Updated IAAs and LAAs will become available in 2016 and should then replace the ...... The decision tree shown in Figure
CSIRO LAND AND WATER
Decision Support System for Investigating Gas in Water Bores and Links to Coal Seam Gas Development
Dirk Mallants, Matthias Raiber, and Phil Davies August, 2016 For: Queensland Department of Natural Resources and Mines
Copyright and disclaimer © 2016 CSIRO To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO.
Citation This report should be cited as: Mallants D, Raiber M, and Davies P (2016) Decision Support System for investigating gas in water bores and links to coal seam gas development. Project report prepared by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) for the Queensland Department of Natural Resources and Mines.
Important disclaimer CSIRO advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it. CSIRO is committed to providing web accessible content wherever possible. If you are having difficulties with accessing this document please
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Table of Contents Decision Support System for Investigating Gas in Water Bores and Links to Coal Seam Gas Development i Table of Contents ......................................................................................................................................... iii Figures .......................................................................................................................................................... v Tables ....................................................................................................................................................... viii Acknowledgments ......................................................................................................................................... x Executive summary
xi
1
Introduction
1
2
Desktop process to assess causes for increased gas in bores
2
2.1
Properties of methane and associated risks ................................................................................... 2
2.2
Gas sources, release mechanisms, and transport pathways .......................................................... 4
2.3
Elements of the Decision Support System ...................................................................................... 7
3
LEVEL 1: desktop assessment
3.1
Methodology ................................................................................................................................. 10
3.2
Observed gas in water bores ......................................................................................................... 13
3.3
Aquifer protection level ................................................................................................................ 13
3.4
Immediately and long‐term affected areas .................................................................................. 18
3.5
Distance to gas pathway ............................................................................................................... 21
3.6
Proximity to CSG production wells ................................................................................................ 26
3.7
Bore construction and age ............................................................................................................ 28
4
LEVEL 2: Hydrochemical analyses
4.1
Previous studies in Australian coal basins .................................................................................... 34
4.2
Assigning hydrostratigraphic units at bore screens ...................................................................... 39
4.3
Normal distribution testing for hydrochemical data .................................................................... 41
4.4
Derivation of trigger levels for key hydrochemical parameters ................................................... 42
4.5
Trend analysis of hydrochemical data .......................................................................................... 45
4.6
Additional cluster analysis of hydrochemical parameters ............................................................ 49
4.7
Summary of Level 2 decision tree ................................................................................................. 52
5
LEVEL 3: Methane analyses
5.1
Previous studies in Australian coal basins .................................................................................... 54
5.2
Normal distribution testing for methane data .............................................................................. 55
5.3
Derivation of trigger levels for methane ....................................................................................... 58
10
34
54
iii
5.4
Trend analysis of methane data .................................................................................................... 60
5.5
Geographic mapping of methane concentrations ........................................................................ 61
5.6
Summary of Level 3 decision tree ................................................................................................. 62
6
Summary
63
Glossary
64
References
65
Appendix 1 Statistical analysis of hydrochemistry
69
Appendix 2 Hierarchical cluster analysis
80
Appendix 3 Geographic maps of methane concentration
92
Appendix 4 Hypothetical examples
iv
100
Figures Figure 1 Methane gas solubility as function of temperature and salinity at atmospheric pressure (based on data from Wiesenburg and Guinasso (1979)). ........................................................................................ 3 Figure 2 Methane gas solubility as function of pressure and temperature (based on data from Duan et al. 1992). Right plot provides a magnified view of the shaded area in the left plot. ........................................ 4 Figure 3 Groundwater level decline for water bore in the Gubberamunda Sandstone – levels in m below surface (top) and m AHD (Australian Height Datum) (bottom) (Coal Seam Gas Globe 2015). .................... 5 Figure 4 Three‐level Decision Support System to screen water impaired bores or bores with increased gas. For LEVEL 1 trigger levels, see Table 4. Sampling for microbiological analysis is described in Smith‐ Comeskey (2015). ......................................................................................................................................... 9 Figure 5 Conceptual Model of the Groundwater Systems in the Surat Cumulative Management Area (QWC 2012) ................................................................................................................................................ 14 Figure 6 Stratigraphy of the Surat basin (QWC, 2012). .............................................................................. 15 Figure 7 Stratigraphic table of the Surat Basin with indication of main aquifers, aquitards and APL score. .......................................................................................................................................................... 17 Figure 8 Groundwater level variation for water bore in the Condamine River Alluvium (Coal Seam Gas Globe 2015) ................................................................................................................................................ 18 Figure 9 Extent of immediately affected areas (Coal Seam Gas Globe 2015). ........................................... 20 Figure 10 Extent of the long‐term affected areas (Coal Seam Gas Globe 2015). ....................................... 21 Figure 11 Schematic representation of groundwater‐driven gas migration from a gas pathway to a water bore. ........................................................................................................................................................... 22 Figure 12 Top: Conceptual model for simulating 3D solute transport in groundwater based on cylindrical coordinates. Bottom: time history of chemical breakthrough at different bores. .................................... 