Coalbed methane: From hazard to resource
Introduction
Coalbed gas has been considered a major mine hazard since the early to mid 19th century when the first documented coal mine gas explosions occurred in the United States in 1810 and in France in 1845. Since the late 20th century coalbed methane has received increased emphasis as a potential energy resource. Coal beds contain a mixture of gases in which methane makes up 80–99% and varies from 0.0003–18.66 m3/metric ton (0.01 to >600 ft3/ton) with heat combustion ranging from 8455 to 9345 cal/m3 (950–1050 Btu/ft3) (Kemp and Petersen, 1988). Minor amounts of carbon dioxide, nitrogen, hydrogen sulphide, and sulphur dioxide make up the other components of coalbed gas. A minimum amount of gas is contained as `free gas' in fractures (cleat system), but it mainly occurs as sorbed gas on micropore surfaces in the matrix of coal beds. In addition, some of the coalbed gas migrates from the coal into adjacent sandstones. Early research investigations of coalbed gas have concentrated on technology to control and manage gas outbursts and explosions (Lama, 1995). Methane research in the United States was separate from health and safety research (Deul and Kim, 1986). Since the 1970's when coalbed methane was determined to be an economically viable energy source, investigations have focused on understanding its origin, occurrence, distribution, availability, producibility, and recoverability.
Investigations of coalbed gas in the United States underground bituminous coal mines in the mid 1970's showed a production of 7×109 m3/d (>200 million cubic feet per day or mmcfgpd) of methane per day (Skow et al., 1980). Degasification of underground coal mines by draining methane either by horizontal boreholes at the base of mine shafts or by outside mine entries and vertical boreholes in advance of mining have led to concerns about the contribution of methane to the `greenhouse effect' of the atmosphere. During the course of investigations concerning the gas emissions into the atmosphere, it has become apparent that methane from underground coal mines is a viable energy source. In the United States this effort was enhanced by pioneering work of government agencies (e.g. US Bureau of Mines or USBM, US Department of Energy or DOE, and Gas Research Institute or GRI) developing the technology to recover coalbed methane. However, it was not until the vertical wells unrelated to mining operations, were drilled in the central Appalachian, Black Warrior, and San Juan Basins in mid 1970's that coalbed methane was pronounced a viable commercial energy commodity.
The most important factor that has influenced the producibility of coalbed methane in the United States is enactment of the `Crude Oil Windfall Profit Tax of 1980' (Soot, 1988). This tax-credit incentive was proposed for production of unconventional fuels from: (1) oil shale and tar sands, (2) gas from biomass, geopressured brines, or Devonian shale, (3) liquid, gaseous or solid synthetic fuels from coal, (4) some processed wood fuels, and (5) steam from some agricultural by-products. This production tax credit permitted coalbed methane producers to receive $0.75 per million Btu of gas sold in 1986, which rose to $0.78 per million Btu in 1987. This tax credit was projected to increase $1.34 per million Btu in 2001. However, in order to qualify for the tax credit, wells must have been drilled by 1990; production from these wells is eligible for credit until 2001.
Thus, the current focus on coalbed methane is on providing safe mining operations, utilization of methane as a unconventional energy source (and resulting tax writeoff), and its effect on the environment. In the past this progression of interests provided a vehicle for research investigations that led to coalbed methane becoming a significant part of the energy resource and a target for exploration and development worldwide. This paper summarizes the historical and geological perspectives of the state of knowledge and research advances in coalbed methane as a conventional energy resource.
Section snippets
Historical perspective in coal mining
Historically, coalbed methane was vented to conduct safe mining operations in order to increase mine productivity. Coalbed methane was not a problem when coal was mined from outcrops by stripping and shallow shafting in the United Kingdom and other European countries. As shallow coal resources were slowly exhausted at the end of the 18th century and technology was improved to permit construction of large deep mines, coalbed methane in these mines was observed. In the early 19th century, coal
Geological perspectives in coalbed methane development
Gas dilution by ventilation during active mining operations and post- and pre-mining methane drainage activity mainly has been conducted in order to improve mine safety, increase productivity, and improve mining economics. However, ventilation and drainage efforts related to underground mining has also led to production of coalbed methane by conventionally drilled vertical wells (Diamond et al., 1989; Diamond, 1994; Dunn, 1995). Production was followed by successful recovery and sale of coalbed
Summary
Although the detrimental effects of coalbed gas on mining and development has been known for more than 100 years, harnessing the associated coalbed methane as an energy resource is still in its infancy. Release of coalbed gas during underground mining and its migration into the network of tunnels or mine workings have been the focus of investigations because of safety and productivity. This research towards safe and productive mining has led to understanding the origin, mechanism, prediction,
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