Oil and gas production processes result in the venting of a portion of the produced natural gas. Vent gas management is a continuing issue for oil and gas operators. Companies must balance permitting requirements, regulatory compliance and corporate responsibility to minimize pollution, waste and liabilities.  Recovery of the vent gas increases revenue to the facility while releasing the gas to the atmosphere or combusting the gas results in air pollution. This makes an understanding of vent gas and a practical management strategy important.

Below is a description of the characteristics of typical vent gas from oil and gas operations and some proven options to minimize air emissions.

 

Sources of Natural Gas Venting

Oil and gas industry VOC emission sources include:

• Storage Tanks – Crude Oil, Condensate and Produced water
• Emergency/Process Vents
• Fugitive Emissions from Equipment Leaks
• Glycol Dehydrators Still Column & Flash Tank
• Natural Gas Pneumatic Devices (e.g., Pressure/Level Controllers)
• Natural Gas Driven Pneumatic Pumps (e.g., Wilden/Aro/Texsteam)
• Reciprocating and Centrifugal Compressor Seals
• Loading/Unloading Facilities – Tank Trucks, Barges
• Amine Gas Sweetening Units
• Stuck Dump Valves
• Well Venting from Well Liquids Unloading
• Facility Blowdown Vent Stacks
• Well Casing Head Gas Venting

Chemical Composition

Typical vent gas from oil and gas processes will be a natural gas that contains hydrocarbons and other inorganic components in varying concentrations. Organic components of natural gas can include:

• Methane C1
• Ethane, C2
• Propane, C3
• Butanes, C4
• Pentanes, C5
• Hexanes, C6
• Heptanes, C7
• Octanes, C8
• Nonanes, C9
• Decanes plus, C10+
• n-hexane
• Benzene
• Toluene
• Ethylbenzene
• Xylenes
• 2,2,4-trimethylpentane

Inorganic components of vent gas include:

• Nitrogen (N2)
• Hydrogen sulfide (H2S)
• Carbon dioxide (CO2)

Ranges for Typical Vent Gas Physical Parameters

Air Pollutant	Molecular Weight (lbs/lb-mole)	VOC Content (mole %)	Energy Content (BTU/SCF)
Storage tank vent gas	25 to 60+	35 to 50+	1500 to 2500+
Sweet field gas, sales gas, blowdown gas	16 to 20+	80 to 98+	900 to 1200+
BTU/SCF = BTU per standard cubic feet
Standard conditions of 60°F and 14.7 psia
Mole % = volume %

 

Air Pollution from Vent Gas

Vent gas can contain the following types of air pollutants:

• Volatile organic compounds (VOCs)
• Hazardous air pollutants (HAPs)
• Greenhouse gases (GHGs)
• Toxic gases

VOCs are defined as nonmethane, nonethane hydrocarbons. VOCs in the atmosphere are a precursor to the formation of smog (ozone).

HAPs of concern for oil and gas operations include:

• N-Hexane
• Benzene
• Toluene
• Ethylbenzene
• Xylenes
• 2,2,4-trimetylpentane

GHGs of concern to oil and gas operations include:

• Methane
• Carbon dioxide (CO2)

Toxic component that may be present in sour gas:

• Hydrogen sulfide (H2S)

Each of these air pollutants may have air permit and State or Federal emission standards that must be met on an ongoing basis. At this time there are no limitations on carbon dioxide emissions from oil and gas operations.

Reducing Venting Emissions

Using a systematic method to reduce venting emissions ensure greater success. The following is HY-BON/EDI’s IQR proven method to reduce venting emissions from an oil and gas facility.

1. Identify vent gas emission sources of concern during a site visit. This service uses visual observations and optical gas imaging (OGI) camera.
2. Quantify vent gas emissions using direct measurement with a meter.
3. Rectify the emissions by recovering the vent gas, combusting the vent gas or fixing leaks.

Direct measurement of vent gas yields quantitative data on the flowrate and chemical makeup of the gas. This data is used to determine vent gas monetary value and mass (lbs/hr). Direct measurement data is input for sizing the emissions controls (e.g., VRU) that can be used.

Reasons to Recover Vent Gas

1. Recover valuable natural gas product
2. Reduce air emissions
3. Compliance with State and Federal (NSPS OOOO/OOOOa) regulations and air permits
4. Reduce liabilities

Vent gas from storage tanks contains natural gas that has a higher energy content (BTU/SCF) than separator/sales gas. Based on the energy content, vent gas with BTU values of 1500 to 2500 can have a monetary value of 1.5 to 2.5 times the value of a typical pipeline natural gas that contains around 1000 BTU/SCF. Sending vent gas to the sales pipeline shortens the payout time for the VRU.

Vent Gas Recovery

A properly sized and designed vapor recovery unit (VRU) for the application can reliably recover vent gas. The added benefit is that this dramatically reduces venting and flaring of natural gas. Also, using VRUs as process equipment can reduce storage tank VOC potential to emit (PTE) to ensure compliance with NSPS OOOO/OOOOa and various State VOC regulations.

Vent Gas Combustion

Vent gas that is not economic to recover can be combusted in an enclosed combustors (ECD) or using an open-tipped pipe flare. ECDs can have destruction efficiencies that exceed 99%. Pipe flares are often permitted in air permits using a 98% destruction efficiency. Enclosed combustors (ECD) are often used for aesthetic reasons since the flame is not visible to the public.

Storage Tank Leak Monitoring

Facilities should ensure that thief hatches are closed and sealed when not used to gauge/sample storage tank contents. Leaking hatches from faulty gasket seals or hatches that are not closed properly will reduce the capture/control efficiency of VRUs, flares and enclosed combustors (VCU).

Cimarron Services and Products

Cimarron is a primary solution provider that engineers and manufactures production, process, and environmental equipment for the upstream, midstream and downstream energy industries. Cimarron also engineers and provides environmental control solutions for biogas at wastewater facilities, digester tanks, landfills or breweries.

As a long-standing industry supplier with international market reach, Cimarron brings decades of engineering, design, manufacturing and service experience to its customers with product capabilities including: separation, treating, conditioning, and environmental compliance. As a process systems specialist, Cimarron ensures its customers prepare and transport oil and natural gas for end use, while treating and disposing of water and vapor by-products in a safe and environmentally-conscious manner.