Coalbed Methane

Coalbed Methane

What is coalbed methane?

Coalbed methane (CBM) is natural light hydrocarbon found in coal beds as gas. CBM is composed mostly of methane (CH4) and minor amounts of nitrogen, carbon dioxide, and heavier hydrocarbons like ethane. It forms naturally as a by-product of the geological process where high temperature and pressure turns vegetal organic matter to coal.

Coal beds that generate CBM are considered unconventional reservoirs because the coal acts both as the source of the gas and the storage reservoir. The methane 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 naturally migrates out of a coal seam and becomes trapped in adjacent porous rock such as sandstones, it is no longer considered CBM, but 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 freed to flow to a well borehole. The amount of gas liberated from a given coal seam is a function of many factors, such as the chemical composition of the coal, history of the organic matter, thermal maturity of the organic matter, and whether the coal had been previously depressurized. The gas content of a coal bed can be estimated by collecting drilling samples and measuring the volume of gas released as a function of pressure in laboratory conditions, assessing the gas adsorption/desorption isotherm capacity.

Coal Rank (Thermal Maturity)
With increasing depth of burial, the temperature and pressure increase and the vegetal organic matter undergoes coalification, releasing volatile matter (water, carbon dioxide, and light hydrocarbons, including methane) as it begins to transform into coal. With ongoing coalification, the coal becomes progressively enriched in carbon and continues to expel volatile matter. Generation of methane is a result of organic matter thermal maturation in coals, and it is named ‘thermally generated methane.’ This process begins around the sub-bituminous A to high volatile bituminous C rank stage. The generated amounts of methane increase significantly throughout the medium to high volatile bituminous coal ranks. Further increase of burial temperature above the high volatile bituminous rank leads to a reduction of the amount of thermally generated methane.

In some areas where the coal beds are shallow, CBM can be produced by methanogenic bacterial processes.

Coal Cleats (Permeability)
Particular to coal beds is the degree of natural permeability pathways for fluid migration, known as cleat systems (i.e., butt and face cleats or joints).

Coal cleat systems occur in response to regional stress and are important in enhancing CBM potential. If cleats are not infilled by minerals or naturally squeezed or closed by geological factors, they provide natural pathways through the coal for CBM migration. The more interconnected the fracture networks are, the more fluids (water and gas) can move through the coals and ultimately into a wellbore for production. The widths or openings of cleats are generally wider near the surface, but with increasing depth, the weight of the overlying rock compresses or closes the cleats, reducing their width and connectivity (reducing reservoir permeability). In areas of faulting, fracturing, or where coal seams are draped over sandstone channels, coal cleats may widen.

What is unconventional about CBM production?

CBM wells differ from conventional gas wells during production development because a reservoir depressurization step is needed to release the gas from the coal matrix and make it flow to a production well. The depressurization can be accomplished by allowing free gas within the naturally occurring or induced 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 a combination of formation water and free gas. The natural cracks and fractures in the coal also provide the pathways for the CBM to migrate to adjacent porous beds such as sandstones, where it is no longer considered CBM.

It is common practice to stimulate a well to assist CBM production through artificial hydraulic fracturing, commonly called “fracking.” Hydraulic fracturing involves pumping large volumes of fluids (commonly nitrogen) into the wellbore to create fractures that allow better connection between the well and the natural coal cleat system.

Because CBM reservoirs require depressurization before 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 Regulator (AER). 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. Directive 056: Energy Development Applications and Schedules directs that all CBM wells, pipelines, and facility licences receive a special code for tracking purposes. Also, any requests for commingling (i.e., combining production across several thin zones in a single well) and for produced water disposal are governed by Directive 065: Resources Applications for Oil and Gas Reservoirs. 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. The AER actively performs surveillance on all produced water in oil and gas wells.

AER directives dictate that any wells drilled and completed below Alberta’s designated base of groundwater protection (BGP) must have cement behind surface casing or production casing from surface to below the BGP. This practice protects all groundwater aquifers in formations known to have groundwater quality up to 4000 mg/l total dissolved solids (TDS), which is generally deemed usable without treatment.

Alberta Environment and Parks’ 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.

Other CBM producers are the U.S. states of Montana, Wyoming, Colorado, Utah, New Mexico, and Alabama. All of these CBM-producing states regulate their coalbed methane development regarding well locations, well spacing, dewatering effects, and produced water.

How much CBM is in Alberta?

Coal seams with CBM potential are found underneath much of Alberta, especially in central and southern Alberta. When measurements of gas content from coal samples of the various coal seams are mapped along with the thicknesses and areal extents, estimates of Alberta’s gas-in-place (GIP) volumes can be made. The AGS has estimated that as much as 14 trillion cubic metres (about 500 trillion cubic feet or Tcf) of coalbed methane is held in Alberta coal beds. For comparison, a joint study by the AER and the National Energy Board in 2006 estimated that the ultimate potential of marketable conventional natural gas in Alberta is between 5.7 and 7.1 trillion cubic metres (205–253 Tcf).

Horseshoe Canyon coal zones have been successfully producing CBM in east-central Alberta due to complementary cleating/fracturing systems. Mannville Group coals and correlative stratigraphic coal-bearing units have shown high gas content in the north-central to central Plains, with reasonable permeability pathways (cleat systems). Kootenay coals, which also contain high gas content, have been evaluated in the southern Foothills. Ardley CBM production has been initiated in the Pembina area, where the coal seams show moderate gas content.

Shallow coal seams also contain CBM, including those seams at mines or used as aquifers by water wells. 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 coal by methane-generating bacteria where conditions are favourable; even non-coal aquifers can store migrated biogenic methane. For instance, water wells drilled in shallow coals may encounter products of methanogenetic processes.

The AGS’s Earth Sciences Report 2002-05: Regional Evaluation of the Coalbed Methane Potential of the Foothills/Mountains of Alberta describes the geology and CBM potential of the foothills and mountains of Alberta.

Earth Sciences Report 2002-06: Coalbed Methane Potential of Upper Cretaceous-Tertiary Strata, Alberta Plains and Earth Sciences Report 2003-03: Production Potential of Coalbed Methane Resources in Alberta describe the geology, chemistry, and CBM potential of the major coal zones in the Alberta plains.

How much CBM is being produced in Alberta?

Alberta's production for natural gas from coal reservoirs steadily increased, up to ~1 billion cubic feet per day (Bcf/day) in 2009 as the new unconventional exploration technology was stimulated by favourable economic factors. Although Alberta's extensive development of CBM attracted numerous exploration companies in the past, in 2014 the production declined to 0.68 bcf/day (see the AER’s ST98-2015). Nevertheless, the CBM resources are far from exhausted and will remain a secure source of methane, with production depending on economic factors.