ANALYSIS AND PREDICTION 

OF LONGWALL METHANE EMISSIONS: 

A CASE STUDY IN THE POCAHONTAS NO. 3 COALBED, VA 

William P. Diamond,' Fred Garcia,l George N. AuI,~ and Richard E. Ray213 

'National lnstitute of Occupational Safety and Health (formerly U.S. Bureau of Mines), P.O. Box 18070, Pittsburgh, PA; 

2Consolidation Coal Co., Drawer L, Oakwood, VA; 3Currently, Parsons Brinckerhoff, NY 

ABSTRACT STUDY AREA 

Increasing longwall panel dimensions, particularly face width in 

gassy coalbeds, may result in unexpected increases in methane 

emissions. To investigate this problem, continuous longwall face 

emission monitoring studies were conducted at two adjacent 

mines operating in the Pocahontas No. 3 Coalbed, where longwall 

faces were to be extended from 229 to 305 m (750 to 1,000 ft). It 

was predicted by regression analysis of methane emissions 

data from 229 m (750 ft) wide longwall faces that extending 

faces to 305 m (1,000 ft) would increase methane emission 

rates by only 7 percent (0.6 m3/min (20 cfm)), to 8.6 m3/min 

(304 cfm) at the VP-1 Mine. In contrast, it was predicted that 

extending faces at the VP-3 Mine could increase methane 

emissions by as much as 13 percent (1.8 m3/min (65 cfm)) to 

16.1 m3/min (567 cfm). The geologic and mine design factors influencing the variation in face emissions between the two mines 

will be discussed. 

INTRODUCTION 

As part of its Coal Mine Health and Safety research program, 

National lnstitute of Occupational Safety and Health (formerly 

U.S. Bureau of Mines) Pittsburgh Research Center, has been investigating methane emissions associated with longwall mining 

(Diamond, et al., 1992, McCall, et al., 1993, and Schatzel, et al., 

1992). The general goals of this research program include the development of a fundamental understanding of the factors influencing gas migration so that techniques can be developed to predict 

underaround methane emission levels. An understanding of the 

interrelationship of geology, mining methods, gas storage, and 

migration is also required to develop methane control strategies 

to cope with the increased levels of methane emissions sometimes resulting from the use of high productivity, advanced mining technology (Diamond, et al, 1994). 

The primary purpose of this study was to predict the methane 

gas emission consequences of mining longwall panels of greater 

face width in the Pocahontas No. 3 Coalbed. The mine operators 

were considering increasing longwall panel face width from 229 to 

305 m (750 to 1,000 ft). However, since historically high methane 

emissions from the longwall face and gobs were already being 

experienced, there was a concern for further increases in methane emission rates. If higher emission rates were encountered on 

wider faces, it was preferable from a safety perspective to be prepared in advance, either with increased ventilation airflow, or additional methane drainage. As part of this work, the emission characteristics and factors influencing emission rates on the two study 

panels were also investigated. 

The longwall methane emission studies were conducted in the 

VP-1 Mine, and adjoining VP-3 Mine, located in Buchanan Co. VA 

(Fig. 1). Additional mines in this complex, both active and inactive, 

are located to the east and south. The two study panels are approximately 1,600 m (1 mile) apart. The depth of cover ranged 

from 485 m (1,590 ft) at the VP-1 site to 668 m (2,190 ft) at the 

VP-3 site. 

INSTRUMENTATION AND MONITORING STRATEGY 

The gas emission monitoring instrumentation consisted of a 

methane sensor to measure gas concentration and a data logger 

to continuously collect and store information. Airflow was periodically measured at the monitoring locations by standard mine ventilation survey methods using a hand held anemometer. Methane 

gas emissions were monitored at four locations at each mine site. 

Two instruments were attached to shields over the pan line, the 

first close to the headgate and the second close to the tailgate. 

Two additional locations were monitored in the bleeders to determine the total volume of methane gas liberated from each panel, 

as well as the adjacent mined areas. Gas production data from 

vertical gob gas ventholes and underground horizontal boreholes 

were also obtained for evaluation. A production time study was 

conducted to relate the methane gas emission data to mining activity on the face. 

