What is coal seam gas?
Coal seam gas (CSG), also known as coal bed methane, is a mixture of a number of gases but is mostly made up of methane (generally 95-97 per cent pure methane). It is typically attached by adsorption to the coal matrix, and is held in the coal by the pressure of formation water in the coal cleats and fractures.
[Music plays and text appears: Unearthing coal seam gas – What is coal seam gas, how is it extracted and what are some of the challenges involved?]
[Image appears of a farmland landscape on the surface of a cross section of land. Camera zooms down the cross section of land to a map of Australia and text appears: 1997 over Queensland]
Narrator: Coal seam gas has been part of Australia’s energy mix since it was first produced in Queensland in 1997, and development of the resource has been steadily increasing since then.
[A blue coal seam line appears on the rotating cross section of land and text appears: Coal seam]
Coal seam gas is mainly methane found within coal deposits trapped underground by water pressure.
[A line appears on the cross section of land moving through the rock to the coal seam and text appears: Surface, 300 m, 1000 m and Aquifer Aquitard]
To access the gas, a well is drilled – anywhere from 300 to 1000 metres deep through various layers of rock – to the coal seam.
[Camera zooms in on the well in the cross section of land and then a small block pops out of the side to show the cement and steel casing of the well]
To protect groundwater from being contaminated the well is lined with cement and steel casings.
[Camera zooms down to show water in the coal seam, text appears: Formation Water]
Water already in the coal seam is pumped out to release the trapped gas.
[Text appears: Hydraulic Fracturing and camera zooms in on well shaft to show perforations in well shaft]
If water and gas don’t flow freely, hydraulic fracturing, also known as fracking, may be used to increase the rate of flow. Hydraulic fracturing involves perforating the casing at different levels along the well, to gain access to the coal.
[Image shows water moving down the well shaft and into the coal seam. A single water drop appears and text appears: 1% chemical additives, 99% water & proppant]
Water containing chemical additives is pumped under high pressure down the well, opening up existing fractures and creating new ones.
[Camera zooms in on the coal seam to show sand in the water and then zooms out to show the well shaft and the water and sand moving up the well shaft]
Proppant, such as sand is then added to the water that flows through to the fractures. The sand keeps the cracks open allowing the gas to flow to the well and up to the surface.
[Camera zooms up the well shaft to the well head at the surface. Image shows the well head with a truck and a pumping station. Text appears: Produced Water = hydraulic fracturing fluid + formation water]
Produced water and gas are pumped to the surface, and separated at the well head.
[Camera zooms out to reveal the whole cross section of land with arrows pointing left to three hexagons showing what happens to the gas and arrows pointing right to six hexagons to show what happens to the water]
The extracted gas is processed and transported for domestic and international use. Produced water is treated to remove salts and other chemicals and then either re-used or disposed of according to state government regulations.
[Camera zooms in on the cross section of land again and shows the well. Image appears of a cube of the Aquifer and Aquitard layer: text appears: On one cube Aquifer and the other cube Aquitard . Over decades and thousands of years
A source of concern is that hydraulic fracturing fluids may leave the coal seam and enter fresh water aquifers, which are layers of porous permeable rock that allow water to flow through easily.
This risk is reduced by layers of rock with low permeability, known as aquitards, which limit water flow and can act as a barrier.
[Camera zooms back to surface of cross section of land and image shows a truck with fluid spilling from the rear]
Contamination of groundwater is more likely to occur as a result of accidental surface spills or leaks of produced water and hydraulic fracturing fluids.
[Camera zooms out to show the coal seam and the layers either side. Text appears to label the layers: Aquifer, Aquitard, Coal Seam, Aquitard, Aquifer]
Another impact is the lowering of water levels in aquifers. Removing large amounts of water from the coal seam decreases the water pressure within the rock layer containing coal deposits. Water in the aquifers can then move towards the coal seam. Just how fast and far this happens depends on the type of and connectivity between the aquifers and aquitards.
[Camera zooms up the well shaft to the surface again and then zooms in on a chimney spewing flame]
[Camera zooms out to show the cross section of land]
Other potential environmental impacts include the industry’s greenhouse gas footprint, fragmenting of local habitat, changes to agricultural landscapes and rural communities.
[Text appears: Research to inform decisions, Visit the CSIRO and GISERA websites for more information and latest research. www.csiro.au, www.gisera.org.au’]
CSIRO is conducting research to better understand the impacts of coal seam gas development and develop sound technologies and practices to ensure socially and environmentally responsible development.
