Introduction
Composition of coalbed methane :
Coalbed methane (CBM) primarily composed of methane (CH4)- the main component of natural gas. It may also include other hydrocarbon gases and small amounts of non-hydrocarbon gases, with its exact composition depending on the coal seam and surrounding geological conditions.
1. Methane (CH4) : Methane forms the major part of CBM, often accounting for 80% to more than 95% of the total gas present.
2. Ethane (C2H6): Ethane is the second most common hydrocarbon found in CBM. Its concentration can vary but is generally lower than methane, typically ranging from a few percent to around 10% or less.3. Propane (C3H8) and Butane (C4H10) : Propane and butane are also present in coalbed methane, but in lower concentrations compared to methane and ethane. They are usually present in trace amounts, typically less than a few percent.
4. Carbon Dioxide (CO2) : Carbon dioxide is often present in CBM and can be found in varying concentrations. Its content is lower than that of methane but can range from a few percent to over 30% depending on the coal deposit.
5. Nitrogen (N2) : Nitrogen is a non-hydrocarbon gas that can be present in coalbed methane. Its concentration can vary but is generally lower than methane and carbon dioxide.
6. Trace Elements : CBM can also contain trace amounts of other gases such as hydrogen (H2), helium (He), oxygen (O2) and sulfur compounds. The presence of these elements depends on the specific coal deposit and its geological characteristics.
Classification :
Depending on the phase of the methane within the coal seam and its relationship to water CBM can classify into sorbed or adsorbed methane and dissolved methane.
1. Sorbed or Adsorbed Methane:
This type of CBM is characterized by methane gas that is physically attached or adsorbed onto the surface of the coal particles within the coal seam. It occurs when the methane molecules are trapped within the micropores and fractures of the coal matrix. Sorbed methane is typically the dominant form of methane in coal seams. It is released when the pressure within the coal seam is reduced, such as during the extraction process, and the methane desorbs from the coal, flows into the wellbore, and is produced.
2. Dissolved Methane: Dissolved methane refers to methane gas that is dissolved in the groundwater within the coal seam. This occurs when methane dissolves into the water that saturates the coal matrix. The dissolved methane is typically found in coal seams that have a high water content. When the pressure is reduced, such as during the production of CBM, the dissolved methane is released from the water and flows into the wellbore along with the produced water.
Formation of coalbed methane :
Coalbed methane (CBM) is formed through a complex process that involves the organic matter in coal deposits undergoing transformation over millions of years.
1. Accumulation of Organic Matter : CBM formation begins with the accumulation of organic matter, mainly from plants, in ancient swamps, marshes, or peat bogs. These environments provide the necessary conditions for the accumulation of plant material without complete decomposition.
2. Burial and Coal Formation : Over time, the organic matter is buried by layers of sediment such as sand, silt and clay. The increasing pressure from the overlying sediments and the heat generated by the Earth's geothermal gradient leading the organic matter to undergo physical and chemical changes and transforming it into coal. This process, known as coalification, occurs over millions of years.
3. Methane Generation : As coal forms, the organic matter undergoes chemical changes. One of the byproducts of these changes is the generation of methane (CH4) gas. Methane is produced through the thermal breakdown of organic matter under high temperature and pressure. The methane is then stored within the coal matrix, adsorbed onto the coal surface or trapped in fractures and pore spaces of the coal seams.
4. Adsorption and Storage : Methane is adsorbed onto the surface of the coal particles and is trapped within the coal matrix due to the high surface area and microporous nature of coal. The coal acts as a reservoir for the methane, which is stored within the coal seams.
5. Geological Factors : The formation and storage of CBM are influenced by various geological factors. These include the depth and thickness of the coal seams, the rank and composition of the coal, the permeability of the coal matrix and surrounding rocks, the presence of natural fractures or faults and the pressure and temperature conditions.
Faults and fractures in the coal seam can increase its permeability, allowing methane to flow more freely and gather in specific areas.
Advantages of coalbed methane :
Some of the advantages of CBM include:
1. Energy Resource : Coalbed methane (CBM) serves as an alternative energy source, useful for generating heat, producing electricity and supporting various industrial processes. Also reducing dependence on conventional fossil fuels.
2. Methane Emissions Reduction : Extracting methane not only provides a usable energy source but also prevents its direct release into the atmosphere that lower the emissions of greenhouse gas and impacts of climate change.
3. Utilization of Coal Deposits : CBM extraction allows for the productive use of coal deposits that may otherwise remain untapped. It provides an additional economic benefit from coal resources.
4. Local Economic Development : CBM production can stimulate local economies through job creation, generating revenue for landowners and governments, and supporting related industries such as drilling and pipeline construction.
