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

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

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

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

Pressure drop at choke Hydrates can form

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

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

Glycol injection ports marked by arrows

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)

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

Formation of convection currents Pressure Drop occurs HERE

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)

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

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

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

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

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