Impact of an historic underground gas well blowout on the current methane chemistry in a shallow groundwater system Gilian Schouta,b,c,1, Niels Hartogb,c, S. Majid Hassanizadehb, and Jasper Griffioena,d aCopernicus Institute of Sustainable Development, Utrecht University, 3584 CS Utrecht, The Netherlands; bEarth Sciences Department, Utrecht University, 3584 CD Utrecht, The Netherlands; cGeohydrology Unit, KWR Water Cycle Research Institute, 3433 PE Nieuwegein, The Netherlands; and dGeological Survey of the Netherlands, Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek (TNO), 3584 CB Utrecht, The Netherlands Edited by Susan L. Brantley, Pennsylvania State University, University Park, PA, and approved November 27, 2017 (received for review June 27, 2017) Blowouts present a small but genuine risk when drilling into the deep (15) and Aliso Canyon (16) blowouts. In some cases, the pres- subsurface and can have an immediate and significant impact on the sures generated during a blowout do not escape at the surface surrounding environment. Nevertheless, studies that document their but form a fracture network that allows the well to blow out long-term impact are scarce. In 1965, a catastrophic underground underground (17). When these fractures reach the surface, they blowout occurred during the drilling of a gas well in The Netherlands, may negatively impact the chemistry of shallow groundwater by which led to the uncontrolled release of large amounts of natural gas the massive introduction of methane (18). from the reservoir to the surface. In this study, the remaining impact In this study, we investigated the long-term effect of an un- on methane chemistry in the overlying aquifers was investigated. derground blowout on the current methane chemistry in a shallow Methane concentrations higher than 10 mg/L (n = 12) were all found groundwater system. On December 1, 1965, an underground blow- 13 out with a catastrophic outcome occurred near the village of Sleen, to have δ C-CH4 values larger than −30‰, typical of a thermogenic 13 The Netherlands. At a depth of 1,944 m below ground level, serious origin. Both δ C-CH4 and δD-CH4 correspond to the isotopic compo- sition of the gas reservoir. Based on analysis of local groundwater well control issues were experienced while drilling well SLN-02 (Fig. flow conditions, this methane is not a remnant but most likely the S1) as a result of an unexpectedly high downhole pressure. Even- SCIENCES result of ongoing leakage from the reservoir as a result of the blow- tually, gas started erupting from a crater that formed several tens of ENVIRONMENTAL 13 meters away from the spud point. Within 40 min (during which all out. Progressive enrichment of both δ C-CH4 and δD-CH4 is observed with increasing distance and decreasing methane concentrations. The personnel were evacuated), numerous similar eruptions appeared within a 350-m radius that coalesced to form a crater that engulfed calculated isotopic fractionation factors of eC = 3andeD = 54 suggest anaerobic methane oxidation is partly responsible for the observed the entire well pad. Half an hour later, the drilling rig collapsed and eventually sank completely into the ground (Fig. 1). On December decrease in concentrations. Elevated dissolved iron and manganese ∼ concentrations at the fringe of the methane plume show that oxida- 16, a relief well (SLN-03) was spudded 600 m northwest of the blowout (Fig. S1). Using deviated drilling, this well was drilled to- tion is primarily mediated by the reduction of iron and manganese ward the underground location of the blowout. At a depth of oxides. Combined, the data reveal the long-term impact that under- 1,924 m, hydraulic fracturing was successfully carried out to establish ground gas well blowouts may have on groundwater chemistry, as a connection with well SLN-02. On February 18, after more than well as the important role of anaerobic oxidation in controlling the 2.5 mo of near-continuous leakage of large amounts of natural gas, fate of dissolved methane. 760 m3 of heavy mud, followed by 390 tons of cement, was injected into the relief well, which eventually caused the blowout to die out. groundwater contamination | well blowouts | methane | isotopic fingerprinting | anaerobic methane oxidation Significance ncontrolled subsurface leaks of natural gas resulting from The rapid increase in shale gas production in recent years has human underground activities have been shown to occur for U led to increased attention to its potential negative environ- several decades (1, 2). However, they became a highly debated mental effects, including the risks of contaminating ground- subject in recent years after elevated methane concentrations in water with methane and other substances. In this context, the groundwater were attributed to well failure of nearby shale gas – uncontrolled gas migration that is triggered during well wells in an area of intense shale gas development (3 5). In addition blowouts is an understudied