Managing groundwater impacts of coal seam gas
The coal seam gas (CSG) industry is expanding rapidly in the Surat and Bowen Basins of Queensland. CSG production involves pumping water from the coal seams to release the gas adsorbed to coal particles. The reduction in water pressure in the coal seams will cause some reductions in water pressure in overlying and underlying aquifers because there will always be some interconnectivity between the formations. The Surat Basin is a sub-basin of the Great Artesian Basin which contains aquifers of high economic, environmental and cultural value. The Condamine Alluvium is also an important water resource that overlies parts of the eastern margin of the Surat Basin.
Under the Queensland regulatory framework, an area of intensive CSG development involving multiple CSG operators can be declared to be a Cumulative Management Area. For such areas, the Office of Groundwater Impact Assessment develops assessments of future impacts on aquifer water pressures and develops and manages arrangements. The results are published in Underground Water Impact Reports.
The Surat Underground Water Impact Report was approved in December 2012. This article summarises the place of the report in the Queensland regulatory framework, content of the report and progress in its implementation.
The Petroleum and Gas (Production and Safety) Act 2004 (Qld) and Petroleum Act 1923 (Qld) authorise petroleum tenure holders to undertake activities related to the exploration for, and production of, petroleum and gas, including coal seam gas (CSG). The authorisation includes the right to take or interfere with groundwater. The provisions exist because water is found in association with petroleum and gas and it is not practicable to manage the water separately. The provisions apply to both conventional production and CSG production. As a result, the exercise of these water rights, private water supply bores and environmental assets such as springs can be affected.
The Water Act 2000 (Qld) establishes obligations for petroleum tenure holders in relation to the management of these impacts. It places the following obligations on petroleum tenure holders:
- Make good any impairment in the adequacy of a private water supply bore for its intended use, that is caused by petroleum and gas water extraction;
- Collect baseline data about private water supply bores before petroleum and gas production commences; and
- Monitor and assess impacts on water pressures in aquifers and springs.
Office of Groundwater Impact Assessment
Water pressure reductions from petroleum and gas development propagate horizontally. In areas where there are multiple operations, the impacts from the individual operations can overlap. As a result, more than one operator can contribute to a reduction in the water level in a water supply bore. Under the Queensland regulatory framework where these circumstances exist, a cumulative management area (CMA) can be established.
When a CMA is established, the Office of Groundwater Impact Assessment (OGIA) becomes responsible for the preparation of an Underground Water Impact Report (UWIR) and updating UWIR every three years to incorporate new knowledge about the system. The UWIR must include the following:
- Predictions of impacts on water pressures in all affected aquifers. This involves constructing a regional groundwater flow model and using it to assess the impact of the cumulative planned petroleum and gas development.
- Specification of integrated regional management arrangements. This includes a water monitoring strategy and spring impact management strategy.
- Assignment of responsibilities for implementing separate parts of the management systems to individual petroleum tenure holders. For example individual petroleum tenure holders are assigned responsibility for implementing separate parts of the regional monitoring network.
OGIA is an independent body housed within the Queensland Department of Natural Resources and Mines. It has powers to obtain data from petroleum tenure holders in order to carry out its assessments. It is fully funded through a levy on petroleum tenure holders.
OGIA has existed since 1 January 2013. Prior to that time the functions were carried out by the Queensland Water Commission. As a part of wider reforms to institutional arrangements, the Queensland Water Commission was abolished and the OGIA was established to continue the functions.
A major CSG expansion is in progress in the Surat and southern Bowen Basins involving multiple operations. As a consequence, the Surat CMA was established in March 2011. A Surat UWIR was prepared with approval of that report having effect from 1 December 2012. The UWIR is a statutory instrument under the Water Act 2000 (Qld). Obligations for individual petroleum tenure holders for activities arising from the UWIR are legally enforceable. The Department of Environment and Heritage Protection is responsible for ensuring petroleum tenure holders comply with their obligations.
The wider regulatory framework
There is a wide range of activities which could be carried out by petroleum tenure holders that have the potential to affect surface or groundwater resources. These include: injection of treated water into aquifers; hydraulic fracturing of formations; and discharge of water to streams. These activities are regulated through environmental authorities issued under the Environmental Protection Act 1994 (Qld). Approvals may also be required under the Environmental Protection and Biodiversity Conservation Act 1999 (Cth).
