Coalbed Methane (CBM): Origin, Evaluation and Production

Coalbed Methane (CBM) is methane (CH4) or simple natural gas that was generated during the coalification process and is still hosted or trapped in those same coal beds; the source rock and the reservoir are the same. For natural gas hosted in conventional sandstone and limestone reservoirs, the source rock and the reservoir are generally distinct and separated .

The recovery of CBM in commercial quantities was pioneered in the United States and Canada during the 1980's. Early work in Canada was promising but did not produce quick commercial success. Relatively low gas prices combined with good success in the development of conventional gas in the 1990's created an atmosphere of disinterest in coal-hosted gas resources. Interest in Canadian CBM has recently reawakened, however. Strong gas prices and growing demand, combined with the continued commercial success of CBM in the Unites States, have stimulated many energy companies to reexamine the vast CBM potential of Canada's coal deposits. Retread Resources personnel are involved in all aspects of CBM exploration and evaluation. The discussion below is based on a paper entitled "The Origin and Evaluation of Coalbed Methane" by Dennis Nikols of Retread Reseouces. It formed part of the "What's it All About" seminar series, and was presented to industry by Dennis Nikols in 1990-91, during his tenure at the Alberta Geological Survey. The Origin, Evaluation and Production of Coalbed Methane (CBM)

Introduction

Methane trapped in coal seams (coalbed methane or CBM) is a vast but relatively unexploited energy resource. In the United States, CBM resources are estimated at more than 400 trillion cubic feet (TCF) [11.3 X 1012 m3] and in Canada, speculative resources are estimated at 2000-3000 TCF [57 to 85 X 1012 m3]. CBM has a high heating value, generally around 900 -1050 British Thermal Units per standard cubic foot (BTU/SCF) [33534-39122 kJ/ m3]. It is clean, sulfur-free, pipeline-quality gas that can constitute an economically viable exploration and development objective in Canada and many other parts of the world.

The origin of CBM starts with the formation of the basic source rock, coal, which also acts as the CBM reservoir. An understanding of coal therefore provides insight into the generation, storage and recovery of CBM. By examining the characteristics of coal we can compare this reservoir to conventional reservoirs and see what makes it such a unique and prolific producer.

Origin of Coal

Coal is a sedimentary rock formed from accumulated plant debris that has been metamorphosed due to burial beneath substantial thickness of other sediments.

The physical and chemical properties of the coal are determined by the types of original plant material, and the geological history during and after deposition, especially the maximum depth of burial.

Coal consists of microscopic particles called macerals that are derived from the different types of original plant material (wood, bark, spores, algae, waxes, etc.). Macerals are analogous to the individual minerals that make up inorganic rocks like sandstones. Each type of maceral contains characteristic proportions of carbon, hydrogen, sulphur, nitrogen, oxygen and ash-forming elements that contribute to the character of the resulting coal. The chemistry of the macerals is altered by the degree to which the plant materials decay before and during deposition, and by coalification processes that occur in response to burial.

Coalification and Coal Rank

When the plant material becomes deeply buried, the resulting increase in pressure (and to some extant, temperature) causes the more volatile, hydrocarbon-rich components (mostly methane) to be released, so that the material becomes progressively richer in carbon. That process is called coalification.

The degree of coalification, or maturity, is commonly expressed in terms of rank, which progresses from peat immediately after deposition, to lignite, subbituminous coal, bituminous coal, anthracite, and finally graphite. As rank increases, the percentage of carbon increases, and the percentage of hydrogen decreases; graphite is 100% carbon. The optical reflectance of the macerals (especially vitrinite) is very sensitive to changes in rank, so measurements of vitrinite reflectance are often used in an indicator of rank.

Porosity in Coal

Coal is characterized by two types of porosity: microporosity, which consists of pores within the coal substance itself; and fracture porosity, which is comprised of sets of regularly spaced joints or fractures called cleats. Most of the gas storage capacity is related to the microporosity, but the fracture porosity, with its higher permeability, is significant for gas production.

At the beginning of coalification, peat has an interconnected network of very large pores. As coalification increases that network collapses, and when bituminous rank is reached the coal is characterized by very small micropores. The porosity of higher-rank coals is low, usually less than 5%, but the open molecular structure of the micropores provides Angstrom- to nanometer-sized interstices that can accommodate small molecules such as methane. At that scale, the gas molecules exhibit weak attractive forces with the coal substance, which gives the coal a high apparent "surface area" for gas storage. Consequently, high-rank coals can contain surprisingly large volumes of gas. Volume for volume, high-rank coals can store several times more gas than porous sandstones at similar pressures (depths), due to the extremely high internal surface areas within the coal microporosity. Shallow coals (1500 ft or 450 m depth) in the San Juan Basin of New Mexico have a storage capacity of about 17:1 (SCF or m3/m3 of reservoir), and the deeper coals (3000 ft or 900 m depth) have a storage capacity of about 28:1. In contrast, for conventional sandstone reservoirs, the capacities are about 7:1 and about 12:1 for shallow and deep reservoirs, respectively.

Internal Surface Area of Coal

The percentage of carbon is a useful indicator of coal rank, and it can therefore be related to the changes of the internal surface area of the coal microporosity. There is a general trend of increasing surface area with increasing carbon content. The amount of internal surface area is astonishing. Coals with a carbon content of 76% - 86% have values ranging from 500,000 -1,000,000 square feet/pound [6422-12,846 m2/kg] of coal. As carbon content increases above 88%, the internal surface area increases beyond 1,000,000 square feet/pound [12,846 m2/kg].

Releasing and Recovering Coalbed Methane

The theory of how methane is retained in the coal microporosity has not yet been proven in detail, but the general consensus is that the methane is somehow adsorbed onto the internal surface area of the coal. One explanation is that the aromatic ring structures within the coal substance are each just large enough to hold a molecule of methane. It is believed that a water molecule, which is larger than the gas molecule, provides enough pressure to ensure that the gas molecule does not escape.

In order to release the gas molecules, the hydrostatic pressure must be reduced. In practice, this is accomplished by drilling a well into a coal seam and pumping out water, which causes a "cone of depression" to form around the well-bore. When the water pressure has been sufficiently reduced, the methane molecules start to desorb. They then diffuse through the coal matrix and the micropores. Finally, they reach a fracture system (either natural or induced), through which they travel to the well-bore and then to the surface, along with the water.

The best CBM production appears to come from wells intersecting coal seams that have:

large in-place reserves of gas;
relatively low water saturations; and
well-developed systems of open fractures.

The trick is to identify the most likely "plays" or targets, given the abundance of coal seams in North America and some other parts of the world. Retread Resources has a strong track record in identifying such targets for our clients. We have developed several geological, geotechnical and geophysical techniques that, when combined, become a powerful tool-kit for CBM exploration and development.

To learn more how Retread Resources can help your company with its CBM objectives, simply contact us. For more on-line information about coal, coalification and coal rank, check out some of these links:

Coal Education Site at: www.coaleducation.org
The Coal Association of Canada www.coal.ca