1	PETE 310 Lectures # 32 & # 33 Chapter 17
2	Gas Hydrates Prediction & Control
3	Basics Of Gas Hydrates General Description Crystal Structure Thermodynamic inhibition Hydrate Phase Behavior And Inhibition Kinetic inhibition Prediction of the hydrate phase behavior Remedial Actions
4	Hydrates Definition “Natural gas hydrates are ice-like structures composed of water and natural gas molecules. Under favorable conditions of high pressure and low temperature, water molecules form cages which encapsulate gas molecules inside a hydrogen-bonded solid lattice”
5	Why Study Gas Hydrates? Hydrates have potential as a future energy resource Related to climate change Affect strength of sediments in which they are found (care in constructing underwater structures) Hydrates currently cause blocking in some underwater natural gas pipelines Hydrates may be an alternative to pipeline transmission as a way to move natural gas from deep water to the terminals of existing offshore pipelines
6	Hydrates Occurrence in Petroleum Engineering Operations Production More common, est. $500 million just for inhibition in offshore pipelines Drilling Typically during well control situations external to BOP at seafloor Drill Stem Testing operations Typically in deeper water, but definitely possible in shallow water operations
7	Serious Effects of Gas Hydrates in Drilling Operations Plugging of choke and kill lines Formation of a plug at or below BOP, preventing monitoring of pressures below BOP Plugging tubing, downhole tools and wireline during DST operation Free water tied up with hydrates can cause thickening of the mud
8	World Wide Locations of Natural Methane Hydrate
9	Storage Capacity of Hydrates
10	A source of clean burning fuel…?
11	Natural Gas Hydrate on the Sea Floor
12	Gas hydrates may cause landslides on the continental slope
13	Pressure, Temperature Profiles – Sea Water
14	Crystal Structure Cavities, guest, host
15	Crystal Structure of Gas Hydrates Gas Clathrates are crystalline compounds that occur when water forms a cage-like structure around smaller guest molecules.
16	Definitions Host - water molecules Guest - gas molecules
17	Methane Hydrate Molecular Structure
18	Hydrates vs Ice Different dielectric constant Different thermal conductivity (2.23 W/m-K à ice, vs 0.5 W/m-K hydrate)
19	Hydrate Forming Conditions Hydrates can form when 4 ingredients are present: free water natural gas (N₂, H₂S, CO₂, C₁, C₂, C₃, iC₄) reduced temperature increased pressure
20	Elements Necessary for Hydrate Formation
21	Hydrate Modeling How much hydrate forms and when?
22	Hydrate Modeling Three-phase V-L-S equilibria Need models for fugacity coefficients (solid, liquid,gas) Hydrate formation curves f(P,T,composition) à saturation boundary
23	Hydrate vs No-Hydrate
24	Hydrate Equipment
25	Typical Hydrate PT Curves
26	Typical Gas Compositions (mol %)
27	Light Components Effect
28	Inhibition/ Dissociation of Hydrates Remove One Component i.e. Water, Gas IncreaseTemperature Decrease System Pressure Use an Inhibitor in the Water Phase (Thermodynamic)
29	Remove one of the components needed for hydrates to form
30	Hydrates: Remedies Once hydrates have formed what can be done to remove them? reduce pressure increase temperature chemical (thermodynamic) inhibition kinetic inhibitors mechanical removal
31	Remedial Actions (Hydrate melting schemes ) Mechanical Depressurization Chemical Thermal
32	Hydrates: Prevention Remove any of the 4 ingredients Thermodynamic inhibitors electrolytes (salts) form ionic bonds with free water polar compounds (alcohols, glycols) compete with hydrates for hydrogen bonding
33	Thermodynamic Inhibitors Salts Alcohols Glycols
34	Hydrates: Prevention Salts Normally sodium chloride, 20-24% by wt. Potassium chloride can be used but it is significantly more expensive, and saturated KCl muds have performed poorly in offshore environments Calcium Chloride, very expensive and not as effective as NaCl for hydrate suppression
35	Salt Inhibitors Salt Ionizes In Solution And Interacts With The Dipoles Of The Water Molecules And Causes Clustering This Clustering Also Causes A Decrease In The Solubility Of Potential Hydrate Guest Molecules In The Water These Combine To Require Substantially More Subcooling To Cause Hydrates To Form Examples: Sodium Chloride And Calcium Chloride.
36	Alcohol Inhibitors The Hydroxyl Group Hydrogen Bonds The Water Molecules. In Direct Competition With The Dissolved Apolar Molecules Inhibition Ability - Decreases With Volatility Examples: Methanol Ethanol Isopropanol
37	Glycol Inhibitors More Hydrogen Bonding Opportunity With Water Through One More Hydroxyl Group Than Alcohols Glycols Generally Have Higher Molecular Weights Which Inhibit Volatility Examples Ethylene Glycol Triethylene Glycol
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39	Thermodynamic Inhibitors Summary Glycols Alcohols Salts Alcohols & Glycols when dissolved in aqueous solutions form hydrogen bond with the water molecules and make it difficult for the water molecules to participate in the hydrate structure.
40	Effect of NaCl
41	Effect of Methanol
42	Hydrate Inhibitors Salts - Effectiveness NaCl > KCl > CaCl₂ > NaBr > Na Formate > Ca Nitrate
43	Typical Sample Formulations Equilibrium DT F° psi 20% NaCl + 10% Aqua-Col 45 5360 36.5 20% NaCl + 10% HF-100 44.9 5067 37.1 20% KCl 66.1 5580 16.7 10% KCl + 10% Aqua-Col 71.2 5245 11.1 10% KCl + 10% HF-100 70.2 5460 12.4 10% KCl + 10% NaCl 61.6 4936 20.2 10% KCl + 10% NaCl + 10% Aqua-Col 51.0 4825 30.6 10% Aqua-Col 79.2 5500 3.5 20% NaFormate + 10% Aqua-Col 50.6 4570 30.5 Seawater 80 5500 2.7
44	Inhibition of Hydrates
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47	Hydrate References (with full articles)
48	Hydrate Forming Gases For a given T Hydrate formation P increases as HC size decreases
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57	Hydrates Summary Lots of research issues to pursue Be aware of hydrates Be prepared to prevent hydrate problems materials procedures contingencies site/job investigation