22 Figure 13 Calculated breakthrough curves at five different times since gas release (v = 20 m/y). ........... 23 Figure 14 (a) Calculated dilution factor (Cmax/C0) for three values of pore velocity (v). (b) DGP score derived from dilution factor for three values of pore velocity. ................................................................. 23 Figure 15 Conceptual diagram of gas migration in the Surat Basin near Roma due to pressure gradient and buoyancy, and migration pathways (APLNG 2010). ............................................................................ 25 Figure 16 Conceptual model of potential gas flow towards CSG well and water bore. Gas pathway 1 is potentially due to groundwater abstraction, gas pathway 2 is potentially due to CSG extraction. .......... 26 Figure 17 Minimum separation distance required to avoid neighbouring bores impacting one another. 27 Figure 18 Design of multiple aquifer bore (left) and flowing aquifer bore (NUDLC 2012). ....................... 28 Figure 19 Potential pathways for leakage along a bore with poor integrity, including flow along the material interfaces (a, b, f) and through well cements and casings (c, d, e). (Nordbotten et al. 2005) .... 29 Figure 20 Schematic representation of potential coal seam gas preferential pathways via leaky bores and faults. Preferential pathway (1) considers migration of methane into the water bore via corroded bore casing while pathway (2) considers gas flow through fractured/degraded bore seal (for details of pathway, see Figure 19 a, c, f). ................................................................................................................... 30 Figure 21 Bores in the Surat Basin, by age and depth and divided in age classes (source: SKM 2013). .... 33 Figure 22 Bores by depth age and casing material (source: SKM 2013). ................................................... 33
v
Figure 23 Generic representation of aquifer interactions along the recharge flow path and methane generation. Note: These processes are shown along the flow path, but they do not necessarily occur sequentially (Dahm et al. 2014). ................................................................................................................ 35 Figure 24 (a) and (b): Na/Cl ratios versus alkalinity/Cl ratios and Na/Cl versus Na/alkalinity ratios respectively for CSG groundwater samples from (a) and (b) Surat Basin (Roma and Dalby). (c) and (d): Total chloride concentrations for CSG groundwaters from the Surat Basin (Roma and Dalby, QLD), the Illinois Basin (USA) , and the Bowen Basin (QLD) versus: (a) residual alkalinity, where residual alkalinity is defined as (HCO3+CO3) − (Ca + Mg); and (b) pH (Source: Owen et al. 2015). ............................................ 36 Figure 25 Surat Basin stratigraphic and hydrologic units (Hamilton et al. 2014) ....................................... 37 Figure 26 Schoeller plot of water quality data from the Roma (solid lines) and Dalby (dotted lines) field studies (source: Papendick et al. 2011). ..................................................................................................... 38 Figure 27 Cross‐section through western Clarence‐Moreton Basin/eastern Surat Basin showing a hypothetical example where multiple screens occur in different formations (Hutton Sandstone and Evergreen assigned). In this example, the methane concentration cannot be included in the baseline assessment. ................................................................................................................................................ 40 Figure 28 Negatively skewed distribution, normal distribution and positively skewed distribution. ....... 42 Figure 29 Truth table used in hypothesis testing. ...................................................................................... 43 Figure 30 EPA reference power curves for three typical yearly statistical evaluation schedules — quarterly, semi‐annual, or annual (modified from US EPA 2009). ............................................................. 44 Figure 31 Normal distribution plot with indication of sigma (σ) levels and corresponding percentage of outcomes within 1, 2, etc. sigma levels from the mean (µ). ...................................................................... 45 Figure 32 Time series of water quality parameters for bore # 22372 (data source: DNRM, 2015). .......... 48 Figure 33 Frequency of cluster membership for major aquifers................................................................ 50 Figure 34 Distribution of clusters in the Surat and western Clarence‐Moreton basins and simplified surface geology for all aquifers. Major characteristics such as water type and median electrical conductivity and methane concentrations are also shown. ...................................................................... 51 Figure 35 Level 2 decision tree regarding hydrochemical and microbiological analyses........................... 53 Figure 36 Box‐Whisker plots of methane gas concentration in eight aquifer groups. Caps or whiskers at the end of each box indicate extreme values (10/90 percentiles), the box is defined by the lower and upper quartiles, and the line in the centre of the box is the median (values indicated). Aquitard formations in between aquifers are included. BMO = Bungil‐Mooga‐OralloOrallo Formation; WCM = Walloon Coal Measures. ............................................................................................................................. 57 Figure 37 Cumulative probability plots for dissolved methane concentration in major aquifers in the Surat. Vertical lines 3D and 4D represent mean concentration to detect a 3, respectively 4 standard deviation increase above the true mean background concentration. ....................................................... 59 Figure 38 Map of the Surat Cumulative Impact Area with dissolved methane measurements in the Bungil‐Mooga‐Orallo Formations (source data from the baseline surveys (CH4 concentrations) and DNRM (2014) for CSG wells). ...................................................................................................................... 61 Figure 39 Level 3 decision tree regarding methane analyses (FA = forensic analysis). .............................. 62 Figure 40 Dendrogram of cluster analysis identifying seven major clusters. The separation threshold could be lowered further to increase the number of clusters. The clustering is based on the major ions, EC and methane concentrations. pH was not included into the clustering procedure as many groundwater chemistry records did not have a measured value for pH. Furthermore, many samples had either field pH or lab pH, but not both. All input parameters were log‐transformed prior to the clustering procedure, and outliers were removed. .................................................................................................... 80
