OR/18/020 Soil gas

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R S Ward1, G Allen2, B J Baptie1, L Bateson1, R A Bell1, A S Butcher1, Z Daraktchieva3, R Dunmore4, R E Fisher5, A Horleston6, C H Howarth3, D G Jones1, C J Jordan1, M Kendall6, A Lewis4, D Lowry5, C A Miller3, C J Milne1, A Novellino1, J Pitt2, R M Purvis4, P L Smedley1 and J M Wasikiewicz3. 2018. Preliminary assessment of the environmental baseline in the Fylde, Lancashire. British Geological Survey Internal Report, OR/18/020.

Introduction

The soil gas element of the project sought to establish baseline conditions for the concentrations of gases in the soil, flux of key gases from the soil to the atmosphere and near-ground atmospheric levels of gases. There is therefore some overlap with the atmospheric monitoring and, since radon was measured at a subset of the surveyed locations, there is also some linkage to the additional radon work (see Section 8).

Baseline soil gas measurements, like those for the other parts of the project, provide a basis against which to assess any future changes that might result from shale-gas activities. Although of low probability, there is the potential for gas to escape from depth along geological pathways (faults, fractures and other higher permeability zones) or man-made features, especially wells (either pre-existing or drilled for shale gas exploration, evaluation or development).

Whilst large faults may be known from existing geological maps and/or data acquired during hydrocarbon exploration (e.g. 3D seismic data), or become apparent from seismicity or ground motion studies, smaller faults and fractures may be present but unknown. The completion (plugging and abandonment) of existing deep boreholes could be of variable quality depending on the age of the well; there are wells in the Fylde that are more than 50 years old. New wells also represent a potential pathway.

It is very difficult to predict where fluid migration from depth might reach the surface whether it follows natural or man-made pathways. Natural seepage of gas along faults tends to occur at limited sites, metres to tens of metres across, along only a very small proportion of the fault length (e.g. Annunziatellis et al., 2008[1]; Johnson et al., 2017[2]; Ziogou et al., 2013[3]). Borehole leaks can occur at the wellhead or, if fluid escapes from the annulus of the well, can reach the surface up to several kilometres away (e.g. Allison, 2001[4]).

Monitoring site selection and supporting information

The general principles of the approach were set out in the Site Selection report (Smedley et al., 2015[5]). The aim was to acquire a representative dataset that reflected the spatial and temporal variability of baseline soil gas conditions in the Fylde in the vicinity of the proposed shale gas activity at both Preston New Road and Roseacre Wood. This was carried out within the constraints of logistical requirements and budgetary limits. For example, landowner permissions are needed for access and continuous monitoring needs to be in secure locations, safeguarded against human or animal interventions, where mains power is an advantage.

This soil gas study included field measurement of methane, CO2 (which could be produced from methane oxidation or present in reservoir gas), O2 (useful in helping determine the source of CH4 and CO2) and Rn (a possible tracer of gas migration pathways). The trace gases H2S and H2, were also included.

A mix of survey mode (single point and mobile) and continuous measurements at selected sites was carried out. Surveying large areas for discrete surface gas outlets is conducted best with mobile equipment to identify locations of specific interest. However, due to dilution in air, sensitivity is reduced. Single-point measurements provide the highest sensitivity as the gas is extracted from the soil or soil surface where concentrations are highest, and a sufficient number of analyses over a site provides a good indication of the range of baseline conditions. Continuous measurements at a small number of sites provide information on temporal variations (e.g. diurnal or seasonal changes).

It was the intention to supplement field measurements with a subset of duplicated laboratory determinations of soil gas concentrations. This would have provided information on additional gases, such as other light hydrocarbons, and verified field determinations with higher precision data. However, this would have significantly reduced the amount of field data, through diversion of effort and has therefore not yet been undertaken.

Monitoring and data processing activities

The study included:

  • detailed coverage of near-ground atmospheric methane and CO2 using mobile open path lasers;
  • broad-scale grids of point measurements of soil gas (CO2, CH4, O2, H2, H2S, Rn) and flux (CH4 and CO2) in the field with closer spaced coverage to investigate a major fault at one of the sites;
  • at one specific location, continuous measurements using eddy covariance techniques to derive local CO2 flux where other atmospheric measurements are being undertaken.

