The borehole monitoring experiment: field measurements of 

reservoir conditions and responses in longwall panel overburden 

during active mining 

S.J. Schatzel, C.Ö. Karacan, G.V.R. Goodman, R. Mainiero 

National Institute of Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, PA, 

USA 

F. Garcia 

Retired, formerly of U.S. Department of Health and Human Services, Centers National Institute of 

Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, PA, USA 

ABSTRACT: In order to better understand the effects of longwall mining on the methane reservoir overlying the 

panel, the National Institute for Occupational Safety and Health/Pittsburgh Research Laboratory (NIOSH/PRL) 

designed and implemented a vertical borehole monitoring experiment (BME) on an active longwall panel. The 

mine operates in the Pittsburgh Coalbed in Greene County, Pennsylvania. The BME site was located away from 

either end of the panel to avoid mechanical influences caused by the draping of overburden in this region and was 

positioned at the same distance from the tailgate gateroad as the mine’s gob gas venthole sites. The borehole 

drilling depths and monitoring intervals were chosen to address a different stratigraphic zone in each of the three 

planned holes. The monitoring boreholes were positioned perpendicular to face advance using a close spacing 

pattern of about 15 m (50 ft) between collars. Boreholes were instrumented downhole and on the surface for 

various measurements. It was observed from the field data that the gob gas venthole fracture network formed 24­

46 m (80-150 ft) ahead of the mining face. Overburden permeability’s within the same test zones increased 100 to 

500 times following undermining. The borehole test interval within the fractured rock responded to overburden 

gas pressure while the test interval in the caved zone responded to mine ventilation conditions. Eight months after 

the boreholes were undermined, slug testing indicated permeabilities had increased further in the fractured rock 

zone. Measurements of surface movement indicate that the permeability development observed in the boreholes is 

a product of vertical subsidence without lateral movement within the tensional rock mechanics zone on a 

supercritical longwall panel. Results from this study suggest that maximum permeability in the longwall panel 

overburden is achieved 58 to 190 m (190 to 620 ft) behind the longwall following maximum compaction of the 

overburden. 

1 Introduction 

Methane control in underground coal mines is an active 

area of research at NIOSH/PRL. The liberation of methane 

gas into underground coal mines is impacted by 

geotechnical variables, as well as those arising from 

mining method and mine design. A primary goal of 

NIOSH methane control research is to improve control 

technology to diminish the potential for an explosive 

mixture of methane in air which poses a risk to the 

underground work force. Increased face advance rates, 

increased productivities, increased panel sizes, and more 

extensive gateroad developments have challenged existing 

designs for controlling methane on longwalls. 

Studies have shown that methane contributions from 

the subsided strata (gob) generally account for 80% to 94% 

of the methane present in the ventilation system of an 

operating longwall (Curl, 1978; Schatzel et al., 1992). 

Consequently, controlling methane emissions from 

longwall gobs is critical to maintaining statutory and safe 

underground methane concentrations. Methane control in 

coal mines relies on methane drainage via boreholes. 

During longwall mining, most US operators utilize vertical 

boreholes or gob gas ventholes (GGVs) drilled in advance 

into the panel. 

The basic operating principles controlling many 

aspects of GGV performance are identical to conventional 

coalbed methane wells. The theoretical basis for coalbed 

methane well behavior and the key factors influencing 

production performance have been discussed in the 

literature (Ertekin et al., 1986; King and Ertekin, 1991; 

Young et al., 1991; Young et al., 1993; Zuber, 1998). In 

addition to the parameters controlling coalbed gas wells 

gas production, the performance of a GGV is influenced by 

additional borehole design factors (Schatzel et al., 1992; 

Diamond et al, 1994; Thakur, 1997; Karacan et al., 2005; 

Karacan et al., 2007). These factors include: the position of 

the borehole relative to the start and completion of the 

panel, the position of the borehole relative to the panel 

margin or gateroad, the height of the borehole completion 
 

 

 

 

 

 

 

over the mined coalbed, the diameter of the borehole, the 

length of open and/or slotted hole, and the surface 

exhauster configuration. In general, coalbed gas is released 

during overburden deformation associated with mining and 

the fractures created from this process become pathways 

for gas migration. The rate, quantity, concentration and 

duration of gas movement produced by GGVs are largely 

controlled by the aforementioned parameters. 

