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Occurrence and fate of methane leakage from cut and buried abandoned
gas wells in the Netherlands
Gilian Schout
a,b,
⁎, Jasper Griffioen
a,c
, S. Majid Hassanizadeh
b
, Guillaume Cardon de Lichtbuer
a
, Niels Hartog
d,b
a
Copernicus Institute of Sustainable Development, Utrecht University, 3584 CB Utrecht, the Netherlands
b
Earth Sciences Department, Utrecht University, 3584 CB Utrecht, the Netherlands
c
TNO Geological Survey of the Netherlands, 3584 CB Utrecht, the Netherlands
d
KWR Water Cycle Research Institute, 3433 PE Nieuwegein, the Netherlands
HIGHLIGHTS
•Monitoring for gas leakage at cut and
buried wells is complicated by several
factors.
•Leakage was detected at 1 of 29 cut and
buried abandoned wells in the
Netherlands.
•Dispersion and oxidation in soils can
prevent detection by surficial measure-
ments.
•Subsurface and isotopic measurements
needed to confirm and quantify meth-
ane leakage.
•Cut and buried wells may constitute an
explosion hazard that needs to be
considered.
GRAPHICAL ABSTRACT
abstractarticle info
Article history:
Received 21 November 2018
Received in revised form 21 December 2018
Accepted 22 December 2018
Available online 24 December 2018
Editor: Frederic Coulon
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 sur-
face. The estimated total flux from the well (443 g CH
4
hr
−1
) was calculated by extrapolation of the individual
flow rate measurements at 2 m depth and should be considered an indicativevalue 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 methaneemissions (e.g. b1% of emissions from the Dutchenergy sector). Further-
more, 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.
© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://
creativecommons.org/licenses/by/4.0/).
Keywords:
Abandoned wells
Gas migration
Methane leakage
Well integrity
Static chamber measurements
Methane isotopes
Science of the Total Environment 659 (2019) 773–782
⁎Corresponding author at: Copernicus Institute of Sustainable Development, Utrecht University, 3584 CB Utrecht, the Netherlands.
E-mail address: g.schout@uu.nl (G. Schout).
https://doi.org/10.1016/j.scitotenv.2018.12.339
0048-9697/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Contents lists available at ScienceDirect
Science of the Total Environment
journal homepage: www.elsevier.com/locate/scitotenv
1. Introduction
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). Not only does leakage from
oil and gas infrastructure appear to contribute significantly to rising an-
thropogenic methane emissions and the associated greenhouse effect
(Miller et al., 2013;Rice et al., 2016), gas migrating into surrounding
geological strata adversely impacts groundwater in aquifers overlying
oil and gas reservoirs (Harrison, 1983;Kelly et al., 1985) and may
pose an explosion hazard (Chilingar and Endres, 2005). In recent
years, elevated levels of thermogenic methane in groundwater have
also been attributed to wellbore leakage from newer, unconventional
oil and gas wells (Darrah et al., 2014;Sherwood et al., 2016). Although
water containing methane is not considered a health hazard (Vidic
et al., 2013), the introduction of methane in a groundwater system
may result in changing redox conditions that in turn may lead to
water quality changes, such as increased sulfide concentrations
(Gorody, 2012) or the mobilization of trace elements (Cahill et al.,
2017).
Recent conservative estimates suggest that at least four million on-
shore hydrocarbon wells have been drilled worldwide (Davies et al.,
2014). A well may leak if a conduit for fluid migration exists from the
wellbore system to its surroundings, a situation that is referred to as
well integrity failure (King and King, 2013). 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). Other
attributes that play a role in determining leakage frequency are differ-
ences in well type (conventional versus unconventional), geographic lo-
cation, well age (Ingraffea et al., 2014), well depth (Watson and Bachu,
2009), and plugging status (Kang et al., 2016). Hence, observed failure
frequencies vary greatly between different basins, with reported values
rangingfrom1.9%toupto75%(Davies et al., 2014).
In addition to these well attributes, observed leakage frequencies
may also differ as a result of the monitoring method used. Detection
and quantification of gas leakage caused by well integrity failure is typ-
ically assessed at the wellhead using measurements of either sustained
casing pressure (SCP) (Brufatto et al., 2003;Lackey et al., 2017), surface
casing vent flow (SCVF) (Dusseault et al., 2014;Watson and Bachu,
2009) or visual observation of leaking gas bubbles. Furthermore, gas
leakage from oil and gas wells has also been assessed by measuring
the concentration and isotopic composition of dissolved gasses in
nearby groundwater wells (e.g. Osborn et al., 2011;Van Stempvoort
and Jaworski, 1995). However, the ability to detect leakage using
groundwater monitoring strongly depends on the distance between
the leakage point and the groundwater sampling location (Jackson
et al., 2013;Molofsky et al., 2011), the direction and velocity of ground-
water flow and the occurrence of microbial methane oxidation (Schout
et al., 2017;Van Stempvoort et al., 2005).
At abandoned oil and gas wells, monitoring options are often limited
by the lack of an intact wellhead. With the number of abandoned wells
growing rapidly (Dusseault et al., 2014), effective monitoring at these
wells will be crucial to identify and remedy methane leakage. At 88
abandoned oil and gas wells in Pennsylvania, USA, methane flow
could be measured by sealing a chamber over the remainder of aban-
doned wellbore systems protruding from the surface (Kang et al.,
2016). 90% of these wells were a positive source of methane, with gas
wells and unplugged wells emitting relatively large amounts of meth-
ane compared to oil wells and plugged wells. Besides plugging, some
oil and gas jurisdictions require that wells are cut to below the ground
surface and buried as the final step in the abandonment process
(Davies et al., 2014). At such wells, detecting and quantifying a gas
leak at surface is complicated as it originates from an uncertain depth
below the surface.
