Auer Advances in Energy Systems and Technology

Volume 3
1. Auflage 2013
ISBN: 978-1-4831-9127-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 324 Seiten

Advances in Energy Systems and Technology: Volume 3 present articles that provides a critical review of specific topics within the general field of energy. It discusses the energy conservation in transportation. It addresses all modes of transportation in its discussion. Some of the topics covered in the book are the role of natural gas in domestic energy supplies, the tight gas formation, Devonian gas resources, and the theory of gas flow in porous media, a review of gas production mechanism, and the estimates of economically recoverable gas. The analysis of the methane from coal seams is covered. The technology and economics of geopressured energy production is discussed. The text describes the geopressured aquifers. The Mexican gas imports are presented. A chapter is devoted to the production of synthetic gas. Another section focuses on the conversion of methane from coalbeds and the effect of gas porosity. The book can provide useful information to scientists, engineers, students, and researchers.

Autoren/Hrsg.

Auer, Peter

Leseproben

VI METHANE FROM COAL SEAMS

A Background

Since the inception of underground coal mining, the release of methane from coal beds (“coal gas”) has been a hazard to mining safety. When methane combines with air, it forms a combustible mixture which has been the cause of countless mine explosions. In response to this hazard, considerable investment has been directed toward disposing of the methane in coal mines.

Recovering the now vented methane could provide an important augmentation to local supplies of natural gas. Moreover, these supplies could be made rapidly available since the technology is relatively simple and is commonly used in several European countries. Of the European countries, Germany has been most aggressive in use of coalbed methane, capturing nearly 10 Bcf of gas per year, as shown in Fig. 35.

Fig. 35 Methane recovery from coal seams in Europe. (from von Schonfeldt, 1979)

In comparison, the U.S. has had little success, until very recently, in commercially using coalbed methane. The research and development being sponsored by the Bureau of Mines, Pittsburgh Mining and Safety Research Center, and the Department of Energy, Morgantown Energy Technology Center, is seeking to change this condition by increasing mine safety and adding to domestic gas supply through methane recovery from coal seams.

The methane released from coal mining stems from three sources: (1) from the coal seam itself where the methane is held by adsorption in the structure of the coal and released when the coal is mined; (2) from the thin sand lenses adjacent to the coal seams that also serve as a reservoir for desorbed gas; and (3) from fractures where methane has accumulated by desorption.

Since the coal is impermeable, the gas must flow either through the natural fracture system in the coal (the butt and face cleats) or through the microporous structure of the coal. Unless an area is naturally highly fractured, such as in portions of the Appalachian Basin in West Virginia and the Black Warrior Basin in Alabama, unstimulated vertical holes drilled into the coal will not release appreciable amounts of methane.

Although several approaches have been tried and have produced gas, none as yet have demonstrated economic feasibility as purely a gas recovery project. Each approach must rely upon the safety benefits and mining production efficiencies that accrue from lowered emissions and possibly instantaneous gas explosions. The production rates and duration of production have, to date, been too low or too uncertain to offset the considerable costs of well drilling, water removal, compression, piping, stimulation, gas purification, and gathering costs associated with commercial recovery of methane from coal.

In addition to methane being vented from mined coal seams, a considerable amount of the coalbed methane resource is in formations too thin or too deep for economic mining. Should these unminable coalbeds have geological characteristics that are favorable to efficient methane recovery, e.g., high intensity of natural fractures, high gas content, and uniform beds and seams, they could be commercial sources of natural gas.

At this time, however, rapid commercialization is hampered by an inadequate definition of the resources, uncertainties in recovery technologies, and the marginal economics of collecting and marketing the resource. These limitations are further described in the following sections.

Several estimates have been made for the size of the coalbed methane resource base and the portion that could be economically recovered. These estimates vary widely reflecting the different portions of the resource studied, the different assumptions used in the analysis, and, most importantly, the limited scientific data that has been gathered on the methane content and the variables that govern the flow of methane in coal seams. The main estimates are reviewed in Table XXVI.

TABLE XXVI

In-Place and Recovery Estimates for Methane from Coal Seams (Tcf)

Source: Duel and Kim, 1978; Wise and Skillern, 1978; Lewin and Associates, 1978; National Petroleum Council, 1979.