2 Description of the Borehole Monitoring 

Experiment on an Active Longwall Panel 

In order to better understand the effects of longwall mining 

on the methane reservoir overlying a longwall panel, a 

borehole monitoring experiment (BME) was designed and 

implemented. The BME provided recommendations to 

operators for improving methane control measures through 

field observations and through enhancements of reservoir 

modeling techniques. The borehole drilling depths and 

monitoring intervals were chosen to address a different 

stratigraphic zone in each hole. The experimental design 

specified that drilling was to be completed two months in 

advance of undermining the borehole location. 

2.1 Borehole Monitoring Test Site 

The mine operates in the Pittsburgh Coalbed in Greene 

County, Pennsylvania. The boreholes were drilled in a new 

mining district. The drill site was selected based on an 

active panel and the BME site location was chosen to be 

away from either end of the panel to avoid mechanical 

influences caused by the draping of overburden in this 

region. 

The BME was planned for a panel that was 442 m 

(1450 ft) in width (Figure 1). The three test boreholes were 

arranged in a line parallel to the tailgate gateroads. The 

distance to the tailgate gateroads was 101 m (330 ft), the 

same distance which the mine used for its GGVs so that 

they would be in the same mechanical behavior zone and 

in a similar stress field. One of the operator’s GGVs was to 

be drilled 1930 m (6325 ft) from the completion end of the 

panel, and was to be included in the BME monitoring 

activities. The distances between each of the monitoring 

boreholes was 15 m (50 ft), and the third borehole (BH-3) 

was 76 m (250 ft) from the nearby GGV. 

The three test boreholes were drilled and completed at 

different depths to monitor initial reservoir and mechanical 

properties in different strata horizons and subsequent 

property changes during the mining of the longwall panel. 

The top of the Pittsburgh Coalbed was 252 m (827 ft) from 

the surface (Figure 2). According to this plan, the first, or 

shallowest, borehole (BH-1) was drilled to a total depth of 

220 m (721 ft). The second, or middle-range borehole 

(BH-2), was drilled to a depth of 230 m (755 ft). The 

deepest borehole (BH-3), which was also closest to the 

GGV, was drilled to 245 m (803 ft). 

NIOSH 

BME test 

site 

Gob 

Tailgate gateroads 

76m 

101m

(330 ft) 

(250 ft) 3 2 1 

Study panelwidth, 442m 

Mining direction GGV

(1450 ft)

NIOSH monitoring 

boreholes and Parallel instrument locations to face Perpendicular to face 

NIOSH test mon toring interval, 

335 m (2100 ft) 

Headgate gateroads 

Figure 1. Field site for NIOSH’s borehole monitoring 

experiment (BME) (not to scale).test 

Based on the local geology and the depths of the 

boreholes shown in Figure 2, BH-1 was intended to 

monitor the Sewickley Coalbed, BH-2 monitored mostly 

shale sequences below the Sewickley Coalbed and above 

the Pittsburgh Coalbed, and the deepest (BH-3) monitored 

the shale and sandstone horizons that would be retained in 

the gob after undermining. Boreholes were drilled with a 

15-cm (6.0-in) diameter bit. After the completion of

drilling, the deepest borehole (BH-3) was logged open-hole 

with density, gamma ray, and sonic tools to identify 

formations, to refine drilling depths, and to calculate 

porosity, density, and some of the mechanical properties of 

the rock. The drilling of the boreholes was started and 

completed when the longwall face was 760 m (2500 ft) 

away from the BH-1 location. 

The boreholes were cased with 13-cm (5.0-in) steel 

casing. They were cemented using conventional grout and 

cement baskets, except for the bottom 6.1 to 9.1 m (20 to 

30 ft). These sections were cased with slotted casing and 

were the primary monitoring zones for each hole. The 

length of slotted casing was 9.1 m (30 ft) in BH-1 in order 

to monitor both splits of the Sewickley Coalbed. The 

slotted section of BH-2 was 6.1 m (20 ft) long. The last 2.4 

m (8.0 ft) of the slotted casing of BH-3 was cut and left 

open-hole in order to keep the casing as high as possible 

above the gob (Figure 2). 