Methaneflow rates at 138 abandoned wells from fourUS states were
measured by Townsend-Small et al. (2016), including a subset of wells
that had been cut and buried. To measure leakage at these locations,
the surface above the well sites was first scanned using mobile methane
detection instruments. When anomalous concentrations were detected,
methane fluxes at the soil-atmosphere interface were measured using
flux chamber methodology and gas samples were collected for source
identification. Overall, the study showed that 42% of unplugged wells
compared to only 0.8% of plugged wells were a positive source of meth-
ane. Boothroyd et al. (2016) measured methane concentrations at the
soil-atmosphere interface above 102 plugged, cut and buried wells
that had been decommissioned according to best practice in the UK.
By comparing the measured concentrations to that at nearby control lo-
cations, ongoing leakage caused by well integrity failure was concluded
for 30% of these wells. However, the wells being the actual source of the
methane were not corroborated by gas sampling and isotopic analysis.
In addition, gas leaking from cut and buried wells has to migrate up-
ward through the vadose zone by diffusion and buoyancy, where it is
dispersed and may be partially oxidized before it can be detected at
the surface (Erno and Schmitz, 1996). Furthermore, preferential flow
paths may cause leaking gas to emerge offset with a considerable dis-
tance (N10 m) from a leaking wellbore (Forde et al., 2019).
Overall, both the environmental hazard of gas leakage at cut and
buried wells and the complexities of detecting and quantifying such
leakage have been understudied. So far, studies on methane leakage at
cut and buried abandoned gas wells did not account for the possibility
that significant portions of leaking gas may be dispersed and oxidized
in the vadose zone, which could have led to an underestimation of the
actual occurrence and leakage rate. In the Netherlands, there are cur-
rently roughly 900 onshore oil and gas wells that have already been
abandoned (Table 1) and the effectivenessof past and present abandon-
ment procedures is largely unknown. Therefore, we selected 29
plugged, cut and buried wells for field investigation into the occurrence
of gas leakage at these sites and to assess the importance of gas disper-
sion and oxidation in the vadose zone using static flux measurements
above holes drilled into the surface, in addition to methane flux mea-
surements and thorough concentration scanning at the surface. The
aims of the study were to evaluate (1) the potential methane leakage
from abandoned wells in the Netherlands and (2) the suitability of var-
ious methods for detecting and quantifying gas migration from aban-
doned wells that have been cut and buriedto below the ground surface.
2. Materials and methods
2.1. Oil and gas wells in the Netherlands
The Netherlands has a long history of oil and gas production, and has
been one of the major hydrocarbon producing countries in Europe. It
owes this mainly to the discovery of two large reservoirs: the
Schoonebeek oil field in 1943 and the Groningen gas field in 1959. In
combination with the discovery of numerous smaller fields, this has
led to the drilling of around 2500 hydrocarbon wells, of which N1700
onshore (Table 1). Gas production and drilling of new onshore wells
in the Netherlands has slowed down in recent decades, because of de-
pletion of reserves and induced seismicity in the Groningen area (van
Thienen-Visser and Breunese, 2015). Currently, around 85% of all
Table 1
Breakdown of oil and gas well status in the Netherlands (NLOG, 2018).
Active Closed-in Plugged/abandoned Other/ unknown Total
Onshore –oil 29 64 706 27 826
Offshore –oil 24 16 97 12 149
Onshore –gas 439 212 191 64 906
Offshore –gas 179 105 270 55 609
Total 671 397 1264 158 2490
774 G. Schout et al. / Science of the Total Environment 659 (2019) 773–782
onshore oil wells and 20% of onshore gas wells have already been aban-
doned (Table 1). The Dutch mining law regulates abandonment proce-
dures of oil and gas wells. Besides prescribing detailed plugging
procedures (Van Der Kuip et al., 2011), it also mandates that onshore
wells are cut to at least three meter below the surface and buried
(6 m below the seafloor for offshore wells). While this lawdoes not pre-
scribe any post-abandonment monitoring, operators still carry respon-
sibility for the well according to Dutch civil law, should any deviation
from the abandonment regulations be uncovered later (i.e., if leakage
is detected) (Ministry of Economic Affairs, 2017).
Publicly available well integrity data from Dutch wells is limited. A
2011 assessment carried out by the State Supervision of Mines
(SodM) uncovered well barrier failure in 1 out of 26 production wells
and 3 out of 5 water injection wells for produced water (Vignes,
2011). However, well barrier failure does not necessarily equate to
well integrity failure (King and King, 2013), given that there are often
multiple barriers in place (e.g. several layers of casing and cement).
Hence, the risk that these wells pose collectively for both groundwater
quality and anthropogenic GHG emissions is unknown. Recently, ongo-
ing gas leakage was detected at one borehole where a catastrophic
blowout occurred while drilling this well in 1965, and as such it was
not abandoned according to protocol (Schout et al., 2017). Thus far,
this is the only known instance of methane leakage to groundwater in
the Netherlands.
2.2. Well selection
Hydrocarbon well locations and other meta-data were obtained
from the governmental ‘Netherlands Oil and Gas Portal’(NLOG, 2018).
Gas wells were prioritized over oil wells. No differentiation is made in
the database between wells that have only been plugged and those
that also have been cut and buried. Hence, a subset of such wells was
identified by visual inspection of satellite imagery, at which p oint acces-
sibility for fieldwork was also estimated. For example, locations that
were built over already could not be included. Wells were selected
from the various gas reservoirs spread throughout the Netherlands. Fi-
nally, as most abandoned well locations are situated on private land, ac-
cessibility depended on obtaining permission from the landowner
during fieldwork. Ultimately, measurements were carried out at 29 lo-
cations (Fig. 1) during June and July of 2017: 28 gas wells and 1 oil
well (RWK-14).
