Coalbed methane

Coalbed methane, coalbed gas, or coal seam gas is a form of natural gas extracted from coal beds. In recent decades it has become an important source of energy in United States, Canada, Australia, and other countries.
The term refers to methane absorbed into the solid matrix of the coal. It is called "sweet gas" because of its lack of hydrogen sulfide. The presence of this gas is well known from its occurrence in underground coal mining, where it presents a serious safety risk. Coalbed methane is distinct from a typical sandstone or other conventional gas reservoir, as the methane is stored within the coal by a process called adsorption. The methane is in a near-liquid state, lining the inside of pores within the coal. The open fractures in the coal can also contain free gas or can be saturated with water.
Unlike much natural gas from conventional reservoirs, coalbed methane contains very little heavier hydrocarbons such as propane or butane, and no natural-gas condensate. It often contains up to a few percent carbon dioxide. Coalbed methane is generally formed due to thermal maturation of kerogen and organic matter, in contrast to coal seams with regular groundwater recharge where methane is typically generated by microbial communities living in situ.

History

Coalbed methane grew out of venting methane from coal seams. Some coal beds have long been known to be "gassy" so, as a safety measure, boreholes were drilled into the seams from the surface, and the methane allowed to vent before mining.
Coalbed methane as a natural-gas resource received a major push from the US federal government in the late 1970s. Federal price controls were discouraging natural gas drilling by keeping natural gas prices below market levels; at the same time, the government wanted to encourage more gas production. The US Department of Energy funded research into a number of unconventional gas sources, including coalbed methane. Coalbed methane was exempted from federal price controls, and was also given a federal tax credit.
In Australia, commercial extraction of coal seam gas began in 1996 in the Bowen Basin of Queensland.

Reservoir properties

Gas contained in coal bed methane is mainly methane and trace quantities of ethane, nitrogen, carbon dioxide, and few other gases. Intrinsic properties of coal as found in nature determine the amount of gas that can be recovered.

Porosity

Coalbed methane reservoirs are considered as a dual-porosity reservoirs, where porosity related to cleats are responsible for flow behavior and reservoir porosity of the matrix is responsible for the storage of gas. The porosity of a coalbed methane reservoir can vary from 10%-20%; However, the cleat porosity of the reservoir is estimated to be in the range of 0.1%-1%

Adsorption capacity

Adsorption capacity of coal is defined as the volume of gas adsorbed per unit mass of coal usually expressed in SCF gas/ton of coal. The capacity to adsorb depends on the rank and quality of coal. The range is usually between 100 and 800 SCF/ton for most coal seams found in the US. Most of the gas in coal beds is in the adsorbed form. When the reservoir is put into production, water in the fracture spaces is pumped off on first. This leads to a reduction of pressure enhancing desorption of gas from the matrix.

Fracture permeability

Fracture permeability acts as the major channel for the gas to flow. The higher the permeability, the higher the gas production. For most coal seams found in the US, the permeability lies in the range of 0.1–50 milliDarcys. The permeability of fractured reservoirs changes with the stress applied to them. Coal displays a stress-sensitive permeability and this process plays an important role during stimulation and production operations. Fracture permeability in Coalbed methane reservoir tends to increase with depletion of gas; in contrast to conventional reservoirs. This unique behavior is because of shrinking of coal, when methane is released from its matrix, which results in opening up of fractures and increased permeability. It is also believed that due to shrinkage of coal matrix at lower reservoir pressures, there is a loss of horizontal stress in the reservoir which induces in-situ failure of coal. Such a failure has been attributed to sudden decrease in the fracture permeability of the reservoir

Thickness of formation and initial reservoir pressure

The thickness of the formation may not be directly proportional to the volume of gas produced in some areas.
For example, it has been observed in the Cherokee Basin in Southeast Kansas that a well with a single zone of of pay can produce excellent gas rates, whereas an alternative formation with twice the thickness can produce next to nothing. Some coal formations may have high gas concentrations regardless of the formation's thickness, probably due to other factors of the area's geology.
The pressure difference between the well block and the sand face should be as high as possible as is the case with any producing reservoir in general.

