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Shallow gas migration along hydrocarbon wells – An unconsidered, anthropogenic source of biogenic methane in the North Sea Lisa Vielstädte, Matthias Haeckel, Jens Karstens, Peter Linke, Mark Schmidt, Lea Steinle, and Klaus Wallmann Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02732 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 5, 2017
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Shallow gas migration along hydrocarbon wells – An unconsidered,
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anthropogenic source of biogenic methane in the North Sea
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Lisa Vielstädte1,2,*, Matthias Haeckel1, Jens Karstens1, Peter Linke1, Mark Schmidt1, Lea
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Steinle1,3, and Klaus Wallmann1
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GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany
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Department for Earth System Science, Stanford University, USA
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Department of Environmental Sciences, University of Basel, Switzerland
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Keywords: methane emissions │ abandoned wells │ oil and gas │ North Sea
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Abstract
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Shallow gas migration along hydrocarbon wells constitutes a potential methane emission
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pathway that currently is not recognized in any regulatory framework or greenhouse gas
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inventory. Recently, the first methane emission measurements at three abandoned offshore wells
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in the Central North Sea (CNS) were conducted showing that considerable amounts of biogenic
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methane originating from shallow gas accumulations in the overburden of deep reservoirs were
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released by the boreholes. Here, we identify numerous wells poking through shallow gas pockets
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in 3-D seismic data of the CNS indicating that about one third of the wells may leak, potentially
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releasing a total of 3-17 kt of methane per year into the North Sea. This poses a significant
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contribution to the North Sea methane budget. A large fraction of this gas (~42 %) may reach the
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atmosphere via direct bubble transport (0-2 kt yr-1) and via diffusive exchange of methane
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dissolving in the surface mixed layer (1-5 kt yr-1), as indicated by numerical modeling. In the
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North Sea and in other hydrocarbon-prolific provinces of the world shallow gas pockets are
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frequently observed in the sedimentary overburden and aggregate leakages along the numerous
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wells drilled in those areas may be significant.
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Introduction
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Gas leakage from hydrocarbon infrastructure is a major concern because the primary fugitive
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component of petroleum and natural gas is methane (CH4), which has a significant global
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warming potential1. However, regular monitoring of wells is only mandatory during the active
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life time of the well and is solely targeted at the leakage of thermogenic gas and formation fluids
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from the deep reservoir2-3. As such, emissions reported by the industry currently can only provide
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a lower estimate of atmospheric CH4 emissions because not all leakages are identified and
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quantitative data on release rates are rare. This is supported by growing evidence for increased
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CH4 emissions in hydrocarbon production areas4-11 that are generally attributed to leakage of the
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produced fossil fuel from several infrastructure parts during various operation steps4,11 and after
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well abandonment12-15.
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Conventionally, the petroleum industry considers the potential failure of well material (e.g.
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casing steel, cement), typically identified by sustained casing pressure (SCP)16, that is expected to
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lead to uncontrolled migration of reservoir gas to the surface installations (wellhead, pumps,
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controllers etc.). Available data on such well integrity issues provides an uncertain estimate of 2-
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75% of all wells2 being compromised and potentially at risk for leakage (Fig. 1). Additional
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emissions of reservoir gas arise through the millions of abandoned wells10-15 and/or via gas ACS Paragon Plus Environment
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migration along the well paths which is known to occur due to certain soil/sediment conditions or
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cementing issues in both active and abandoned sites. Both of these emission sources are not
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addressed in conventional well integrity surveys, so that the number of leakages and their
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emission rates are highly uncertain. In the Province of Alberta Canada, the only region where
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testing for gas migration through the soil is obligatory17, 0.6-45% of wells showed soil gas
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leakage18-19, but solely the discharge of reservoir fluids is targeted.
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Here, we focus on the so far unconsidered migration of biogenic CH4 gas along the borehole
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originating from shallow gas accumulations that are penetrated when drilling into the underlying
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deep hydrocarbon reservoirs20. Drilling disturbs and fractures the sediment around the wellbore
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mechanically, thereby creating highly permeable pathways for the buoyancy-driven migration of
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the gas21. We present new and important data on the probability of shallow gas migration along
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hydrocarbon wells in the North Sea, including the extensive analysis of an industrial 3-D seismic
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data set that covers an area of ~ 2,000 m2, and provide the first estimates on its regional
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relevance, including comprehensive modelling, literature review and GIS analysis. Based on this,
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we reinvestigated the CH4 budget of the North Sea, indicating that shallow gas migration is likely
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of regional significance.