METHANE EMISSION DATA 

To predict the methane gas emission consequences of mining 

longwall panels of greater face width, it was first necessary to 

determine if a general increase in methane emission levels occurs 

with time (staircase effect), both during a longwall pass, and from 

day to day. To characterize any changes in the rate of methane 

gas emissions across the longwall face, each pass on the 229 m 

(750 ft) wide faces was divided into three equal length segments 

of 76 m (250 ft). Average methane emission rates of the second 

and third segments of a pass were compared to the respective 

previous segment to evaluate emission rate trends during the 

mining of an individual pass. Emission trends established by this 

methodology were then extended to forecast the expected methane emission rate of an additional 76 m (250 it) longwall pass 

segment, i.e., a 305 m (1,000 ft) face. 

Baseline emission data were collected over the weekend prior 

to the start of production shifts on Monday. Three full days and 

two partial days of monitoring (three production shifts per day) 

were completed in the VP-1 Mine. During this time the face was 
VP-3 MlNE 

Figure 1. Map of VP-1 and VP-3 study areas. 

advanced approximately 79 m (260 ft). Methane emission monitoring data and production time study data were collected on 83 

individual passes, of which 77 had complete data sets foranalysis. 

Two full days and one partial day (two production shifts per day), 

and 37 m (120 ft) of face advance, were monitored at the VP-3 

Mine site. A total of only 38 passes were monitored (23 with complete data sets) due to a substantial number of delays. Delays accounted for 42 percent of the time to complete a pass during the 

VP-3 study. In contrast, delays only accounted for 13 percent of 

the time to complete a pass during the VP-1 study. 

VP-5 MlNE 

Legend 

P77. Mined-out lonqwall panel 

Tc Solid coal 

*240 Gob gas venthole 

Due to the expected influence of delay time on face emission 

rates, a data analysis method was devised that would facilitate a 

direct comparison of results from the two study sites. Methane 

emission data were evaluated on three bases related to the relative amount of delay time (Table 1). The first basis included all the 

passes with complete methane emission and time study data. The 

second basis eliminated any pass with delay time equal to or 

greater than 50 percent of the total time to complete the pass, and 

any pass that had a delay equal to or greater than 15 minutes on 

any pass segment. Any pass that experienced a reported methTable 1. Average methane emissions summary, VP-1 and 3 Mines 

Average pass segment methane flow rate 

Number of Calculation basis passes used lsl 'h. 2nd H. 2ndll st. 3rd 'h. 3rd12nd. Total. 

m3/min (dm) m%in (cfm) percent difference m'hin (dm) percent difference m'hin (cfm) 

VP-1 Mine 

All with data . . . . . 77 of 83 6.6 (233) 7.4 (262) +12.4 7.8 (276) +5.3 7.1 (250) 

<50% delay. . . . . . 69 of 77 6.6 (232) 7.7 (271) +16.8 8.0 (281) +3.7 7.2 (254) 

<25% delay. . . . . . 60 of 77 6.7 (235) 7.8 (276) +17.4 8.0 (284) +3.1 7.3 (257) 

All with data . . . . . 27 of38 10.2 (359) 11.7 (415) +15.6 14.0 (494) +19.2 11.4 (403) 

<50% delay . . . . . . 11 of 27 10.6 (374) 12.4 (438) +17.4 14.2 (502) +14.6 12.1 (427 

Baseline flow rate VP-1 Mine = 1.4 m3/min (50 cfm). 

Baseline flow rate VP-3 Mine = 4.2 m31min (1 50 cfm). 
Banline DO; I DO; 2 

TlME OF DAY, h 

DOG 3 DO; 4 hi 5 

TlME OF DAY, h 

Figure 2. Methane emissions for the baseline and days 1 and 2, VP-1 Mine. Figure 4. Methane emissions for days 3-5. VP-1 Mine. 

Figure 3. Average methane emissions by pass segment, VP-1 Mine. Figure 5. Methane emissions during floor gas event, pass 8, day 2, VP-1 Mine. 

24 I I I I I 

- MY - 800 

i 

ane delay that would skew the comparison of emission levels 

between the pass segment with the methane delay and the pass 

segments before and/or after the delay, were also eliminated. For 

the VP-1 Mine site, 69 of the 77 passes with "complete datan were 

evaluated for this second delay time basis, compared to only 11 

of 23 passes for the VP-3 Mine site. 

