Differences Among Coalbed Methane, Coal-rock Gas, and Shale Gas
1. Composition
Coalbed Methane (CBM):
Its primary component is methane, typically with a purity exceeding 90%. It contains almost no heavy hydrocarbons, giving it a relatively simple composition—similar to “pure juice.”
Coal-Rock Gas:
This gas has a more complex composition. In addition to methane, it contains carbon dioxide, nitrogen, and small amounts of heavy hydrocarbons. It is more like a “mixed beverage,” with a diverse set of components.
Shale Gas:
Shale gas is mainly composed of methane (CH₄), with smaller amounts of ethane, propane, butane, and trace gases such as carbon dioxide, nitrogen, and hydrogen sulfide.
2. Formation Process
Coalbed Methane (CBM):
Formed directly within coal seams, the gas exists mainly in an adsorbed state on the surface of coal matrix particles. The formation process is relatively simple, and the gas generally does not undergo migration.
Coal-Rock Gas:
This gas is either generated within the coal and surrounding rocks or transported from other gas sources and trapped in the coal-bearing strata. It exists in both free and adsorbed states, and its formation process is more complex, showing a “cross-boundary” nature.
Shale Gas:
Formed primarily through thermogenic or biogenic processes, shale gas exists in both adsorbed and free states within the pores and fractures of shale formations.
3. Extraction Methods
Coalbed Methane (CBM):
CBM is typically extracted through vertical wells, relying on the coal seam’s natural permeability and pressure differences to allow the gas to flow out. The extraction process is relatively simple.
Coal-Rock Gas:
Because the reservoir conditions are more complex, special stimulation measures are often required—such as horizontal wells and multilayer commingled production. The extraction process is more complicated.
Shale Gas:
Shale gas is relatively difficult to extract due to the extremely low permeability of shale reservoirs. The main methods are horizontal drilling and multi-stage hydraulic fracturing.
4. Reservoir Characteristics
Coalbed Methane (CBM):
CBM is mainly stored in the coal matrix pores and cleat systems. In other words, the gas “lives” primarily inside the coal itself.
Coal-Rock Gas:
Coal-rock gas is stored not only within the coal but also in the surrounding rock layers adjacent to the coal seams, giving it a wider distribution range.
Shale Gas:
Shale gas is an unconventional natural gas hosted in organic-rich shale formations. It consists of continuously generated biogenic gas, thermogenic gas, or a mixture of both. It can exist in a free state within natural fractures and pore spaces.
5. Application Fields
Coalbed Methane (CBM):
CBM is mainly used as a residential fuel and for gas-fired power generation, providing energy for daily life and industrial production.
Coal-Rock Gas:
In addition to being used for energy and gas-fired power generation, coal-rock gas can also serve as a feedstock for producing high value-added chemical products, such as synthetic rubber and fibers.
Shale Gas:
As a clean and efficient energy resource, shale gas is widely used in residential gas supply, district heating, gas-fired power generation, chemical production, and as a fuel for vehicles.
6. Future Outlook
With technological advancements and increasing market demand, the prospects for power generation from coalbed methane (CBM), coal-rock gas, and shale gas in China are highly promising. The geographical distribution of these gas fields aligns well with major power load centers. Localized power generation using these gases can effectively meet the growing energy demand while reducing pressure on energy conservation and emission reduction efforts.
Looking ahead, as more large gas fields are developed, power generation from CBM, coal-rock gas, and shale gas is expected to become an indispensable part of China’s energy mix.