2.3. Surficial scanning
In the Netherlands, the locations of cut and buried hydrocarbon
wells are not visually marked at the ground surface. Hence, they had
to be located using GPS based on available coordinates. To decrease
the chance of missing methane emissions from a leaking well, methane
concentrations at the soil-atmosphere interface were first scanned in a
circular area above the abandoned wells. A minimum radius of 15 m
was chosen, which was estimated to be much larger than the uncer-
tainty of the GPS and the given coordinates of the well. Scanning was
performed using a Gazomat TDL-500 Inspectra Laser, which measures
CH
4
concentrations real-time with a resolution of 0.1 ppmv and was
checked for accuracy every day using a calibration gas cylinder
(50 ppmv). To scan each site thoroughly and in a repeatable manner,
the operator of the device was connected with a rope to a cylinder
(1 m circumference) placed at the coordinates of the well and had to
walk in circles around this cylinder while keeping the rope tight. The
resulting spiraling pattern (known as the ‘involute of a circle’,Fig. 2)
places the operator closer to the well every lap with a distance equal
to the circumference of the cylinder.
A rod with a suction cup was attached to the inlet of the device
(Fig. 2) that was held just above the soil surface during the scanning
and moved from side to side while walking slowly. Additionally, the
suction cup was placed directly on the soil at regular intervals and left
for 5–10 s, after which methane concentrations and GPS coordinates
were recorded. The locations of any deviating values were marked for
subsequent analysis with static chamber flux measurements. The dis-
tance along the spiral in between measurements was decreased to-
wards the well, such that the density of measurements was highest
near the well location. Scanning in this manner yielded an overall aver-
age of around 160 data points per well. Maps of the recorded values for
each well location are shown in Fig. S1.
2.4. Static chamber measurements
Static chamber measurements were carried out directly above the
buried remains of the well and, for 22 of the 29 well sites, a control
Fig. 1. Locations of the 29investigated abandoned wellstogether with theoil and gas fields
in the Netherlands (NLOG, 2018).
Fig. 2. Photo of the surficial scanning in action. Operator is attached to a wooden cylinder
placed on the estimated location of the buried well, as marked with a red dot. The black
line schematically shows the remaining path to be wa lked. (For interpr etation of the
references to color in this figure legend, the reader is referred to the web version of this
article.)
775G. Schout et al. / Science of the Total Environment 659 (2019) 773–782
location. For consistency, controls were taken at the starting pointof the
surficial scanning, i.e., a minimum of 15 m away from the well. This dis-
tance was expected to be larger than the uncertainty range associated
with locating the well. However, the soil at this location would still be
similarly affected by the abandonment procedure of the well (pad),
local land use and soil conditions. Measurements wereadditionally con-
ducted at locations that had elevated methane concentrations during
the scanning.
A plexiglas cylindrical chamber (Fig. 3) with a height of 56 cm and a
radius of 11.5 cm was used (volume of 23.25 L). To limit gas exchange
with the atmosphere, the cylinder was placed on a plastic ring that
was pushed 10 cm into the ground. Methane concentrations in the cyl-
inder were recorded at regular intervals using the Gazomat TDL-500,
also used for the surficial scanning. Both the inlet and outlet of this de-
vice were connected to the chamber with flexible tubing, such that
measured gas was returned to the chamber (Fig. 3). The pumping rate
of the device was 70 L hr
−1
, which ensured mixing inside the cylinder.
After completion of the measurements at the soil surface, 1 m deep
open holes were drilled using a hand auger (5 cm diameter). Measure-
ments were then directly repeated above the newly drilled holes (with
the ring still in place).
The mass flow rate into the chamber was calculated using linear re-
gression on the concentration versus time data (Eq. 1).
F¼dC=dt∙Vð1Þ
where F is the CH
4
flow rate (mg hr
−1
), dC/dt is the rate of change in
CH
4
concentration in the chamber [mg cm
−3
h
−1
] and V the chamber
volume [cm
3
]. Similar to previous studies (Kang et al., 2016, 2014),
only linear fits with an R
2
value N0.8 were considered. R
2
values below
0.8 only occurred when the concentration in the chamber remained
close to the atmospheric methane concentration. Hence, the methane
flow at these locations was assumed to be zero. Measurements were
discontinued if no trend or change in concentrations inside the chamber
was observed after 5 min. While the measurement resolution was
0.1 ppmv, experience learned that measured concentrations sometimes
drifted slightly, going up and down within a bandwidth of around
0.5 ppmv. Assuming a temperature of 18 °C and a pressure of 1 atm,
the minimum mass flow that would result in a positive determination
within 5 min (ΔCH
4
≥0.5 ppmv) was 0.09 mg hr
−1
. In some cases the
measurement period was extended to lower this detection limit
slightly.
2.5. Analysis of gas samples for stable methane isotopes
Gas samples for isotopic analyses were collected upon completion of
the static chamber measurements by placing 1 L tedlar bags in a sealed
box attached directly to the static chamber.Vacuum was created in this
box using a foot pump, causing the sample bags to fill with gas from the
chamber. As measured fluxes were generally higher at 1 m depth, it was
decided to collect samples after these measurements rather than those
at the surface. Samples were collected at additional locations if methane
leakage from the abandoned well was suspected. Analyses of the sam-
ples were carried out at the Institute for Atmospheric and Marine Re-
search (IMAU) of the Utrecht University. Methane was extracted as
described in Röckmann et al. (2016): methane carbon (δ
13
C-CH
4
)and
hydrogen (δ
2
H-CH
4
) stable isotope ratios were determined in separate
measurements by Isotope RatioMass Spectrometry (IRMS) on a Thermo
Finnigan Delta plus XP. The sample mixing ratio and isotopic composi-
tion were determined by comparing the measurements to that of a
known reference air that was extracted in the same way.
