Coalbed methane potential in the Appalachian states of Pennsylvania,West Virginia, Maryland, Ohio, Virginia, Kentucky, and Tennessee--An overview

Paul C. Lyons
Open-File Report 96-735

CBM composition and desorption data

The composition of CBM has been generally treated by Rice (1993). These data come from sampling of underground mines, desorption tests of coals, and samples from active reservoirs. These gases are of both biogenic and thermogenic origin, the latter originating during coalification beginning at high volatile C bituminous coal and increasing into low volatile bituminous coal and anthracitic ranks. Methane is usually the major component, but carbon dioxide, ethane, and higher hydrocarbon gases are important components of some coals (Rice, 1993). There are reports of up to 10% CO2 in the CBM of the Appalachian basin (Rice, 1995)

In Virginia, CBM contains an average of 96.6% methane and has a calorific value of about 990 Btu/cf (Nolde, 1995). Rice (1995) reported CBM composed of 97.0% methane, 2.5% ethane and heavier gases, and 0.5% CO2 in this same state; he also reported as much as 2% CO2. In Greene County, Pennsylvania, CBM contains 94% methane with a similar calorific value of 979 Btu/cf was reported from a CBM well (Markowski, 1993; WVGES and PTGS, 1993; Bruner et al., 1995); the remaining 6% consists of ethane, propane, butane, and pentane, carbon dioxide, and nitrogen.

As much as 98% of the CBM is adsorbed in the micropores of coal, which generally have diameters less than 40 angstroms (Rogers, 1994), rather than being in intergranular pores as in conventional gas reservoirs. Methane and ethane have molecular diameters of 4.1 and 5.5 angstroms, respectively (Rogers, 1994, p. 169). The micropores in high volatile A/B bituminous coal to anthracite are mainly less than 12 angstroms in diameter; the percentage of these less than 12 angstroms micropores increases with rank to 75% in anthracite (Gan et al., 1972).

The volume of gas contained in a core sample (i.e., total gas content) is the sum of three measured components--desorbed gas, residual gas, and lost gas (Rice et al., 1993). The desorbed gas is measured in a sealed canister over days, weeks, or months, and the residual gas is measured after the desorption tests by crushing the sample to a very small size and measuring the volume of evolved gas. The residual gas in some northern Appalachian coals may be relatively high and, in some cases, exceeds 50 percent of the total gas content (Hunt, 1991). Finally, the lost gas, which represents the amount of gas lost from the core sample before it was placed in the canister, is determined by linear extrapolation. Most of the water in the cleat system of coal must be removed before the CBM can be desorbed (Rogers, 1994).

The average amount of total gas by rank for bituminous and anthracitic coals ranges from about 39-430 cf/ton (Eddy et al., 1982). The highest average is for low volatile bituminous coal, and the lowest average is for high volatile C bituminous coals.

CBM samples have seldom yielded more than 600 cf/ton and estimates of the amount of methane generated during the coalification process exceeds 5,000 cf/ton through the rank of low volatile bituminous coal (Rightmire and Choate, 1986). This implies that the bulk amount of CBM has escaped or has been lost into the surrounding strata. Kelafant et al. (1988) reported the following desorption data for high volatile bituminous A coal beds of the northern Appalachian basin, which shows a general increase of CBM with depth:

135 cf/ton at 500 ft
196 cf/ton at 1,000 ft
231 cf/ton at 1,500 ft

At the same depths, the gas values are about twice as much for low volatile bituminous coal from the central Appalachian basin (see data in Kelafant and Boyer, 1988). This partly explains the greater productivity of CBM wells in the central Appalachian basin where the principal CBM producing coals are mainly of low volatile bituminous rank.