[CSIRO logo and text appears: Big ideas start here, www.csiro.au]
Unearthing coal seam gas
Different forms of natural gas
The different forms of natural gas are generally categorised into conventional and unconventional gas. The difference between conventional and unconventional gas is the geology of the reservoirs from which they are produced.
Conventional gas
Conventional gas is obtained from reservoirs that largely consist of porous sandstone formations capped by impermeable rock, with the gas trapped by buoyancy. The gas can often move to the surface through the gas wells without the need to pump.
Unconventional gas
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. There are several types of unconventional gas such as CSG, shale gas and tight gas, which are outlined below.
CSG
CSG is entirely adsorbed into the coal matrix. Movement of CSG to the surface through gas wells normally requires extraction of formation water from the coal cleats and fractures. This reduces the pressure, allowing methane to be released from the coal matrix. Over time, water production decreases and gas production increases. CSG production normally requires a higher density of wells than conventional gas production but CSG wells are typically shallower than conventional wells and cost much less to drill.
Shale gas
Shale gas is generally extracted from a clay-rich sedimentary rock, which has naturally low permeability. The gas it contains is either adsorbed or in a free state in the pores of the rock. [Note: the US documentary ‘Gasland’ refers to coal and shale gas; there are important differences between the two in terms of the geological location and characteristics of the reservoirs they are found in, and the processes employed to extract them].
Tight gas
Tight gas is trapped in reservoirs characterised by very low porosity and permeability. The rock pores that contain the gas are minuscule, and the interconnections between them are so limited that the gas can only migrate through it with great difficulty.
Why use natural gas as an energy source?
Natural gas extracted from coal seams can offer a number of benefits as an energy source:
- natural gas typically burns more efficiently than coal or oil and can emit less greenhouse gas at the points of extraction and combustion
- natural gas has a role in supporting the journey towards lower or zero emission renewable energy sources
- natural gas has direct use for a range of purposes such as heating and for powering fast-response, electricity-generating turbines
- Australia has abundant resources of natural gas.
Regulation of onshore gas developments
Regulation of onshore gas operations is undertaken by relevant state and environmental authorities. These authorities establish regulatory frameworks based on the evaluation of potential environmental risks and hazards of proposed developments.
Undertaking comprehensive scientific research can give insights into the likely risks and impacts associated with individual onshore gas operations, and feed into policy and regulation of the industry.
Monitoring and management of CSG sites
Characterising CSG sites for production and drilling wells is important in assessing the potential of CSG production. Technologies such as three‐dimensional geophysical surveying techniques, mathematical based modelling and imaging of underground reservoirs can be used to observe subsurface aquifers and geological strata, determine how coal seams are connected to aquifers and assess the potential for groundwater contamination.
Groundwater modelling can assist in indicating the extent to which coal seams are connected to aquifers, and to predict whether drawing water from one can impact levels in the other. Seismic mapping technologies can be used to map fracture locations and channels for water movement underground.
Although absolute guarantees about potential impacts are not possible, existing knowledge from research on aquifers and groundwater models make it possible to estimate the risks and uncertainties of adverse impacts.
How is a site determined as suitable for gas operations?
A number of detailed evaluation tests and analyses can be used to help determine the suitability of a site for drilling and extraction of onshore gas.
These analyses can include:
- geological site descriptions from well data – to characterise the rock layers associated with each coal seam well and their distribution, deposition and age
- seismic surveys – to define the geological structure beneath the ground surface and identify faults or fractures that could potentially create leakage pathways that may also be associated with subsurface water movement
- hydrodynamic assessments – to map the rate and direction of groundwater movement and to determine the connectivity of aquifers in the subsurface
- analysis of water quality samples – to measure barriers to flow between the deep and shallow groundwater zones or areas
- analysis of groundwater samples – to determine the existing water quality levels at the site before onshore gas production, and to use as a baseline to monitor any changes during and after production.
- Information gathered from all the analyses and geological characterisations can be used to build computer models of the site. These models can then be used to make predictions of the impacts of onshore gas production and groundwater systems.
Infrastructure footprint
Wells may be typically sunk to depths of 300-1000 metres below the surface. They are often laid out on a grid separated by about 750 metres, connected by a network of roads, pipelines and compressor stations.
The surface footprint of onshore gas infrastructure is generally less intensive than other industries such as mining. However, the distribution of developments may fragment local habitat and agricultural landscapes and may compromise the scenic and aural quality of the landscape.