The final basis for evaluating the emission results was the 

most stringent, reducing the previous delay time criteria by one 

half. This basis eliminated all passes having a delay time equal to 

or greater than 25 percent of the total time to mine a complete 

pass, or a delay equal to or greater than 7.5 minutes on any segment, and any pass that experienced a methane delay. This basis resulted in 60 of 77 passes with "complete data" being evaluated for the VP-1 Mine site. Only 5 of 27 passes met this more 

stringent criteria for the VP-3 Mine site, a number judged to be 

insufficient for meaningful comparisons. 

0 

g 5.5 

w. 

W 3 4.5 r. 

Y 3.5 

2.5 

VP-1 Mine Results 

Baseline methane emissions for the VP-1 Mine were 1.4 m3/ 

min (50 cfm) (Fig. 2). Methane emissions during the VP-1 study 

increased about 12 to 17 percent, depending on the delay time 

basis, from the first to second pass segment (Table 1, Fig. 3). As 

would be expected, the evaluation basis with the least amount of 

delay time generally had the highest methane emission levels. 

Emission rates were 6.7 and 7.8 m3/min (235 and 276 cfm), respectively, for the first and second pass segments for the <25 

percent delay evaluation basis. Emissions increased only slightly 

from the second to third pass segment, averaging from 3 to 5 percent higher on the final third of the pass as compared to the secIs1 Third 2nd Third 3 rd Third 0 I! I I I I I 

5100 5,lO 5.20 530 5 40 5 6-00 O 

LONGWALL PASS SEGMENT TlME OF DAY, h, min 

- .-L. All w~th dola - ---- z50 pcl delay 

- - c25 pct delay - - 

Basellns ernisslons . l.4rn3/rnm(50 cfm) - - 

- - I 

ond third. The average methane emission rate for a complete 

pass ranged from 7.1 to 7.3 m3 (250 to 257 cfm), depending on 

the delay time basis. 

Delays were generally minor during the VP-1 emission study, 

with the exception of one major delay on day 3 that lasted approximately 16 hours (Fig. 4). This delay was caused by a problem with 

220 g 

E 

5 180, 

: 140 

Z 

100 1 

- Pou 7 1 PASS 8 (HEAD-TAIL) ; has9 

- 100 

the aerial tram used to remove mine refuse material from the 

preparation plant. Reported delays of 13 minutes and 1 minute on 

day 2 were attributed to methane (Fig. 2). Methane levels actually 

began to climb early on day 2, before the reported methane gas 

delays. Emission rates increased from about 7.1 - 8.5 m3 (250 - 

300 cfm) to over 19.8 m3/min (700 cfm). High methane gas levels 

experienced during day 2 of the VP-1 study (Figs. 2 and 5) were 

attributed to a floor gas bleeder, as evidence by gas bubbling in 

2.5 . . . . ... . . I 00; 5 ] 2.0 

Is1 Third 2nd Third 3rd Third 

LONGWALL PASS SEGMENT 

Figure 6. Average daily emissions, all passes with data. VP-1 Mine. 
Easellne Day I Day 2 Doy 3 Day 4 Ooy 5 

TlME OF DAY, h 

Figure 7. Methane flows and emissions during the mining of the VP-1 Mine study 

site. 

standing water near the face. 

Average daily methane gas emissions for each pass segment, 

for all passes with data, during the VP-1 study are shown on Figure 6. The first day of emission monitoring had the lowest methane emission levels, probably due to the two idle days prior to the 

start of mining on Monday. The average methane gas emission 

rate for a complete pass on day 1 was about 5.7 m3/min (200 cfm). 

Day 2 showed a marked increase in methane gas emission rates 

to an average of 8.5 m3/min (300 cfm) due to the floor gas event. 

After this steep rise, average daily emissions declined on days 3 

and 4 (Fig. 6). This decline is probably due to a diminishing influence of the floor gas bleeder that occurred on day 2, the 16-hour 

delay early on day 3, as well as numerous shorter delays on day 

4. The evaluation of emission trends with time, i.e., a day to day 

"staircase effect," was essentially lost due to the impact of the 

floor gas event and major delay that interrupted any longer term 

trends developing early in the study. However, a visual inspection 

of the methane emission curves (Figs. 2 and 4) suggests several 

time intervals during continuous mining operations, prior to the floor 

gas event, where the general emission rates were trending upward. 

