Processing of Natural Gas at the Wellhead (Field Processing)

Gas wells produce raw natural gas along with other low molecular weight hydrocarbons, known as "natural gas liquids". These include ethane, propane, butane, iso-butane and pentanes. Certain of these hydrocarbons that are that are liquid or semi-liquid at ambient conditions (i.e., pentane and heavier) may be present in the gas; such wells are called condensate wells and the gas is called "wet" gas. Wet gas requires processing to remove condensable hydrocarbons. Once the liquefiable hydrocarbon portion has been removed from the gas stream (in the field or at a separation plant) the gas is called dry gas. Dry natural gas may also be found directly at the gas well. "Wet" and "dry" gas, either, may require dehydration to remove water or sweetening to remove excessive levels of carbon dioxide (CO2) and hydrogen sulfide (H2S). Field processing involves two main processes:

• Oil and Condensate (light crude oil or liquid hydrocarbons) Removal (separation)
• Water removal

The need and type of process used to separate any associated oil and condensate from natural gas varies based on composition and physical conditions of the natural gas. The most common type is the "heater-treater," which is a combination of a heater, free-water knockout, and condensate and gas separator. The heater helps to break up the condensate and gas emulsion while the separator operates on simple principal of differentials in density. A significant source of air emissions occur when the condensate is transferred from the heater-treater, which operates under pressure, to an atmospheric storage vessel. These emissions are called "flash emissions" and can dwarf breathing and working losses from a condensate storage tank. In addition to removing any condensate from the natural gas, it is also necessary to remove most of the associated water. Most water associated with extracted natural gas is removed by simple separation methods. However, the removal of the water vapor that exists in solution in natural gas requires a more complex treatment. Three main approaches toward dehydration are the use of a liquid or solid desiccant, and refrigeration. Of these three approaches, glycol dehydration is the most common when processing occurs in the field (i.e., at or near the well). With glycol dehydration, a liquid desiccant dehydrator serves to absorb water vapor from the gas stream. The water can be evaporated from the glycol by a process called heat regeneration, and the glycol can then be reused after being put through a specialized boiler designed to vaporize only the water out of the solution.

Potentially Applicable Regulations

Clean Air Act (CAA)

• Condensate Storage
  • 40 CFR Part 60 Subpart A. General provisions, including control device and work practice requirements for flares and compliance with standards and maintenance requirements (maintain air pollution control equipment in a manner consistent with good air pollution control practice for minimizing emissions).
  • 40 CFR Part 63 Subpart A. General provisions, General provisions and compliance with standards and maintenance requirements (maintain air pollution control equipment in a manner consistent with good air pollution control practice for minimizing emissions).
  • 40 CFR 60 Subpart K, Ka or Kb. Standards of Performance for Volatile Organic Liquid Storage Vessels
  • 40 CFR Part 60 Subpart OOOO and Subpart OOOOa. Standards for Performance for Crude Oil and Natural Gas.
  • 40 CFR 63 Subpart HH. National Emission Standards for Hazardous Air Pollutants from Oil and Natural Gas Production Facilities.
  • 40 CFR 98 Subpart W. Petroleum and Natural Gas Greenhouse Gas (GHG) Reporting Rule.
• Dehydration at Well Site
  • 40 CFR Part 63 Subpart A. General provisions, General provisions and compliance with standards and maintenance requirements (maintain air pollution control equipment in a manner consistent with good air pollution control practice for minimizing emissions).
  • 40 CFR 63 Subpart HH. National Emission Standards for Hazardous Air Pollutants from Oil and Natural Gas Production Facilities.
• Gathering Lines
  • 40 CFR Part 60 Subpart A. General provisions, including control device and work practice requirements for flares and compliance with standards and maintenance requirements (maintain air pollution control equipment in a manner consistent with good air pollution control practice for minimizing emissions).
  • 40 CFR Part 63 Subpart A. General provisions, General provisions and compliance with standards and maintenance requirements (maintain air pollution control equipment in a manner consistent with good air pollution control practice for minimizing emissions).
  • 40 CFR Part 60 Subpart OOOO and Subpart OOOOa. Standards for Performance for Crude Oil and Natural Gas 40 CFR 63 Subpart HH. National Emission Standards for Hazardous Air Pollutants from Oil and Natural Gas Production Facilities.
  • 40 CFR 60 Subpart JJJJ. Standards of Performance for Stationary Spark. Ignition Internal Combustion Engines.
  • 40 CFR 60 Subpart GG. Standards of Performance for Stationary Gas Turbines.
  • 40 CFR 60 Subpart KKKK. Standards of Performance for Stationary Combustion Turbines.
  • 40 CFR 63 Subpart ZZZZ. National Emission Standards for Hazardous Air Pollutants for. Stationary Reciprocating Internal Combustion Engines (RICE).
  • 40 CFR 98 Subpart W. Petroleum and Natural Gas Greenhouse Gas (GHG) Reporting Rule.