The soil gas surveys (mobile and point measurements) were carried close to the two proposed shale gas sites near Preston New Road and Roseacre Wood (Figure 61). They included areas of glacial till, peat and minor tidal flat deposits on Triassic mudstones (Kirkham and Breckells Mudstone Formations) and also the inferred surface location of a major NE–SW to NNE–SSW fault near Roseacre Wood.

An eddy covariance system was installed at Preston New Road (Site 1) to provide continuous CO2 flux information in conjunction with the atmospheric monitoring.

Results

Spatial surveys

Two separate surveys were carried out in the Fylde in August 2015 and September 2016. In general, the soil was dry at these times enabling soil gas data to be obtained at most sites except for a few low-lying waterlogged locations at Preston New Road.

Equipment availability and some instrument problems meant that obtaining full datasets was not possible with all techniques on each visit. Additional instruments were available in 2016, which widened the range of measurements possible significantly, especially of CH4 concentration and flux but also to include mobile laser measurements of both CH4 and CO2. The data obtained from all the baseline surveys is summarised in Table 14.

Table 14    Summary of survey soil gas data acquisition.
Technique

Survey period

August 2015

September 2016

Mobile CH4 laser

Mobile CO2 laser

CH4 in soil gas

CO2 etc. in soil gas

Rn in soil gas

CH4 flux

CO2 flux

Figure 61    Soil gas study areas within the red circles with solid geology (top) and drift geology (below). Site 1 is Preston New Road and Site 2 is Roseacre Wood. Includes mapping data licensed from Ordnance Survey; © Crown Copyright and/or database right 2017. Licence number 100021290 EUL.

The soil gas and flux results are summarised in Figure 62. The CO2 flux data show general seasonal trends with higher fluxes due to enhanced biological activity in the summer compared with the autumn. There were also a greater number of outlying values in August 2015, predominantly at Site 2. Outliers aside, there was little difference in flux between the two sites on each visit.

The soil gas concentrations of CO2 were generally higher in September 2016. This could be the result of higher soil moisture inhibiting the (relatively low) flux from the soil and creating a build-up of gas in the soil pores. The Rn concentrations were lower in 2016 although similar between the two sites. Both moisture and temperature affect CO2 biological production in the soil. Thus, under moist conditions more CO2 is generated in the soil but may be retained if the soil is capped by a relatively wet surface layer. Concentrations of CO2 above 10% are at the higher end of values for biogenic CO2 in soil but not outside the observed range.

Figure 62    Boxplots summarising soil-gas data for the two sites in August 2015 and September 2016 (data for CH4 from September 2016 only).

Methane in the soil showed a narrow range of concentrations in September 2016 with little difference between the two sites (Figure 62) and an upper limit below 3 mg/kg except for one value of 6.5 mg/kg at Site 1. Methane fluxes were very low with maxima of 0.013 and 0.008 g/m2/d at sites 1 and 2 respectively. The median methane concentration was below the atmospheric level of 1.9 mg/kg. Taken together this suggests limited methane exchange with the atmosphere and that any methane being produced is being oxidised to CO2.

Spatial variations in soil gas and flux are compared in Figure 63 to Figure 66. Whilst there are broad patterns of relatively high and low CO2 concentrations in the different areas of measurement (Figure 63), individual points do not tend to match well between surveys with a few exceptions. The precise re-sampling of the same site is not possible and points could differ by a few metres between surveys. Thus the differences seen probably reflect small-scale variations of the soil in terms of biological production as well as physical properties such as permeability and moisture content superimposed on seasonal effects. There is a suggestion, from the highest CO2 concentrations for the 2015 survey, that the inferred fault at Site 2 might lie some 60–130 m to the west. However, this is not corroborated by the 2016 data.