The experimental boreholes were configured to be 

completed in a manner similar to the mine operator’s 

GGVs. Both borehole designs included flame arrestors, 

shut-in valves, and long, vertical PVC pipe stacks. 

However, unlike the experimental boreholes, the GGVs 

were cased with 61-m (200-ft) slotted casing at the bottom 

of all boreholes. The operator used a dump grouting 

procedure on the GGVs instead of the circulating grout 

method used on the test boreholes. The test boreholes were 

also different from the operator’s GGVs in that the BME 

boreholes were kept shut-in throughout the mining 

duration and a powered exhauster was not attached. 
 

 

 

 

 

 

 

 

 

Similar to the GGVs, test borehole wellheads were 

equipped with 15-cm (6.0-in) diameter flange for 

installation of the flame arrestor and the wellhead valve. 

Borehole monitoring instrumentation and associated 

hardware were also attached to the test borehole stack. 

Figure 2. Stratigraphy and downhole configurations for the 

NIOSH BME. 

2.2 Methodology and Instrumentation 

Formation permeability is a key variable influencing 

coalbed methane borehole production from both coalbed 

gas wells and GGVs. An important portion of this study 

performed measurements of insitu permeability in the 

monitoring boreholes before, during, and after the 

undermining of the NIOSH drill sites. To make these 

measurements, an experimental method was designed. 

For the initial slug test prior to mining, boreholes were 

equipped with submersible, downhole transducers which 

were positioned within the downhole monitoring zones. 

The boreholes were filled with water so that the change in 

water head could be observed. The downhole transducers 

were installed underwater in the boreholes. The water head 

drop was monitored for about a week on each borehole 

downhole transducer until the rate of water head change 

reached a steady state. After the conclusion of the initial 

slug tests, the downhole pressure transducers were 

repositioned just above the slotted sections. 

Water levels remaining in the boreholes following the 

initial set of slug tests were monitored to determine the 

change in formation permeabilities as mining progressed 

underneath. In order to quantify permeability changes in 

the overlying strata in response to mining, a slug test 

model for confined, anisotropic aquifers of infinite or 

semi-infinite radial extent was utilized (Dawson and Istok, 

1991). The model was used for calculating instantaneous 

permeabilities that are represented by two consecutive data 

points recorded in 1-minute intervals and also for 

calculating the average permeabilities during intervals that 

can be recognized by abrupt changes in the rate of water 

head drop. The borehole monitoring intervals were selected 

to provide input on the initiation of strata disturbances and 

the changing degree of rock damage in terms of fracture 

permeability with respect to the position of the longwall 

face. The submersible transducers recorded changes of 

water head until the water drained completely from the 

boreholes. A final set of slug tests for determining final 

permeabilities was to be run after the face had left the test 

zone and neared completion of the panel. 

The wellheads on BME boreholes were equipped with 

surface pressure transducers for continuous data recording 

of pressure changes at the surface. Methanometers were 

also installed to measure methane concentrations at the 

tops of the three experimental boreholes and to monitor 

any changes in the wellbores as a result of fracturing of 

strata. 

A total of three tiltmeters were installed on the BME 

wellhead stacks to define the timing of ground movement 

(Figure 3). Prior research has shown that GGV production 

is strongly influenced by the position of the borehole 

relative to the subsidence trough developed at the surface 

during undermining of a longwall panel face (Jeran et al., 

1986; Adamek et al., 1987; Diamond et al., 1994; Ingram 

and Trevits, 1995). Conventional positional surveys were 

scheduled to quantify the vertical and horizontal movement 

of the surface. 

Figure 3. Wellhead arrangement for monitoring boreholes. 

The primary experiment test zone extended a distance 

of 305 m (1000 ft) on either side of BH-1 and BH-3. This 

distance was chosen based on the overburden depth and 

typical subsidence profiles for a supercritical panel in the 

Northern Appalachian Basin. An additional 30.5 m (100 ft) 
 

 

 

 

 

 

 

 

was included in the overall test zone, the distance from 

BH-1 to BH-3 (Figure 1). All instrumentation was 

installed, the boreholes shut-in, and monitoring begun 

while the longwall face was 366 m (1200 ft) away from the 

first borehole. The longwall face position was recorded 

daily. 