In addition to the longwall face emission study, the bleeder 

system associated with this area of the VP-1 Mine was monitored 

to obtain a more comprehensive evaluation of the study panel's 

gas contribution to the mine's ventilation system. The bleeder 

system monitoring location included methane contributions from 

the study panel as well as from the gobs of the two previously 

mined panels. Methane volumes above the baseline (established 

over the idle weekend) are attributed to emissions associated with 

mining on the active study panel. The bleeder system had a base 

flow rate of 105 x 10"3/d (3.7 MMcfd) (Fig. 7). As soon as mining began on the study panel, the methane gas flow rate began 

to climb, and generally continued to climb throughout the five days 

of the emission study. The maximum methane gas flow rate of 

184 x lo3 m3/d (6.5 MMcfd) was measured on the final day of the 

study. Therefore, a maximum of 79 x lo3 m3/d (2.8 MMcfd) of 

methane attributed to mining of the study panel was liberated into 

the mine's bleeder ventilation system. This is significantly more 

methane than the average of 11 x lo3 m3/d (0.4 MMcfd) measured on the longwall face during the emissions study, as shown 

in Figure 7. 

VP-3 Mine Results 

Mining delays were substantially more frequent during the VP3 Mine emission study than experienced during the VP-1 Mine 

study. The most stringent delay time basis for which a sufficient 

number of passes were available to establish any meaningful 

ported gas delay A 1 

Baseline DO; l Doy 2 

TlME OF DAY. h 

Figure 8. Methane emissions for the baseline and day 1. VP-3 Mine. 

Bowline emissions -4 2m'/m~n(l50cfrnl 

9 1 I I320 

Isl Third 2nd Third 3 rd Thlrd 

LONGWALL PASS SEGMENT 

Figure 9. Average methane emissions by pass segment for all bases. VP-3 Mine. 

trends is the c50 percent delay time data (Table 1) for average 

total emissions and average emissions by pass segment. Due to 

the small number of passes available for analysis of daily emission trends at the more stringent delay time bases, these trends 

were only evaluated on the all passes with data basis. Baseline 

methane emissions (Fig. 8) were 4.2 m3/min (1 50 cfm), or 2.8 m3/ 

min (100 cfm) higher than that of the VP-1 study. Methane emissions during the VP-3 study increased by about 17 percent from 

the first to the second pass segment on the c50 percent delay 

time basis (Table 1, Fig. 9). This is essentially the same percentage increase in emission levels noted in the VP-1 study on the 

same delay time basis. However, the actual VP-3 average emission rate for the second pass segment was substantially higher, 

12.4 m3/min (438 cfm) vs. 7.7 m3/min (271 cfm) for VP-I. 

There is one significant departure in the emission trends between the two mine sites. The emission rate at the VP-3 study site 

continued to increase from the second to third pass segment. This 

increase of 14.6 percent is in marked contrast to the minimal 3.7 

percent average increase observed at VP-1. The average methane emission rate for the third pass segment reached 14.2 m3/min 

(502 cfrn), and the average emission rate for the VP-3 longwall 

passes was 12.1 m3/min (427 cfrn). Average daily pass methane 

emission rates progressively increased from 10.6 to 12.4 m3/min 

(373 to 438 cfm) over the three days of the study, indicating a 

general "staircase effect" (Fig. 10). A visual inspection of the 

emission curves for the VP-3 study (Figs. 8 and 11) suggest several shorter time intervals where emissions were also trending 
I I 250 1st Third 2 nd Thlrd 3 rd Third 

LONGWALL PASS SEGMENT TIME OF DAY, h 

Figure 10. Average daily emissions, all passes with data, VP-3 Mine. Figure 13. Methane flows and emissions during mining of the VP-3 Mine study area. 

I I I I 1 I I 0 

0 6'00 I2200 18;OO 0 6:OO 12'00 1000 

Day 2 007 3 

TlME OF DAY, h W\C. 

Figure 11. Methane emissions for day 2 and 3. VP-3 Mine. 

