Prior to 2008, DEQ did not, for the most part, estimate and report to EPA the emissions from the vast majority of oil and gas exploration and production sites. In 2008, EPA promulgated the Air Emissions Reporting Requirements (AERR) which established new thresholds for individual (point) facilities whose emissions must be reported to EPA during each annual and triennial NEI reporting period. 2009 was the first year that the AERR went into effect. 2008 was a triennial NEI year and, for the triennial NEI, states are required to submit more comprehensive emissions data than is required during intervening years. In 2008, a multi-state consortium called CenRAP (Central States Regional Air Partnership) developed a report and a simple tool to estimate nonpoint emissions from well sites and lateral gathering compressor stations using well counts and production data by county. DEQ used this Tool to estimate nonpoint oil and gas emissions for the 2008 NEI. At that time, Oklahoma was one of only a handful of states to estimate emissions from oil and gas well sites and lateral gathering compressor stations.

In an effort to improve the 2008 Tool developed by CenRAP, CenSARA (a companion organization which overlapped with CenRAP) contracted with ENVIRON and ERG (its subcontractor) to develop a better approach using available data sources, surveys of oil and gas industry companies, and input from CenSARA state permitting and emissions inventory groups, yielding the 2011 Oil and Gas Emissions Inventory Project. A key output of that project was an Emissions Calculator Tool developed in Microsoft Excel. This tool included new input factors for major oil and gas basins. These factors were developed using information from survey results from major oil and gas producers and from drilling and production data from an industry database called HPDI.

Prior to 2011, DEQ midstream sources with emissions below the AERR Type A and Type B reporting thresholds were not submitted to the NEI as point sources. (In 2008, an attempt was made to aggregate emissions from permitted midstream oil and gas facilities, with emissions below the AERR reporting thresholds, and to report their emissions as nonpoint sources, but that effort was less systematic than later efforts.)  Because all facilities with DEQ air quality permits are required to submit emissions inventories to the DEQ (as point sources), those data represent a valuable resource for characterizing emissions from these facilities. Common midstream facilities include natural gas compressor stations, production tank batteries, and natural gas liquids extraction plants. To have a more complete estimate of the emissions in the midstream sector, DEQ began submitting midstream facilities below AERR Type A and Type B thresholds to the National Emission Inventory in 2011.

The following resources were developed in support of the data gathering efforts for the 2011 NEI submission:

CenSARA Survey Final Report – Includes detailed descriptions of the methods used to collect the data as well as the equations and approaches used to perform the emissions estimates.

Example Industry Survey Forms – These were sent to the largest oil and gas producers in Oklahoma in the Anadarko and Arkoma Basins.

2011 Oil & Gas Area Source Emissions Calculator Tool for Oklahoma – An Excel spreadsheet which uses activity data by county and basin-specific emission factors to estimate emissions from different types of equipment for each county in Oklahoma.

Concurrent with the work performed under a CenSARA contract, EPA initiated the development of a National Nonpoint Oil and Gas Emissions Estimation Tool (the Tool). The Tool used data gathered for the CenSARA states, additional information provided by other regional and state data collection efforts, and state-specific exploration and production data (well spuds, well counts, oil and gas production, etc.) to estimate emissions from nonpoint oil and gas sectors for the entire country. States were able to either provide their own nonpoint emissions estimates for the oil and gas sector or use the Tool. If a state did not provide its own submission, EPA ran the Tool for the state.

In 2014, DEQ sought to improve oil and gas nonpoint emissions estimates by modifying the existing National Nonpoint Oil and Gas Emissions Tool and by incorporating state point inventory data for permitted well sites. To improve the emissions estimates generated by the Tool, DEQ focused on improving the gas composition factors, estimation of pneumatic device emissions, and the refinement of well site emissions estimates. The Nonpoint Oil and Gas Tool provides gas composition data for major oil and gas basins. DEQ wished to use more specific county level gas composition data in the nonpoint Tool. DEQ used gas composition data from existing permits and permit applications in order to create county level gas composition factors. DEQ substituted these county-specific gas compositions for the broader basin-specific compositions only where at least 10 unique gas composition analyses where available for a given county. For the 2014 NEI submission, DEQ substituted 15 county-specific gas compositions. These counties tended to represent the most active counties with regard to new drilling and new production.

