W8- Gas Treatment & Specifications Flashcards
Well Types
Oil wells: NG called associated gas
Gas wells: NG called non-associated gas
Condensate wells: NG called non-associated gas
What is wet gas?
Methane composition less than 85%, might be out 2-phase region at reservoir, but crosses dew point curve and enters 2-phase region at separator
What is dry (lean) gas?
Contains few/no liquefiable HCs (outside 2-phase region), and minimal hydrate formation risk.
What is Sour Gas?
Contains more than 4ppm H2S (sweet gas for lower concentrations)
What is acid gas?
Contains high composition of acidic gases (CO2/H2S) can be sour/sweet
What are condensates?
Contains HC mixture which leaves well as gas but condensates during production
Natural Gas (NG)
Methane content at 90%, processed to specific composition/energy content
Natural Gas Liquids (NGLs)
mixture of ethane, propane, butane and pentane
Liquefied Petroleum Gas (LPG)
propane-butane mixture compressed to liquid at room temperature
Liquefied Natural Gas (LNG)
NG liquefied by refrigeration (T=162, P=1atm)
Compressed Natural Gas (CNG)
NG compressed at 200bar.
Cheaper vs LNG
NG Quality Specifications
Volume: measured in standard cubic meters (scm) or feed (scf)
Calorific Value: total energy per unit gas produced during combustion
Wobbe Index: measure of fuel gases interchangeability of fuel gases
Methane Number: indicates knock tendency
H2S+S: max 5+10mg respectively
Mercury: extremely corrosive- must be below 0.001ppb
Dew Point: temp at which each gas condenses at pipeline pressure
Particles: No particulate solids/liquids causing erosion/corrosion
Additives: tetrahydrothiophene to give smell for domestic use (leak detection)
NG Treatment Methods
Gas Cooling
Compression
Acid Gas (CO2/H2S) removal
Dehydration
Mercury, Nitrogen or NGLs removal
NG Compression Goals
Gas for export;
Reinjection for well;
Gas lift to improve production;
Provide fuel for gas turbines
NG Compression
Must comply with dry gas specification (no H2O/gas condensed)
Multistage used as gas compression produces heat so gas coolers are needed after each compressor.
Intercooling reduces compressor load
NG Compression Types
Positive Displacement (increase pressure by operating gas in confined space)
Dynamic (high velocity to induce pressure increase)
Hydrates
Crystalline structure stabilised by guest (often HC) molecules.
Form in high P, low T environments
More frequent in start-up/shutdown due to pressure changes.
Controlling Hydrate Formation
High Flowrate (less time for formation)
Pipe Scanning (hydrate plug detection_
Pressure monitoring (high P = hydrate presence)
Use of inhibitors/dehydrators (desiccants)
Advantages/Disadvantages of solid+liquid desiccants (Hydrate)
Solid desiccants: +less affected by P,T, flowrate; -higher capital cost & P drop, high space & weight req.
Liquid desiccants: +lower installation costs and P drop; -glycol susceptible to contamination
Advantages/Disadvantages of Hydrate Inhibitors
Thermodynamic: +more efficient; -safety considerations
Kinetic: +low cost, non-toxic; -limited experience
Antiagglomerants: +low volume req., non-toxic; -limited experience
Glycol-based NG Dehydration
Remove moisture in NG as water can cause problems such as corrosion, freezing, and reduced heating value, which can lead to operational and safety issues
Water Absorption Column Design
Kremser-Brown Equation:
Number of theoretical column stages
N=ln[((yN+1-y0)/(y1-y0))(1-(1/A))+1/A](1/lnA)
Acid Gas Removal
CO2/H2S react with water forming strong acids, which corrode equipment + pipes.
Sweetening: removal H2S or CO2 (reduction of sour character).
Methods: absorption, adsorption, membranes, cryogenic processing
Natural Gas Liquids (NGLs) Removal
Separate CH4 from NGLs (propane, butane, pentane); these heavier HCs (NGLs) are absorbed into an oil using an absorption tower.
Cryogenic processing: gas blown into expansion chamber, so vapour pressure and temp reduced.
Cooling forces NGLs to liquefy while CH4 still gas
