1. Natural Gas Production Chapter 4 Hydrate Formation
PTRT 2323
Natural Gas Production
Chapter 4
Hydrate Formation

2. Hydrates Composition Crystals resembling snow sp=0.98 Formation
90% Water
10% Hydrocarbon
Crystals resembling snow
sp=0.98
Float on water
Sink in hydrocarbon liquid
Formation
Water ALWAYS necessary
Turbulence often required as well

3. Formation of Hydrates
Temperature at which hydrates form depends on the composition of the gas
Typical behavior shown in Fig 4.1
Saturated water content (lbs/MMscf) vs T
Many different pressures shown
Dotted line shows probable hydrate formation
1500 psia – hydrates can form at 70 ⁰F
200 psia – hydrate do not form until 39 ⁰F
Saturation value of the gas at any temperature and pressure
100 psia and 60 ⁰F will contain 130 lbs/MMscf at saturation
Same gas at 20 ⁰F will only hold 30 lb/MMscf
Pipeline specs typically require LESS than 7 lb/MMscf to help avoid hydrate formation

4. Contains
130 LBS/MMscf
At 100psia and 60 ⁰F
Pressure Values
Hydrate
Formation
Line
Only 30 LBS/MMscf
At 100psia and 60 ⁰F
Saturated
Water Vapor
Content of Gas

5. Pressure drop at choke
Hydrates can form

6. Ground Temperature Effects
Movement of gas saturated with water
Temperature drops at choke due to pressure decrease
Further temperature decreases can occur in gathering system
Data from East Texas
Ground temps vary in different parts of the country
Note: This is exactly why you increase the length of pipe in the line heater

7. Controlling Hydrate Formation
Hydrate inhibitors
Additives that reduce the freezing temperature of water vapor
Ammonia
Brines
Glycol
Methanol
Low volume injection pumps used to feed the system with inhibitor fluid
Glycol and methanol most common
Used when cost of heater is not warranted by infrequent risk of freezing

8. Glycol injection ports marked by arrows

9. Flow-line Heaters
Primary means of protection from hydrates is dehydration (Chapter 5)
In many field applications dehydration is too expensive
Line heaters are a cost effective alternative
Low capital cost
Fuel is easily available (runs off well gas)
Controls allow operation with minimal attention (lease operator)

10. Indirect Heaters w/water bath
Flow lines run at pressures as high as 10,000 psig but are then transferred to pipelines operating at 1200 psig.
Pressure drop results in temperature decrease “natural refrigeration”
Line heater has 3 basic parts
Heater shell
Fire tube/burner assembly
Removable coil assembly

11. Formation of convection currents
Pressure Drop occurs HERE

12. Thermosiphon Baffle Two distinct advantages
Circulating water carries heated water away from the fire tube more rapidly. This prevents steam generation and scaling
Overall heat transfer efficiency is improved in the upper section.
Both are the result of avoiding stagnant water (boundary layers)

13. Long-nosed Choke
Used to ensure that pressure drop occurs INSIDE the heated zone
Avoid hydrate formation and plugging

14. Field Gas as Fuel Supply
Pre-heated
Pressure regulator (25-40 psig)
Fuel gas scrubber
Strainer
Control valve (thermostatically controlled)
Low-pressure regulator
Burner assembly (pilot supplied gas upstream of control valve)
Main gas valve shuts in if pilot flame fails

16. Corrosion and Scaling
Whenever untreated water is heated and cooled the chance of scale and corrosion is present
As water cools it can’t hold onto dissolved minerals
Dissolved minerals can react with metal structures
Suspended solids can also cause erosion issues

17. Safety Drilling Create a deliberate thin spot on elbows and bends
Turbulence causes erosion in these areas
The drilled areas will leak FIRST in a controlled manner
Provide early warning of corrosion problems

18. Alternate Bath Solutions
CaCl2 solutions
Inhibitor added to prevent fouling of the coils from scale
Prevents water from freezing
Glycol/water solution
Reduces heat transfer (20% lower at 50/50 mix)
Increased boiling temperature (200 ⁰F)
Steam Bath
245 ⁰F (must have ASME stamp)
Molten Salt
600 ⁰F