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Material and Methods
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Data analysis in the Central North Sea (CNS). Evidence for shallow gas migration along
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offshore hydrocarbon wells was first reported by Vielstädte et al.20 who investigated three leaky
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abandoned wells (15/9-13, 16/4-2, and 16/7-2) located in water depth of 81-93 m in the CNS. For
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the extrapolation analysis of CH4 leakage to the North Sea scale presented here, we used relevant
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data on shallow gas migration that has been published in this earlier study20 including
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geochemical data that characterized the origin of the emanating gases, leakage rates, initial gas
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bubble size distributions (Tab. 1), and the analysis of industrial 3-D seismic data (ST98M3,
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Statoil ASA) for shallow gas pockets in the area around the three wells by mapping high
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amplitude anomalies22 in the upper 1,000 m of sediment using Petrel (Schlumberger). We
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additionally analyzed CH4 oxidation rates in the water column at wells 15/9-13 and 16/7-2 (Tab.
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1) and total organic carbon and sulfate concentrations in near surface sediments obtained during
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cruise CE12010 (July-August 2012; Fig. S4). In addition, we estimated the leakage probability of
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wells in an area of ~2,000 km2 in the Norwegian Sector of the CNS by identifying 18 out of 55
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wells (i.e. 33%) that poke through shallow gas pockets in 3-D seismic data (ST98M3, Statoil
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ASA). Assuming a binomial distribution from a Bernoulli process, a sample size of 55 wells,
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investigated during seismic analysis, produces an uncertainty in the leakage frequency of wells of
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6% (1-σ). A summary of relevant results published in an earlier study20 and additional
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geochemical and seismic data are provided in the Supporting Information (SI Appendix, Section
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2.1).
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Extrapolation of CH4 leakage to the North Sea scale. CH4 leakage from wells into the North
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Sea and the atmosphere, was calculated by applying results of a numerical bubble dissolution
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model20 to the EMODnet North Sea bathymetry (available at http://www.emodnet-
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bathymetry.eu) and combining publicly available data on drilled wells (see SI Appendix, Table S3
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for details) using the geographical information system software ArcGIS 10.1. In total, 11,122
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active and inactive wells were selected for the CH4 flux quantification excluding sidetracks of
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wells (see SI Appendix, Table S4). The North Sea was subdivided into equal area polygons of 25
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km2 using a Cylindric Equal Area projection and the seabed CH4 flow (QSF) was calculated for
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each of these polygons multiplying the determined leakage probability of 33±6% for the wells,
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the number of wells located inside each polygon, and the range of per-well CH4 leakage rates of
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1-4 t yr-1 (sample range, N=2) observed in the CNS20. Note, we only consider the two lower gas ACS Paragon Plus Environment
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flows measured at wells 15/9-13 and 16/4-220 because they are believed to be more typical for
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shallow gas migration than the larger gas flow measured at well 16/7-2, which has been drilled
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through a seismic chimney20 (for details see discussion and SI Appendix, Section 2.1.1). For each
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polygon, the resulting CH4 flow from the surface water into the atmosphere was then estimated
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applying a transfer function describing the CH4 bubble transport efficiency to the sea surface and
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to the SML of the North Sea as a function of the seabed CH4 flow and water depth (SI Appendix,
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Section 2.2.2). All determined flow estimates and the effective uncertainty in the leakage
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frequency of wells were added to calculate the potential range of total CH4 ebullition from the
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seafloor and into the atmosphere. Reported values of CH4 leakage into the North Sea and the
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atmosphere are expressed in kilo tonnes of CH4 per year (kt yr-1 of CH4), considering the
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potential range of per-well leakage rates of 1-4 t yr-1 of CH4 (sample range, N=2)20 and an
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uncertainty in the leakage frequency of 6% for the wells (N=55). Full methodology and a
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discussion of associated uncertainties are provided in the Supporting Information.
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Results and Discussion
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In the North Sea, Pleistocene and Pliocene organic-rich sediments are the most prominent
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stratigraphic units containing biogenic gas accumulations that are widespread in 300-750 m
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sediment depth23-24. Vielstädte et al.20 identified these units as potential source areas for the CH4
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emitted along three leaky, abandoned wells investigated in the Norwegian Sector of the CNS.