3 Discussion of Results 

3.1 Fracture Permeability 

The calculated initial permeabilities determined by the slug 

tests for BH-1, BH-2 and BH-3 were 2.8 md (millidarcies), 

0.1 md, and 0.2 md for the Sewickley Coalbed, shale and 

limestone, and for the shales, respectively. These data were 

measured immediately after the borehole stacks were 

assembled and prior to the formation of mining-induced 

fractures in the BME test zone. 

During mining, fracture permeabilities were calculated 

using the data recorded by the downhole transducers as 

water-head dropped during undermining of all three 

boreholes. However, soon after starting to record data from 

the deepest borehole, communication with the downhole 

pressure transducer was lost. Thus, none of the figures 

include downhole pressure data from BH-3. Water head 

data suggest that mining-induced disturbances forming the 

GGV fracture network can occur 24-46 m (80-150 ft) 

ahead of the mining face. In monitoring water levels in the 

boreholes, the loss of water can be rapid up to about 32 m 

(100 ft) above the mined coalbed, limiting the duration of 

monitoring. The rate of water loss was not related to the 

depth of the borehole. Shearing and deformation in the 

overburden is typically severe in near margin GGVs where 

much of the annulus is generally modified. 

Figure 4 shows water head and permeability evolution 

averaged for some distinctive segments in BH-1. The data 

show that as soon as the strata are affected from mining 

disturbances, an initial permeability increase response. 

The averaged value of this initial permeability increase is 

~400 md. After this initial increase, permeability decreases 

to 100 md. This permeability change behavior may 

indicate that initial reservoir disturbance is due to shearing 

and fracturing of the strata or bedding planes. After the 

borehole is undermined, averaged permeability increases to 

larger values (~400 md) due to larger-scale fractures. The 

highest instantaneous permeabilities measured in the 

borehole occur just after disturbances first affect the 

coalbed (over 800 md), during initial ground movement, 

and during the undermining of the borehole location. The 

highest averaged fracture permeability for any of the 

segments in the Sewickley horizon occurred after the face 

had passed the BH-1 location as last of the water was 

leaving the borehole (Figure 4). Borehole 2, designed to 

monitor a 6.1-m (20-ft) section in the shale and limestone 

zone between Sewickley and Pittsburgh Coalbeds, was 

analyzed using the same approach. 

Figure 4. BH-1 average permeabilities and water head drop 

rate (height of water/original water height). 

Figure 5 presents average permeabilities (utilizing 

short-term, rapid instrument response data) within different 

segments based on the changes in the rate of water head 

drop for BH-2. Although this borehole is deeper and closer 

to mining than BH-1, this interval did not show the sudden 

water head loss as observed in BH-1 when mining 

influence reached the borehole location. This behavior may 

be due to the combination of different structural and 

mechanical properties of this horizon compared to the BH1 test horizon which included the Sewickley Coalbed. 

The data in Figure 5 show that permeabilities in BH-2 

gradually increased to the 100-200 md range, possibly due 

to bedding plane movements. A sudden drop in water head 

occurred with an associated permeability increase occurred 

on 6/21 where the segment averaged about 400 md. This 

increase coincides with the approach of the longwall face 

(15 m (50 ft) away) and may suggest communication 

between these two wells. Figure 5 shows that permeability 

starts increasing as the mining face advances towards the 

BH-2 location until it is undermined. Shortly after 

undermining, large-scale horizontal and vertical fractures 

are created and the permeability increased to about ~1600 

md. Permeabilities measured prior to undermining were in 

the ~1 md range. Permeability increases following 

undermining were dramatic with increases of about 100 to 

500 times and instantaneous increases of up to about 1000 

times. Formation and fracture permeabilities gradually 

increase as mining-induced disturbances progress to the 

borehole locations. However, the biggest average change 

occurs following the interception of borehole locations. 

The advantage of the test method applied is to allow 

monitoring of water-head changes as mining progresses. 

