A coal seam is a black banded or dark brown coal deposit evident in rock layers. These deposits are below the ground level and, based on their distance from the surface, can be extracted using techniques of either deep or strip mining. Such seams undergo natural formation of coal and function as a typical supply of coal.
Coal seams can also serve as an alternative natural gas source. It is recognized as coal seam gas or termed coal bed methane when natural gas is collected from a coal seam. The gas sticks to the surface of deposits of underground coal, normally lined with water, and forms a thin film on the coal surface because of the water’s pressure. The coal seams’ level of gas fortified is dependent on factors such as the coal’s thickness, depth, and penetrability.
Formation of Coal Seams
Coal seams are shaped from dead and rotting natural matter, which, by and large, is from antiquated peat lowlands that died and fell into shallow, stagnant waters, rotting over time to become peat. Increasing temperature and pressure in the region changed the peat into harder lignite coal. The burial and pressure, accompanied by rising temperatures, resulted in the brown lignite coal turning into the final form of anthracite coal after changing to sub-bituminous coal, and then bituminous coal. Coal seams are formed in this identical manner, irrespective of where they’re located or how big they are.
The measure, area, and availability of coal seams can change broadly. Due to extraordinary weights included in the peat compression for the coal formation, approximately ten meters of peat will shape around one meter of coal, but coal bands differ in the level of thickness based on the initial quantity of peat buried. The size of coal seams differs greatly, with coal seams of 30 centimeters having been found, as well as coal seams that measure 30 meters wide. Coal seams also differ in their depth, available both at deep and surface levels, with different mining techniques being used to mine seams depending on the depth of the seams. Deep mining techniques need to be used for seams of great depth while surface area seams can be subjected to strip mining.
Techniques for Mining Coal from Coal Seams
Mining coal from coal seams is divided into two techniques: surface or opencast mining and underground mining.
Surface or Opencast Mining
Surface mining is the technique used for coal seams that lie near the surface because, in such cases, minimum overburden removal will be needed to extract every ton of coal being mined. In other words, the surface or opencast mining technique has a minimum stripping ratio for the area.
Several surface mining methods exist for mining of coal seams, and of all the methods, the mountaintop removal mining method, with its environmental impact, has gained the maximum amount of attention in the central Appalachian region. The mountaintop removal mining method consists of the removal of a mountain top to gain access to the coal deposits that exist below the surface. The mountaintop removal mining method generates excessive spoil, which is deposited into the valley fills along the mountainsides.
If coal seams are located at greater depths within the earth or have a steep incline, underground mining of coal serves as a better option. The two primary methods included in underground mining are room and pillar mining and longwall mining.
In the first method, that is, room and pillar mining, the coal seam is cut into pillar form to develop panels from it, with every panel having pillars of two types: panel pillar and barrier pillar.
For every panel, in case of a structural failure of panel pillars, a substantial amount of coal is kept in the manner of a barrier pillar to provide support to the panel. Continuous mining equipment is used to extract coal from the mining face. With the advancement of the mining face, panel pillars get formed, and sometimes, to promote recovery, extraction of the coal that gets stuck in the panel pillars is done during retrieval from the mining face. Thin coal seams are usually subjected to the room and pillar method of mining.
The mining method that’s followed for thick, smoothly flat, and fault-free coal seams is longwall mining. Longwall mines make use of artificial hydraulic supports to provide roof support rather than making use of intermediate panel pillars, and these are bigger than room and pillar mines in general.
The Gas Content of Coal Seams
The biochemical decay and metamorphic transformation of the initial plant matter resulted in the creation of coal seams over millions of years. The above process, referred to as coalification, generates large amounts of by-product gases rising with the grade of coal, and is the highest for anthracite, with methane alone being approximately 27,000 ft3/t. Throughout the process of coalification, many of these gases are released into the atmosphere, with a tiny proportion being maintained in coal.