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Figure 41 Distribution of clusters in the Surat and western Clarence‐Moreton basins and simplified surface geology for the alluvial aquifers. Major characteristics such as water type and median electrical conductivity and methane concentrations are also shown. ...................................................................... 84 Figure 42 Distribution of clusters in the Surat and western Clarence‐Moreton basins and simplified surface geology for the BMO Group. Major characteristics such as water type and median electrical conductivity and methane concentrations are also shown. ...................................................................... 85 Figure 43 Distribution of clusters in the Surat and western Clarence‐Moreton basins and simplified surface geology for the Gubberamunda Sandstone. Major characteristics such as water type and median electrical conductivity and methane concentrations are also shown. ....................................................... 86 Figure 44 Distribution of clusters in the Surat and western Clarence‐Moreton basins and simplified surface geology for the Springbok Sandstone. Major characteristics such as water type and median electrical conductivity and methane concentrations are also shown. ....................................................... 87 Figure 45 Distribution of clusters in the Surat and western Clarence‐Moreton basins and simplified surface geology for the Walloon Coal Measures. Major characteristics such as water type and median electrical conductivity and methane concentrations are also shown. ....................................................... 88 Figure 46 Distribution of clusters in the Surat and western Clarence‐Moreton basins and simplified surface geology for the Hutton Sandstone. Major characteristics such as water type and median electrical conductivity and methane concentrations are also shown. ....................................................... 89 Figure 47 Distribution of clusters in the Surat and western Clarence‐Moreton basins and simplified surface geology for the Precipice Sandstone. Major characteristics such as water type and median electrical conductivity and methane concentrations are also shown. ....................................................... 90 Figure 48 Distribution of clusters in the Surat and western Clarence‐Moreton basins and simplified surface geology for the Clematis Group. Major characteristics such as water type and median electrical conductivity and methane concentrations are also shown. ...................................................................... 91 Figure 49 Map of the Surat Cumulative Impact Area with dissolved methane measurements in the Condamine alluvium (source data from the baseline surveys (CH4 concentrations) and DNRM (2014) for CSG wells). .................................................................................................................................................. 93 Figure 50 Map of the Surat Cumulative Impact Area with dissolved methane measurements in the Gubberamunda Sandstone (source data from the baseline surveys (CH4 concentrations) and DNRM (2014) for CSG wells). ................................................................................................................................. 94 Figure 51 Map of the Surat Cumulative Impact Area with dissolved methane measurements in the Springbok Sandstone (source data from the baseline surveys (CH4 concentrations) and DNRM (2014) for CSG wells). .................................................................................................................................................. 95 Figure 52 Map of the Surat Cumulative Impact Area with dissolved methane measurements in the Walloon Coal Measures (source data from the baseline surveys (CH4 concentrations) and DNRM (2014) for CSG wells). ............................................................................................................................................. 96 Figure 53 Map of the Surat Cumulative Impact Area with dissolved methane measurements in the Hutton Sandstone (source data from the baseline surveys (CH4 concentrations) and DNRM (2014) for CSG wells). .................................................................................................................................................. 97 Figure 54 Map of the Surat Cumulative Impact Area with dissolved methane measurements in the Precipice Sandstone (source data from the baseline surveys (CH4 concentrations) and DNRM (2014) for CSG wells). .................................................................................................................................................. 98 Figure 55 Map of the Surat Cumulative Impact Area with dissolved methane measurements in the Clematis Group (source data from the baseline surveys (CH4 concentrations) and DNRM (2014) for CSG wells). .......................................................................................................................................................... 99 vii
Tables Table 1 Linkage between gas source/pathway and assessment parameters regarding gas in bores. ........ 6 Table 2 Parameters and their scores used to calculate SCORE1 of the Level 1 desktop assessment. High scores mean higher likelihood that bore impairment is caused by CSG extraction................................... 11 Table 3 Likelihood levels, descriptors and their corresponding parameter score. Grey shaded cells will trigger Level 2 assessment. ........................................................................................................................ 12 Table 4 Trigger level for Step 2 investigations and likelihood levels included (SCORE1≥324) or excluded (SCORE1 0.05 (at the 95% confidence level). ......................................................................................... 47 Table 13 Descriptive statistics of methane concentrations (mg/L) for eight aquifers in the Surat Basin (StDev=standard deviation). 3D = mean concentration to detect a 3 standard deviation increase above the true mean background; 4D = mean concentration to detect a 4 standard deviation increase above the true mean background. SW = Shapiro‐Wilk test for normality (p‐values > 0.05 indicate data is normally distributed). ................................................................................................................................. 56 Table 14 Action levels for dissolved methane in water wells (source: Eltschlager et al. 2001; Environmental Consultants 2012) .............................................................................................................. 60 Table 15 Descriptive statistics for calcium (Ca) concentration (mg/L) in groundwater bores. .................. 69 Table 16 Descriptive statistics for magnesium (Mg) concentration (mg/L) in groundwater bores. .......... 70 Table 17 Descriptive statistics for potassium (K) concentration (mg/L) in groundwater bores. ............... 71 Table 18 Descriptive statistics for sodium (Na) concentration (mg/L) in groundwater bores. .................. 72 Table 19 Descriptive statistics for chloride (Cl) concentration (mg/L) in groundwater bores. .................. 73 Table 20 Descriptive statistics for bicarbonate (HCO3) concentration (mg/L) in groundwater bores. ...... 74 Table 21 Descriptive statistics for sulfate (SO4) concentration (mg/L) in groundwater bores. ................. 75 Table 22 Descriptive statistics for total dissolved solids (TDS) concentration (mg/L) in groundwater bores. .......................................................................................................................................................... 76 Table 23 Descriptive statistics for fluoride (F) concentration (mg/L) in groundwater bores. .................... 77 Table 24 Descriptive statistics for pH in groundwater bores. .................................................................... 78 Table 25 Descriptive statistics for alkalinity (mg/L) in groundwater bores. ............................................... 79 Table 26 Median major ion concentrations, EC, methane, pH and major ion ratios ................................. 81 viii
Table 27 Cluster membership of major hydrostratigraphic units. ............................................................. 82
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Acknowledgments The authors would like to acknowledge the funding from the Queensland Government CSGCU. Useful discussions were held with a number of people. In particular, David Free and Ross Carruthers gave much useful information on the topic and provided data, technical reports and scientific papers. This report was subject to peer review by Dr. Allison Hortle (CSIRO) and Mr. Stan Smith (CSIRO).