B Description of the Resource Base

1 Basic Evidence for Methane Adsorption

The bulk of methane released during coal production cannot be from the pores of the coal. Assuming an average of 200 standard cubic feet of gas per ton of coal (8 SCF/CF), a seam pressure of 500 psi, and a temperature of 80°F, a porosity of 28% would be required. With 600 standard cubic feet of gas per ton of coal (24 SCF/CF) and a seam pressure of 500 psi, the porosity must be 85%. Since coal is not that porous, the habitat of the methane in coal seams must be gas adsorbed to coal facies or held as a sorbed “liquid” in the coal itself.

The gas content of coal is believed to be dependent upon rank and depth of burial, although further work is required to develop statistically reliable correlations of gas content and geological proporties.

The coalbed methane is distributed among three distinct resource areas: in deep, unminable coal seams; in thin coal seams; and in shallow, minable coal seams. Each of these resource areas is further examined in Sections VI,B,2–4.

2 Deep, Unminable Coal Seams

a Gas in Place

A significant coal reserve is buried too deep for economic mining. Although volumetric data on this unminable coal is sketchy, the available data can be used to make gross estimates of the gas in place21:

The Bureau of Mines has identified 388 billion tons of coal as being too deep (3000–6000 ft) to be mined economically. About 45% of this coal is bituminous or of higher rank; the balance is subbituminous coals and lignite.

Gas content of bituminous and higher-rank coals averages 480 cf/ton, accounting in total for 80 Tcf.

Gas content for subbituminous coals is substantially lower than that of bituminous and higher-rank coals. Their high moisture content tends to limit their gas adsorption capacity. Assuming a gas content of 100 cf/ton,22 about one-fifth that of bituminous coals, deep subbituminous coals would contain about 20 Tcf.

In total, the 388 tons of deep, unminable coal contain an estimated 100 Tcf of gas.23

b Key Reservoir Properties

. Distribution of bituminous coalbed thickness was estimated based on data from Colorado and aggregated data from the three states of Colorado, New Mexico, and Utah. Colorado has about 70% of the bituminous coal and the three states combined have about 90% of the bituminous coal in the Western Basins. For Colorado and these three states together, the distribution of thickness is estimated as follows:

These data are plotted as a cumulative percentage of the total resource versus coalbed thickness in Fig. 36. The graph can be used to estimate the resource size as a function of coalbed thickness.

Fig. 36 Estimated distribution of total bituminous coal resources by coalbed thickness for Colorado and three Western states. (from Booz et al., 1977)

(ii) . Assuming a methane content of bituminous and higher-rank coals of 480 cf/ton, and a methane content of subbituminous coals and lignites one-fifth that of bituminous coals, it is possible to estimate the gas content of deep, unminable coal seams for various acreages as shown in Table XXVII.

TABLE XXVII

(iii) . The deep, unminable but thick coalbeds may possibly be an economic resource. However, the data available on the essential properties (e.g., permeability, fracture density, diffusion con- stants) is too limited to enable a reliable estimate of the economically productive capacity of the deeply buried coals. Therefore, the in-place resource of 100 Tcf is classified as speculative until further resource assessment is completed.

3 Thin Coal Seams

Approximately 290 billion tons of coal are in seams too thin (averaging 15 inches) to be economically mined. This coal contains an estimated 260 cf/ton of methane, or a total of about 70 Tcf.

The 70 Tcf of methane in thin coal seams cannot be produced economically under any reasonable set of technological assumptions. At 260 cf/ton and an average well drainage area of 80 acres, the drainage area will contain only 45,000 Mcf of methane.24 Assuming deviated wells costing $300,000 (2 wells drilled to 800 ft), operating, compressing, separating, and gathering costs of $0.50 per Mcf, and 20% royalty and severance tax, the coal seam must be at least 10 ft thick for economic recovery.25 Thin coal seams were therefore eliminated from further analysis as a potential resource of methane.

4 Shallow, Minable Coal Seams

The best source of coalbed methane in the near-term consists of gas emissions from minable coal seams. Although this gas can be produced independently of mining, it is...