However, the disadvantage is the data can be collected 

only as long as water is present in the borehole. 

After longwall mining had neared completion of the 

study panel, the final slug tests were performed on BH-1 

and BH-2. When adding known amounts of water to the 

boreholes, the rate of water head loss was rapid and the 

interval above the transducers was emptied in a matter of 

minutes. In both holes, a second slug of water was added to 
 

 

 

 

 

 

 

 

 

 

 

 

retest borehole permeability. Borehole 1 measured 63 and 

65 Darcys in the two tests. Consequently, the increase in 

permeability for BH-1 compared to the maximum 

measured immediately following undermining increased 

by a factor of about 40 to 60 times. When performing the 

slug test in BH-2, no water head build-up was achieved 

due to water leaving the borehole at such a rapid rate that a 

permeability value could not be determined. The final 

permeability level for BH-2 was apparently far above what 

was measured in BH-1. 

Figure 5. Average permeabilities and water head drop rate 

(height of water/original water height). 

3.2 Borehole Gas Pressures and Concentrations 

The changes in methane concentrations and static pressures 

in the boreholes were monitored at the surface after the 

boreholes were shut-in. These concentration and pressure 

measurements are shown in Figures 6A and 6B, 

respectively, during the monitoring period. The vertical 

shaded area in these figures shows the dates for miners’ 

vacation. All three boreholes were intercepted during the 

week just before vacation, during which time the longwall 

did not operate. The nearby GGV was intercepted after 

longwall mining resumed. The coal production delay 

associated with miners’ vacation and the date when the 

nearest GGV began gob gas production is also marked on 

Figure 6. 

The monitoring of shut-in pressures began before 

mining disturbances reached the borehole locations. This 

period (until the shaded area) is characterized by almost 

consistent changes in methane levels near about June 12 

(Figure 6A) and static borehole pressures (Figure 6B) in 

BH-1 until the mining-related disturbances reach the 

borehole locations. The data show that until shearing 

occurred, resulting in an initial water head drop, methane 

concentration was 75% in BH-1. Gas concentration then 

increased to 85% methane within a few days and pressure 

increased to 20 cm (8.0 in) of water gauge in the borehole. 

These increases suggest that the coalbed was producing 

methane that was migrating into the borehole. However, 

when mining disturbances reached the BH-1 borehole 

location, methane concentrations decreased to 40% and 

borehole pressure dropped to 10 cm (4.0 in) of water 

gauge. Interception by mining resulted in a further 

decrease. However, after this borehole location was 

intercepted by the longwall face and all the water was 

drained out, methane concentrations started to increase to 

90%-100% due to coalbed fracturing. 

The interception of BH-2, which monitored shale and 

limestone layers, showed a different behavior. Initial 

methane readings in this borehole before undermining 

were around 10% and the shut-in pressures were low, 

indicating that there was not significant methane flow into 

the borehole, probably due to low permeabilities and the 

presence of water in the borehole. However, after it was 

undermined and the water drained out, methane 

concentrations increased to 90%-100% in the borehole 

with a sudden pressure fluctuation. This borehole response 

may be the result of new horizontal and vertical fractures 

associated with the permeability increase in the monitoring 

zone. 

It is also interesting to note the similar behavior of 

methane concentration change in BH-1 and BH-2 after 

undermining. This behavior suggests that these two 

boreholes started to communicate through horizontal and 

vertical fractures even though they monitored two different 

horizons about an 11 m (35 ft) vertical distance from each 

other. This observation shows that the layers within 24 m 

(80 ft) of the top of the coalbed being mined are 

sufficiently fractured or there is enough shearing or 

opening of natural fractures so that the formations can 

interact with each other. However, the fact that the mine 

ventilation pressures (-8 to -10 cm water gauge (-3 to -4 

in)) have not been continuously recorded at these boreholes 

suggests that there was either no direct communication 

with the mine or that the positive gas pressure into the 

boreholes from the monitored zones was high enough to 

compensate the negative pressure influence of mine 

ventilation. 

