The commercial production of coalbed methane or coal seam gas (CSG) as it is also known began in Australia in the 1990s and has grown from around 2 petajoules (PJ) per year in 1997–1998 to 1502 PJ/year as of December 2019 with a further 31,803 PJ of remaining CSG reserves in Queensland [1
]. Gas wells provide a potential pathway for introduced and/or geogenic contaminants to enter intersected aquifers [2
]. Prediction of contamination pathways is challenging in data sparse areas such as deep coal-bearing sedimentary basins where pre-development monitoring is rare. There is usually high uncertainty in hydrogeological parameters for aquifers and aquitards. To account for uncertainty a probabilistic approach is useful for producing a range of plausible outcomes but is time consuming and costly for contaminant transport modelling. For efficient regional scale screening level assessment of receptor vulnerability and identification and prioritisation of source-receptor combinations that may need detailed analyses, a probabilistic particle tracking method was developed and applied for a CSG project near Narrabri, in northern New South Wales (NSW), Australia, approximately 500 km north-east of Sydney.
This study assessed the vulnerability of specific receptors to contamination from a set of point source locations (CSG wells) the pathways from which could vary according to conceptual and empirical understanding of hydrogeological characteristics and is similar to previous studies in this respect [4
]. However, the study presented here applied a stochastic approach. This is also distinct from groundwater vulnerability assessments that assume the inherent vulnerability of a groundwater system is dependent on a group of factors that are spatially overlaid to generate a type of vulnerability index. The most common examples of these are based on the ‘DRASTIC’ method originally developed by the United States Environmental Protection Agency [6
]. These methods assess relative groundwater vulnerability across an area and are suited to examining diffuse pollution sources particularly those from the ground surface through the vadose zone and into aquifers, as opposed to point sources of contamination [7
In Australia, the majority of CSG development in Queensland has occurred in the Surat Basin and Bowen Basin, both of which form part of the wider Great Artesian Basin (GAB) with the number of new wells drilled peaking at 1634 in 2013–14; 901 additional wells began producing in the 2018–2019 period [1
]. The Narrabri Gas Project (NGP) in NSW is within the Gunnedah geological basin which underlies part of the younger Surat Basin and is a sub-unit of the GAB. Overlying the CSG targets, the GAB comprises extensive aquifers with good water quality that have a long history of use for irrigation, stock watering, industrial, domestic and town water supply [9
] and support a wide range of groundwater-dependent ecosystems including streams and riparian habitats, springs and wetlands [10
]. The sustainability of the GAB aquifers has been increasingly challenged since the beginning of land development with historical over-extraction leading to chronic declines in water tables, associated surface water flows and cases of spring extinction. Further demands from extractive industries are expected to increase pressure on this important asset [10
CSG-associated contaminants can be introduced externally through drilling muds and hydraulic fracturing fluids where they are used and can also originate as geogenic compounds from within coal formations. The physical and chemical composition of water from within CSG target formations typically has high ionic concentrations (e.g., sodium to 300 mmol/L, chlorine to 100 mmol/L) [11
] and various organic compounds that are hazardous to human health and the environment [12
]. The health and environmental risks of these chemicals depends largely on their respective load, persistence and toxicity, all influenced by local hydrogeological conditions, e.g., travel times and distances, minerology, and factors affecting chemical and biological degradation potential in the aquifer.
Contaminants can potentially be released directly through well casing or cementing failures, or through inter-aquifer or aquitard leakage particularly near faults and shears as a result of fracture stimulation or over-pressurisation, or through depressurisation of coal seams and compaction that compromises well integrity [2
]. CSG well barrier or casing failure rates vary internationally. In the United States, failure rates as defined by casing pressure thresholds were estimated at 3.4% from about 1000 wells over 10 years of annual testing. In Australia, compliance auditing from 2010 to 2015 reported no subsurface leaks, and subsurface casing failures rates were determined to be effectively zero after remediation of suspicious cementing during construction that was detected at a rate of 0.3%; surface well head leaks were reported at a rate of 3% and subsequently repaired [2
Development of contaminant transport models that simulate advection, dispersion, attenuation and transformation processes are computationally expensive. It is also challenging to deploy such models when dealing with numerous, spatially distributed contaminant sources and receptors. Additional complexity arises where there are uncertainties about hydrogeological characteristics; in such circumstances model simulations undertaken for risk assessment should consider a wide range of plausible realizations of groundwater system characteristics. Models then need to be run over multiple iterations to produce a defensible risk assessment. The probabilistic particle tracking method implemented in this study can be used as a screening analysis for assessing the vulnerability of receptors in the region from specific point sources and identify source-receptor combinations that warrant detailed contamination risk assessment.