: X) Shisld No Ishcorar - loc0li0"1 

I I00 Pas 2 PASS 3 (HEAD-TAIL ) I bas4 

0 I I I I ! 0 8150 9m00 940 9.20 9.39 9'40 9250 

TlME OF DAY. h, rnin 

Figure 12. Methane emissions for pass 3, day 1, VP-3 Mine. 

upward. There were four methane delays noted during the VP-3 

longwall emission study. Methane delays of 4 minutes (Figs. 8 

and 12) and 1 minute (Fig. 8) occurred on day 1. Emission rates 

at these points reached approximately 17.0 m3/min (600 cfm), 

similar to the rates associated with methane delays during the VP1 study. However, emission rates prior to the methane delays 

were approximately 9.9 to 11.3 m3/min (350 to 400 cfm), or 2.8 

rn3/min (100 cfm) higher than those during the VP-1 study. Since 

methane delays were observed at about the same emission rate 

at each mine, there was less margin for methane emission increases at the VP-3 site. Additional methane delays of 4 minutes 

and 8 minutes were reported on days 2 and 3, respectively, at 

emission rates of about 18.4 m3/min (650 cfm) (Fig. 11). 

Methane flow rates in the bleeder system associated with the 

study panel were also monitored (Fig. 13). In addition to the study 

panel, ten other previously mined panels to the north were part of 

I I I I 0 

0 1 2 3 4 

762 m (250 fl) LONGWALL PASS SEGMENT 

Figure 14. Methane emission prediction curve for 305 m (1.000 ft) face. VP-1 and 

VP-3 Mines. 

this same bleeder system, complicating meaningful analysis of 

the data and comparisons to the VP-1 study. Baseline monitoring 

indicated a minimum methane flow rate of about 136 x lo3 m3/d 

(4.8 MMcfd), approximately 31 x lo3 m3/d (1.1 MMcfd) greater 

than that for the VP-1 study. 

Methane flow rates in the VP-3 bleeder system peaked at 

about 176 x 10W3/d (6.2 MMcfd), slightly lower than observed at 

the VP-1 Mine. The difference of 40 x lo3 m3/d (1.4 MMcfd) between the maximum and base methane levels is inferred to be an 

estimate of the methane liberation into the bleeders as a result of 

mining on the active study panel, and is significantly less than the 

2.8 MMcfd differential on the VP-1 study panel. This lower differential in the VP-3 bleeders, in spite of the higher face emissions, 

may be related to the large associated gob area, or to the higher 

base level bleeder flow rates. As was seen with the VP-1 study 

site, the maximum increase in methane flow rate in the bleeder 

system is higher, but in this case, not substantially higher, than 

the 17 x 1 O3 m3/d (0.6 MMcfd) of methane measured on the active 

longwall face (Fig.13). 

PREDICTION OF METHANE EMISSIONS 

FOR 305 M (1,000 FT) FACES 

Predictions of methane emission levels for faces extended 

from 229 to 305 m (750 to 1,000 ft) were made by regression 

analysis of the average longwall pass segment emission values 

from the 229-m (750-11) wide faces at each mine. The minimum 

"delay time" basis with sufficient data for meaningful analysis was 
selected for regression analysis for each mine to best represent 

a "typicaln pass with optimum mining rates. The predicted emission values for the additional 76 m (250 ft) of face width are not 

necessarily the "worst case" scenario, which would include sporadic high methane emission events such as the floor gas bleeder 

observed at the VP-1 Mine. 

VP-1 Mine 

Longwall pass segment methane emission data for the <25 

percent delay time basis (Table 1) was selected for regression 

analysis at the VP-1 Mine. A power curve in the form 

Y = 236.889X0.170889, where Y = average pass segment methane 

emission rate, and X = individual 76-m (250-ft) pass segment 

number, fit the measured data best (R2= 0.95). Extending the regression curve to a 4th, 76-m (250 A) longwall pass segment (Fig. 

14). i.e., to a face width of 305 m (1,000 ft), yields an average 

methane emission rate of 8.6 m3/min (304 cfm) for the additional 

segment. This predicted value is a 7.1 percent (0.6 m3/min [20 

cfrn]) increase over the average measured methane emission 

value of 8.0 m3/min (284 cfm) for the third 76 m (250 ft) segment 

of the 229 m (750 ft) wide faces. At this relatively low predicted 

increase in methane emissions at the face, it is unlikely that methane emissions would be a limiting factor for the increased coal 

production expected from a 305 m (1,000 ft) wide longwall face. 

