Peculiarities of unconventional coal seam gas production

Abstract

Unconventional natural gas, Coalbed Methane (CBM), is formed in the bowels of the Earth under relevant
thermobaric conditions during the coalification process, when peat turns at first to brown coal, black coal, then to anthracite. There are next types (ranks) of coal according to the international classification: lignite (brown coal), subbituminous coal, bituminous coal, semianthracite, anthracite. The porosity of coal bed reservoirs is usually very small, ranging from 0.1 to 10%. The permeability of a coal bed is usually considered to be due to system of cleats (natural fractures), since the permeability of coal matrix is less than 1mD. CBM consist mainly of methane (88-98%). Coal stores 6–7 times as much gas as conventional reservoirs of equal rock volume. Gas content ranging from 0,5-30 m3 /t in black coal to 35-40 m3 /t in anthracite.
About 98 % of the gas is adsorbed in the coal matrix and about 2 % occurs like “free” gas in coal fractures or
as solute in groundwater in the coal. There are three ways for CBM production: before, during and after coal mining. The main method of CBM production in worldwide practice is previous degassing of coal beds by means of directional drilling, hydraulic fracturing, withdrawing of formation water with following gas extraction, which is desorbed from coal matrix. Besides it, experts suggest displacing of CBM by nonhydrocarbon gases, as carbon dioxide, and accomplishing of gravity decompaction of coal-bearing rock by means of creation of burned-out space in thin coal seam lying under the main commercial bed. Coal mining begins just after pre-degassing of coal seams. Moreover CBM is pumped out with the help of vacuum pumps through a same system of the degasification wells during and after coal mining.
It is expedient a comprehensive application of various methods of CBM production and degassing works.