Central Appalachian Basin

The Pocahontas No. 3 coal bed was previously reported to be one of the gassiest coals in the United States (Irani et al., 1977). In 1985, The Pocahontas No. 3 mines of Virginia ranked in the top 15 for having the highest methane liberations in the United States (Grau, 1987). Methane emissions of 135-304 Mcf/day were reported from the Beckley Mine in Raleigh County, West Virginia (Adams et al., 1984). In 1985, the Beckley coal mines of West Virginia and a mine in the Jawbone coal bed of Virginia ranked in the top 25 for methane liberation among U.S. coal mines (Grau, 1987).

For desorption tests for 109 samples from 12 coal beds in the central Appalachian basin (Diamond and Levine, 1981), a range of 6-573 cf/ton was determined. In their study area in the central Appalachian basin, Kelafant and Boyer (1988) reported a minimum of 86 cf/ton The highest desorption values reported were for the Pocahontas No. 3 coal bed, which ranged from 285-573 cf/ton at depths of 778-2143 ft; Hunt and Steele (1991a) reported a high value of 660 cf/ton for this coal bed. In Virginia, the gas content of the target beds for CBM development range from 249 to 408 cf/ton (Nolde, 1995). The Sewell coal bed in Raleigh County, West Virginia, had total gas contents of 130-296 cf/ton at depths of 680-981 ft, as compared to considerably lower values of 6-143 cf/ton at depths of 684-1,037 ft and an average total gas content of 51 cf/ton for the L. Cedar Grove coal bed (high volatile A bituminous coal) in Mingo County, West Virginia (Adams, 1984). Desorption tests for three coal samples from Clay County, Kentucky, indicated 25 and 45 cf/ton (after 3-4 months) from depths from 643 to 869 ft (Adams, 1984), which indicates poor potential for CBM development in that area. For the Jawbone coal bed (see Fig. 3), approximately 280 cf/ton was reported by Adams et al. (1984). The Pond Creek coal bed in Pike and Martin Counties in eastern Kentucky, at depths of 125-500 ft, showed very low total gas contents of 38 to 67 cf/ton). Such low gas contents would be expected at depths less than 500 ft unless there were enhanced structural conditions for CBM retention.

In Tennessee, there are very scanty data on gas contents of coal beds. (Diamond et al., 1986). In Morgan County, the total gas for three samples from the Sewanee coal bed (low volatile bituminous coal) at depths from 821-825 ft varied from 32 to 83 cf/ton. The sample set is very inadequate to be able to predict the CBM potential in Tennessee.

Northern Appalachian Basin

In 1985, The Lower Kittanning, Lower Freeport, Upper Freeport, and Pittsburgh coal beds of West Virginia and Pennsylvania were among the 10 highest methane liberating coal beds from coal mines in the United States(Grau, 1987). In general, desorption and total gas values for the northern Appalachian basin are lower than those for the central Appalachian basin. These data probably reflect higher ranks and greater depths for coal beds of the central Appalachian basin. According to Rice (1995), coals in the northern Appalachian basin have much longer desorption times (as much as 600 days); in contrast, CBM in southwestern Virginia in the central Appalachian basin desorbs in a few days probably due to lower hydrostatic pressure.