2.6. Detailed characterization of methane leakage at the MON-02 site
Thermogenic methane was observed at well MON-02 following the
previously described procedures. This site was therefore characterized
in more detail. Well MON-02 produced from the ‘Monster’gas field
(Fig. 1), a small gas accumulation in the West Netherlands Basin, and
was abandoned in the 1990s. The reservoir consists of a series of Triassic
layers known together as the Main Bundsandstein Subgroup, and is sit-
uated at a depth around 2800 to 3000 m. Starting at the coordinates of
the abandoned wellbore, static chamber measurements were carried
out in each cardinal direction with a 2 m spacing. Once a negligible
flux was identified at 1 m depth, measurements were continued in the
next direction. This procedure led to 14 measurement locations that
form a grid like pattern, with each measurement representing an area
of 2 by 2 m. In addition to the procedure described in Section 2.4,static
chamber measurements were also conducted at 2 m depth, slightly
above the groundwater table at this location (2.2 mbgl). This was
done by extending the already drilled holes to 2 m once the 1 m deep
measurements were completed. Furthermore, PVC tubes with the
same length and diameter (5 cm) as the drilled holes were placed inside
the holes and connected directly to the chamber so that flow into the
chamber could only occur from the bottom of the tube. A sandy soil pro-
file was encountered throughout the site. Samples for isotopic analysis
were collected at the end of seven measurements of a single transect:
three at 1 m depth and four at 2 m depth.
In total, 28 static flux measurements were carried out (Table S.2).
Regression was performed over a linear part of the data (R
2
≥0.8) of
at minimum 50 s in order to calculate the flow rates. To estimate the
total leakage flux from the well the methane flow rates were converted
to a measured mass flux (Eq. 2):
Jm¼F=Atð2Þ
where J
m
is the measured CH
4
flux [mg cm
−2
h
−1
]andA
t
is the surficial
area of the opening of the tube [cm
2
]. Due to the lack of a porous me-
dium inside the tube, the diffusive flux towards the chamber was en-
hanced compared to the initial, undisturbed soil condition. To account
for this, measurements were scaled by the ratio of the diffusion
Fig. 3. Schematic representation of the static chamber measurements carried out above
plugged, cut and buried wells. Chamber dimensions were 56 cm height, 11.5 cm radius,
23.25 L volume. Static chamber measurements were first carried before drilling the 1 m
hole on which additional static chamber measurements were conducted. Figure not to
scale.
776 G. Schout et al. / Science of the Total Environment 659 (2019) 773–782
coefficient of methanein sandy soil to that in air,yielding an approxima-
tion of the undisturbed flux:
Ju¼De=Da
Jmð3Þ
where J
u
is the estimated undisturbed methane flux [mg cm
−2
h
−1
]and
D
e
and D
a
the effective diffusion coefficient in sandy soil and the diffu-
sion coefficient in air, respectively [cm
2
h
−1
]. A D
a
of 726 cm
2
h
−1
was
assumed according to the method of Fuller et al. (1966) for CH
4
at 18
°C and 1015 mbar. The effective diffusion coefficient in sandy soil was
calculated as follows (e.g. Scanlon et al., 2002):
Deff ¼ϕτSgDað4Þ
where ϕis the porosity [−], S
g
is the saturation of the gas phase [−]and
τthe tortuosity factor (determined according to Penman, 1940a, 1940b.
A porosity of 0.3 and soil gas saturation of 0.9 were assumed. Finally, the
total leakage flux from the well was calculated by means of a nearest
neighbor interpolation, at 1 and 2 m depth separately: first, the esti-
mated undisturbed flux for each measurement location was extrapo-
lated to the 2 × 2 m square area surrounding it and then all data
points were summed.
3. Results
3.1. Surficial scanning
Surficial scanning of methane concentrations at the soil-atmosphere
interface was carried out fully at 19 and partly at 5 of the 29 selected
well sites (Fig. S1). At 10 locations obstructions such as canals or
dense undergrowth either partially or fully limited the required mobil-
ity (see for example Fig. 4B). During the course of most measurements,
observed concentrations tended to drift around the atmospheric meth-
ane concentration (~1.8 ppmv) between roughly 1.1 and 2.5 ppmv.
Thus, observed patterns of both increasing and decreasing concentra-
tions could not be related to locally enhancedmethane emissions or up-
take by the soil as long as the concentration changes remained within
this bandwidth (e.g. Fig. 4AandFig. 4B). Isolated locations with concen-
trations N2.5 ppmv were observed at6 well sites (Fig. S1). In most cases,
these observations were temporary concentration spikes and could not
be reproduced on subsequent measurements. However, these locations
were marked for laterinvestigation using the static chambersetup. Only
while scanning well site NKK-01elevated methane concentrations up to
several hundred ppmv were observed that also sustained over a longer
time period (Fig. 4C).
