With the depleting conventional oil and gas reserves in the world on one hand and increasing guzzling energy needs on the other, the energy industry is constantly looking out for unconventional gas sources. This has led to the discovery of shale gas, tight gas, gas hydrates and coal seam gas as alternative sources of energy. (Learn more about shale gas in the article Is Shale Gas Losing Its Sheen?)
Coal seam gas (CSG), also known as coal bed methane (CBM), coal mine methane (CMM) or coal bed gas, is a form of natural gas extracted from coal beds. It is primarily made up of methane gas and found trapped in coal seams underground in the depths of 300-1000 m (1000-3300 ft) surrounded by water and ground pressure. Its single largest component is methane (95%-97%). It is used for domestic cooking and heating, power generation and for industry purposes to make a wide range of products.
Coal Transformation Phases
It is interesting to note that the coal goes through several stages before it develops into a solid shiny substance called hard coal. The journey of coal starts from something called peat, which is nothing but the wastes of the earth and plants. Over a long period of time with the heat and burial pressure acting on the peat, it passes through various maturity levels to become tough coal at the end. Peat matures into states like lignite, sub-bituminous, bituminous and finally hard anthracite, which is commonly known as coal. These stages are called ranks. Lignite and sub-bituminous coals are considered to be low rank coal while bituminous and anthracites are considered to be high rank coal. Gaseous hydrocarbons are found in greater quantities in hard coal than in soft coal like lignite.
Biogenic vs Thermogenic Coal Seam Gas (CSG) Formation
Methane formation in coal happens mainly in two ways, one through a biogenic process and the other through a thermogenic process. Biogenic process-enabled methane is produced by bacteria as they acquire nutrition from the coal source and produce methane as a by-product. The thermogenic process occurs as the result of a chemical reaction in the coal due to high pressure and temperature over a period of time. This results in the production of a significant amount of methane gas. (Learn more about these processes in The Significance of Biogenic and Thermogenic Gases in the Energy Space.)
Extraction Process of CSG
Coal seam gas is extracted using unconventional methods such as horizontal drilling or hydraulic fracturing (fracking). It is extracted by initially drilling a well vertically through the rock strata until reaching the coal seam; at this point the horizontal drilling process is adopted for increased access to the methane gas trapped inside. Similar to conventional oil extraction, a vertical steel encased well is drilled to reach the coal bed.
If the water and methane gas is not flowing freely then hydraulic fracturing is used. In this method, sand along with chemical additives is powerfully injected on the coal cleat to create a fracture so that the trapped gas can escape through the outlet to the surface. This is also called horizontal drilling. The sand helps to expand the fractures and enlarge them to facilitate gas extraction.
The extracted gas is processed further and taken to storage facilities before being used for domestic and international purposes. CSG is a capital-intensive process and the extraction process involves tens of thousands of gas wells, with roads, pipelines, compressor stations, wastewater dams and other infrastructure spread over a huge land area.
Water Plays a Major Role
Because coal seams contain water and methane, water must be pumped out of the coal seam to lower the pressure and allow the gas to flow to the surface. The amount of water pumped out depends upon the type of well/coal seams and can vary between a few thousand to hundreds of thousands of liters per day. The wastewater is generally toxic, salty, containing heavy metals and radioactive materials that have to be stored in tanks or holding ponds and transported through pipelines for further treatment.
The main challenge organizations face while extracting CSG is the water disposal. As stated earlier, the water pumped out during the production process is contaminated and chemically toxic in nature. Various experiments have indicated that the produced water is of very high salinity and hence the high sodium content is detrimental to the environment. This water is not suitable for irrigation because it endangers the quality of the soil, which could negatively affect the crop growth.
Handling of water disposal is an important part of a CSG project. Years ago, the water was channeled into the stream ways to be flushed into the ocean. But continued use of the stream channels resulted in contamination and hence environmentalists raised their concern. In recent times the water is collected in infiltration ponds where it is treated to eliminate the salinity. Once the water meets the accepted levels of quality it is channeled into the stream.
Another key challenge in extracting CSG is that methane is a greenhouse gas and has the potential to release carbon dioxide into the environment. This can have an adverse effect on climate change and global warming. Organizations expend a lot of effort to minimize greenhouse gas emissions to acceptable levels.
World Production and Reserves
Major producers of CSG are the United States, Canada, Australia, the United Kingdom, Kazakhstan and India. CSG extraction and production was pioneered by the US in the 1970s when it encouraged the extraction of unconventional sources of natural gas. Australia, which is one of the major explorers and users of CSG, started its commercial operations during 1996 at Bowen Basin of Queensland. CSG accounts for 27% of Australian gas reserves. It is estimated that by 2030, 30% of the country’s domestic needs will be met by CSG with 50% of the gas demand in eastern Australia. All CSG reserves are in NSW and Queensland states.
Prognosis for CSG
With lots of questions raised from environmental and health perspectives, CSG has to stand on its own merits by effectively addressing those concerns and emerging as a clean fuel for the future.