So, what is coal?
When organic matter is laid down in a swamp and covered over it decomposes without oxygen (anaerobic) to form coal. Depending on the dominant type of organic matter, different macerals of coal will form (inertinite, exinite, vitrinite etc). Depending on the burial depth (ie temperature, pressure and time), the coal will be more or less mature.
The coal industry talks about maturity (or rank) ranging from peat to brown coal (lignite), subbituminous, bituminous and finally anthracite ( although graphite is sometimes the next step). These definitions are based on water and ash content and hence usage.
Coals can form at any time in geological history, but are generally confined to periods of intense biological growth. The first period was during the Carboniferous about 300-350 my ago. The period was named for its coal content in the geological record. This period extended through to the end of the Permian Period, when massive extinctions wiped out many species, leaving little vegetation. During the Triassic period (200 – 250 my ago) no coals were laid down ( at least none have been discovered). Coals were again laid down during Jurassic and Tertiary periods when vegetation was again rife.
Permo Carboniferous coals are likely to be the highest rank (Cooper basin and Hunter valley coals). Tertiary coals are likely to be the lowest rank (Victorian Latrobe Valley and SA Leigh Creek).
Low rank coals (lignite and sub bituminous) have less calorific value and less actual carbon by percentage (ie more ash) and are used for power generation and some industrial processes, including manufacture of cement. Bituminous coal can also be used in power generation (thermal) or cooked up to make coking coal used in steel manufacture. It is this coking coal that is hardest to replace with current technology.
In the oil and gas business, coal maturity and maceral content are looked at more closely as these will define how much oil and gas is cooked out of the coal as it matures. The method used is to measure Vitrinite Reflectance (VR) – or simply how shiny the vitrinite maceral is. Biogenic gas (methane) is first released as the coal matures, then working up through the more complex hydrocarbons (oil) as the thermal processes get going (VR = 0.5 – 0.8). Finally methane gas is produced in large quantities as the coal reaches total Maturity (VR = 0.9 -1.3). Methane may be released into surrounding rocks (eg Cooper Basin sandstone reservoirs) or it can be adsorbed to the coal molecule. In this process very large amounts of methane can remain attached to coal molecules.
When burned, coal produces the most CO2 of all the hydrocarbons, and is likely to produce associated gases from impurities within the coal ( Sulphur dioxide etc)
Coal Seam Gas (CSG)
CSG (or sometimes Coal Bed Methane – CBM) is the methane that is adsorbed to the coal. When coal is mined, the pressure is reduced on the methane and it is released to the atmosphere. Coal mines always have an issue with escaping methane. Collection of this escaping methane from a coal mine, was the first CSG produced.
Today, CSG is produced by separate, competing companies to the coal interests. Here, wells are drilled to intersect coal seams, generally 100- 300 m below ground. These are dewatered (ie let the water flow out), to reduce the pressure on the coal seam and hence release the gas. The gas is captured in flow lines and distributed.
In Australia about 10 percent of CSG wells require further stimulation (fraccing) to release the gas. Notably the Permian coals around Narrabri in NSW do not need fraccing and the local producer Santos has stated that they will not frac these coals.
Coal Mining versus CSG
These are competing businesses and each will tell you about the other’s issues. Let’s try to look at them scientifically (if that is possible?)
A coal mine will fracture all the coal completely releasing all gases to the atmosphere. Aquifers will be removed (open cut) or dewatered (underground mine). Remaining aquifers may have increased associated gas.
CSG fraccing will attempt to keep the coal seam isolated, but may create a leak into another partially connected aquifer. Should be avoided where this is likley. Drilling the well may also create a connection between aquifers if not done effectively.
Water well fraccing may create a connection between previously separate aquifers. Should be avoided near coal seams. Water well drilling may do this if aquifers are not kept separate.
Underground coal mines will create subsidence as the earth settles around the created space. Open cut coal mines will cause isostasy as the earth moves to balance the weight removed. If the mine is filled with water then it will subside due to the weight added.
Gas and water production will cause minimal subsidence in consolidated sediments (measured in millimetres by modern Lidar techniques). If sediments are unconsolidated, massive subsidence is likely (eg modern deltas).
Coal mines will release all associated gas as fugitive gases unless collected (as I understand this is done in three mines in Australia). Transported coal will continue to release adsorbed methane (desorption) as it is transported. All the gas will escape from open cut mines. Hydrocarbon liquids (eg Benzene) may be associated with coal mines and may also be released to the atmosphere or be washed into local water. This depends on the coal maturity and will be less in mature Permian coals
Gas from CSG is collected in a flow line system. These systems should be checked regularly and should not leak (they are much the same as your home gas system). There may also be leaks during the actual drilling and connection phase. These again should be kept to a minimum.
Flowing water from water wells or from dewatering CSG wells is likely to release dissolved gases. Note that Benzene is a volatile liquid that does not dissolve in water (or very minimally). It does however float on the surface ( as does oil). Water and CSG wells need to be regularly monitored when being dewatered for contaminants.
Note that BTEX (Benzene, Toluene, Ethylbenzene and Xylene) compounds are not used in drilling or frac fluids in Australia. They are naturally present in many places and are constituents of petrol (gasoline) and diesel. If parts are washed in petrol or diesel these may cause contamination. BTEX chemicals are used as solvents (paint thinners etc)
Coal mines produce significant coal dust, release gases to the atmosphere and leave large holes in the ground. They may also contaminate local water supplies with hydrocarbon pollutants ( benzene etc) and any associated minerals ( uranium etc). Burning coal creates the most CO2 per unit of energy of the hydrocarbons and depending on the rank of the coal will have significant ash residue.
CSG wells produce no coal dust ( unless cavitation processes are used) and leave no holes. They do have a network of pipes attaching each gas well to the system and may pollute ground water. Burning methane produces CO2 and water.
We will look at this in more detail in another blog. Any mine will produce subsidence and isostasy affects and in doing so will create small earthquakes. Many thousands of small earthquakes occur daily around the globe as the tectonic plates jostle around. Large mines / dams will create larger earthquakes (eg China recently).
Fraccing can create small earthquakes in areas under stress (eg alps, Himalayas, Appalachians). This is more likely from deep wells than shallow CSG wells. Deep fracs are monitored to measure any seismic activity created.
Pumping water or gas into or out of aquifers will create earth movement and hence minor earthquakes. Pumping gas from CSG wells is unlikely to be an issue as the gas is adsorbed and takes little space. Pumping water is more of an issue.
Pumping significant amounts of waste water under pressure into the subsurface will create minor earthquakes. In Oklahoma this practice has been seen to create significant earthquake activity.
Underground Coal Fires
There are thousands of uncontrolled coal fires burning underground around the world. Some are created by coal mining opening up vast expanses of coal and others are natural. The oldest is near Singleton in NSW and is estimated to have been burning for the last 6000 years. Recent examples from opening up large areas of coal have been highly polluting and have devastated local communities.
Impact on rural land
This is probably the major bugbear of both coal and CSG (after Greenhouse gases). While coal mines have generally been around for a long time, local communities are used to them and often rely on them for employment.
CSG is the new kid on the block and is putting more rural land into an industrial setting.
In Australia coal mining is progressing essentially unabated. At the same time NSW and Victoria have moratoriums on CSG and all forms of onshore gas drilling.
I think ( very unscientifically) that the coal lobby might be winning!