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CH4 in the seep gas was isotopically light (δ13CCH4 < -70‰ VPDB) and contained only minor
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amounts of higher hydrocarbons (C1/ΣC2+ > 1,000), clearly pointing towards a biogenic origin20
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(Tab. 1, SI Appendix, Figure S2). The shallow origin is supported by bright spots (e.g. reverse
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polarity high-amplitude seismic anomalies) and zones of chaotic signatures in the seismic data
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surrounding the three well paths20 (Fig. 2b).
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Here, we further hypothesize that leakage from existing wells in the North Sea is likely to
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constitute an important part of the respective regional CH4 budget due to the large number of
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wells (i.e. ~11,122, discounting extra sidetracked wellbores) and ubiquitous gas accumulations in
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the shallow subsurface23,
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assess CH4 leakage along wells into the North Sea and estimate the resulting emission into the
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atmosphere. Tracking the subsurface well paths of 55 wells in an area of ~2,000 km2 in the
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Norwegian Sector of the CNS, where shallow gas accumulations have been identified by bright
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spots in industrial 3-D seismic data, we examine the likelihood of wells to leak shallow gas: 18
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out of 55 wells in this area (about 33±6%) were drilled through shallow gas accumulations and
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are thus believed to leak CH4 (Fig. 2). Individual leakage rates along three wells (15/9-13, 16/4-2,
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and 16/7-2) in this area were highly variable, amounting to 1, 4, and 19 t yr-1 of CH4,
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respectively20 (Tab. 1, SI Appendix, Table S1). These rates are in the upper range of gas emissions
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measured from soils around wells in Alberta (i.e. ~0.03-16 t yr-1 of CH4)18. The highest release
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rate in the North Sea was measured at well 16/7-2, which was drilled through a seismic chimney
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(Fig. 2b), typically believed to be more permeable than the surrounding sediment20. Assuming
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that the high gas flow at this well is representative for only a small fraction of leaky wells in the
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North Sea and that the two lower rates of wells 15/9-13 and 16/4-220 are more typical for shallow
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gas migration, we solely consider the two lower rates in our extrapolation analysis (i.e. sample
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range, N=2). Given the small sample size of only three measured wells in the North Sea and the
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unknown spatial and temporal variability of the ebullition (i.e. CH4 flow measurements at the
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three wells covered only a few minutes)20, we emphasize that the uncertainties in our per-well
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leakage rates are high.
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Using publicly available wellbore data and extrapolating our results to the North Sea scale, we
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estimate that shallow gas migration along wells may release around 3-17 kt of CH4 from the
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(Fig. 2, SI Appendix, Table S4). In the following, we will thus,
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North Sea seafloor per year, assuming that 33±6% of the 11,122 wells in the North Sea are
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leaking. In comparison to other major sources of CH4 in the North Sea, i.e. rivers (0.5 kt yr-1)27-30,
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the Wadden Sea area (1.1-2.1 kt yr-1)27, and known natural seep sites (0.2 kt yr-1)31-34, leaky wells
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may constitute a significant input to the CH4 budget of the North Sea (Fig. 3 and SI Appendix,
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Table S5). It should, however, be noted that also numerous additional natural gas seeps have been
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observed at the seabed of the North Sea31, but their abundance and contribution to the CH4
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budget are poorly constrained23,35. Despite the poor characterization of the North Sea CH4
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sources, the patchiness and high spatial variability of CH4 super-saturation in the surface mixed
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layer (SML) of the open North Sea with respect to atmospheric partial pressure, i.e. 103-
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50,000%27,30,36, suggest that ubiquitous point sources at the seabed (wells and natural seeps)
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dominate the regional CH4 budget. The high super-saturation of the North Sea surface waters
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drives a diffusive CH4 loss to the atmosphere of 10 - 50 kt yr-1 30,36, which constitutes the major
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sink in the marine CH4 budget (Fig. 3). An additional amount of 8 kt yr-1 is exported to the
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Atlantic30.
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To examine the extent to which emissions from leaky wells may contribute to the CH4 budget of
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the North Sea, we apply a numerical bubble dissolution model to the North Sea scale. Each of the
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three key fates of leaking CH4 is considered: 1) dissolution in the deep stratified layer (>50 m
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water depth)37, 2) dissolution in the surface mixed layer (SML,