x
Executive summary
Methane in water bores is a major concern in areas of coal seam gas (CSG) development. There are risks associated with ignition and asphyxiation in closed spaces around bores that create real concern. There are also other risks, such as gas lock in pumps, colour and odour impacts from water quality changes, toxicity due to other gases and build‐up of gases affecting the integrity of the bores. In Queensland, increasingly the complaints are related to increased gas in bores causing problems with the operation of pumps in sub‐ artesian bores and causing blockages in distribution lines from artesian bores. A desktop based assessment tool and guidelines or Decision Support System (DSS) have been developed on which to base the determination of coal seam gas operations as a likely cause of increased gas. The DSS allows easy entry of bore information and is underpinned by a knowledge base that is derived from state‐of‐ the‐science in the Surat and southern Bowen basins. For instance, the current conceptual framework for the regional model of the Surat Cumulative Management Area (CMA) and calculated groundwater impacts from CSG has been used as an underpinning knowledge platform. Links to data from ongoing monitoring programs such as the Coal Seam Gas Globe online tool embedded in Google EarthTM can also be built‐in. The DSS involves a three‐level assessment, where the first level is a desktop assessment, the second level involves hydrochemical and microbiological analyses, and level three involves methane analyses possibly followed by further forensic investigations to identify the source of methane. The desktop assessment (Level 1 of the DSS) involves an evaluation of site‐specific parameters relevant to assess whether or not bore impairment is likely caused by nearby coal seam gas extraction. Six parameters are considered in the assessment: i) observations of gas in bores, ii) number of aquitards between a bore screen and a coal seam gas target formation, as an indication of the isolation of water bores from a depressurised coal seam, iii) horizontal distance of the bore to the Immediately Impacted Area (IIA) as calculated with the groundwater model for the Surat CMA, iv) distance to a gas pathway, v) whether or not the bore intersects a coal seam gas target formation and the proximity to a CSG well, and vi) the bore construction material and age as indicators of bore integrity. Scores assigned to each of these six parameters are combined in an overall score and interpreted as a likelihood that the bore impairment may be caused by coal seam gas extraction. If the score exceeds a threshold, Level 2 of the DSS is triggered. The hydrochemical and microbiological analyses (Level 2 of the DSS) involve testing the hypothesis whether the hydrochemical data fall within the range of natural variability and therefore are not an indication of altered geochemical processes triggered by increased methane gas. Such processes can occur in a zone distant from a water bore and requires such water to be transport to the bore (‘off‐site’ processes), and/or in the bore field (‘on‐site’ processes). Typical ‘off‐site’ processes associated with (biogenic) methane (CH4) formation under anaerobic conditions in the presence of organic matter (coal, peat bogs and lignite deposits) include microbial sulfate (SO42‐) reduction, and bicarbonate (HCO3‐) enrichment which subsequently causes calcium (Ca2+) and magnesium (Mg2+) precipitation. Additional ‘on‐site’ biogeochemical processes may occur at the bore field, including sulfate‐to‐sulfide reduction (affecting the sulfur cycle) and methane‐to‐carbon dioxide oxidation (affecting the carbonate system equilibrium). Natural variability in hydrochemistry has been established for key aquifers in the Surat Basin using up‐to‐date data. A rigorous statistical analysis of about 560 water samples collected between 2010 to 2013 was undertaken to determine natural variability of hydrochemical parameters. If more than four samples are available for testing and the data are not significantly different from the background, a trend analysis is undertaken to test if the data may be showing indications of geochemical processes triggered by increased methane concentrations. If the hydrochemical parameters exceed their trigger values (the 3D or 4D statistics, i.e. mean concentration to detect a 3 or 4 standard deviation increase above the background mean), then methane analyses are recommended. xi
The methane analyses (Level 3 of the DSS) involve measurement of methane in water bores and comparison of methane concentrations with trigger values (3D or 4D statistics). Trigger values are based on pre‐ development methane concentrations and account for spatial and, to a lesser degree, on temporal variability. In developing the trigger values, statistical methods have been used that are scientifically rigorous. Exceeding these trigger values is an indication of increased methane gas in groundwater, possibly related to coal seam gas extraction. Additional forensic analysis to determine the source of methane is then recommended.
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1 Introduction
The rapid development of coal seam gas in Queensland has led to a number of issues, including complaints associated with increased gas in bores and the problems it causes in operation of pumps. The Coal Seam Gas Compliance Unit (CSGCU) in the Queensland Department of Natural Resources and Mines (DNRM) is responsible for investigating complaints associated with impacts to water bores from coal seam gas (CSG) development in the Surat and Bowen basins in Queensland. Increasingly the complaints are related to increased gas in bores causing problems with the operation of pumps in sub‐artesian bores and causing blockages in distribution lines from artesian bores. The CSGCU had previously contracted CSIRO to undertake a literature review to support decision making around the issue. The review addressed the issue of an accepted methodology for sampling, analysis, and data interpretation to address risks associated with gas in water bores (Walker and Mallants, 2014). The report includes i) the occurrence of gas in water bores prior to the commencement of the coal seam gas industry in Queensland, ii) methods for undertaking investigations into gas in water bores, iii) methods for determining methane gas migration potential including gas migration processes and mitigating factors affecting vertical/lateral gas migration, and (iv) Investigations undertaken into gas in water bores to date in Australia and in particular the Surat and Bowen basins. As a follow‐on from the literature review study, the CSGCU has currently contracted CSIRO to develop 1) guidelines and assessment tool (desktop based) on which to base the elimination of coal seam gas operations as a likely cause of increased gas; and 2) an operational procedure for undertaking field investigations and analysis of the gas produced by a water bore (reported separately). The terms of reference are specific with respect to Stage 2, namely:
1) Development of a desktop process to identify bores where increased gas is not associated with CSG operations (Activity 1). Given the number and type of gas complaints received to date, there is a need to develop a desk top procedure to determine the likelihood of coal seam gas operations as a cause of increased gas in water bores. This might consist of a decision support system (DSS) that could assess information available such as bore construction and bore history, hydrogeological relationship between an ‘increased gas’ aquifer and CSG target formation, distance from bore to gasfield, presence of other sources of methane, etc. This desktop process should put the CSG Compliance Unit in a position to undertake or direct complex investigations required to quantify and analyse the presence of gas in bores if required.