The behavior of pressure and concentration data 

produced by BH-3, 6.1 m (20 ft) above the Pittsburgh 

Coalbed, is completely different from the other two 

boreholes. In BH-3, the decrease in methane 

concentrations and shut-in pressures after undermining 

showed that this borehole started to communicate with the 

mine atmosphere. After undermining, the shut-in pressure 

decreased to mine ventilation pressures and stayed as such 

for the rest of the monitoring. Methane concentrations 

were in the 40% to 50% range before undermining then 

decreased to about 5% methane during and immediately 

following undermining. Concentrations remained close to 

5% methane after undermining. Then the GGV exhauster, 

76 m (250 ft) away, began to operate with a high flow rate 

following undermining. This resulted in an initial decrease 

in borehole methane concentration followed by a 

concentration rise in the gob, as recorded in BH-3, to about 

35% (Figure 6A). 

The start of the nearby GGV operation is also 

noticeable in the other two BME boreholes (Figure 6). 

Although this venthole did not operate continuously and 
 

 

 

 

 

 

 

successfully due to mechanical problems after an initial 

high production rate, the data confirms that the venthole 

was communicating with the monitoring boreholes through 

fractures and bedding plane separations (Mucho et al., 

2000). The explanation for the methane concentration 

increase in the gob (or in BH-3) may be due to an increase 

in methane height in the gob as a result of the negative 

pressure generated by the venthole blower. This rise may 

also be due to drawing gas from other horizons into a more 

permeable gob following undermining. This may also 

explain the decrease in BH-1 and BH-2 shut-in pressures 

and the decrease in measured methane concentrations 

associated with a concentration increase in BH-3, before 

the recovery again to 70% methane. Downhole pressure 

and methane concentration data suggest that BH-1 and BH2 interact and behave similarly to overburden conditions. 

BH-3 responds to mine ventilation pressure and later to 

gob pressure conditions. 

Figure 6. Methane concentration (A) and static shut-in 

pressures (B) measured in the boreholes during progress of 

mining. 

3.3 Survey Measurements for Evaluation of Subsidence 

During Mining 

Figure 7 illustrates the ground elevation survey 

information and the data collected by the three surfacemounted tiltmeters. The plot shows there was an 

instrument adjustment on about June 12, which is usually 

performed when the instrument is bumped or reaches its 

range limit. By June 13, all instruments were recording tilt 

perpendicular to the longwall face, well before 

undermining occurred (Figure 1). This tilt response appears 

to precede rock fracturing recorded by the slug tests 

(Figures 4 and 5) but corresponds to gas concentration and 

pressure changes in the boreholes (Figure 6). These data 

show an interval of strata bending preceding fracture 

formation. On June 21, the longwall undermined BH-1. 

Major fracturing in BH-1 on June 18 (Figure 4) is 

represented by slight tilting with fracturing at BH-2 

occurring on June 21 during continued tilting 

perpendicular to the longwall face. From June 24 through 

July 8, operations on the longwall stopped for miners’ 

vacation. The gob stopped collapsing during this period but 

began again after longwall mining resumed. A much larger 

tilt response perpendicular to the longwall face occurred on 

about July 8 at all three boreholes after the resumption of 

mining. Tilt perpendicular to the face reached a maximum 

on July 11 to 13 at all three boreholes and then returned 

close to the original position. 

Tilt parallel to the face (Figure 1) began at BH-1 and 

BH-2 from June 13 to 15 and very slight movement was 

seen at BH-3. A period of movement was indicated at BH3 parallel to the face during the non-mining period from 

June 30 to July 8 (Figure 7). The instrument measured offscale on the axis corresponding to the parallel to the face 

direction during this time. Since no other surface 

movement was recorded by the tilt meters and due the lack 

of induced surface movement during the period of nonmining, these data are likely instrumentation problems. 

Instrument data from BH-3 is back within scale on July 9 

and appears to be responding to surface movement for the 

remainder of the monitoring period. 

Maximum movement parallel to the face was reached 

from July 18 to July 21 at all boreholes. This period of 

movement may correspond to the formation of higher 

fracture permeabilities measured in the final series of slug 

tests months later. This time corresponds to a face position 

about 160 to 190 m (520 to 620 ft) past the borehole 

locations. Tilt parallel to the longwall face did not return to 

level (pre-mining position). 