SURAT UWIR – PREPARATION & CONTENT
The Surat CMA covers part of three geologic basins: the northern Surat Basin; the southern Bowen; and the western Clarence-Moreton Basin. The Bowen Basin is the deepest and oldest and runs north-south through the centre of the region. Overlying this is the Surat Basin, which covers most of the central and southern parts of the CMA. The Clarence-Moreton Basin interfingers with the Surat Basin across the Kumbarilla Ridge to the east. Overlying these basins are areas of unconsolidated younger alluvial sediments and volcanics.
The Great Artesian Basin (GAB) is a roundwater system comprising all or parts of several geologic basins.
Within the Surat CMA, the GAB includes the Surat Basin, the equivalent formations in the Clarence-Moreton Basin and the upper parts of the Bowen Basin. The GAB is a multilayered system. The target coal formation in the Surat Basin is the Walloon Coal Measures, which is one of the formations of the GAB. Major aquifers in the sequence include the Hutton Sandstone and the Springbok sandstones which underlie and overlie the Walloon Coal Measures. The Bandanna Formation is the target coal formation in the Bowen Basin. The coal formations comprise many thin lenses of coal in a non-carboniferous matrix.
There are younger alluvial and volcanic quifers overlying the basins. The most important of these is the Condamine Alluvium which is used extensively for irrigation.
Within the Surat CMA some 87 000 megalitres per annum are extracted from the GAB and deeper sediments for mostly stock supply but also for other uses. A further 127 000 megalitres per annum are extracted from shallow alluvial and volcanic aquifers mostly for irrigation purposes. In total there are some 20 000 private water supply bores in the CMA.
Water extraction as part of conventional petroleum and gas operations commenced in 1962. CSG production from the Bandanna Formation commenced in the Bowen Basin in 1993, and from the Walloon Coal Measures in the Surat Basin in 2003. Water extraction associated with conventional operations is steady at some 2 000 megalitres per annum. Water extraction associated with CSG operations is currently about 18 000 megalitres per annum and rising as the industry expands.
The coal formations supporting CSG roduction are part of a multilayered groundwater flow system. Water pressures in the coal formations are substantially reduced in the process of producing gas. As the industry develops, these pressure impacts will be transmitted to some extent to overlying and underlying aquifers where there is hydraulic interconnectivity between the formations. Appropriate representation of interconnectivity between the formations was a priority in the development of the regional groundwater flow model. Important areas of interconnectivity in the area are as follows:
The Condamine Alluvium overlies the eastern margin of the area of planned development. Basal alluvial clays and the upper weathered horizon of the underlying Walloon Coal Measures together comprise a transition zone that functions as an aquatard of variable thickness. This area of interconnectivity is of particular importance because of the economic importance of the Condamine Alluvium.
The upper and lower non-coal bearing parts of the Walloon Coal Measures form aquatard layers separating the gas bearing horizons of the Walloon Coal Measures from the overlying and underlying aquifers. The lower aquatard is generally thicker and of more consistent thickness than the upper aquatard.
For the most part the Bandanna formation is isolated from aquifers by thick low permeability formations. However, there is a relatively small area north of Roma where the Precipice Sandstone, a basal aquifer of the Surat Basin, rests unconformably on the Bandanna Formation.
UWIR predicted impacts
A regional groundwater flow model was constructed using the MODFLOW 2005 finite difference code. It covers an area of 550km by 600km, has 19 layers and a grid size of 1.5 km. It was calibrated against existing data and uncertainty analysis techniques were used to improve confidence in use of the model. The model was externally reviewed and found to be fit for purpose. Planned development scenarios were obtained from each of the tenure holders in the area to produce a cumulative development profile that was used for assessment purposes. The planned development scenarios will change over time and the model will be used periodically to review predicted impacts.
Representation of affected areas:
The UWIR contains a map of the extent of the Immediately Affected Areas in accordance with regulatory requirements. The Immediately Affected Area for an aquifer, is the area where the water pressure in the aquifer is predicted to fall by more than 5 metres within three years, as a result of petroleum and gas operations. These maps are of legal significance as within 12 months of the approval of the UWIR the responsible petroleum tenure holder is required to proactively enter into an agreement with the owners of bores in the area that tap the aquifer about any measures necessary to make good impairment of supply, before impairment occurs.