2) Development of an operational procedure for undertaking a field investigation and analysis of the gas produced by a water bore (Activity 2). The key issue raised by landholders is that the gas being produced by their bore is increasing and is believed to be a result of CSG development. It is suggested that an operational procedure be developed for undertaking a field investigation and analysis of the gas produced by a water bore, both artesian and sub‐artesian. This should include the development of a method, field testing, risk assessment and workers health and safety (WHS) requirements, a fully documented procedure and training. The CSG Compliance Unit is available to assist with sourcing field testing sites and other resources if necessary. An information sheet, “Methane Gas in Water Bores – Recommendations for Field Sampling” (CSIRO, 2015) will be developed in conjunction with the review. The word ‘bore’ has been used in this report to refer collectively to both agricultural bores, domestic bores and industrial bores.
1
2 Desktop process to assess causes for increased gas in bores
2.1 Properties of methane and associated risks Methane is a primary component of natural gas along with other light hydrocarbons. Natural gas is typically accumulated in a subsurface reservoir ‐ any rock formation with adequate porosity, fractures, or sorption potential that can store liquid or gas hydrocarbons. The different forms of natural gas are generally categorised into conventional and unconventional gas. Conventional gas is obtained from reservoirs that largely consist of porous sandstone formations capped by impermeable rock. The gas can move to the surface through the gas wells without the need to pump. Unconventional gas is generally produced from complex geological systems that prevent or significantly limit the migration of gas and require innovative technological solutions for extraction. The difference between conventional and unconventional gas is the geology of the reservoirs from which they are produced (Cook et al. 2013). There are several types of unconventional gas such as coal seam gas, shale gas and tight gas. Coal seam gas is entirely adsorbed onto the coal matrix. Movement of coal seam gas to the surface through gas wells normally requires extraction of formation water from the coal cleats and fractures. Shale gas is generally extracted from a clay‐rich sedimentary rock which has naturally low permeability. Tight gas is trapped in ultra‐compact reservoirs characterised by very low porosity and permeability (Cook et al. 2013). Methane is the largest component of the gas causing concern in water bores in the Surat and Bowen basins. It is a colourless, odourless and non‐toxic gas, but poses an asphyxiation hazard if it collects in an enclosed space (e.g. basement) at concentrations high enough to displace existing air and create an oxygen‐deficient environment (at a concentration of over 50% in air) (ATSDR 2001; Gov. Canada 2004). Many of the specific properties of methane can be found in Stalker (2013). Methane in water bores may be present as ‘dissolved gas’ or as ‘free gas’ and ‘dissolved gas’ (if methane is present in a bore, some of it will always be dissolved, even in the presence of free gas.). One of the analogies used to differentiate these two forms is that of the soda bottle. While the lid is sealed, pressure keeps the gas dissolved in the liquid. Removing the lid causes a drop in pressure, allowing the previously dissolved gas to form bubbles (exsolve1) and rise to the liquid surface as free gas. Agitation due to pumping and movement through samplers can lead to free gas release at under‐saturated conditions. Methane usually only exsolves from a still solution, if the concentration of methane in the fluid exceeds its dissolved gas saturation point or solubility (Jackson et al. 2013). For a sample at the land surface, the solubility at normal levels of atmospheric pressure is 24.7 mg/L (or 34.6 mL/L) at 20 °C and 20.7 mg/L (or 29 mL/L) at 30 °C (Wiesenburg and Guinasso 1979; Hirsche and Mayer 2009). Gas solubility decreases with increasing temperature and salinity and increases with increasing pressure. The effects are non‐linear in all cases. A temperature difference of 20 °C (e.g. between 10 and 30 °C) for fresh water (zero salinity) results in a difference in solubility of 10 mg/L/atm. At 20 °C, methane solubility ranges from 25 mg/L for fresh water to 19.3 mg/L at 40,000 mg/L salinity (Figure 1). 1
Gas to separate out from groundwater and form a free phase
2
Hirsche and Mayer (2009) cite the example of a 360 m column of water leading to a methane solubility of 863 mg/L at 25 °C. Pressure effects can lead to water degassing as it is brought from depth to atmospheric pressure at the surface. This is similar to removing the lid of a soda bottle resulting in free gas coming to the surface. The combined effect of pressure and temperature on methane solubility is shown in Figure 2. In the pressure range relevant to coal seam gas extraction in Australia, i.e. from 0 to approximately 100 bar2 (=10 MPa or approximately 1020 meter of head), the solubility at 30⁰ C increases from 20.7 mg/L at 0 bar to 1520 mg/L at 100 bar, respectively. In other words, methane is 73 times more soluble at depths corresponding to 1020 m of head compared to atmospheric conditions.
Figure 1 Methane gas solubility as function of temperature and salinity at atmospheric pressure (based on data from Wiesenburg and Guinasso (1979)).
