Abstract
Methane leakage caused by well integrity failure was assessed at 28 abandoned gas wells and 1 oil well in the Netherlands, which have been plugged, cut and buried to below the ground surface (≥3 mbgl). At each location, methane concentrations were thoroughly scanned at the surface. A static chamber setup was used to measure methane flow rates from the surface as well as from 1 m deep holes drilled using a hand auger. An anomalously high flow rate from 1 m depth combined with isotopic confirmation of a thermogenic origin revealed ongoing leakage at 1 of the 29 wells (3.4%), that had gone undetected by surficial measurements. Gas fluxes at the other sites were due to shallow production of biogenic methane. Detailed investigation at the leaking well (MON-02), consisting of 28 flux measurements conducted in a 2 × 2 m grid from holes drilled to 1 and 2 m depth, showed that flux magnitude was spatially heterogeneous and consistently larger at 2 m depth compared to 1 m. Isotopic evidence revealed oxidation accounted for roughly 25% of the decrease in flux towards the surface. The estimated total flux from the well (443 g CH4 hr−1) was calculated by extrapolation of the individual flow rate measurements at 2 m depth and should be considered an indicative value as the validity of the estimate using our approach requires confirmation by modelling and/or experimental studies. Together, our findings show that total methane emissions from leaking gas wells in the Netherlands are likely negligible compared to other sources of anthropogenic methane emissions (e.g. <1% of emissions from the Dutch energy sector). Furthermore, subsurface measurements greatly improve the likelihood of detecting leakage at buried abandoned wells and are therefore essential to accurately assess their greenhouse gas emissions and explosion hazards.
Highlights
Leakage of natural gas from oil and gas wells has been identified as an environmental hazard for several decades (Dusseault et al, 2000; Erno and Schmitz, 1996; Harrison, 1983)
Our findings show that total methane emissions from leaking gas wells in the Netherlands are likely negligible compared to other sources of anthropogenic methane emissions (e.g. b1% of emissions from the Dutch energy sector)
As shown by the results of our study, the occurrence of gas leakage at cut and buried abandoned wells is more missed when relying on surface measurements only, as compared with abandoned wells that still have existing wellhead infrastructure protruding to the surface
Summary
Leakage of natural gas from oil and gas wells has been identified as an environmental hazard for several decades (Dusseault et al, 2000; Erno and Schmitz, 1996; Harrison, 1983). In recent years, elevated levels of thermogenic methane in groundwater have been attributed to wellbore leakage from newer, unconventional oil and gas wells (Darrah et al, 2014; Sherwood et al, 2016). Whether leakage to outside the well occurs depends on the presence of a driving force. This explains why buoyancy driven gas leakage is a more common observation than leakage of liquids (Davies et al, 2014) and that the leakage risk appears to be higher for gas wells than for oil wells (King and King, 2013). Observed failure frequencies vary greatly between different basins, with reported values ranging from 1.9% to up to 75% (Davies et al, 2014)
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