Hunt and Steele (1991a) postulated CBM values of 100-150 cf/ton for the Pittsburgh coal in the northern Appalachian basin. A low gas value of less than 50 cf/ton at a depth of 520 ft was reported for the Pittsburgh coal (WVGES and PTGS, 1993). An average gas content of 140 cf/ton for the Pittsburgh coal bed, as compared with 192 cf/ton and 252 cf/ton for the Freeport and Kittanning coal beds (Fig. 3), respectively, was reported (WVGES and PTGS, 1993; Bruner et al., 1995). These values reflect increased CBM with depth. Markowski (1993) reported 95-216 cf/ton for seven Monongahela samples in this part of the basin, which is in general agreement with previous reports. Adams et al. (1984) reported 100 cf/ton for the western part of the northern Appalachian basin and 150-200 cf/ton for the eastern part. In Ohio County in the panhandle of West Virginia, Hunt and Steele (1991a) reported 112 cf/ton for the Pittsburgh coal bed at 722 ft, which may have been affected by some CBM depletion from nearby coal mining; Hunt and Steele (1991c) reported a reservoir pressure of only 75 psi in this well, which is now shut in. In Greene County, Pennsylvania, three CBM coal tests were staked (Petroleum Information Corporation, 1991). Twenty-one coal core samples for desorption measurements were taken from six drill holes in Beaver, Lawrence, Somerset, and Washington Counties, Pennsylvania, but the results were not reported (Markowski, 1995). In Ohio, there are a limited amount of desorption data (Couchot et al., 1980; Diamond et al., 1986). For 23 core samples of the Brookville, Middle Kittanning, Lower and Upper Freeport, and Pittsburgh coal beds of Belmont, Guernsey, Monroe, Noble, and Washington Counties, Ohio, the desorption values ranged from 11 to175 cf/ton) at depths as much as 786 ft. The highest value (175 cf/ton) was for the Upper Freeport was from a depth of 667 ft. Diamond et al. (1986) reported similar low desorption values ranging from 9.5 to 95.4 cf/ton for the Upper Freeport and Kittanning coal beds of Harrison County, Ohio.

There is a lack of information on methane emissions from Maryland coal mines. However, Maryland coal beds are not known to be gassy (R.H. Grau and W.P. Diamond, Bruceton Research Center, Department of Energy, Pittsburgh, personal commun., March, 1996). This information is consistent with mine-safety information from bottled gas samples taken quarterly at fans in the Mittiki A, B, C, and D mines (all mining Upper Freeport coal bed) in the southern part of the Upper Potomac coal field, the largest mines in Maryland; the Mittiki mines show generally low CBM emissions (less than 100,000 cf/day, March 1, 1996; Barry Ryan, Mine Safety and Health (Department of Labor), mining inspector, Oakland, Maryland, personal commun., March,1996). However, from the Mittiki C Mine (circa 1989) there were a few quarters that year when the C mine, which is now sealed, in the southernmost part of the Upper Potomac coal field had high emissions in the range of 250,000-300,000 cf/day and was put on a 15-day spot check (Barry Ryan, personal commun., March, 1996). Another deep mine in Garrett County near Steyer and owned by the Patriot Mining Company (Permit DM-90-109), which mines the Bakerstown coal bed (Fig. 3), also has low methane emissions (Barry Ryan, personal commun., March, 1996). These data do not represent mined coal beds with the greatest amount of overburden, so they are probably misleading with respect to the CBM potential of deeply buried beds in the Maryland coal fields.

In the Anthracite region of eastern Pennsylvania there are limited known gas-content data (Diamond and Levine, 1981; Diamond et al., 1986). However, the data available from these two sources suggest very high amounts of CBM in some parts of the Anthracite region. For the Peach Mountain coal bed (Llewellyn Formation) in Schuylkill County in the Southern Anthracite field, at a depth of 685 ft, the total gas content was measured at 598 to 687 cf/ton, the second highest total gas content known to me for Appalachian basin coal beds. For the Tunnel coal bed at depths of 604-608 ft in Schuylkill County, the total gas content of three samples ranged from 445 to 582 cf/ton. These gas contents can be contrasted with very low total gas contents of 6 to 29 cf/ton for the Orchard coal bed and 13 cf/ton for the Mammoth coal bed in Schuylkill County (Diamond et al., 1985). Similar low total gas contents of 16 to 70 cf/ton were reported for the New County coal bed in Lackawanna County (Diamond et al., 1986) in the Northern Anthracite field These extreme differences in total gas contents may represent structural and permeability problems due to the absence of cleats or mineral-filled cleats (Law, 1993) and other local factors. These will be an important consideration that may prevent development in some areas. Nevertheless, the very high total gas contents of some coal beds in the Anthracite region indicate that CBM exploration should be carried out in this region.

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