Because it is odourless, methane can accumulate undetected in bores and bore enclosures that are not properly vented. Methane is extremely flammable and can be easily ignited by heat, sparks or flames. Methane is explosive at volumes of 5 per cent to 15 per cent (50,000 to 150,000 mg/L) in air. Methane is also an asphyxiant at a concentration of over 50 per cent in air. Although methane will rise, it can displace oxygen in confined spaces and hence such spaces can become vulnerable. Such risks can be mitigated through monitoring and proper ventilation. There are a number of useful sources of information on this (National Groundwater Association (NGWA) 2013a; NGWA 2013b; Indiana Department of Natural Resources; Pennsylvania Department of Environmental Protection (DEP) 2011; Griffiths 2007). Gas may also leak from the bore into the shallow sub‐surface and then leak into closed buildings (Pennsylvania DEP 2013). Some water quality issues can be treated with some form of treatment plants. The bubbling of gas in water bores can also lead to total bore impairment. For example, it can affect pumps as the gas bubbles can lead to a “gas lock”, in which the gas bubbles adhere to the impeller and impede the water flow. Harris et al. (2012) reported on the need to replace bore pumps due to the motors burning out as a result of “cavitation” when the dissolved gas comes out of solution. Pump shrouds or sleeves could be 2
Gauge pressure = absolute pressure minus atmospheric pressure 3
used or the type of pump changed (NGWA 2013a). The shroud or sleeve is a tube open only at its base enclosing the submersible pump. Gas bubbling can affect water quality in at least two ways. First, bubbles cause sediments that accumulate at the bottom of water bores to move through the water column, which in turn leads to water being used going from being clear to being “coloured, turbid, slimy, and smelly”. Secondly, in certain circumstances, it can lead to the conversion of dissolved sulfate into “odiferous, noxious, and toxic” sulfides due to sulfate‐ reducing bacteria (Gorody 2012).
Figure 2 Methane gas solubility as function of pressure and temperature (based on data from Duan et al. 1992). Right plot provides a magnified view of the shaded area in the left plot.
2.2 Gas sources, release mechanisms, and transport pathways An assessment of the possible causes of increased gas in water bores involves an analysis of primary gas sources, primary release mechanisms, transport pathways, secondary gas trapping or storage and secondary release mechanisms (Walker and Mallants 2014). Moreover, the presence of gas in water bores may be due to a combination of mechanisms, which work sequentially or in parallel. Because understanding of these sources, mechanisms and pathways is critical to assess causes of gas in bores in a particular hydrogeological setting, a summary is provided that will form the basis for developing the Decision Support System. Primary gas sources refers to the location where gas originated from, by desorption of methane to the free gas phase due to depressurisation. For the Surat Basin, this mainly refers to coal seams in the Walloon Coal Measures, although there exist shallower coal seams in the upper portion of the Springbok Sandstone that may form a primary source of gas (Baldwin and Thomson 2014). For the Bowen Basin, the Bandana Formation is the target formation. Secondary gas sources exist where migrating gas has been trapped by physical barriers, such as faults or changes in geological facies (e.g. transition from coarse to fine particles with corresponding decrease in pore size). Gas migration may have occurred in earlier times due to desorption, followed by upward migration by buoyancy through permeable sandstones, faults and fractures. For example, where the lower Springbok Sandstone had eroded into the Walloon Coal Measures, the sandstones can become charged with methane from the underlying Walloon Coal Measures (Scott et al. 2006). It is believed such secondary 4
gas sources have been responsible for some of the historical blowouts that have occurred during drilling water bores (Walker and Mallants 2014). Depressurisation of coal seams is the primary gas release mechanism responsible for releasing sorbed methane. Depressurisation may be due to a combination of reasons, including i) water extraction from water bores for domestic use and stock (Figure 3) and CSG wells, ii) drilling for bore construction which will release pressure from below confining layers, iii) water migration from deeper to shallower formations through preferential pathways (abandoned coal exploration bores, conventional gas and petroleum wells, and water bores), and/or iv) natural water pressure changes following droughts (pressure decrease) and floods (pressure increase). In case of depressurisation due to groundwater abstraction from water bores or water loss via pathways provided by abandoned bores and wells, the gas may be transported via such bores or via other pathways such as faults, fractures or more permeable zones.
Figure 3 Groundwater level decline for water bore in the Gubberamunda Sandstone – levels in m below surface (top) and m AHD (Australian Height Datum) (bottom) (Coal Seam Gas Globe 2015).
In addition to the above anthropogenically enhanced mechanisms, both short‐term and long‐term natural mechanisms exist. The short‐term mechanisms include pressure changes in shallower formations due to i) flooding causing trapped gas to dissolve and migrate with the groundwater, and ii) infiltration of nutrient‐ and bacteria‐rich floodwaters triggering production of biogenic methane. An example of a long‐term mechanisms is long‐term climate change causing groundwater levels to drop which reduces the hydrostatic pressure in coals seams and thus reduces its capacity to retain the gas. Transport pathways refers to the routes that may be taken by the free methane gas from its primary or secondary source to a CSG well, a productive water bore, abandoned bores or overlying aquifers. Free gas will migrate vertically upwards through the formation due to buoyancy. This pathway can be quite tortuous or winding, depending on the vertical and horizontal permeabilities of the formations through which it migrates, dipping geological strata, or by sudden changes in permeability due to conduits or barriers (e.g. faults).
