Coalbed Methane (CBM)

What is Coalbed Methane?

Coalbed methane (CBM) is natural gas found in coal. CBM is composed mostly of methane (CH4) but may have minor amounts of nitrogen, carbon dioxide and heavier hydrocarbons like ethane. It forms naturally as a byproduct of the geological process that turns plant materials to coal.

CBM is considered an unconventional form of natural gas because the coal acts both as the source of the gas and the storage reservoir. As well, the gas is primarily adsorbed on the molecular surface of the coal rather than stored in pore spaces, as occurs in conventional gas reservoirs. If the CBM gas ever naturally migrates out of a coal seam and becomes trapped in adjacent porous rock, it is no longer considered CBM, but deemed to be conventional gas.

The gas adsorbed within coals is held there mostly by pressure. If the pressure is reduced, the gas is released from the coal and free to flow to a well. The amount of gas liberated from a given coal seam is a function of many factors, such as the chemical composition of the coal, the geological history of the coal, and whether the coal had been previously depressured. The gas content of a coal can be estimated by collecting drilling samples and measuring the volume of gas released as a function of pressure in a laboratory.

How Much CBM is in Alberta?

Coal seams with CBM potential are found underneath much of Alberta, especially in southern and central Alberta. When measurements of gas content from coal samples are mapped along with the thicknesses and areal extents of the various coal seams, estimates of Alberta’s in-place volumes of CBM can be made. The Alberta Geological Survey of Alberta recently estimated there could be as much as 14 trillion cubic metres (about 500 trillion cubic feet or Tcf) of coalbed methane held in Alberta coal. For comparison, a joint study by the Alberta Energy and Utilities Board (ERCB) and the National Energy Board in 2006 estimated the ultimate potential of marketable conventional natural gas in Alberta to be between 5.7 and 7.1 trillion cubic metres (205-253 Tcf), with 2.8 trillion cubic metres (101 Tcf) being the estimate of remaining ultimate potential after consideration of past gas production.

Shallow coal seams also contain CBM, including those seams at mines and those seams used as aquifers by water wells. Due to low pressure, this CBM is not viewed as a natural gas resource. However, shallow CBM can be a nuisance and even a safety hazard for miners and water-well owners. Water wells completed in coals or coaly aquifers need appropriate venting to manage methane released during water production. Methane can also be produced in noncoal aquifers and in water wells by methane-generating bacteria, where conditions are favourable.

How Much CBM is Being Produced in Alberta?

Though the amount of CBM in-place that will ultimately reach market is presently unknown, the sheer potential size of the in-place resource estimates strongly suggests that CBM will make a significant contribution to Canada’s energy security in the future. In 2005, the ERCB estimated the initial established reserves of CBM to be 22.9 billion cubic metres, but this estimate only considered the areas of commercial CBM production up to that time. This number is expected to increase with further evaluation to include areas of known resources drilled but not yet producing. Production of CBM in Alberta was 2.9 billion cubic metres in 2005 and is forecast to increase to 19.6 billion cubic metres in 2015. This would represent an increase from less than 2 per cent in 2005 to about 16 per cent in 2015 of total Alberta marketable gas production.

As of December 31, 2006, 10,723 CBM wells were drilled in Alberta, with approximately 2,000 CBM wells drilled in 2005. The total number of producing conventional gas wells in Alberta in 2005 was 97,900, with 13,248 conventional gas wells drilled in 2005, slightly less than 12,000 of these were connected and producing.

What is Unconventional About CBM Production?

CBM wells differ from conventional gas wells because a reservoir depressurization step is needed during development to release the gas from the coal matrix and make it flow to a well. The depressurization can be accomplished by allowing free gas within the naturally occurring cracks and fractures in the coal to flow to the surface, or by pumping out any natural fluids occupying the connected cracks and fractures. This fluid can be formation water and/or free gas. The cracks and fractures in the coal also provide the pathways for the CBM to migrate to the production well. Coals usually have a blocky set of natural fractures called cleats, which form during coal formation. Provided they are not infilled by minerals or squeezed shut by geological processes, the cleats provide the natural pathways through the coal for CBM production. It is common practice to stimulate a well to assist CBM production through artificial hydraulic fracturing, commonly called “fracing.” Fracing involves pumping large volumes of fluids (commonly nitrogen) into the wellbore to create fractures that allow better contact between the well and the natural coal seam cleats.

Because CBM reservoirs require depressurization prior to production, there can be a start-up phase of free gas or formation-water production before gas reaches the production well. Most current Alberta CBM wells are dry or produce very low volumes of water, even during start-up.

How is CBM Production Regulated in Alberta?

CBM production in Alberta is regulated by the Alberta Energy and Utilities Board. All existing regulations for natural gas development apply to CBM in Alberta. CBM operations not covered by existing gas regulations are handled by special applications. ERCB Directive 056 directs that all CBM wells, pipelines and facility licenses receive a special code for tracking purposes. As well, any requests for commingling (i.e. , combining production across several thin zones in a single well) and for produced water disposal are governed by ERCB Directive 065. All produced water from CBM wells in Alberta requires disposal by deep-well injection. No surface disposal of produced water from CBM is allowed nor is disposal above Alberta’s designated base of groundwater protection. ERCB actively performs surveillance on all produced water in oil and gas wells.

ERCB directives dictate that any wells drilled and completed below Alberta’s designated base of groundwater protection (BGP) must have cement behind surface casing and/or production casing from surface to below the BGP. This practice protects all groundwater aquifers in formations known to have groundwater quality up to 4,000 mg/l total dissolved solids, which is generally deemed useable without treatment.   

Alberta Environment’s proposed Code of Practice for water diversion from CBM wells will require registration for water production rates between 5 and 30 cubic metres per month and a more formal regulatory approval at water withdrawal rates greater than 30 cubic metres per month for any CBM wells completed above the BGP.