VP-3 Mine 

The <50 percent delay time basis (Table 1) was selected for 

regression analysis at the VP-3 Mine. 

Due to the small number of passes meeting the more restrictive <25 percent delay time basis for the VP-3 Mine study, this 

basis could not be used for prediction of emissions from a "typical" 305-m (1,000-ft) pass. A linear regression curve in the form 

Y = 309.6 + 64.25X, where Y = average pass segment methane 

emission rate, and X = individual 76 m (250 ft) pass segment 

number, fit the measured data best (R2 = 0.99). Extending the 

regression curve to a longwall face width of 305 m (1,000 ft), 

yields an average methane emission rate of 16.1 m3/min (567 

cfm) for an additional 76-m (2504) pass segment. This predicted 

value is a 12.9 percent (1.8 m3/min [65 cfrn]) increase over the 

measured methane emission value of 14.2 m3/min (502 cfm) for 

the third 76 m (250 ft) pass segment of the 229-m (750-11) wide 

longwall faces at the VP-3 Mine. Face emissions in the 17.0 to 

19.8 m3/min (600 to 700 cfm) range at the VP-3 Mine had occasionally resulted in short delays during the study. These higher 

emission rates may require additional ventilation airflow on the 

face, or increased levels of methane drainage in advance of 

longwall mining. 

VARIATION IN METHANE EMISSION LEVELS 

The methane emission characteristics were quite different at 

the two study sites. Baseline methane emission levels were 2.8 

m3/min (100 cfm) higher, and average longwall pass emissions 

(<50 percent delay basis) were approximately 4.8 m3/min (170 

cfm) higher at the VP-3 site. Methane emission levels were higher 

at the VP-3 Mine in spite of the fact that there were only two daily 

production shifts compared to three at the VP-1 Mine. There were 

also significantly more mining delays (non-methane related) at 

VP-3, which should have further allowed for additional "natural" 

bleed-off of methane. Additionally, there was a floor gas bleeder 

at the VP-1 Mine, which generally raised the average methane 

emission levels for about two days. Everything else being equal. 

lower methane emission rates would have been expected at the 

VP-3 Mine. 

At first glance these methane emission observations are perhaps somewhat unexpected for two sites located only about 1,600 

m (1 mile) apart in the same coalbed. However, a closer look at 

the mine design factors associated with the sites may to a large 

extent explain the observed differences. Probably the most significant mine design factors influencing the variation in methane 

emission levels at the two sites are their relative exposure to the 

virgin coalbed gas reservoir and gas bleed-off time prior to 

longwall mining. At the time of the studies, the VP-1 study panel 

was the third in a series of five longwall panels that were progressively mined to the north (Fig. 1). This block of five panels in the 

southwest corner of the mine had been outlined by main entries 

three and one-half months prior to the mining of the first panel. 

The area to the north of this five panel block was mined 3 to 12 

years prior to the outlining of the block. This part of the VP-1 Mine 

is also bordered by the VP-5 Mine to the south and the VP-3 Mine 

to the west. The adjacent areas of the VP-5 and VP-3 Mines were 

mined approximately one to three years prior to the mining of the 

study panel. The extensive mine development surrounding the 

VP-1 study panel has very effectively isolated the area from methane migration from the downdip virgin coalbed gas reservoir. 

This mining sequence is optimum from a methane control 

viewpoint. In addition to the general isolation of the southwest 

corner of the VP-1 Mine from gas migration, the in situ gas volume 

within the five-panel block had a substantial period of time to 

bleed-off into the surrounding entries. Gas was able to bleed-off 

for about six months prior to mining of the first panel in the block, 

and for eighteen months prior to mining of the study panel. This 

relatively long period for in situ gas content reduction for the isolated five-panel block, in particular the later-mined panels, should 

have resulted in progressively lower gas emissions at the face. 

Company engineering staff reported that the first panel mined in 

this isolated block was one of the gassiest at the mine, and that 

supplemental horizontal methane drainage boreholes had to be 

drilled to control the higher emissions. As each new panel in the 

block was mined, it is reported that methane emissions generally 

decreased. However, even with the additional bleed-off time prior to 

mining, it was still necessary to utilize horizontal methane drainage 

boreholes on the study panel to further control methane emissions. 