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It is important to recognise that depressurisation due to CSG wells can result in some of the gas not being captured by the wells. For example, the depressurisation may be propagated from a producing CSG well to coal seams not in the vicinity of the well, but still within the “cone of depression”; gas released from such distant coals may not be captured by that CSG well and will migrate in a direction controlled by the reservoir properties. When gas desorption occurs relatively close to a producing CSG well, gas migrates generally updip due to buoyancy. In this case, the effect spreads generally updip at a rate governed by the geologic dip and the reservoir properties. In case the cement bond between the casing and the borehole is incomplete, channels have developed through the cement allowing gas to migrate from the production zone up and into overlying formations, aquifers, and potentially into water bores (Baldwin and Thomson 2014). While the effect of depressurisation, gas mobilisation and migration may be observed at a considerable distance from a gas source, especially under conditions of a geological dip, assessments by Baldwin and Thomson (2014) seem to indicate that methane would probably not travel beyond a distance of 10‐15 km from a CSG well. The discussion on gas source, gas pathway and release mechanism is relevant to understanding the causes of gas in bores and helps define practical parameters that capture one or several features of gas source and pathway and link them to the hydrogeological settings of the water bore. Table 1 identifies six parameters that will be used to develop a decision support system to assess the possible causes of gas in bores, and links each parameter with a gas source or gas pathway as a means to evaluate the likelihood of a cause‐ effect relation. Table 1 Linkage between gas source/pathway and assessment parameters regarding gas in bores.
Gas source, gas Relevance to gas in bores pathway, release mechanism, impact
Assessment parameter in DSS
Primary gas source
Immediately Affected Area (IAA)
Free gas will mainly be present in depressurised production areas of the SCMA, identified as Immediately Affected Area (IAA) and Long‐term Affected Area (LAA) (QWC 2012). Bores present in these areas may be directly affected if water is taken from within the target coal seam formation, or indirectly when water is taken from aquifers above or below the depressurised coal seams, provided the depressurisation has propagated into these aquifers. For overlying aquifers, bores may also be impacted if gas has been transported through preferential pathways from the primary source area into a beneficial aquifer. The 2012 IAA and LAA will be updated once the new OGIA groundwater impact study becomes publicly available. This will also include changes in CSG development scenarios.
Primary gas source
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The screened section of a water bore residing within the primary gas source is a strong indication for the likelihood for gas in water bores being related to this gas source.
For bores outside the IAA the likelihood for gas entering a bore being related to CSG extraction is small but not impossible because depressurisation may still occur due to other reasons, e.g. groundwater abstraction for agricultural and stock and domestic use. The likelihood is evaluated on the basis of comparing the observed head decrease in an impacted bore with annual head variations. For bores inside the IAA the likelihood of gas entering a bore being related to CSG extraction is determined by comparing the observed head decrease with threshold values and with annual head variations (not related to CSG extraction).
Aquifer protection level (APL) As the number of aquitards between aquifers and target coal seam formation increases, so does the degree of protection
Gas pathway
Gas pathway
Impact
Bores within a primary gas source have no physical protection against gas migration towards the bore; the likelihood that bores separated from the gas source by physical barriers (typically an aquitard) will contain this particular gas is much smaller, and in most cases probably negligible.
of the water bore. The higher the protection level the lower the likelihood for gas‐related bore impairment being caused by CSG extraction. Aquitards are effective barriers between a primary gas source and an aquifer, although short‐circuiting may occur via preferential flow paths. The likelihood for short‐circuiting to be effective across multiple aquitards is much smaller than for a single aquitard.
Several preferential gas pathways such as abandoned coal exploration bores, leaky conventional gas and petroleum wells and water bores, and faults may provide a pathway for gas transport from the primary or secondary gas source towards aquifers used for groundwater. When the gas pathway intersects the aquifer, it becomes a source itself with groundwater now becoming the main transport mechanism potentially taking the gas towards a nearby bore.
Distance to gas pathway (DGP)
Whenever gas is not captured by CSG wells, it spreads generally updip and may be intercepted by a water bore.
This captures several ways free gas can affect a bore’s capacity to supply water.
The farther away a bore from a potential gas pathway, the smaller the probability for this gas to enter a bore. The conceptual model adopted in the DSS assumes horizontal groundwater flow and gas transport from a preferential pathway to the bore. Bore construction and age (BCA) The degree of bore integrity defines whether or not there is a likely pathway for gas migration from a CSG target formation into the water bore. Bore construction material and age may be used to assess bore integrity. Proximity to CSG production wells (PTP) For water bores located within coal seam target formations, the greater the distance between CSG well and water bore, the less likely migrating gas will reach the bore. Observed gas in water bores (GIB) Impacts range from zero impact (no observation of gas), to reported gas issues causing bore operation problems to gas and water blow‐outs.