Maximum tilt movement may indicate the onset of 

maximum subsidence at the surface and an increase in 

permeability underground in the monitored borehole 

interval. The permeability increase was evident in BH-1 

and BH-2 but can only be assumed in BH-3 due to the 

downhole instrumentation failure in this borehole. In 

directions both parallel and perpendicular to the face, 

surface tiltmeters indicated movement had ceased 1 to 2 

weeks after undermining, not including work stoppages. 

Two mechanisms are recognized as potentially 

contributing to the increase in fracture permeability during 

overburden compaction. The first is the draping effect in 

the tensional zone of the overburden. As the final phase of 

overburden compaction over a longwall panel came to an 

end, the adjacent overburden over the gateroads 

experienced very limited surface movement. The 

overburden connecting the compacted zone over the 

longwall panel to the largely uncompacted zone over the 

gateroads is in tension and achieves maximum fracture 

permeability during maximum compaction of the gob. 

The compaction-related increase in permeability may 

be partly produced by a loss of coal volume between the 

coal cleats as coalbed gas migrates out of the coal matrix in 
 

 

 

 

 

 

 

response to the decrease in gas pressure within the coal 

fracture system. In other words, the data indicate swelling 

of the coal matrix due to coalbed gas retention and 

shrinkage of the matrix during the loss of that gas. 

Diamond et al. (1992) measured gas content in longwall 

panel overburden before and after undermining to 

determine the primary sources of gas in the zone of 

emission. Diamond et al. estimated that the coalbed 

sequences contributed 91% of the overburden gas released 

during undermining. It is considered likely that both 

tensional strata fractures and coal matrix shrinkage 

mechanisms are contributing to the permeability increase 

measured in the near-margin GGV monitoring intervals. 

Five conventional surface surveys were conducted at 

different dates during the monitoring period. These surveys 

registered the positions of a bolt at the wellhead and 

ground locations for each borehole to define the lateral and 

vertical movements. A portion of the subsidence data is 

also shown in Figure 7. The full set of vertical movement 

data at the borehole sites is shown in Figure 8. Three 

surveys were conducted within the primary test zone. 

These surveys were conducted just before, during, and just 

after the borehole locations were intercepted by the 

longwall face. The other two surveys were conducted 

outside the primary test zone as the longwall face 

approached the boreholes and after it passed beneath them. 

Elevation measurements of the landmarks during these 

surveys revealed the amount of subsidence at the borehole 

locations. The magnitude of the subsidence was 0.99 m 

(3.25 ft) at BH-1, 0.93 m (3.05 ft), at BH-2, and 0.86 m 

(2.82 ft) at BH-3. These values are within the range of 

expected subsidence values observed in Northern 

Appalachian basin due to longwall mining. The 

measurements show that there was no appreciable lateral 

movement of either the ground or the wellhead. 

Permeability development observed in the boreholes is 

product of vertical subsidence without any component of 

lateral movement. 

Figure 7. Surface movement over longwall panel study site 

during undermining as measured by tilt meters. 

Figure 8 shows that as of July 13, surface subsidence 

had ceased. The tilt meter data shows that by this date, 

most borehole movement parallel to the longwall face had 

already occurred on boreholes 1 and 2 and some movement 

was still occurring on borehole 3. Perpendicular to the 

longwall face, maximum movement had already been 

achieved by July 13 although the surface contour was in 

the process of returning to the pre-mining configuration. 

These results suggest that although the bulk of the surface 

subsidence had occurred by July 13, surface movement and 

tilting associated with subsidence trough formation was not 

complete. Maximum tilt movement parallel to the face 

from July 18 to July 21 did not produce a significant 

component of vertical movement. The high permeability’s 

measured or indicated in the final series of slug tests were 

Probably reached once these tilting and vertical 

movement events were completed and subsequent surface 

movement had diminished (Figures 7 and 8). If the end of 

vertical surface movement indicates essentially the end of 

permeability increases, then maximum permeability was 

achieved 58 to 88 m (190 to 290 ft) behind the longwall 

face. If permeability increases were still being realized 

during surface tilting without vertical movement, then the 

increases occurred 160 to 190 m (520 to 620 ft) past the 

borehole locations. Distances are approximate due to the 

limited quantity of conventional survey measurements and 

the termination of tilt meter monitoring outside the primary 

test zone. 
 