In addition, DEQ incorporated modified pneumatic device counts and bleed rates based on work performed by the Oklahoma Independent Petroleum Association (OIPA) to better refine the emissions from these sources. In 2011 the pneumatic device total VOC emissions were 68,614 tons, which represented 42% of total VOC emissions from the nonpoint oil and gas exploration and production sectors. The 2011 Tool incorporated device counts based on a survey of oil and gas operators in the CenSARA states and pneumatic device bleed rates based on a 1996 study performed for EPA by the Gas Research Institute. OIPA members shared their concerns with DEQ personnel that the data incorporated into the Tool did not accurately reflect current industry practices in Oklahoma. OIPA proposed to conduct a detailed study of oil and gas well sites in the state to update information on equipment types, counts, and operations. After conferring with DEQ personnel, OIPA performed an engineering analysis and an observation of facility operations for pneumatic devices at 172 oil and gas production sites. Based on that work, OIPA generated device counts and bleed rates characteristic of well sites in Oklahoma. With input from OIPA, DEQ staff developed (1) an approach using mean device counts and mean bleed rates to estimate emissions from normally-operating pneumatic devices and (2) a separate approach informed by the OIPA study and work lead by researchers at the University of Texas to estimate emissions from malfunctioning devices (the so-called “heavy tails” of the emissions distribution). The new approach reduced pneumatic device emissions estimates to 37,940 tons in 2014.

In 2014, DEQ began incorporating facility point source inventories from permitted well sites into the nonpoint oil and gas sector emissions data submitted to the NEI. The DEQ point source emissions inventories supplied by oil and gas operators are a more accurate representation of well site emissions than the estimate provided by the Nonpoint Tool. However, the number of permitted well sites represented only a fraction of the total number of wells active in the state. To get a better estimate from the entire universe of active wells, DEQ integrated wellsite emissions data from the DEQ point source inventories with data from the Nonpoint Tool representing emissions from wells without air quality permits. The point inventory data were aggregated by source type and “crosswalked” to nonpoint source categories. (A crosswalk is a translation, of sorts, which correlates point source categories to nearly-equivalent nonpoint source categories of emissions.)  To ensure that there is no double-counting, DEQ uses the American Petroleum Institute (API)/US well number(s) associated with each permitted well site, with assistance from EPA’s contractor (ERG), to reduce activity in the Nonpoint Tool to eliminate activity associated with permitted facilities. To perform this activity subtraction, the API well numbers are sent to EPA so their contractor can subtract the production associated with those wells from the Nonpoint Tool. This allows the Tool to estimate emissions from the remaining unpermitted sources.

The following resources informed the work done to generate the DEQ nonpoint oil and gas submission to the 2014 NEI.

1996 GRI-EPA Study on Pneumatic Device Emissions

Pneumatic Controllers Study (Allen et al.) – University of Texas

OIPA Pneumatic Controller Emissions Study

In 2017, DEQ continued the practices from 2014. Exploration emissions were estimated using the National Nonpoint Exploration Tool without any modifications by Oklahoma DEQ.  For production emissions, DEQ collected point-source-style inventories from over 7,000 permitted well sites. To avoid double counting, activity data associated with the API/US Well numbers at these sites were sent to EPA’s contractor, ERG, for subtraction from the National Nonpoint Oil and Gas Production Emissions Estimation Tool (Nonpoint Tool). Then, DEQ staff used the Nonpoint Tool to estimate the emissions from remaining (unpermitted) well sites.  Emissions data from the permitted well sites (aggregated by point source category and cross-walked to nonpoint source categories) was combined with emissions data generated by the Nonpoint Tool (for the unpermitted well sites) for final submittal to EPA.

Previous work to better characterize the gas composition, pneumatic device emissions, and well types was used again to replace default values in the Nonpoint Tool, as these Oklahoma-specific approaches represent a more accurate picture of this sector for the state.

As in previous NEI cycles, DEQ submitted midstream sources, even those below AERR reporting thresholds, to EPA as point sources. DEQ gathered available data to estimate the percent of lateral/gathering compressor stations that were included in the inventory of permitted sources. A conservative estimate suggests that 28% of lateral/gathering compressor stations in Oklahoma are not accounted for in the inventory of permitted sources.  So, DEQ used the Nonpoint Tool to estimate the emissions of the 28% of lateral/gathering compressor stations not accounted for in the inventory of permitted sources and submitted those emissions to EPA as part of the nonpoint submittal.

In 2020, no major changes to the methodology of emissions estimations occurred. Refer to 2014 and 2017 NEI for methodology details.

DEQ collected point-source-style inventories from over 10,000 permitted oil and gas sites, including over 9,000 well sites. As was done previously, these point-source-style data were aggregated and cross-walked to nonpoint source categories for submittal to EPA.

Although the number of permitted lateral/gathering compressor stations increased from 2017 to 2020, DEQ still felt that the estimate of 28% of lateral/gathering compressor stations being unpermitted was reasonable, and this value was still used for the emissions estimates that came from the Nonpoint Tool.