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The secret of adsorption: How does coalbed methane store and release methane?
Coalbed Methane (CBM) is a natural gas derived from coal seams, which is different from traditional natural gas resources. As energy demand rises, especially in the United States, Canada and Australia, coal seam methane has become an important energy source. The special feature of this energy source is that methane is stored in the coal seam in the form of adsorption rather than directly flowing in the form of gas.
Coalbed methane is not only an interactive energy source, but also an important example of the adsorption process in the oil and gas industry.
The storage process of coalbed methane utilizes the porous structure of coal, and methane is adsorbed in the tiny pores of coal in a state close to liquid. This makes coalbed methane different from traditional sandstone or other conventional gas reservoirs. These open cracks in the pores, called fissures, may also contain free gas or be saturated with water.
The formation process and history of coalbed methane
The formation of coalbed methane is mainly due to the thermal maturation process of organic matter. Unlike common coal seams that are refilled with groundwater, coal seam methane is often generated by fermentation of microbial communities within the coal seam. This means that the existence of coalbed methane is closely related to the inventory and quality of coal, and the internal conditions of coal seams in different regions will affect the generation and storage of gas.
In the late 1970s, the U.S. government's promotion of coalbed methane not only promoted research in the field, but also stimulated interest in commercial development.
Storage properties of coalbed methane
The storage and release process of coalbed methane involves various physical properties, such as porosity and adsorption capacity. The porous structure of coalbed methane reservoirs allows coal seams to be considered as dual-porosity aquifers, where the porosity of the fractures affects the flow behavior of the gas, while the coal matrix porosity determines the amount of gas that can be stored. Generally speaking, the porosity of coalbed methane water storage ranges from about 10% to 20%, while the porosity of fractures is between 0.1% and 1%.
In addition, the adsorption capacity of the coal reservoir is also crucial. The capacity depends on the quality and type of coal. If the coal seam is of superior grade and quality, the adsorption capacity can reach 100 to 800 standard cubic feet of gas per ton of coal.
Methods of coalbed methane extraction
Coalbed methane extraction methods typically involve drilling wells into coal seams, a process that can go 100 to 1,500 meters underground. As the pressure in the coal seam decreases, the gas and produced water rise to the surface through pipes. Treatment of produced water depends primarily on its chemical composition, which may contain dissolved solids such as sodium hydroxide and sodium chloride. The generated gas can be further sent to a compression station and transported to a natural gas pipeline.
Due to the lower greenhouse gas emissions of coal-bed methane during its combustion process compared to coal, it has become a clean energy source that is gradually gaining attention.
Environmental impacts and challenges
The exploitation and use of coal-bed methane is not without environmental impact, especially the impact of methane released on global warming cannot be ignored. Various studies have shown that the greenhouse effect of methane is far greater than that of carbon dioxide, so effective gas management during the mining process is crucial to reducing greenhouse gas emissions.
In addition, the quality of water produced during the production process has also caused concern, as it contains a large amount of salt and heavy metals. Different regions need to explore appropriate methods to deal with these by-products to protect local water resources.
Global Coalbed Methane Production Areas
Currently, the main production areas of coalbed methane include the United States, Australia and Canada. With the advancement of technology, many countries such as India and Kazakhstan have also begun to research and develop the potential of coalbed methane. According to recent data, annual coal seam methane production in the United States still accounts for 3.6% of total gas production, while Australia and Canada also reported stable production growth.
In the face of the increase in global energy demand, the market potential and development sustainability of coalbed methane are still worthy of our in-depth consideration. Therefore, how to balance the environmental impact while expanding the use of coalbed methane should become an important issue for governments and energy companies.
