CBM stands for coal bed methane where gas (typically CH4 and CO2) is sorbed under pressure into the large internal surface area of coal. The methane can have up to 6 to 7 times the volume of methane found in conventional gas reservoir.
Basics of CBM production can be given as follows
1. Relatively low gas production rate in early production time, which is called de-watering stage.
2. By the time, gas production rate increases and more stable as water is produced, pressure lowered, and gas desorbs and flows into fracture network.
3. Subsequently, gas production will reach hyperbolic/harmonic decline rates.
4. Generally it requires closer well spacing and hydraulic fracturing.
To support CBM production, mainly CBM well needs artificial lift right away to pump-off water before gas will flow at optimal rates. PCP (progressive cavity pump), ESP (electric submersible pump) and SRP (sucker rod pump) are production lifting options in the early stage. However for better solid handling (e.g. coal, shale, sand), some operating companies prefer to use PCP rather than the others.
As water production drops in time and gas production increases, the operating company may change the production lifting to plunger lift to support high gas liquid ratio production.