The volume of gas preserved in coal depends on several factors, such as coal rank, burial depth, the coal seams’ immediate roof and floor, geological anomalies, tectonic tensions, and prevailing temperature at the end of the coalification cycle. Generally speaking, the higher the coal rank and the higher the coal seam depth, the higher is the natural gas output from the coal seam. Coal seam gas content ranges from a few ft3/t to 800 ft3/t for depths of up to 3000 ft. Coal seams are the origin of all gases, and also the reservoir. Methane accounts for 80%-95% of the full gas content, while the balance consists of butane, ethane, propane, hydrogen, argon, carbon dioxide, and oxygen.
Coal Seam Degasification
Techniques for coal seam degasification are dependent on the specific coal seams’ reservoir properties. Strong methane control preparation hinges on correct information about the coal seam reservoir properties and the total space made available by the process of mining. Reservoir resources can be divided into two classes for the release of methane from coal seams:
- Properties which establish the seam capacity for the total production of gas, such as adsorbed gas and porosity.
- Properties that assess gas flow levels, like permeability, pressure in the reservoir, and coal diffusiveness.
The properties of the reservoir depend heavily on the depths and grade rank of the coal seam, with the seam gas quality being the most significant of these properties.
Coal Seam Gas
Coal seam gas (CSG), otherwise identified as coal bed methane, is primarily made up of methane, the colorless and odorless gas utilized in businesses and homes. Coal seam gas gathers from compressed organic matter in underground seams of coal built over several millions of years. The resulting coal seam gas binds to the coal particle’s surface within fractures of underground coal deposits, usually 200 to 1,000 meters below the surface; a mixture of water and ground pressure captures the gas.
Coal seams are typically full of water, and it’s the water pressure that holds the gas adsorbed on the coal’s surface, in the form of a thin film. Coal seam gas is obtained from deep deposits of coal that can still be mined in a cost-effective manner.
Coal seam gas had first been obtained in the early 1900s at Balmain’s Sydney Harbour Colliery and was sold as a motor and industrial fuel then, with its production peaking in 1994 as over 11 million cubic feet of gas was produced.
Difference Between Conventional Natural Gas and Coal Seam Gas
Conventional natural gas reserves comprise mainly porous, impermeable rock-capped sandstone formations. The main distinction between conventional natural gas and coal seam gas is the reservoir type from which it is produced. Coal seams have natural fractures as well as porous blocks of matrix. Coal seam gas is found in naturally existing coal seam fractures.
Benefits of Coal Seam Gas
CSG is a part of the supply of gas piped into various businesses and homes for usage in daily cooking, heating, and production. It is used in gas-fired power stations for electricity generation as a low-emission alternative to coal-fired electricity.
The benefits of CSG go much further than using it as a fuel. The CSG industry also produces economic opportunities for people and local businesses, contributing to the revitalization of neighborhoods and reversing youth migration to urban areas.
CSG is highly cost-effective and efficient, offering guaranteed results immediately. Even though several ways of using energy are available, few ways are as economical as seam gas from coal, making CSG ideal for nations where there is not much money to produce energy.
It helps developing nations to change their power generation capacities significantly to a high level. CSG is among the main ways that energy production occurs in areas such as the Middle East and Asia.
Extraction of Coal Seam Gas
Natural gas from deposits of coal (CSG) is generated from underground depths by making use of practices that also protect the population and our climate. Various ways to extract natural gas from deposits of coal exist. These include vertical drilling, as well as horizontal/directional drilling. Often hydraulic fracturing is used to release gas from a seam of coal. A well is normally fracture stimulated only once in its lifetime.
For the production of CSG, wells are drilled underground into the coal seams, and to make sure they’re separated from other layers like water aquifers underground, steel and concrete are used to surround the wells. The gas and fluid are brought to the surface through a pipe embedded in concrete and metal layers to prevent leakage. The gas can be split from the water on the surface. The saltwater responsible for capturing the coal seam gas is then removed and sent to be treated and reused. A pipe network helps to pump the CSG to stations for compression and purification, before it is sent for end-use.
