantee a hydrate-free
environment
Seawater temperature profile
Effect of gas gravity, mud weight and salt
content on hydrate stability
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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 and BOP,
  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 END