There appears to be little consistency in the spatial patterns of CO2 flux (Figure 64) between visits, but the 2015 survey data are affected by a small number of very high values. Direct comparison between sites for the two surveys shows very little correlation (r2 of 0.0015 for Site 1 and 0.0052 for Site 2).

The one potentially anomalous methane concentration measurement occurs on the northern edge of Site 1 in a lower-lying area close to a stream or drainage ditch. This site also had a high CO2 concentration (17.4%). The higher gas concentrations may be related to the relative wetness of the site. There were also higher CO2 concentrations along the northern margin of Site 1 in 2015. This part of Site 1 generally has low Rn concentrations.

Radon values (Figure 66) also show little consistency between the two surveys (r2 of 0.0409 for the 32 points common to both surveys) despite not being affected by seasonal biological effects. They may however be subject to small-scale variability and are known to be influenced by soil moisture and temperature, atmospheric pressure and wind speed (e.g. Klusman, 1993[6]).

Figure 63    Spatial plots of CO2 in soil gas for the different surveys.



Figure 64    Spatial plots of CO2 flux from the soil for the different surveys.



Figure 65    Methane concentrations in soil gas for September 2016.



Figure 66    Radon in soil gas for August 2015 (top) and September 2016 (below).

The mobile laser data for CH4 (Figure 67) generally have a similar range of values to the soil gas with an upper limit only marginally above the base atmospheric level of 1.9 mg/kg. The CO2 laser data (Figure 68) also show a fairly restricted range with maxima around 530 mg/kg, only a little above the global atmospheric average of around 400 mg/kg. Apparent spatial variability probably reflects diurnal changes and is more likely temporally than spatially controlled. This is suggested by gradual changes as the traverses progressed from one end of a field to the other. Seepage of gas through the soil, of geological or anthropogenic origin, is typified by relatively rapid short-term changes in gas concentration over the scale of seconds to a few minutes, at particular locations, rather than such longer-period variations. No such features were seen in either of the datasets.

Gas ratios can be a useful tool in source attribution, especially CO2/O2 and CO2/N2 plots. These have been used successfully in a number of studies related to geological CO2 storage (Beaubien et al., 2013[7]; Jones et al., 2014[8]; Romanak et al., 2012[9]; Romanak et al., 2014[10]; Schroder et al., 2016[11]). Examples from the Fylde are shown in Figure 69. This shows points plotting close to the ideal biogenic CO2 line (where one mole of O2 is consumed for every mole of CO2 produced) but with scatter, most likely caused by dissolution of a proportion of CO2 into soil pore water, more apparent at Site 2. The Site 1 data show less scatter and define a trend somewhat to the leakage side of the biogenic line. The well-defined trend (r2 of 0.8622) suggests a slight departure from perfect calibration of the instruments rather than a component of deep CO2. The smaller number of data points from site 1 in 2015 that had reliable O2 data (the O2 sensor failed on one instrument) lay on the biogenic trend, supporting this conclusion.

Other possible methods of source attribution include the use of stable or radiogenic carbon isotopes in CO2 and CH4, or noble gas isotopes. These approaches have yet to be applied to our baseline soil gas studies in Lancashire.

Figure 67    Mobile open-path laser data for CH4 for Site 1 (top) and Site 2 (below) from September 2016.



Figure 68    Mobile open-path laser data for CO2 for Site 1 (top) and Site 2 (below) from September 2016.



Figure 69    CO2/O2 ratio plot for soil gas data for Site 1 (top) and Site 2 (below) from September 2016.

Eddy covariance

A CO2 eddy covariance system was installed at the Preston New Road atmospheric monitoring site in January 2016 and has recorded data continuously (with minor breaks) for over 18 months.

The Eddy Covariance (EC) system collects meteorological information and CO2 observations. Post-processing allows CO2 flux to be determined and the covariance of vertical and horizontal wind statistics and CO2 flux to be calculated. The system ran continuously from the 19th Jan 2016 to the present with a short down-period between 4th May 2016 and 19th May 2016.

