Interferences caused by the microbial methane cycle during the assessment of abandoned oil and gas wells

Abstract. In the global effort to reduce anthropogenic methane emissions, the millions of abandoned oil and gas wells are suspected to be prominent but so far often overlooked methane sources. Recent studies highlighted the hundreds of thousand undocumented abandoned wells in North America as sometimes strong methane emitters with up to several tons of methane per year. However, the majority of studies focused on abandoned wells with their surface installations still in place. Only a few studies examined cut and buried wells as their exact location are often unknown. In Germany, approximately 20,000 abandoned wells are described, which are well documented, and the data is publicly available. Here we present a methodological approach to assess methane emissions from such cut and buried abandoned wells. We sampled eight oil wells in a peat rich setting with four wells in a forest, three wells in an active peat extraction site, and one well on a meadow. All three areas have peat deposits underneath. At each site, we sampled a 30 x 30 m grid and a corresponding 20 x 20 m reference grid. Three of the eight well and reference sites showed net methane emissions. The highest emissions with up to ~110 nmol CH₄ m^–2 s^–1 were observed at one of the reference sites. All three methane-emitting sites were located within the active peat extraction area. Detailed soil gas characterization revealed no methane, ethane, and propane ratio typical for reservoir gas, but instead showed a typical biogenic composition and isotopic signature (mean δ¹³C-CH₄ –63 ‰). Accordingly, the escaping methane did not originate from the abandoned wells or the connected oil reservoir. In addition, isotopic signatures of methane and carbon dioxide suggest that the peat extraction site’s methane was produced by acetoclastic methanogens, whereas methane at the meadow site was from hydrogenotrophic methanogens. However, our genetic analysis showed that both types of methanogens were present at both sites and thus other factors were controlling the prevailing methane production mechanism. Subsequent molecular biological investigations highlighted that aerobic methanotrophs were also important and that they had the highest relative abundance at the peat extraction site. Furthermore, the composition of the methanotrophic community varied across sites and depth. The aerobic methane oxidation rates were highest at the peat extraction site potentially oxidizing a multiple of the emitted methane. Our findings underscore the necessity to combine methane emissions with the characterization of soil gases in comparison with a suitable reference site to survey cut and buried abandoned wells as a solely emission-based surveillance could misinterpret natural occurring emissions.