Although there is an underlying general requirement that impairment of supply be made good irrespective of the amount of pressure reduction that has occurred, a predicted pressure reduction of 5 metres within three years is a trigger for proactive action. There are Immediately Affected Areas of any significant extent only for the coal formations as impacts in overlying and underlying aquifers will tend to occur later in time. There are 85 bores in the Immediately Affected Areas.
The UWIR also contains maps of the extent of the Long-term Affected Areas in accordance with regulatory requirements. The Long-term Affected Area for an aquifer, is the area where the water pressure in the aquifer is predicted to fall by more than 5 metres at any time in the future, as a result of CSG operations. There are 529 bores affected. The UWIR includes maps showing the distribution patterns of expected impacts for each aquifer. The following section summarises the expected impact in each aquifer.
Long-term impact in individual aquifers:
The Walloon Coal Measures is the CSG target formation in the Surat Basin. For most of the impacted area, the long-term impact is expected to be less than 150 metres. Within the production area, the magnitude of impact reflects the depth of the top of the coal formation because operational practice for CSG production is to lower the pressure in the coal seams to approximately 35 to 40 metres above the top of the uppermost coal seam. As a result, in the more westerly areas, where the coal formation is deep, the pressure reduction is expected to be large. There are 400 private water bores that source water from Walloon Coal Measures formation in its Long-term Affected Area. Most of these are located to the east where the formation is shallow and impacts are smaller. Half of the affected bores are expected to experience an impact of less than 21 metres.
“Water extraction associated with CSG operations is currently about 18,000 megalitres per annum and rising as the industry expands.”
The Bandanna Formation is the CSG target formation in the Bowen Basin. For most of the impacted area the long-term impact is expected to be less than 200 metres. As for the Walloon Coal Measures, the pressure reduction will be greater in areas where the coal formation is deep. There are no private water supply bores that source water from the formation in its Long-term Affected Area.
The Springbok Sandstone overlies the Walloon Coal Measures. For the most part, the aquifer is separated from the productive coal seams by an upper, low permeability layer of the Walloon Coal Measures, although this layer is thin or absent in some areas. Over most of the affected area the maximum impact is expected to be less than 20 metres, although there is a small area south of Miles where the impact is expected to reach 90 metres. There are 104 bores that source water from the formation in its Long-term Affected Area. It is expected that the impact will not exceed 20 metres in any of those bores and to be less than 10 metres in more than half of them.
Hutton Sandstone underlies the Walloon Coal Measures. It is separated from the productive coal seams by a low permeability layer of the Walloon Coal Measures. Over most of the affected area the maximum impact is expected to be less than 5 metres, although there are small areas where maximum impacts may reach 18 metres. There are 23 private bores that source water from the formation in its Long-term Affected Area. The maximum impact in one bore is expected to be 13 metres but more than half of the bores will experience a maximum impact of less than 7 metres.
“Although there is an underlying general requirement that impairment of supply be made good irrespective of the amount of pressure reduction that has occurred, a predicted pressure reduction of 5 metres within three years is a trigger for proactive action.”
Precipice Sandstone is a basal aquifer of the Surat Basin that has some interconnectivity with the Bandanna Formation of the underlying Bowen Basin. Near the contact area the long term maximum impact is expected to reach 10 metres. There are no private bores that source water from the formation in its Long-term Affected Area.
The Gubberamunda Sandstone and Mooga Sandstones are shallow aquifers that are not well connected to the coal formations. Generally, impacts will be less than 3 metres and only small areas will be affected.
The Condamine Alluvium partially overlies the eastern margin of the area of planned CSG development. There is no Long-term Affected Area for the Condamine Alluvium as the maximum water level impact is expected to be smaller than the threshold. The maximum impact is expected to be 1.2 metres in a small area on the western edge of the alluvium. Over most of the area the maximum impact is expected to be approximately 0.5 metres.
Water Monitoring Strategy
The UWIR includes a Water Monitoring Strategy. The Strategy specifies an integrated regional monitoring network for both water pressure and water chemistry. Over time, the network will provide data about the response of the groundwater flow system to the stresses imposed through CSG development.
The network incorporates and builds on existing monitoring bore networks. It utilises existing monitoring bores managed by the Department of Natural Resources and Mines as well as monitoring bores that have already been established by individual petroleum tenure holders. At many of the specified monitoring sites, individual monitoring points will be established in several aquifers at different depths. The UWIR requires that 392 new monitoring points be installed to complete a network of 498 monitoring points at 142 monitoring sites. Petroleum tenure holders are constructing and operating individual parts of the monitoring network and reporting results to OGIA.