2.3 Elements of the Decision Support System The primary objective here is to develop a Decision Support System (DSS) that is able to assess the likelihood for coal seam gas operations to be the cause of increased gas in water bores. The DSS will be more broadly applicable to any impairment of bore supply caused by the extraction of groundwater, whether by petroleum tenure holders, for agricultural uses or for industrial use. Note that the supply from a water bore may be impaired because of the cumulative impacts from water extraction by multiple groundwater users (CSG related and non‐CSG related). The methodology involves a three‐level assessment where the first level is a desktop assessment, the second level involves hydrochemical and microbiological analyses, and level three involves methane analyses possibly followed by a forensic analysis (Figure 4). Whereas the first level is mainly a desktop assessment using existing geological and hydrogeological information accessible from existing databases and geological and hydrogeological models, the second and third step involve actual measurements and analysis of hydrochemical and microbiological data and methane, respectively. In some instances the level 7
1 assessment will suffice to exclude coal seam gas extraction as the likely cause of bore impairment; there will thus not be a need to sample groundwater for hydrochemical and microbiological analyses. In other cases there will be sufficient site‐specific evidence to trigger the level two hydrochemical and microbiological analyses. On the basis of groundwater hydrochemical and microbiological analyses, it may then be possible to exclude coal seam gas extraction as the most likely cause of bore impairment. If, however, the concentrations of major ions or microbiological indicators exceed their trigger values, the level three investigation will be activated; this requires methane gas measurements in water bores. Finally, in case methane concentrations exceed their trigger values, a forensic analysis can be undertaken to identify the source of methane, and thus the cause for bore impairment. In undertaking this stepwise assessment, the amount and breadth of site‐specific information increases from one step to another, effectively decreasing the uncertainty or increasing the confidence in the assessment. In the subsequent sections each of the three assessment levels will be discussed in detail. In the current three‐level assessment the hydrochemical analysis (Level 2) is undertaken prior to measurement and testing of methane concentrations (Level 3). The reason for choosing this sequence of analysis is that standard hydrochemical parameters generally require a simpler sampling protocol than dissolved gases such as methane, allowing sampling even when accessibility to the well is poor (e.g. due to presence of a pump that limits use of down‐hole samplers). Furthermore, methane analysis requires a very careful and slightly more technical sampling protocol (Smith 2015). For these reasons methane sampling is recommended only if the hydrochemical (and microbiological) parameters give evidence of changes potentially related to increased methane. Finally, exceptions to this general rule may be invoked (and samples taken for gas analysis) when there is clear evidence of presence of gas in a bore. While quantitative information will provide the most reliable input for a robust assessment, other sources of information may be utilised to help explain the reasons for bore impairment. For example, anecdotic historical information about gas in bores during drilling or soon after bore completing may be insightful. Many of the water bores drilled in the Surat that access water from the Walloon Coal Measures (many of them are not deeper than 100‐200 m) have had gas during their lifetime (DNRM 2012; Walker and Mallants 2014). In one instance a bore started producing gas only after 3 years of pumping, probably the time needed to depressurise groundwater around the bore sufficiently for the gas to be released.
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Uncertainty
Confidence
Figure 4 Three‐level Decision Support System to screen water impaired bores or bores with increased gas. For LEVEL 1 trigger levels, see Table 4. Sampling for microbiological analysis is described in Smith‐Comeskey (2015).
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3 LEVEL 1: desktop assessment
3.1 Methodology The desktop assessment involves an evaluation of different site‐specific parameters relevant to assess whether or not bore impairment is likely caused by nearby coal seam gas extraction. For instance, by considering the proximity of water bores to CSG wells and predicted impacted zones the likelihood of contamination by CSG operations can be assessed. Furthermore, the water bore screen location determines which aquifer or aquifers are being accessed for water production, and if the same aquifer is considered for CSG extraction or whether one or more aquitards protect the bores from being impacted by CSG extraction. Bore screen location also defines the background groundwater hydrochemistry that will be evaluated at a Level 2 assessment if a set trigger value is exceeded. Methane concentrations will be considered in the assessment if the trigger value for the Level 3 assessment is exceeded (Figure 4). Six parameters are considered in the Level 1 assessment (also see Figure 4):
recent observations of presence of gas in water bores (parameter GIB), vertical proximity of the bore screen to a coal seam gas target formation (the aquifer protection level or parameter APL), distance of the bore to the Immediately Impacted Area (parameter IAA) as calculated for SCMA, distance to a gas pathway or source (parameter DGP), if a coal seam gas target formation is intersected, the proximity between bore and CSG well (parameter PTP), and finally the bore construction material and age as indicators of bore integrity (parameter BCA).
These six parameters each are given a score, with a low score indicating a low likelihood that the parameter contributes or is a cause for the bore impairment. The scores are then combined in a multiplicative model to produce an overall score, as shown in Equation 1. The overall score is subsequently interpreted as a likelihood that the bore impairment may be caused by coal seam gas extraction. High values of the total score will trigger the Level 2 analyses of the DSS.
Equation 1
Each of the parameters is given specific scores according to the conditions shown in Table 2. Methods used to derive the different scores and their significance is discussed in the subsequent sections. Using Equation 1, the calculated values for SCORE are then grouped into five likelihood levels according to the SCORE ranges of Table 3. Likelihood level E includes those parameter combinations for which the likelihood that bore impairment is related to coal seam gas extraction is highly unlikely (Table 3). Likelihood level A, on the other hand, represents a combination of parameters that indicate there is a high likelihood (almost certain) that bore impairment is related to coal seam gas extraction, and therefore triggers Level 2 of the DSS.
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Table 2 Parameters and their scores used to calculate SCORE1 of the Level 1 desktop assessment. High scores mean higher likelihood that bore impairment is caused by CSG extraction.
Parameter
Score
Observed gas in water bores (GIB)
1: no observed gas in water bore 2: reported gas issues causing bore operation problems 4: recent gas and water blow‐out
Aquifer Protection Level (APL) [number of aquitards between water bore and target formation]. For details, see Section 3.3.
1: bore screen separated from target formation by 3 aquitards 2: bore screen separated from target formation by 2 aquitards 4: bore screen separated from target formation by 1 aquitards 8: bore screen within CSG target formation
Immediately Affected Area (IAA) [inside IAA: bores with screened interval in affected aquifers, see Figure 9]. For details, see Section 3.4.
1: Outside IAA, head drop mean annual head variation 3: Inside IAA, head drop 2000 m
For details, see Section 3.5.
2: 2000 – 1000 m 4: 1000 – 400 m 6: 400 – 200 m 8: 10 km 2: distance from CSG well 5‐10 km 3: distance from CSG well 2‐5 km 4: distance from CSG well 1‐2 km 5: distance from CSG well
