GHGRP Background
EPA launched the GHGRP for petroleum and natural gas systems under 40 CFR Part 98, Subpart W in 2010. This program requires certain operators to calculate and report their GHG emissions. Last May, EPA expanded the list of emission sources requiring reporting under this rule. Drilling mud degassing operations now fall under the updated regulation.
Drilling mud is a fluid used during the pre-production portion of a well’s life. The fluid lubricates the drill bit, transports cuttings to the surface, and stabilizes the borehole. Drill rigs typically use one of three types of drilling mud: oil-based (diesel), water-based (freshwater or brines), or synthetic (refined fluid compound). Drilling mud is pumped into the wellbore and is continuously circulated to remove solids and stabilize the borehole.
Methane is the primary GHG emitted from this source type. During drilling, gas released from rock mixes with the drilling mud and is circulated out of the wellbore. In addition to methane, the gas entrained in the mud can contain volatile organic compounds (VOC) and carbon dioxide (CO2). Drilling mud is routed to a gas separator which can be sent to a control device or vented directly to the atmosphere. Most of the entrained gas will flash out in these separators with the remaining gas breaking out in cutting traps.
GHGRP Subpart W Updates
Onshore petroleum and natural gas production facilities with an active drilling program will be affected by the newly introduced drilling mud degassing source type. EPA has offered operators the opportunity to develop representative methane emissions estimates or conservative emission factors.
Drilling Mud Degassing Emission Calculation Methodologies
The table below outlines the data collection requirements based on each calculation methodology.
| Data Required | Calculation Methodology 1 | Calculation Methodology 2 | Calculation Methodology 3 |
| Type of Mud | X | X | X |
| Total time that drilling mud is circulated in the representative well in minutes beginning with initial penetration of the first hydrocarbon-bearing zone until drilling mud ceases to be circulated in the wellbore | X | X | X |
| Well ID number (API number) | X | X | X |
| Measured mole fraction of methane in natural gas entrained in the drilling mud | X | | X |
| Average mud circulation rate (gallons per minute) | X | | X |
Approximate total depth below surface
(feet) | X | | X |
| Target hydrocarbon-bearing stratigraphic formation to which the well is drilled | X | | X |
Average concentration of natural gas in the drilling mud as measured by the gas trap
(ppm) | X | | |
| The mole percent of methane gas vented (it may be identical to the average mole fraction of methane in produced gas for the sub-basin, as reported in 98.236(aa)(1)(ii)(I)) | | X | |
Calculation Methodology 1
Methodology 1 involves calculating the drilling mud degassing rate using a representative well and mudlogging measurements. A representative well means a well “...in the same sub-basin (county) and within the equivalent stratigraphic interval (formation).” If a representative well is not available in the same sub-basin and stratigraphic interval, you may select a well within the “facility” drilled into the same formation and stratigraphic interval. Calculation Methodology 1 uses three equations: W-41, W-42, and W-43.
Required data points that need to be collected from representative wells include the following:
Total time that drilling mud is circulated in the representative well in minutes beginning with initial penetration of the first hydrocarbon-bearing zone until drilling mud ceases to be circulated in the wellbore
Average concentration of natural gas in the drilling mud as measured by the gas trap in parts per million
Measured mole fraction of methane in natural gas entrained in the drilling mud
Average mud circulation rate in gallons per minute
Approximate total depth below surface (feet)
Well ID number (API number)
Target hydrocarbon-bearing stratigraphic formation to which the well is drilled
Calculation Methodology 2
Use equation W-44 to calculate drilling mud degassing emissions if you did not take mudlogging measurements (Methodology 2). This uses emission factors based on the type of mud:
Oil-based: 0.0586 MT CH4/Drilling Day
Water-based: 0.2605 MT CH4/Drilling Day
Synthetic-based: 0.0586 MT CH4/Drilling Day
Required data points that need to be collected for every well include the following:
Well ID number
Total number of drilling days
Type of mud used (oil, water, synthetic)
The mole percent of methane gas vented (it may be identical to the average mole fraction of methane in produced gas for the sub-basin, as reported in 98.236(aa)(1)(ii)(I))
Calculation Methodology 3
If mudlogging measurements were taken intermittently from when the wellbore first penetrated the hydrocarbon zone until mud circulation ceased, Calculation Method 1 must be used for the periods when measurements were taken and Method 2 for the periods when they were not (Methodology 3).
Required data points that need to be collected for every well include the following:
Approximate total depth below surface (feet)
Well ID number (API number) or Well ID of representative well
Target hydrocarbon-bearing stratigraphic formation to which the well is drilled
Type of mud used (oil, water, synthetic)
Total time that drilling mud is circulated
Drilling Mud Degassing Emissions Reporting Requirements and Deadlines
Emissions reporting for drilling mud degassing will be required for the first time for calendar year 2025 with reports due by March 31, 2026. In the first year of reporting, you may use measurements from the prior reporting year, if measurements from the current reporting year are not available. EPA-specified data from representative wells is only allowed to be used for two calendar years. EPA allows operators to delay reporting for the “total time drilling mud is circulated” for wildcat or delineation wells.
Questions on Subpart W?
Trihydro's air quality and regulatory specialists can help you manage the complexities of Subpart W compliance. With our expertise in emissions regulations and industry best practices, we can assist you in developing effective strategies for monitoring, repairing leaks, and reporting compliance data.
Contact us to develop your Subpart W compliance strategy.