Other properties

Other affecting parameters include coal density, initial gas-phase concentration, critical gas saturation, irreducible water saturation, and relative permeability to water and gas at conditions of Sw = 1.0 and Sg = 1-Sw irreducible respectively.

Extraction

To extract the gas, a steel-encased hole is drilled into the coal seam below ground. As the pressure within the coal seam declines due to natural production or the pumping of water from the coalbed, both gas and produced water come to the surface through tubing. Then the gas is sent to a compressor station and into natural gas pipelines. The produced water is either reinjected into isolated formations, released into streams, used for irrigation, or sent to evaporation ponds. The water typically contains dissolved solids such as sodium bicarbonate and chloride but varies depending on the formation geology.
Coalbed methane wells often produce at lower gas rates than conventional reservoirs, typically peaking at near per day, and can have large initial costs. The production profiles of CBM wells are typically characterized by a "negative decline" in which the gas production rate initially increases as the water is pumped off and gas begins to desorb and flow. A dry CBM well is similar to a standard gas well.
The methane desorption process follows a curve called a Langmuir isotherm. The isotherm can be analytically described by a maximum gas content, and the pressure at which half that gas exists within the coal. These parameters are properties of the coal, and vary widely. A coal in Alabama and a coal in Colorado may have radically different Langmuir parameters, despite otherwise similar coal properties.
As production occurs from a coal reservoir, the changes in pressure are believed to cause changes in the porosity and permeability of the coal. This is commonly known as matrix shrinkage/swelling. As the gas is desorbed, the pressure exerted by the gas inside the pores decreases, causing them to shrink in size and restricting gas flow through the coal. As the pores shrink, the overall matrix shrinks as well, which may eventually increase the space the gas can travel through, increasing gas flow.
The potential of a particular coalbed as a CBM source depends on the following criteria. Cleat density/intensity: cleats are joints confined within coal sheets. They impart permeability to the coal seam. A high cleat density is required for profitable exploitation of CBM. Also important is the maceral composition: maceral is a microscopic, homogeneous, petrographic entity of a corresponding sedimentary rock. A high vitrinite composition is ideal for CBM extraction, while inertinite hampers the same.
The rank of coal has also been linked to CBM content: a vitrinite reflectance of 0.8–1.5% has been found to imply higher productivity of the coalbed.
The gas composition must be considered, because natural gas appliances are designed for gas with a heating value of about 1,000 BTU per cubic foot, or nearly pure methane. If the gas contains more than a few percent non-flammable gases such as nitrogen or carbon dioxide, either these will have to be removed or it will have to be blended with higher-BTU gas to achieve pipeline quality. If the methane composition of the coalbed gas is less than 92%, it may not be commercially marketable.
"Beyond conventional methods, research institutions like The Energy and Resources Institute in India are investigating microbial techniques to enhance methane recovery from coal beds, with reported successful field trials."

Environmental impacts

Methane

As with all carbon-based fossil fuels, burning coalbed methane releases carbon dioxide into the atmosphere. Its effect as greenhouse gas was first analyzed by chemist and physicist Svante Arrhenius. CBM production also entails leaks of fugitive methane into the atmosphere. Methane is rated as having 72 times the effect on global warming per unit of mass than CO_2. over 20 years, reducing to 25 times over 100 years and 7.5 times over 500 years. Analysis of life-cycle greenhouse gas emissions of energy sources indicates that generating electricity from CBM, as with conventional natural gas, has less than half the greenhouse gas effect of coal.
Globally, coal mining is also a significant source of methane emissions. To identify the volumes, the industry, governments, and NGOs rely on a combination of emissions monitoring and reporting, estimates, and satellite observations. The current methods for estimating emissions at the coal mine level rely on activity data from the operation and the methane gas content of the coal seams. The emissions methodologies for several academic studies, and NGOs like Global Energy Monitor and Climate Trace, are publicly available.
Multiple Australian studies have indicated the long term negative environmental effects of coal seam gas extraction, both locally and globally.
In the United States, methane escaping from coal during mining amounts to seven percent of total methane emissions. Recovery of coal mine methane in advance of mining is seen as a major opportunity to reduce methane emissions. Companies like CNX Resources have methane abatement programs to reduce greenhouse gas emissions from active and closed mines.