From the data it appears that CO2 concentration broadly mirrors temperature which in turn follows diurnal and seasonal trends (Figure 70, Figure 71). Biological controls on natural CO2 production give rise to concentration ranges from 210 to over 600 mg/kg although the majority of readings fall between 350 and 450 mg/kg. It is likely that the extreme concentrations are generated from non-local natural and anthropogenic sources, transported to the EC by the wind. Although it is not possible to distinguish natural and anthropogenic sources using the EC, the fully mixed concentration (around 370 mg/kg) can be considered close to the natural background for the site (see Figure 72).

Using wind direction plotted against CO2 concentration, there is a broad tendency for increased CO2 concentrations when the wind is from the east and lower concentrations from the west (Figure 73). This is likely due to the proximity of the coast to the west of the Preston New Road site, where there are fewer potential biological or anthropogenic sources and relatively clean oceanic air reaches the instrument (see also atmospheric monitoring section). As with the CO2 concentration, CO2 flux shows clear diurnal and annual trends consistent with natural biological processes (Figure 74). CO2 flux increases during the summer months, and during this period there is also the greatest spread in flux values.

Figure 70    Atmospheric temperature at the Preston New Road site.
Figure 71    CO2 concentration (ppm; mg/kg) from EC data at the Preston New Road site.
Figure 72    Fully mixed (background) atmospheric CO2 concentration at the Preston New Road site, determined by plotting CO2 concentration against wind speed from EC data.
Figure 73    Atmospheric CO2 concentrations from EC data related to wind direction. Easterly winds tend to give higher concentrations while westerly winds are associated with lower concentrations.
Figure 74    Atmospheric CO2 flux calculated from EC data at the Preston New Road site.

Summary

Seasonal variability is evident in the soil gas and flux results from the Fylde. Meaningful data are best obtained under the relatively dry soil conditions from spring to autumn. Soil gas concentrations can be higher under wetter conditions, due to surface capping, making them more difficult to interpret. The optimal time for soil-gas surveys in the UK during shale gas operations would seem to be in the autumn, when biological activity (plant and microbial) is reduced but before the soil becomes saturated in the winter. In the autumn, gas concentrations and fluxes are more restricted in range, making any anomalous values easier to detect. Because there is still some plant growth, the effects (visually detectable or through remote sensing techniques) of any gas leakage on the vegetation should still be apparent. The lack of vegetation in harvested arable fields, or those ploughed prior to re-seeding, removes any visual clues of the impact of any gas. This is also true when there is frost or snow cover. Ideally a soil gas survey needs to be carried out under stable conditions with dry soil and no significant rainfall.

In the two Fylde surveys, soil CO2 concentrations covered a wide range, up to almost 18%, although a high proportion was below 2% in August 2015. Soil conditions were inferred to be damper in September 2016 such that probable surface capping caused the majority of readings to be between 3 and 8%. CO2 flux was generally below 40 g/m2/d in August 2015 and 30 g/m2/d in September 2016. There was a small number of much higher measurements in the August visit (up to around 180 g/m2/d at Site 2), whilst the maximum was around 50 g/m2/d in the September survey, consistent with reducing biological activity in the autumn.

Methane concentrations were low in September 2016, both in the soil gas and atmosphere, except for one low-lying wetter site. Radon was relatively variable spatially and temporally. In the autumn it should, therefore, be possible to detect relatively small additional gas emissions through the soil, particularly for CH4, despite this being readily oxidised to CO2 by soil microbes unless flux rates are relatively high.

Continuous eddy covariance monitoring data show clear diurnal and seasonal trends as well as the influence of meteorological events. Wind direction affects the CO2 concentration with cleaner air from over the sea contrasting with more contaminated air from landward sources.

Once a reasonable body of soil gas baseline data have been collected, a fuller geostatistical analysis will be possible. This would allow optimization following the principles set out by Marchant and Lark (2007)[12] (i) to support reliable characterization of space-time mean concentrations and fluxes and their spatio-temporal variation and (ii) to allow the development of a statistical model of the variability of the measurements which can be used to support decisions on sampling requirements for operational monitoring beyond the baseline phase of the project.

References

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