The mod-PATH3DU package [13
] is a modelling library that allows probabilistic predictive analysis of travel paths, times and distances. The mod-PATH3DU package can be used in conjunction with numerical flow models using unstructured grids e.g., MODFLOW-USG [14
]. This differs from previous code that was applicable only to regular grid geometry. The variable resolution and cell geometry that is possible with unstructured grids, from a mathematical perspective, enables more precise calculations to be made in areas of interest without sacrificing computational efficiency across the entire model domain. As mod-PATH3DU only simulates advective processes it is computationally efficient for running multiple iterations, at high resolution, across large spatial extents, and over long time series. This enables rapid identification of pathways between potential sources and receptors and assessing the vulnerability of receptors by estimating the likelihood of interception via a flow path through its distance and time. If receptors are identified as vulnerable by this method, targeted contamination risk assessment could apply dedicated transport modelling approaches that account for dilution, dispersion, degradation or sorption processes that may significantly influence final concentrations of contaminants. The effects of dilution and attenuation, particularly over time scales of 10s to 100s of years could be expected to reduce peak concentrations at receptor locations markedly. It is therefore reasonable to conclude that the current study predictions provide a conservative assessment of receptor vulnerability.
The objective of this study was to apply probabilistic particle tracking analyses to assess the vulnerability of numerous, spatially distributed potential environmental and economic receptors (e.g., water bores, springs, and groundwater dependent ecosystems) for a CSG project with 425 planned well pads across an area of 957 km2 near the town of Narrabri in northern NSW, Australia.
3. Results and Discussion
Probabilistic groundwater model simulations calculated groundwater head drawdown in the Pilliga Sandstone aquifer as a result of CSG well development and defined an impact threshold based on a >5% chance of drawdown of >0.2 m occurring in the Pilliga Sandstone aquifer (Figure 4
). This threshold is consistent with the most conservative minimal impact threshold in the New South Wales Aquifer Interference Policy [24
]. Drawdown estimates at the 95th percentile level indicated the area where the threshold of 0.2 m would be met or exceeded would intersect with 106 water bores and 1437 km2
of GDE areas (including overlapping extents) and no springs (Figure 4
In Narrabri CSG development simulations, modelled CSG water extraction ranged from 4 to 107 million cubic metres over the life of the gas project as there was considerable uncertainty in estimates of CSG water production [21
]. Particle advection in the Pilliga Sandstone was unlikely to be influenced by pumping during CSG well development compared to a no pumping scenario as drawdown in the Pilliga as a result of pumping was <0.3 m at the median level and <1.5 m at the 95th percentile level (Figure 4
). This was considered insufficient to change regional hydraulic gradients and affect contaminant migration paths in the Pilliga aquifer.