Spring Impact Management Strategy
The UWIR includes a Spring Impact Management Strategy. Springs with significant cultural and ecological values fed by GAB aquifers exist in the area. Individual spring vents are often found in close geographic and hydrologic association. Such a group is termed a ‘spring complex’. A field survey was carried out to improve knowledge of springs to support preparation of the UWIR. There are 71 spring complexes comprising 330 individual spring vents within the area. Some of these springs contain species listed under the Environment Protection and Biodiversity Conservation Act 1999 (Cth) and the Nature Conservation Act 1992 (Qld). There are also 43 ‘watercourse springs’ which are sections of watercourse fed by spring flow.
It is expected that at five complexes there will be some impact in water pressures in the source aquifer feeding the spring at the location of the spring. The maximum impact is 1.3 metres. Under the strategy, petroleum tenure holders are required to implement timely actions in response to potential impacts.
Responsible tenure holders
Some water management activities need to be carried out on a collective basis by OGIA with the cost shared between petroleum the tenure holders through the funding levy. A clearexample is the construction of the regional groundwater flow model used for assessment. OGIA has powers to obtain data from all tenure holders to support construction of a model. OGIA also carries out research activities to support improved modelling over time.
Other activities can be carried out by individual petroleum tenure holders in accordance with specifications in the UWIR, rather than by OGIA itself. The UWIR specifies rules for determining a single petroleum tenure holder as the tenure holder responsible for make-good actions relating to impairment of supply from private water bores. The rules seek to balance simplicity and equity between the various petroleum tenure holders.
A widely supported principle that forms the basis for the rules is that the holder of a petroleum tenure be responsible for CSG water extraction impacts on private bores on that tenure, even if the dominant cause of the impact arises from water extraction on a neighbouring tenure. The rules also deal with responsibilities for private water supply bores that are located outside the tenured lands.
The UWIR follows similar principles in assigning responsibility for other activities such as implementation of specific parts of the regional water monitoring network. The responsible tenure holders negotiate with OGIA variations to planned locations for monitoring works that arise from field conditions and report monitoring results to OGIA periodically.
IMPLEMENTATION OF UWIR
Petroleum tenure holders are carrying out their responsibilities under the UWIR. They are entering into Make-good agreements with bore owners within Immediately Affected Areas; implementing the water monitoring network; and carrying out spring impact management actions.
The OGIA groundwater flow model is widely accepted as the model used for the Surat and southern Bowen Basins. OGIA uses the model to test the cumulative impact of possible adjustments to development plans under consideration by individual petroleum tenure holders from time to time. OGIA will report annually to the Department of Environment and Heritage on changes to cumulative CSG development plans and the significance of those changes in relation to future water pressure impacts. If the change to impacts is significant then OGIA could be required to update the UWIR in advance of the regular triennial update.
The current groundwater flow model is based on the knowledge available at the time. OGIA is currently engaged in research to improve the knowledge base in preparation for building a new regional groundwater flow model and then updating the UWIR in late 2015. OGIA maintains a core technical staff which it complements with the purchase of specialist services as required. It leads research on matters that are directly related to improving the conceptualisation of the hydrogeology of the CMA, and seeks to do so in collaboration with experts in other institutions and petroleum tenure holders. The current research focus is: improving understanding about the interconnectivity between the coal formations and other aquifers; determining the best technique for modelling dual phase flow and complex geology within coal formations; and improving understanding of the risk to springs. From mid 2014 outputs from research activities will feed into decision making about the construction of the next generation regional ground water flow model.
RANDALL COX & SANJEEVE PANDEY
Randall Cox is the General Manager, Office of Groundwater Impact Assessment at the Queensland Department of Natural Resources and Mines. Email: randall.cox@ dnrm.qld.gov.au
Sanjeeve Pandey is the Director, Policy and Projects, Office of Groundwater Impact Assessment at the Queensland Department of Natural Resources and Mines. Email: firstname.lastname@example.org
Queensland Water Commission (QWC), 2012, Underground Water Impact Report: Surat Cumulative Management Area, Queensland Government, Brisbane.
The article was first presented at AusIMM’s Water in Mining 2013 conference in Brisbane in November 2013.