Q446. How are greenhouse gas emissions calculated for subpart W (petroleum and natural gas systems) facilities?
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A446. Under this rule, facilities will detect, as applicable, and calculate GHG emissions according to the specified quantification methods. See the table below for a summary of the calculation methodologies by source type. For source types where subpart W allows new calculation method types beginning in RY2024, the methodology type is designated as “RY24” in the table. Where the amended subpart W allows the use of a particular calculation methodology to be more broadly applied beginning in RY 2024, the methodology is designated with “XE” in the table. For example, the engineering calculations methodology for dehydrators is designated as XE because beginning in RY 2024 reporters may use modeling software (as an alternative to the currently required population emission factor) to calculate emissions from glycol dehydrators with a throughput less than 0.4 million standard cubic feet (MMscf) per day as well as those with a throughput greater than or equal to 0.4 MMscf per day. The available calculation method types for source types that are subject to reporting beginning with RY 2025 are designated as “RY25” in the table. Where volumetric emissions are measured, mass emissions of CO2 and CH4 will be estimated based on the annual mole fraction and density of each GHG.
- The engineering calculation methods use monitored process operating parameters and either software models, engineering calculations, or development and use of site-specific emission factors.
- For emissions detection, the rule allows the use of optical gas imaging instruments, organic vapor analyzers (OVA), toxic vapor analyzers (TVA), and infrared laser beam illuminated instruments or acoustic leak detection instruments for accessible components. For inaccessible components, reporters must use an optical gas imaging instrument.
- Direct measurement involves the use of the high-volume sampler; or calibrated bagging; or rotameters, turbine meters, or other meters, as appropriate, depending on the individual component for emissions measurement.
- For the use of leaker factors, the relevant emission factors will be applied to leaking components determined by using an applicable instrument and applying leaker factors. For the use of population factors, the relevant emission factor will be applied to all components.
| Source Type | Engineering Calculations | Direct Measurement | Leak Detection and Leaker Emission Factor | Equipment Count and Population Emission Factor |
| Natural gas pneumatic device venting | RY24 | RY24 | RY24* | X |
| Natural gas driven pneumatic pump venting | RY24 | RY24 | X | |
| Acid gas removal vent stack | XE | X | ||
| Nitrogen removal unit vents | RY25 | RY25 | ||
| Dehydrator vent stacks | XE | X | ||
| Well venting for liquids unloading | X | X | ||
| Gas well venting during well completions with hydraulic fracturing | X | X | ||
| Gas well venting during well workovers with hydraulic fracturing | X | X | ||
| Oil well venting during well completions with hydraulic fracturing | X | |||
| Oil well venting during well workovers with hydraulic fracturing | X | |||
| Gas well venting during well completions without hydraulic fracturing | X | |||
| Gas well venting during well workovers without hydraulic fracturing | X | |||
| Blowdown vent stacks | X | X | ||
| Hydrocarbon liquids atmospheric storage tanks | XE | X | ||
| Produced water atmospheric storage tanks | RY25 | RY25 | ||
| Condensate storage tanks (Transmission storage tanks prior to RY25) | X | |||
| Well testing venting and flaring | X | |||
| Associated natural gas venting and flaring | X | RY24 | ||
| Flare stacks | X | X | X | |
| Centrifugal compressor venting | X | X | X* | |
| Reciprocating compressor venting | X | X | X* | |
| Emissions from equipment leaks | RY24 | RY24 | X** | X*** |
| Above ground meters and regulators at city gate station equipment leaks | X | |||
| Below ground meter and regulator station equipment leaks | X | |||
| Pipeline main equipment leaks | X | |||
| Service line equipment leaks | X | |||
| Offshore petroleum and natural gas venting | ||||
| EOR injection pump blowdown | X | |||
| EOR hydrocarbon liquids dissolved CO2 | X | |||
| Other large release events | RY25 | RY25 | ||
| Stationary and portable combustion emissions | XE | XE | X | |
| Drilling mud degassing | RY25 | RY25 | ||
| Crankcase vents | RY25 | RY25 |
* Applicable to the Onshore Petroleum and Natural Gas Production and Onshore Petroleum and Natural Gas Gathering and Boosting industry segments.
**Applicable to the following industry segments: Onshore Petroleum and Natural Gas Production, Onshore Petroleum and Natural Gas Gathering and Boosting, Onshore Natural Gas Processing, Onshore Natural Gas Transmission Compression, Underground Natural Gas Storage, LNG Storage, LNG Import and Export, and Natural Gas Distribution. Sources with multiple methods indicate options for monitoring.
*** Applicable to the following industry segments: Onshore Petroleum and Natural Gas Production, Onshore Petroleum and Natural Gas Gathering and Boosting, Underground Natural Gas Storage, LNG Storage, LNG Import and Export, and Natural Gas Distribution. Sources with multiple methods indicate options for monitoring.