3.2. Static chamber measurements at surface and 1 m depth
The static chamber measurements carried out at the soil surface di-
rectly above the 29 buried wells revealed only a single location with a
positive methane flow rate: AKM-07 (Fig. 5, Table S.1). In contrast,
flow rateswere negative at thesurface above fourwells, and no measur-
able methane flow was recorded above the remaining wells. The surfi-
cial static chamber measurements also revealed two positive and one
negative flow rate at the control locations; flow could not be detected
at the remaining control locations. Negative flow rates are interpreted
as methane uptake from the soil caused by methane oxidation in the va-
dose zone. At 1 m depth, positive methane flow rates were observed
more frequently than at the surface while negative flow rates were
only observed at two control locations. Overall, 15 of 29 well locations
and 10 of 22 control locations had a positive methane flow rate at 1 m
depth (Fig. 5). With the exception of well NKK-01, measurements car-
ried out at locations that were marked because of concentration outliers
observed during the surficial scanning did not yield appreciably higher
flow rates. Hence, these measurements were not considered for further
interpretation.
Methane flow rates at 1 m depth were larger above the buried wells
than at their respective control for 6 locations. Conversely, control mea-
surements were larger at 8 other locations (Fig. 5). The remaining 8 lo-
cations where control measurements were conducted had undetectable
flow rates both above the buried wells and at the control locations. As
can be seen in Fig. 5, the variability in flow rate between the well and
control locations was such that the difference between them could not
be relied upon to attribute measured flow rates above the buried
wells to well integrity failure. A clear positive outlier was, however, ob-
served at well MON-02 with a flow rate of 1418 mg hr
−1
at 1 m depth.
This is more than two orders of magnitude higher than the nexthighest
flow rate measurement in the entire study. Furthermore, this high flow
rate contrasted with the negative flow rate measured at 1 m depth at
the control location, the undetectable flow rates at the surface and the
lack of elevated concentrations recorded during the surficial scanning
(Fig. 4A).
3.3. Analysis methane stable isotopes
Only the sample collected after the flux measurement at 1 m depth
above well MON-02, where the highest single flux measurement was
observed, had a clear thermogenic origin (δ
13
C-CH4 = −26.6‰and
δD-CH4 = −149.7‰,Fig. 6). This is convincing evidence that leakage
of natural gas caused by well integrity failure was occurring. Samples
from other locations with high flow rates had isotopic compositions
Fig. 4. Three examples of the results obtained during surficial scanning of methane concentrations: A) MON-02, B) WYK-30, where only half the scanning spiral could be executed and
C) NKK-01, where several concentration outliers were identified that were marked for static chamber flux measurements.
777G. Schout et al. / Science of the Total Environment 659 (2019) 773–782
indicative of a biogenic origin, with a δ
13
C-CH
4
b50‰and a δD-CH
4
b
−225‰, regardless of whether the fluxes above the well or at the con-
trol location had given higher fluxes. A biogenic gas source was also con-
firmed at well NKK-01, the only location where high methane
concentrations where recorded during the surficial scanning that also
resulted in positive flow rate measurements at the surface. The bacterial
methane is likely related to the presence of organic-rich formations,
abundant in the shallow subsurface of the Netherlands. Many oil and
gas fields in the Netherlands are found in the coastal provinces, where
Holocene peat and clay deposits are also present (Wong et al., 2007).
Methane emissions from these deposits are well known (Van Den Pol-
Van Dasselaar et al., 1999). Samples from locations with intermediate
flow rates showed isotopic compositions that can be explained by
mixing of biogenic and atmospheric methane, while sites with zero or
low flow rates showed an atmospheric origin with δ
13
C-CH
4
and δD-
CH
4
values of around −47‰and −93‰, respectively (Fig. 6).
3.4. Detailed characterization of methane leakage at the MON-02 site
The detailed characterization carried out at well MON-02 showed
that the estimated undisturbed methane fluxes were significantly
higher at 2 m depth than at 1 m depth (Fig. 7). At the location directly
overlying the coordinates of the abandoned wellbore (A1), the esti-
mated undisturbed flux at 1 m depth was 9.4 g m
−2
h
−1
compared to
19.6 g m
−2
h
−1
at 2 m depth. While the maximum flux at 1 m depth
was located at the coordinates of the well, the maximum flux at 2 m
depth was located almost 3 m to the south (B2) and was 4.5 times larger
than the maximum flux at 1 m depth: 40.4 g CH
4
m
−2
hr
−1
.Consider-
able lateral spreading is observed with fluxes N2gm
−2
hr
−1
measured
up to 4.5 m distance from the well coordinates at 1 m depth and up to
6 m at 2 m depth. If subsurface methane concentrations had solely
been the result of diffusion from a buried point source through a homo-
geneous, sandy porous medium (as encountered at the site), spreading
would have occurred in semi-spherical manner in all directions equally.
However, this is not thecase, as illustrated by the fact that the fluxes just
2 m northeast of the coordinates of the well (O1) were zero at both 1
and 2 m depth. Nearest neighbor interpolation of the data yielded a
total leakage flux estimate of 111 g hr
−1
at 1 m depth and 443 g hr
−1
at 2 m depth. At atmospheric pressure and a temperature of 18 °C,
this is equal to a volumetric flux of 4.0 and 15.8 m
3
day
−1
,respectively.
Furthermore, it suggests that a reduction in flux of 74% had occurred
from 2 to 1 m depth.
Isotopic compositions of the 7 collected samples at MON-02 varied
significantly (Fig. 8). The isotopic composition of methane from the
Monster gas reservoir is known from a publicly available analysis of a
gas sample collected from nearby gas well MON-03 (NLOG, 2018). The
sample collected at the location with the lowest flux (A4-2 m) lies
close to the mixing line of gas from the reservoir and atmospheric meth-
ane. The samples that are isotopically most similar to that of the reser-
voir are three samples collected at 2 m depth at the locations with the
highest flux measurements (A1-2 m, A2-2 m and A3-2 m). Relative to
these three samples, two of the samples from 1 m depth (A1-1 m and
A3-1 m) are enriched in both their carbon and hydrogen isotopic com-
position. This is indicative of the occurrence of aerobic microbial meth-
ane oxidation as the isotopic fractionation factors observed for aerobic
methane oxidation in Dutch and German soils (α
13
C = 1.008 and αD
=1.0039,Bergamaschi et al., 1998) qualitatively match the shift from
the samples collected at 2 m depth to those at 1 m depth (Fig. 8). Fol-
lowing the methods described in Whiticar (1999), the observed shift
can be related to a residual methane fraction of 75%. In other words,
methane oxidation can account for up to 25% of the reduction in meth-
ane flux as observed from 2 to 1 m depth.