Mine design parameters for the area of the VP-3 Mine where 

the study panel was located were quite different, and less desirable, from a methane control viewpoint. The study panel was the 

1 lth in a series of panels mined in the southeast corner of the 

mine. Virgin coal reserves were adjacent to this area of the mine 

both to the west and south. Unlike the isolated block of five panels at the VP-1 study area, the series of panels in the VP-3 Mine 

had not been surrounded by main entries prior to mining of the 

panels (Fig. 1). The panels in the VP-3 study area were progressively developed and mined to the south. Each successive panel 

in the series was therefore connected to the virgin coalbed gas 

reservoir until the development entries for the panel were completed. The VP-3 study panel was surrounded by development 

entries and ventilation airflow just three and one-half months prior 

to the start of mining on the panel, and less than five months prior 

to the emission study. Therefore, there was very little time for in situ 

gas content reduction by horizontal methane drainage boreholes or 

bleed-off into the ventilation system prior to mining of the study panel. 

This gas bleed-off time difference also had an influence on 

gas production from the horizontal boreholes at the two study 

sites. Total horizontal borehole methane production was 66 per- 
cent higher at the VP-3 Mine 13,828.5 x lo3 m3 (135.2 MMcf), or 

226.5 x lo3 m3 (8.0 MMcf)/borehole], than at the VP-1 Mine 

[2,307.8 x lo3 m3 (81.5 MMCF) or 76.5 x lo3 m3 (2.7 MMCF) / 

borehole], where the gas from the panel had a longer time to dissipate into the ventilation system priorto the drilling of the horizontal holes. Methane production from the active gob gas ventholes 

was almost identical [I 16 x lo3 m3/d (4.1 MMcfd) and 1 19 x lo3 

m3/d (4.2 MMcfd)] during the emission studies for the VP-1 (Fig. 7) 

and VP-3 (Fig. 13) Mines, respectively. 

It is probable that at the time of the emission studies, the in situ 

gas content on the VP-3 study panel was higher than that on the 

VP-1 study panel due to the shorter time for bleed-off of gas into 

the mine's ventilation system prior to mining. It is also possible 

that the virgin gas content may have been higher at the VP-3 mine 

site. The VP-3 Mine study panel lies under a depth of cover that 

is about 183 m (600 ft) greater than the VP-1 Mine study panel. 

In general, gas contents of a coalbed increase with increasing 

depth (Diamond, 1982). Based on limited available gas content 

data from the mine complex area, it is estimated that the gas content of coal at the VP-3 study site could be about 1.3 cc/g (40 ft3/ 

t) higher than that at the VP-1 study site. This is probably not a 

significant enough difference to account for the substantial difference in longwall face methane emissions or horizontal borehole 

production observed between the two mines. 

SUMMARY 

The primary purpose of this investigation at the VP-1 and VP3 Mines was to predict the methane emission consequences of 

increasing the longwall panel face width from 229 to 305 m (750 

to 1,000 ft). The predictions were based on emission trends established by continuous monitoring of methane emission rates on 

the existing 229-m (750-ft) wide faces. It was predicted that average emissions would increase from 8.0 to 8.6 m3/min (284 to 304 

cfm) or 7.1 percent on longwall faces extended an additional 76 

m (250 ft) to 305 m (1,000 ft) at the VP-1 Mine. This level of increase alone would probably not present any particular mining 

problem. In contrast, a similar analysis of emission trends from 

the VP-3 Mine study site suggested that average methane emissions could increase from 14.2 to 16.1 m3/min (502 to 567 cfrn), 

or 12.9 percent on faces extended to 305 m (1,000 ft). This level 

of methane emissions predicted for the additional face width at 

the VP-3 Mine is just below the general range of 17.0 to 19.8 mY 

min (600-700 cfm) where methane delays were occasionally experienced. At this level of increase for a "typical" pass, additional 

methane delays could be experienced. It is likely that increased 

ventilation airflow, and/or additional methane drainage would be 

required to safely realize the gain in coal production anticipated 

from mining larger panels. 