environmental hazard. We show to anthropogenically induced leaks of natural gas to groundwater, that the methane chemistry in shallow groundwater overlying conduits for the migration of natural gas may exist naturally, as the site of a catastrophic underground blowout continues to be evidenced by phenomena such as methane seeps, mud volcanoes, impacted 50 y later. The occurrence of anaerobic methane and seabed pockmarks (6, 7). Leakage of natural gas through a oxidation limits the spatial extent to which the dissolved ther- freshwater aquifer can adversely affect groundwater quality by mogenic methane plume could be observed and discerned from changing redox conditions and increasing pH (8, 9). Furthermore, local biogenic methane sources. However, it also highlights the it can result in an explosion and/or asphyxiation hazard (10). requirement to carry out monitoring in close proximity to po- Hydraulically induced fractures have also been investigated for tential gas leakage sources. their potential to act as conduits for gas leakage to groundwater (11). However, groundwater impacted by gas leakage through Author contributions: G.S., N.H., S.M.H., and J.G. designed research; G.S. performed re- hydraulically induced fractures has never been unequivocally search; G.S., N.H., S.M.H., and J.G. analyzed data; and G.S., N.H., and J.G. wrote the paper. identified (12). In contrast, such connections are known to have The authors declare no conflict of interest. been generated as a result of gas well blowouts (13). In general, This article is a PNAS Direct Submission. blowouts occur during drilling when unexpectedly high gas This open access article is distributed under Creative Commons Attribution-NonCommercial- pressures are encountered that can no longer be contained. NoDerivatives License 4.0 (CC BY-NC-ND). While the frequency of blowouts is relatively low [∼1:1,000 wells 1To whom correspondence should be addressed. Email: [email protected]. drilled (14)], their potential environmental impact is huge, as This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. evident from high-profile cases such as the Deepwater Horizon 1073/pnas.1711472115/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1711472115 PNAS Early Edition | 1of6 Downloaded by guest on October 1, 2021 Hydrogeological Setting Currently, the site of the blowout is a small parcel of woodland entirely surrounded by pastures (Fig. 2). The topography is flat, with surface elevations of 16–18 m above sea level. The water table at the location of the blowout varies seasonally at between 1 and 2 m below ground surface. Based on head measurements in the monitoring wells near the blowout area (Fig. 2), the direction of groundwater flow is toward the south and east, with a low hydraulic gradient of roughly 0.25–0.50 m/km. The shallow geology is dom- inated by a thick succession of Quaternary sands and gravels down to a depth of around 120 m (Fig. 2). Three main formations are distinguished with increasing depth: the glacial Peelo Formation (fine- to medium-grained sand), the fluvial Appelscha Formation (coarse sand and gravel), and the marine Oosterhout Formation. The Oosterhout Formation consists of an upper unit of fine- to medium-grained sand and a lower unit that alternates sandy and clayey deposits. The Dutch National Hydrogeological Model, REGIS II (19), describes all but the lower Oosterhout clayey unit as − having hydraulic conductivities between 10 and 100 m·d 1.The Fig. 1. Aerial photograph (39) of the crater formed by the SLN-02 un- Oosterhout Formation is underlain by the highly impermeable derground gas well blowout in 1965. Remnants of the well pad and asso- Breda Formation, a clayey, marine, Neogene unit. Together with ciated trucks and trailers can be seen at the edge of the crater. the aforementioned formations, it is part of the Upper North Sea Group, which extends to a depth of around 225 m (Fig. S1). The Sleen gas field consists of a series of Triassic sand and After the blowout, a network of groundwater monitoring wells claystone layers known together as the Main Bundsandstein was installed by the local drinking water production company to Subgroup, part of the Lower Germanic Trias Group. Gas is cap- monitor for possible adverse effects on groundwater quality at ped in these layers by a more than 100-m-thick deposit of evap- the site of the blowout. For this study, we sampled all relevant orites of the Röt Formation (Fig. S1). In total, seven gas wells monitoring wells. Samples were analyzed for dissolved gas mo- were drilled in the Sleen municipality. The first exploratory well (SLN-01), drilled in 1950, was abandoned at a depth of around lecular and isotopic composition as well as general groundwater i 1,000 m after no hydrocarbons had been encountered. SLN- chemistry. Aims of the study were to assess ( ) the long-term 02 was destroyed in the blowout, and SLN-03 was used as the effects of an underground gas well blowout on dissolved gas relief well.