The isotopic composition of sample A2-1 m, which is isotopically de-
pleted compared to the reservoir gas, represents an outlier that cannot
be explained by leakage of gas from the Monster gas reservoir. Possibly
the leaking gas here does not originate from the reservoir, but rather
from an intermediate formation. The most likely candidate is the Hol-
land Greensand Member, a known gas-bearing formation in the
Netherlands located here at a depth of 1420 to 1481 m. The isotopic
composition of this formation is known from two samples taken from
a nearby small gas field (Fig. 8) and matches that of the sample taken
at A2-1 m relatively well.
Fig. 5. Methane flow rates at thesurface (A) and at 1 m depth (B) as determined by static
chamber flux measurements. Locations are shown that have at minimum one non-zero
measurement (22 out of 29 locations). Grey area is below the detection limit (‘BDL’),
locations where no control measurements were carried out are labelled with ´NC´.
Fig. 6. Carbon and hydrogen isotopic composition of methane in samples collected after
completion of static chamber measurements at 1 m depth. Grey dots show isotopic
composition of samples taken from onshore Dutch gas reservoirs (NLOG, 2018). Isotopic
composition of northern-hemisphere atmospheric methane based on Rice et al. (2016).
778 G. Schout et al. / Science of the Total Environment 659 (2019) 773–782
4. Discussion
4.1. Detecting gas leakage from cut and buried abandoned wells
As shown by the results of our study, the occurrence of gas leakage at
cut and buried abandoned wells is more easily missed when relying on
surface measurements only, as compared with abandoned wells that
still have existing wellhead infrastructure protruding to the surface. Be-
sides additional uncertainties on the exact location of the buried well,
particularly in the absence of surface markers (as is the case in the
Netherlands), leaking methane may become dispersed and oxidized as
it migrates from the leakage point towards the surface, not necessarily
at the well coordinates due to preferential flow paths. When a lea k orig-
inates below the groundwater table, upwardly migrating methane can
be additionally attenuated by dissolution into groundwater (Cahill
et al., 2017). Obviously, when gas leakages result from well integrity
failure at greater depth and the gas migrates along the outside of the
casing these processes would equally affect methane transport to sur-
face (Dusseault and Jackson, 2014), regardless of whether a well is
abandoned, cut and buried, or still protruding to the surface. A combina-
tion of these processes caused leakage from a gas well in Colorado, USA,
to be undetectable by surficial flux measurements and soil gas probes
installed to 60 cm depth, while methane concentrationsin the underly-
ing groundwater were above air-saturation levels (McMahon et al.,
2018). Subsurface (vadose zone) measurements using dedicated soil
gas wells have previously been employed in soils surrounding oil and
gas wells that were notcut and buried by (Lyman et al., 2017). However,
while this allows for continued and precise monitoring of soil gas con-
centrations, our method is lesscostly and time consuming and therefore
enables relatively quick screening at multiple locations.
Methane leakage from a cut and buried wellbore may also be
masked by mixing with naturally occurring biogenic methane. Such
emissions can be highly spatially variable as a result of soil heterogene-
ities and preferentialpathways (Hendrikset al., 2010). This is illustrated
in our study by the detection of hotspots of elevated methane concen-
trations during the surficial scanning at several locations above one
well (NKK-02; Fig. 4C), but where isotopic analysis revealed a shallow
biogenic origin. Furthermore, local variabilities in biogenic methane
fluxes also explain the lack of correlation between the magnitude of
the fluxes measured at surface and 1 m depth as well as their controls
(Fig. 5). Given these conditions, isotopic confirmation of methane origin
is critically important in preventing false negatives or positives in the
detection of subsurface gas leakage from a well. At the one well in this
study for which gas leakage was confirmed, both the surficial scanning
and the surface static chamber measurements did not yield any notice-
able indication of leakage (MON-02; Fig. 4A, Fig. 5). This emphasizes the
importance of subsurface measurements of methane flow rates or con-
centrations, above the level from where leaks could be occurring, as dis-
persion and oxidation will have had less impact on the methane flux
Fig. 7. Results of the detailed investigation at MON-02: values signify the estimated undisturbed fluxes at 1 m depth (left) and 2 m depth (right), contours depict the underground
spreading of leaked methane but are speculative. Grey inlay shows the label for each position, the red line indicates the locations at which samples were collected for isotopic analysis.
(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Isotopic composition and estimated undisturbed flux of seven samples collected as
part of the detailed investigation at MON-02 (sampling positions indicated by red line in
figure inlay). Two thermogenic end members are shown: Monster reservoir gas (MON-
03) and gas from the intermediate Holland Greensand Member (HGM; NLOG, 2018).
Red dashed line shows the estimated effect of aerobic methane oxidation, red labels the
estimated residual methane fraction. Calculation based on isotopic fractionation factors
found in soils above Dutch landfills (ΔD/ΔC = 4.8, Bergamasc hi et al., 1998). (F or
interpretation of the references to color in this figure legend, the reader is referred to
the web version of this article.)