Since this study was completed, the mine engineering staff 

reports that several panels with 305-m (1,000-ft) wide faces have 

been completed at the VP-3 Mine without experiencing the predicted higher methane emission levels. An enhanced methane 

drainage program that provided both increased hole length and 

additional time for the horizontal boreholes to drain gas in advance of mining is credited with maintaining the methane emissions at a lower than predicted level. 

Methane emission levels were significantly different between 

the two mine study sites that were only about 1,600 m (1 mile) 

apart. Baseline methane emission levels at the VP-3 Mine study 

site were 2.8 m3/min (100 cfm) higher than those at the VP-1 

Mine. Average longwall pass methane emission levels were 12.1 

m3/min (427 cfm) at the VP-3 Mine, and only 7.2 m3/min (254 cfm) 

at the VP-1 Mine. Study panels at both mines utilized in-mine 

horizontal borehole and vertical gob gas venthole methane drainage to supplement the mine's ventilation system. The higher level 

of methane emissions at the VP-3 Mine study site is attributed to 

(1) the closer proximity and longer exposure to the adjacent virgin coalbed gas reservoir and (2) to the shorter time for in situ gas 

content reduction by horizontal methane drainage boreholes and 

bleed-off into the ventilation system prior to mining. 

In addition to the influence of isolation time on methane emission levels, it also affected methane drainage rates from the horizontal boreholes at the study sites. The horizontal boreholes on 

the VP-3 study panel produced 66 percent more total gas than 

those at the VP-1 site. In contrast, methane production from the 

gob gas ventholes was actually quite similar at both mine sites, 

indicating that gas bleed-off in the mined coalbed had little if any 

influence on gob gas. 

The final point of interest evident from this study is that methane released at the face during active mining is only a relatively 

small portion of the overall methane liberation from an active 

longwall section. At the VP-1 Mine site, methane emissions from 

the active longwall face averaged about 1 1.3 x 1 O3 m3/d (0.4 

MMcfd), or only about 5 percent of the 210 x 103m3/d (7.4 MMcfd) 

total methane emissions/borehole production attributed to the 

mining of the study panel. However, it was a higher percent of the 

total at the VP-3 Mine where the study panel had been directly 

connected to the virgin coalbed gas rese~oir until shortly before 

mining began. At this study site, methane emissions from the face 

averaged about 17.0 x 1 03m3/d (0.6 MMcfd), or about 9 percent 

of the 18.1 x 1 O3 m3/d (6.4 MMcfd) total methane emissions/borehole production attributed to the study panel. The importance of 

methane drainage in supplementing the face ventilation system is 

quite clear. If even a small fraction of the gas handled by the 

methane drainage systems was diverted to the face area, both 

mine safety and the expected increase in coal production would 

be adversely affected. 

ACKNOWLEDGMENTS 

This research effort was conducted in cooperation with the Virginia Division of Island Creek Coal Company (now Consolidation 

Coal Company), Oakwood, VA. The day-to-day assistance of the 

engineering staff to conduct the underground monitoring and time 

study data collection required for the successful completion of the 

project goals is gratefully acknowledged. 

REFERENCES 

Diamond, W.P., 1982, " Site-Specific and Regional Geologic Considerations for Coalbed Gas Drainage," USBM IC 8898,24 pp. 

Diamond, W.P., P.W. Jeran, and M.A. Trevits, 1994, "Evaluation 

of Alternative Placement of Longwall Gob Gas Ventholes for 

Optimum Performance," USBM R19500,14 pp. 

Diamond, W.P., J.P. Ulery, and S.J. Kravits, 1992, "Determining 

the Source of Longwall Gob Gas: Lower Kittanning Coalbed, 

Cambria County, PA," USBM R1 9430, 15 pp. 

McCall, F.E., F. Garcia, and M.A. Trevits, 1993. "Methane Emissions During Longwall Mining," Conference Papers, Longwall 

U.S.A., Pittsburgh, PA, June 8-10, Maclean Hunter Mining and 

Construction Group, pp. 267-279. 

Schatzel, S.J., F. Garcia, and F.E. McCall, 1992, "Methane 

Sources and Emissions on Two Longwall Panels of a Virginia 

Coal Mine" Proceedings, 9th Annual International Pittsburgh 

Coal Conference, Pittsburgh, PA, Oct. 12-16, pp. 991- 998.