Particle tracking distances for single 3000-year simulations were up to 6.7 km in the forward direction and 8.7 km in reverse with maximum velocities of 2–3 m/year (Figure 5
A,B). These results are similar to maximum groundwater velocities of 3 m/year in GAB aquifers in the region estimated from carbon isotope (14
C) analyses [25
]. The maximum velocity of particles over a 100 year forward tracking simulation using a parameter combination resulting in median predicted drawdown in the Pilliga Sandstone was 1.5 m/year. Velocities were higher along the eastern edge of the project area near the Pilliga Sandstone outcrop (Figure 5
Particle travel velocities from multiple forward tracking simulations over 100 years ranged from <0.5 up to 11 m/year and trajectories also varied (Figure 6
A,B). Path lines from CSG wells intersected areas in the Pilliga Sandstone layer underneath two sections of groundwater-dependent streams located within 200 m of planned well locations (Figure 6
B). Path lines from forward tracking simulations over 100 years intersected areas beneath groundwater-dependent vegetation, mainly grassy woodland vegetation that is extensive across the study area (Figure 6
A). Particle tracking within the Pilliga Sandstone aquifer indicated that impacts are likely to be restricted to receptors in the immediate vicinity of the wells. There was a total of 1437 km2
of groundwater-dependent vegetation within the zone of potential drawdown impact (>5% chance of drawdown >0.2 m in the Pilliga Sandstone aquifer). Monitoring of hydrological response variables associated with these receptors (Table 1
) would inform adaptive management plans to respond to negative changes. The optimisation of the location and design of monitoring bores to improve hydraulic change predictions to inform the establishment of an effective early warning monitoring network was investigated in an associated study that found the addition of 10 multi-level piezometers located in areas with the greatest proportion of variability in the ensemble simulations provided relatively high data-worth for reducing uncertainty [25
Water bore density was highest to the north of the project area (Figure 7
A) and bore proximity to CSG wells was lower toward the south (Figure 7
C). Path line densities (Figure 7
B) followed the velocity trends evident in Figure 5
A,B over 3000-year simulations where velocities and path line densities were highest near the Pilliga Sandstone outcrop. No water bores or springs were intersected by or were within 100 m of particle path lines from multiple 100-year realisations or the single 3000-year forward tracking simulation.
Placement of new monitoring bores with the objective of early detection of hydraulic and water quality impacts before they reach receptor locations could be informed by these results. The groundwater model calculated the areas where a >5% chance of drawdown of >0.2 m occurred in the Pilliga Sandstone aquifer as a result of CSG development so focussing water level monitoring on bores within this area would have additional value. There were 34 monitoring bores in the aquifers of the GAB within the area of potential drawdown in the Pilliga Sandstone equating to an average of 4.2 bores/100 km2
. There were also 106 water bores within this potential zone of hydrological change (Figure 4
). Monitoring bores were sparse in the north-eastern part of the project area where path line density and water bore proximity were high (Figure 7
B,C). As estimated particle travel distances were largely limited to within the project area in the 100-year design period, additional monitoring beyond this extent would be of limited value.
Reporting on oil and gas well integrity and barrier failure rates in Australia over 5 years from 2010 to 2015 show that the frequency of failures and subsequent contamination of surrounding environments including overlying aquifers were very low to near zero [3
]. Regular monitoring of well casing integrity, hydrological response variables and impact metrics associated with potentially impacted receptors (Table 1
), including establishment of baseline (pre-development) conditions and appropriate incident response plans in the event of an incident may be warranted (and potentially required through regulatory approvals) to manage risks.
The specific consequence of contamination in the aquifers that support dependent ecosystems, irrigation, stock watering or potable use, would depend on the persistence, toxicity and bioaccumulation potential of the compounds, and the nature of dilution and attenuation profiles. Given the low groundwater velocities and potentially long travel distances and times to reach many of the receptors (with the exception of grassy woodland GDEs), accounting for dilution and attenuation would likely reduce peak concentrations reached at receptor locations considerably. This study also points to the importance of new data collection particularly during drilling and exploration phases of CSG projects that would further inform the conceptual model understanding of the region and reduce parameter uncertainty. Adaptive management can then be used to adjust operational monitoring plans and responses according to the knowledge gained from new data. A related study applied a spatial optimization technique to identify locations for additional monitoring wells to reduce uncertainty [21
This study therefore presents a conservative screening analysis at regional scale that provides a rapid assessment of plausible particle travel distances and trajectories from planned CSG well locations. It was concluded from this regional scale assessment of particle advection that detailed studies on contaminant transport would be best focused at the well scale to characterise risks for specific contamination scenarios locally. Such studies could also quantify risks from hazardous events e.g., accidental spills or leaks at the surface from chemical transport and storage, and storage and disposal of CSG wastewater.