Inventory of U.S. greenhouse gas emissions and sinks 1990-2016: Abandoned oil and gas wells
Available Emissions Data
EPA identified and reviewed available emissions data to characterize emissions from abandoned wells, including data from the Kang et al. and Townsend-Small et al. studies.
The Kang et al. 2014 study made direct measurements of methane flow rates from 19 wells in Pennsylvania during 2013 and 2014. The wells were not well-documented in state records, so researchers categorized each studied well as plugged or unplugged based on surface observations. The study did not find significantly different emission rates between the two categories.
Table 1 showing these EFs (in units of grams per hour per well, g/h/well) is reproduced from the Kang et al. 2016 study, with minor edits for clarification in the context of this memo.
The Townsend-Small et al. study measured emissions from 138 abandoned wells in the Powder River Basin in Wyoming, Denver-Julesburg Basin in Colorado, Uintah Basin in Utah, and Appalachian Basin in Ohio, during 2015. Townsend-Small et al. developed EFs for categories observed to exhibit significantly different emissions levels: plugged versus unplugged (including inactive, temporarily abandoned, shut in, dormant, orphaned, and abandoned), and eastern versus western U.S. regions. Table 2 showing these EFs is reproduced from the Townsend-Small et al. study.
Based on analysis of various options (refer to the October 2017 memo) and stakeholder feedback, EPA calculated abandoned well emissions using the plugging status-specific EFs presented in Table 2 and Table 3 above. EPA applied “Appalachian” unplugged, combined and plugged, combined EFs (Table 3) to abandoned wells in Appalachian Basin region states (Ohio, Pennsylvania, West Virginia, New York, Kentucky, and Tennessee), and applied Townsend-Small “entire U.S.” unplugged and plugged EFs to all other abandoned wells. Emission estimates are discussed further and presented in Section 4.
Total Abandoned Well Counts
Estimates in the literature for the total national population of abandoned onshore wells in the U.S. in recent years range from over 2.3 million (Townsend-Small et al. 2016) to approximately 3 million (Brandt et al. 2014).
Based on historical records, EPA estimates approximately 2.56 million wells (characterized as oil, gas, or dry) had been drilled in the U.S. by 1973 (and could be assumed to be producing in 1975 if not shutdown/dry). In 1975, approximately 630,000 oil and gas wells were operating in the U.S., based on USGS estimates7 . Therefore, EPA estimates 1.93 million abandoned wells existed in 1975 (2.56 million – 630,000 = 1.93 million). See Appendix A for additional details on how this estimate was developed. Based on querying the DrillingInfo data set’s key date fields as described above, 776,000 wells in the DrillingInfo database could be considered abandoned as of 1975 (i.e., had stopped reporting production prior to 1975 or been installed prior to 1975 and never reported production). Comparing the counts (i.e. 1.93 million abandoned wells from analysis of historical records and USGS data, and 776,000 abandoned wells in the DrillingInfo database), EPA estimates that 1.15 million abandoned wells in the U.S. are not captured in the DrillingInfo-based methodology. EPA added this 1.15 million abandoned well count to the DrillingInfo-based total to develop a complete count of abandoned wells existing in each year of the time series, as shown in Section 4.
Plugging Status Assignment
EPA considered several data sources to generate the estimated split between plugged and unplugged abandoned well counts for each year.
Townsend-Small et al. offer limited observations that might be considered a “snapshot” of plugging status based on wells encountered for testing in year 2015 in the eastern and western U.S. As shown in Table 2 above, 50 percent of the 12 eastern wells tested were plugged; and 90 percent of the 126 western wells tested were plugged. The Kang et al. 2016 Pennsylvania study observed that of the 88 wells sampled, 40 percent were plugged. For both studies, due to the relatively small sample size compared to the national abandoned wells population and other factors of study design (e.g., certain wells could not be located based on records, and certain wells could not be physically accessed), other data sets or approaches might better represent the national population split between plugged and unplugged for purposes of developing GHGI estimates.
69 percent of abandoned wells are considered unplugged, and 31 percent of abandoned wells plugged, in year 2016. Since this data set likely does not include the oldest wells in the U.S., this value might over-estimate the fraction of the well population that is currently plugged. Because this approach is based on the most recent available state data as compiled by DrillingInfo, it reflects impacts of state- or industry-led plugging efforts (e.g., orphaned well plugging programs8 ), assuming that plugging status is generally kept up to date in state databases.