779G. Schout et al. / Science of the Total Environment 659 (2019) 773–782
than at the surface. Also, for conditions of high background biogenic
methane production, the dilution and mixing of the isotopic signature
of the leaking gas would be smaller.
Although oxidation and dispersion decrease the detectability of
methane leakage, isotopic analysis can, in addition to determination of
origin, aid in determining the relative importance of these two pro-
cesses in the total attenuation of the subsurface methane flux from a
cut and buried well. As shown by the enrichment of the methane isoto-
pic composition (Fig. 8) concurrent with the decrease in flux between 2
and 1 m below ground surface for the gas leakage occurring at well
MON-02 (Fig. 7), aerobic methane oxidation accounted for roughly
25% of this decrease. Hence, around 75% of the decrease in methane
flux towards the surface was attributable to dispersion for that well
site. Negative fluxes, resulting from the net oxidation of methane were
indeed observed at several of the investigated locations, including at
the control location of well MON-02 (Fig. 5). Furthermore, the wide-
spread oxidation of subsurface methane fluxes (whether biogenic or
thermogenic) is also suggested by the consistently larger fluxes mea-
sured at 1 m depth than those measured at the surface for the other lo-
cations investigated (Fig. 5). The extent to which dispersion and
oxidation affect the methane flux from subsurface well leakage will
however strongly vary between different well locations, depending on
the depth of the leakage point, as well as a range of local physical (e.g.
water content) and biochemical (e.g. redox conditions, microbialpopu-
lation) conditions and the heterogeneities therein.
4.2. Estimation of the total methane leakage flux from cut and buried aban-
doned wells
At abandoned wells thatare not cut and buried,sealing the wellhead
off from the atmosphere allows for accurate measurements of the total
leakage flux (assuming there is no gas migration outside the casing). At
cut and buried wells however, quantification of the total flux is more
complex as each measured flux represents an unknown portion of the
total flux. Furthermore, while necessary for the detailed characteriza-
tion of leakage at MON-02, carrying out flux chamber measurements
from holes drilled into the soil introduces additional uncertainty by
disturbing soil properties and the initial methane concentration distri-
bution in the vadose zone. The three most notable sources of uncer-
tainty include first that we had to compensate for the lack of a porous
medium in the tube installed into the drilled holes by scaling the mea-
sured flux with the ratio of the effective diffusion coefficient of methane
in soil over the diffusion coefficient of methane in pure air. This requires
knowledge of the porosity, tortuosity and gas saturation of the porous
medium, which we did not determine in the field and hence had to be
approximated. Second, the measurements for the detailed investigation
did not yield strictly linearly increasing concentration versus time data.
Hence, regression analysis had to becarried out on the linear part of the
data for that particular flux measurement. The non-linear behavior was
likely caused by a combination of factors, includingthe alteration of soil
properties due the drilling, non-vertical and possibly some advective in-
flow of methane into the tubes, and disruption of the initial methane
concentration profile in the soil. Third, interpolation of the measure-
ments at 2 m depth assumes that methane fluxes are strictly vertical
whereas in reality there is an important lateral component (as also
shown by the results of our study). Further, the interpolation assumes
that the flux is more or less spatially distributed and not dominated by
fluxes through preferential flow paths. We believe this to be generally
valid heregiven the sandy conditions and the lack of concentration out-
liers observed during the surficial scanning.
Given the number of sources of uncertainty and the assumptions re-
quired under these non-ideal subsurface leakage conditions, our esti-
mation of the flux at 2 m depth (443 g CH
4
hr
−1
or 15.8 m
3
CH
4
d
−1
)
should be considered an indicative value only. Also, vadose zone pro-
cesses have been shown to lead to considerable temporal variation of
methane fluxes measured by static flux chambers (Forde et al., 2018),
and our measurements thus only represent a snapshot of the total leak-
age flux at one moment in time. Longer term monitoring would be rec-
ommended to better constrain the average flux from a leaking well.
Future modelling and/or experimental studies may help to determine
the validity of the approach taken for the estimation of total leakage
flux. In comparison to leakage flux estimates in other studies, our total
flux estimate at two-meter depth is larger than any of the fluxes identi-
fied at plugged wells by both Townsend-Small et al. (2016) and Kang
et al. (2016).However,theflux estimate is in the same order of magni-
tude as the maximum fluxes they observed at unplugged wells. In com-
parison, surface casing vent flow fluxes appear to be much larger on
average: fluxes larger than 10 m
3
day
−1
were observed at 32% of
11,394 conventional wells with reported leakage issues in Alberta and
British Columbia, Canada (Nowamooz et al., 2015). Therefore, our indic-
ative total flux estimate does not seem unrealistic.
4.3. Observed well leakage frequency in context
This study revealed that methane leakage caused by integrity failure
had occurred at 1 out of 28 cut and buried abandoned gas wells (3.6%).
While this value is at the low end of the large range reported for well in-
tegrity failure in the review by Davies et al. (2014), it is considerably
higher than the 0.06–0.15% found by Sherwood et al. (2016) in Colo-
rado,USA.However,theirfindings were based on water chemistry anal-
yses, which is a more indirect measurement and may therefore have
contributed to a relatively low leakage frequency. The observed well
leakage frequency is larger than that found by Townsend-Small et al.,
2016 (0.8%) for plugged and abandoned oil and gas wells, part of
which had also cut and buried, but lower than both the 30% reported
by Boothroyd et al. (2016) for plugged, cut and buried wells in the UK
and the 62% reported by Kang et al., 2016 for plugged wells that had
not been cutand buried. These differences are likely caused by a combi-
nation of factors such as the local geology, well type, well construction,
well abandonment procedures (King and King, 2013) and the type of
monitoring method employed. Here, the amount of wells studies likely
plays a role, too. More studies are needed in order to obtain proper in-
sight in the statistics of well integrity failure for the Netherlands as
well as globally.
