Natural Gas from Coal

Coal can produce large reservoirs of natural gas with commercial potential. Most coals of sufficient rank have generated large volumes of gas that may be trapped by adsorption. Natural gas is generated in coal beds both by biogenic and thermogenic processes.

Natural Gas by Biogenic processes

Gas derived from biogenic processes consists mostly of methane and CO2, and is produced as a by-product during the anerobic microbial decomposition of organic matter (methanogenesis).

• The various type of methanogenic bacteria are the terminal organisms in a consortium of bacteria that degrade biopolymers and geopolymers to low-molecular weight organic compounds via fermentation and finally to methane and CO2.
• Bacteria produce methane via two ways : CO2 reduction and fermentation (demethylation of low-molecular-weight organic acids to produce CO2 and methane)
  • Both CO2 reduction and fermentation produce natural gas in recent peat deposits, but CO2 reduction is normally the dominant metabolic pathway for bacterial methanogenesis in older (more decomposed) organic matter.
• The major requirements for biogenic methane production and accumulation include : water, anoxic environment, a source of metabolizable organic matter, relatively low temperature, a low sulfate concentration, and adequate pore space for natural gas retention.
• Large amount of methane are produced in wetlands during peatification, but most of this methane is probably lost to the atmosphere prior to entrapment during burial. After peat deposits are buried, and during the early stages of coalification (to subbituminous rank), a significant amounts of methane can be produced.

Biogenic methane may also be produced in Late stages, the biogenic methane production occurs during a relatively short period time (10s of thousand to a few Ma) following the intrusion of groundwater into coal beds.

• Coal beds acts as aquifers in some areas. Oxic groundwater may enter the coal bed, sustaining the aerobic metabolism (bacteria and fungi) of the geopolymers of coal. As oxygen is consumed, anoxic conditions develop, and methanogenesis can begin.
• Late stage biogenic methane production can occur in coal of any rank where anoxic conditions, a suitable reservoir of metabolizable organic substrate and relatively low temperature conditions exist.

Natural Gas by Thermogenic processes

Thermogenic production of methane from coal occurs during coalification as increasing temperature and pressure cause devolatilization of methane, CO2, water and other gases from the coal.

• The generation of methane and other hydrocarbon gases from coal occurs at the high volatile bituminous rank and higher.
• Gas production increases rapidly with increasing rank. The amount of natural gas produced by thermogenic processes depends on the composition of coal and its regional variations.
• Other factors influencing natural gas production from coal include the growth of structural features, differences in regional temperature gradients, and basin hydrology.
• Natural gas can either be produced directly from the macerals (liptinite, lipid-rich vitrinite), or by cracking the heavy hydrocarbons present within the coal matrix.

Retention of natural gas in coal beds takes place via adsorption by the coal surface, absorption within the molecular structure of the coal, and storage within the pore spaces or cleats in the coal.

• Presence of liquid hydrocarbon in coal usually reduces its natural gas sorption capacity and natural gas production in coal beds.
• At higher temperature however, liquid hydrocarbons in coal may be cracked to produce natural gas. This process opens up the micropore space for gas storage.

“Methane production from coal increases with rank, and in higher rank coals methane production may exceed the storage capacity of the coal.”

• However, the igneous intrusions may increase the gas reservoir potential of coal by creating microscopic pores that enhance gas adsorption capacity, gas desorption and permeability.

Molecular and Isotopic composition of Gas from coal

Coal beds often contain a mixture of gases, but methane is usually dominant component.