Natural Gas from Coal


What is natural gas?

Natural gas is the cleanest burning fossil fuel. It is a naturally occurring mixture of hydrocarbons comprised of mostly methane. It is colorless, shapeless and odorless. The combustion byproducts of natural gas are carbon dioxide (CO2) and water vapor – the same compounds we exhale. When burned, it produces energy in the form of heat and light. Natural gas is created by both organic and inorganic processes. New technologies have unlocked vast quantities of this resource that have historically been uneconomic to recover.

Some of the environmental benefits of using natural gas are:

  • Natural gas produces 25% lower carbon dioxide emissions than oil and 50% lower emissions than burning coal
  • Particulates from natural gas combustion are 90% lower than oil and 99% lower than coal
  • Natural gas is substantially less polluting than oil or coal
  • Natural gas is supported by the EU for emissions reduction
  • A domestic source of natural gas would reduce Bulgaria’s dependence on foreign gas and coal combustion

What is Coal Bed Methane (“CBM”)?

Coal bed methane (also known as coal seam gas) is the natural gas that is emitted from coals.

Coal originates from the accumulation of organic matter in swamps and marshes that is then buried. In a metamorphic process, the organic matter is transformed into peat; as the matter is buried deeper with increasing temperature and pressure on it, it is transformed into sub bituminous coal and then further matures into bituminous coal and ultimately anthracite coal. The heat and pressure on the organic material cause natural gas to be generated over time, resulting in gas accumulating in the coal and coal seams.

Natural gas produced from conventional sandstone reservoirs is mostly methane with some heavier hydrocarbon gases (such as ethane or propane) or other gases (such as carbon dioxide or nitrogen) mixed in. Natural gas produced from coals is almost entirely methane with a very minor amount of impurities (1-5%). Gas in coal seams is more concentrated than gas in conventional formations.

In CBM, entrapment of gas does not rely on a seal or structural closure. Gas is adsorbed onto the surface of micropores within the coal seam and held in place by molecular attraction. De-pressuring of the coal by drilling and dewatering is required to release gas from the coal matrix. Gas then releases through the natural cleat network in the coal beds, where it flows into wellbore and through the wellbore to the surface. Once produced, coal bed methane is indistinguishable from conventional natural gas except for having a higher purity.

How is natural gas (CBM) extracted from coals?

Coal seams are usually saturated with both water and gas, where the water’s hydrostatic pressure keeps the methane gas adsorbed onto the coal. To release the methane from the coal, the formation must be de-pressured, which occurs when a well is drilled that intersects the coal seam.

The cleats (the internal structure of the coal) form a permeable matrix that allows the natural flow of gas and water. As the pressure is reduced from the seam, the methane gas is released from the cleats for extraction and production.

The cleat system of the coal must be sufficiently permeable to allow the gas to flow through the coal to the wellbore for production of gas to occur at the surface. Generally, the gas flow increases as water pressure decreases so that once enough de-watering has occurred, the quantity of gas flowing into the wellbore increases and the production of water decreases.

Natural Gas is Environmentally Preferred

During the production of any natural gas field, monitoring and testing is undertaken to ensure compliance with strict environmental guidelines. The methane gas is exceptionally pure (greater than 95% methane) and, with minimal treatment, can be transported directly from the wells into the gas pipeline grid. This is unlike most natural gas, which requires further processing to strip impurities such as carbon dioxide. Production of this low emission energy source will help meet environmental commitments to reduce greenhouse gas.