4.4. Environmental impact of abandoned gas wells in the Netherlands
Given the limited amount of wells studied, extrapolation of the re-
sults can only be done tentatively. Using binomial statistics, the lower
and upper probability that could have still resulted in 1 positive detec-
tion within one standard deviation were calculated to convert our find-
ings to a range. This yields a well integrity failure rate for cut and buried
gas wells in the Netherlands of between 1.4% to 9.0%,which is equal to a
range of between 3 and 17 leaking wells when extrapolating to all cur-
rently abandoned gas wells (n= 191) and between 12 and 81 leaking
wells when extrapolating to the total amount of onshore gas wells (n
= 906). Assuming each well leaks with the estimated flux at 2 m
depth at well MON-02, this gives a rough national estimate of between
10 and 66 ton CH
4
yr
−1
emitted for the abandoned gas wells, or be-
tween 48 and 315 ton CH
4
yr
−1
for all gas wells. This is equal to between
0.14 and 0.95% of the total amount of CH
4
estimated to have been emit-
ted by the Dutch energy sector in 2015, and 0.005–0.035% of the total
anthropogenic CH
4
emissions in the country (Environmental Data
Compendium, 2017). In other terms, the maximum estimated value is
equal to the methane produced by roughly 500 Dutch milk cows
(~133 kg CH
4
yr
−1
cow
−1
;Velthof et al., 2016). Concluding, it is improb-
able that the methane leaking from these wells contributes significantly
to current anthropogenic GHG gas emissions in the Netherlands.
In the case of cut and buried wells, part of the leaking methane is
converted to CO
2
as a result of aerobic oxidation. Hence, combining
methane concentration measurements with CO
2
measurements would
be required to fully assess GHG emissions. Given that methane has a
780 G. Schout et al. / Science of the Total Environment 659 (2019) 773–782
global warming potential 24 times that of CO
2
(Myhre et al., 2013), CO
2
emitted in this manner is likely insignificant for our study area. Notably,
leakage from cut and buried oil and gaswells does have to be considered
in the context of explosion hazards. This is illustrated by the MON-02
case reported in this study, where the intended construction of several
new houses was upheld to repair the leaking abandoned well and en-
sure the safety of the new buildings.
5. Conclusions
Methane leakage fromcut and buried abandoned oil and gas wells is
an understudied environmental hazard, and the detection and quantifi-
cation of leakage at such wells is complex. To assess environmental im-
pact and effective monitoring methods, 28 cut and buried gas wells and
1 oil well were investigated in the Netherlands. Leakagewas observed at
1 gas well (MON-02), constituting the first observation of well integrity
failure at a properly decommissioned gas well in the country. Neither
the surficial methane concentration scanning nor the static flux mea-
surements carried out at the surface proved capable of detecting the
leaking gas. However, static flux measurements above 1 m deep holes
drilled into the soil were an effective method. In general, measured
flow rates were consistently larger at depth than at the surface. Control
measurements showed that naturally occurring biogenic methane
fluxes were spatially highly variable. Therefore, the origin of measured
fluxes could not be determined based on flux magnitude alone, and
analyses of methane stable isotopes were required to distinguish be-
tween a biogenic or thermogenic origin.
A detailed characterization at the leaking well, consisting of static
chamber measurements in conducted in a 2 × 2 grid from holes drilled
to 1 and 2 m depth, showed that measured flow rates diminished rap-
idly towards the surface. Enrichment of the isotopic composition con-
current with this decrease confirmed that besides dispersion, aerobic
methane oxidation accounted for roughly 25% of this decrease.
Converting the flow rate measurements in the vadose zone to a total
flux estimate is not trivial, as each measurement represents only a
small portion of the total flux and the drilling of the holes required for
the measurements disturbs the soil properties. Nevertheless, an indica-
tive value wascomputed by converting each measurement at 2 m depth
to estimates of the ‘undisturbed’flux and then interpolating using
nearest neighbor interpolation. This approach yields a value of
443 g CH
4
hr
−1
emitted from the well. Extrapolating this number to
all - abandoned or not - gas wells (n= 906) in the Netherlands and tak-
ing an uncertainty range of 1.4 to 9.0% for the well integrity failure rate
yields a first estimate of 48–315 ton CH
4
yr
−1
. This is negligible com-
pared to the yearly methane emissions from the energy sector
(0.14–0.95%) and overall anthropogenic methane emissions
(0.005–0.035%) in the Netherlands.
Taken together, these findings support the need for subsurface mea-
surements when trying to detect and quantify methane leakage from
cut and buried gas wells, and have important implications for past and
future studies aimed at detecting methane leakage at such wells. How-
ever, more research is required to assess whether point measurements
of vadose zone methane fluxes can be reliable converted to estimates of
the total leakage flux from a cut and buried well. In spite of considerable
uncertainty, our results show that leakages from gas wells are unlikely
to contribute significantly to GHG emissions in the Netherlands. How-
ever, the potential explosion hazard caused by methane leakage from
cut and buried abandoned wells needs to be taken into account when
planning construction projects.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.scitotenv.2018.12.339.
Acknowledgements
We thank KWR Watercycle Research Institute for providing guid-
ance on field methods and field materials, Carina van der Veen for
carryingout the analyses of our samplesat the IMAU lab, and Hans Oonk
for making available his portable methane analyzer for our study. This
work is part of the research program ‘Shale Gas and Water’with project
number 859.14.001, which is financed by the Netherlands Organization
for Scientific Research (NWO).
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