In some situations, a procedure known as hydraulic fracturing is used to promote greater gas release from a well by pumping, at pressure, a combination of sand, water, and some chemicals with the objective of fracturing the coal seam. While water and the chemicals are pumped out from the seam, sand is left behind, and it keeps the fractures open so that greater amounts of gas are released. Using hazardous chemicals such as BTEX is prohibited to protect groundwater, surface water, and the atmosphere.
Horizontal drilling has emerged as a more recent alternative to hydraulic fracturing. Horizontal drilling takes place at deep underground levels and decreases the number of vertical wells above the ground. The wellbore is sealed and pressurized at ground level once the coal seam has been found. The coal seam is drilled with smaller holes horizontally to induce channels by which the gas can flow into the well, removing the need for hydraulic fracturing.
Concerns with Coal Seam Gas
We continue to depend on fossil fuels through the use of CSG. Although good for us in several ways, CSG does produce some by-product pollutants. Agricultural concerns include competing for water resources required during gas production, as well as conflicting needs for CSG development and agricultural activity.
Members of the public also believe that CSG creation is not in line with the objectives of tourism regions and is also worried that their health may be adversely impacted by the creation of CSG.
Environmental groups have expressed fears that the development of CSG could cause:
- Damage to the environment by releasing untreated water
- Damage to wildlife habitat in vulnerable places
- Damage and contamination of underground aquifers through hydraulic fracturing
- Pollution of surface water resources in drinking water catchments
Getting Approval to Carry Out Coal Seam Gas Exploration
A CSG company needs to first apply for an exploration license from the government to carry out seismic surveys and exploratory drilling to determine the quantity and quality of the area’s gas reserves. If the exploration license is approved, the CSG company must identify potential sites for conducting its activities within the licensing area. The company has to make a deal on a land access agreement with the impacted land’s owner and then get government approval for drilling. Upon discovery, if the coal seam gas reservoirs are considered to be viable for business, the company can then apply for development permission from the government and a production license to develop a new gas field.
Regulation for Coal Seam Gas Production
Australian manufacturers are international leaders in the technology and production of CSG, and the NSW government has implemented many regulations for the protection of communities, land, and water resources. For coal seam gas exploration, assessment, and development operations, CSG operators need to have Environment Protection Licenses (EPLs) that have strict checks for the monitoring and prevention of pollution, as well as cleaner production through best practice implementation.
Drilling cannot proceed without a high-pressure test conducted for the well, and it is ensured that government standards are met by using a geophysical logging tool to check the casing by having it lowered into the well. Such steps restrict the potential for exploration operations to conflict with the climate and water resources.
The Environment Protection Authority (EPA) is the regulatory body for gas activities, responsible for ensuring companies comply with the regulations and enforcing all gas activity conditions, while the Division of Resources and Energy tackles issues of health and work safety.
Wells that have exhausted coal seam gas reserves are sealed and plugged as per regulations. The holder of the title is expected to rehabilitate well sites and region around it to their former state or based on what was agreed upon with the landowner. Well rehabilitation can also involve trying to restore the soil’s natural surface contour and planting seeds to make sure the area has sufficient coverage.
Coal Seam Fires
The organic combustion of an outcrop or an underground seam of coal is referred to as a coal seam fire. Many coal-seam fires demonstrate smoldering combustion due to reduced accessibility to oxygen in the atmosphere, especially in the case of underground coal-seam fires. Coal-seam fires can be caused by factors like low-temperature oxidation, wildfires, lightning, and even explosive self-heating.
If it’s a situation of near-surface coal seam fires, concealing the region or setting up gas-tight obstructions can help to disrupt the oxygen flow in the air. Another probability is to prevent the outflow of combustion gasses to quench the fire with its own exhaust fumes. Through cooling, usually through injecting large quantities of water, energy can be drained. If any leftover dry coal retains water, however, the subsequent absorption heat can cause a once-quenched fire to re-ignite as the region dries. That is why it is necessary to remove more energy than the fire produces. Such approaches are mixed in operation, and every situation would depend on the available resources in any given region, particularly for resources like water.