Coalbed methane is produced within a process of oil and petroleum system. A petroleum system usually consists of oil in a sedimentary basin with a hydrocarbon source rock and all related oil and gas sources. Conventional petroleum system comprises of a hydrocarbon-generating source rock slate, with the help of heating and constant burial, resulted gas an oil. Coalbed methane is not same with coal which is originating from coal reservoir. The coal is a hydrocarbon source rock that developed during burial heating into a substance similar in structure to activated carbon. This is an organic substance that is chemically covered or heat-treated to increase micro-pores with a matching increase in the surface to increase gas sorption capacity. Coal produce methane gas and it has the capacity keep gas in place.
Coal as Methane Storage
There are several factors like composition of coal, its porosity and temperature and pressure to determine the amount of methane that can be stored in and recovered from coal.
Gas Sorption Capacity
Gas Sorption is a physical process where gas is kept in internal pore of coal by small electro-static. All solid surfaces are activated and act to deal with small diameter organic compounds like methane. By increasing the pressure working in coal will cause of the increasing quantities of gas in the pore surfaces. Since the electrostatic binding, its force reduce with the square of the distance between the surface and the gas molecule, and the strongest bonds with molecules put in a single layer or monolayer. If we see coal internal surfaces, it shapes is like spherical to irregularly shaped pores with a monolayer of methane gas. The cross-section of a molecule of methane can be easily move within these pores. Distribution of pores that are less than 12 A at the funeral service for heatingduce a net increase of carbon in the surface with an increase in the gas sorption capacity.
Coal Composition
Coal composition in terms of its plant matter, mineral matter, and moisture content is also an important control on coalbed methane gas storage capacity. There are three main organic constituents of coal, called macerals, which are microscopically identifiable plant components. The vitrinite group is formed from woody plant debris; the liptinite group is from waxy, resinous, and oily plant matter; and inertinite is often composed of fossil charcoal but also includes other altered and oxidized plant matter. Increasing vitrinite content enhances coalbed methane storage capacity and the tendency of the coal to develop fracture permeability. Increasing inertinite content enhances matrix permeability, due to the large open pores of this material, but as it is inert, it decreases gas generation potential and reduces storage capacity.
There are two main inorganic constituents of coal, mineral matter and moisture. Mineral matter has a low surface area and low storage capacity; it consequently reduces gas content in direct proportion to its abundance. Mineral matter also inhibits the formation of natural fractures in the coal, called cleats. Moisture competes with gas sorption sites and proportionately reduces the methane storage capacity of a coal.
Temperature and Pressure in Forming Coalbeds
The gas in coalbeds can be of thermogenic or biogenic origin. Burial heating causes thermogenic methane to be generated from cleavage of methyl groups from the coal molecule. One ton of coal to an anthracite rank is estimated to produce up to 180 m3 (6400 ft3) of gas. Biogenic gas generation is the result of a methane waste product produced from anaerobic microbial metabolism. As such, biogenic gas generation is restricted to temperatures less than about 90°C (194°F) because higher temperatures sterilize the rock. Biogenic gas generation can be twofold, occurring during early burial and after exhumation. All of these sources of coalbed-generated gas are readily sorbed into the coal microporosity. This sorbed gas is usually composed of methane, but coal can also contain carbon dioxide as well as nitrogen, ethane, and longer-chain alkane hydrocarbons.
Coalbed methane has a wide occurrence because it tends to be preserved through burial, and even if lost it can be biogenically regenerated late in burial history when cooler temperatures are reached. These properties make it virtually characteristic of coalbeds to contain some methane. The problem is that while the gas is ubiquitous, coals are often impermeable, making the gas inaccessible. Gas must migrate by solid-state diffusion from the coal matrix to open fractures to become the accessible free gas that flows to wells and thus becomes usable. If the natural fractures (cleat) are too widely spaced, the path of solid-state diffusion is so slow that the gas production rate falls below commercial standards. The current geologic paradigm is that coal bed methane production is limited to a 150 m (490 ft) to 2 km (1.2 mi) depth, because at shallow depths low hydraulic force causes the coal to retain too little methane, and at depths greater than 2 km coal cleat is generally tightly closed, reducing permeability and making commercial production difficult.
