|Fig. 1: CBM reserves and activity (in TCF). Data from Al-Jubori et al. |
Gas adsorbed in the coal seams has been commercially produced in many parts of world including US, China, India and Australia. The total estimated gas resource of the world in coal-beds (CBM) is over 9,000 TCF (trillion cubic feet), mainly in North America (3,000 TCF) and former Soviet Union (4,000 TCF). Recent data shows slightly less optimistic data - around 6,700 TCF. [2,3] Fig. 1 demonstrates global distribution of CBM. According to the U.S. EIA Annual Energy Outlook 2013, coal-bed methane contributes about 7.5% of the total US gas production (1.7 TCF dry gas annually). 
Productivity of CBM wells depends primarily upon permeability within micro-fractures. Typical values for cleat-permeability range from a few milli-Darcy to a few tens of milli-Darcy.  Generally, the cleats in coal-beds are initially saturated with water. This water must be produced before gas can start desorbing from the matrix into these micro-fractures. This initial phase of water production is known as "dewatering". In this phase, gas production starts to build-up and reaches a maximum value. This stage requires disposal of huge amount of water, which has a significant impact on environment. Unlike produced water from conventional oil and gas, much of CBM produced water may be put to beneficial use, such as agricultural use (irrigation of crops, hydroponic system), livestock watering, industrial use (dust control, equipment washing, power generation) or some water flood projects (depending on the quality of produced water). [5,6] In other cases it requires to treat produced water using chemical methods. [5,6]
The first phase is followed by a period of relatively stable gas production and then a depletion phase (shown in Fig. 2). 
|Fig. 2: Production behavior of a CBM well.  (Courtesy of the U.S. Geological Survey.)|
Since most of the CBM resides in an adsorbed state, the quality of the reservoir is classified by its adsorption capacity 
Most of CBM wells are vertical and thus have low efficiency without stimulation treatment.  There are several approaches to stimulate CBM reservoirs: hydraulic fracturing, CO2 fracturing, nitrogen fracturing and cavitation.
Fracture simulation is widely used for stimulating CBM reserves. Connection the naturally occurring fracture network to the well-bore provides a conduit through which water and gas are produced. While hydraulic fracturing of coal-beds has been successful in stimulating production, it generally underperformed comparing with fracture-stimulated sand-stone reservoirs.  This is because coal has different physical characteristics that are different from those of conventional rocks. The softness of coal makes fracture propagation difficult. Thus, limited fracture length are achieved even with high treating pressures. 
In cavitation, the well-bore is completed using open-hole techniques, and the target coal seam in under-reamed. Using compression air, the exposed coal is repeatedly pressurized and depressurized. The coal breaks up and is drilled out, forming a cavity around the well-bore. This continuous process results in a donut-shaped area of enhance permeability which increases the productivity of CBM wells. Operators in Australia's rapidly growing CBM industry are investigating this technique.  Other methods of stimulation by CO2 of nitrogen is similar to some extent to water and described in detail by the EPA. 
In the future the CBM industry may take an entirely new direction, becoming an essential player in carbon storage. A number of enhanced coal-bed methane (ECBM) projects have investigated unminable coal seams and depleted CBM fields as candidates for CO2 sequestration. The organic materials that make up coals generally have a stronger affinity for CO2 than for methane. In a process similar to that used for secondary oil recovery, CO2 is pumped into a coal seam and is adsorbed by the coal while displacing and liberating methane. ECBM projects offer the opportunity of removing greenhouse gases from the atmosphere and simultaneously increasing natural gas supplies. The studies have progressed from the data-gathering and analysis phase to implementation, and the results have been encouraging. 
CBM is still under development and only small part of CBM is extracted. Many new approaches have been presented recently and they can make revolutionize CBM as shale gas few years ago in USA.
© Ruslan Iskhakov. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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