 

 

 

 

 

4 

Figure 8.Vertical movements of the survey points of the 

monitoring boreholes during mining. 

Summary 

A primary goal of NIOSH methane control research is to 

improve control technology to diminish the potential for an 

explosive mixture of methane in air which poses a risk to 

the underground work force. Researchers at NIOSH/PRL 

designed and implemented a borehole monitoring 

experiment (BME) on an active longwall panel to measure 

changing reservoir conditions in the overburden. The BME 

provided recommendations to operators for improving 

methane control measures through field observations and 

through enhancements of reservoir modeling techniques. 

The GGV fracture network can form 24-46 m (80-150 ft) 

ahead of the longwall mining face on a Northern 

Appalachian Basin supercritical panel. This distance may 

vary when conventional GGV slotted pipe configurations 

are used. Much shorter lengths were used during 

monitoring. 

The loss of water from GGVs was rapid from 32 m 

(100 ft) to 22 m (72 ft) above the mined coalbed. The rate 

of water loss was not related to borehole depth. 

Overburden permeabilities within the same overburden test 

zones ranged from about ~1 md range prior to undermining 

and increased 100 to 500 times following undermining 

with higher instantaneous peaks. This increase in 

permeability occurred when the longwall face reached the 

monitoring locations 

About 7 months after undermining had occurred, 

additional borehole permeability tests showed the fracture 

permeability had increased again since undermining. 

Borehole 1 (the shallowest in the Sewickley Coalbed 

horizon) had a permeability of 63 to 65 Darcys. Borehole 2 

(limestone and shale interval) had a higher permeability 

but the water loss in the hole was so rapid it could not be 

accurately be measured. Borehole 3 was expected to have 

an extremely high permeability (Pittsburgh Coalbed caved 

zone) but no plans were made to measure this interval after 

mine through. The borehole test intervals within the 

fractured rock responded to overburden gas pressure and 

the test interval in the caved zone responded to mine 

ventilation conditions. 

Surface tilting appears to precede rock fracturing in 

the monitored borehole interval but corresponds to gas 

concentration and pressure changes in the boreholes. 

Additional surface tilting measured 1 to 2 weeks after 

undermining (not including work stoppages) may 

correspond to the onset of maximum permeability 

measured in the boreholes in the final series of slug test 

months later. Permeability development observed in the 

boreholes is product of vertical subsidence without any 

component of lateral movement. 

The period of overburden compaction and tilting 

occurring 58 to 190 m (190 to 620 ft) behind the longwall 

face is assumed to create the final phase of fracture 

permeability measured in the near-margin GGV 

monitoring intervals. Two mechanisms are recognized as 

potentially contributing to the increase in fracture 

permeability during overburden compaction. The first is 

the draping effect in the tensional zone of the overburden. 

The overburden connecting the compacted zone over the 

longwall panel to the largely un-compacted zone over the 

gateroads is in tension and achieves maximum fracture 

permeability during maximum compaction of the gob. 

A second mechanism which may contribute to the 

onset of the final fracture permeability levels in the 

overburden is coal matrix shrinkage. A loss of coal volume 

within the matrix occurs when coalbed gas migrates out of 

the coal matrix in response to the decrease in gas pressure 

within the coal fracture system. In this scenario, there is 

swelling of the coal matrix due to coalbed gas retention 

and shrinkage of the matrix during the loss of that gas. It is 

considered likely that both mechanisms are contributing to 

the permeability increase measured in the near-margin 

GGV monitoring intervals. 

5 Disclaimer 

The findings and conclusions in this report are those of the 

authors and do not necessarily represent the views of the 

National Institute of Occupational Safety and Health. 

References 

Adamek V, Jeran PW, Trevits MA, 1987. Prediction of 

surface deformations over longwall panels in the 

northern Appalachian Coalfield. US Bureau of Mines 

RI9142. 

Curl SJ, 1978. Methane Prediction in Coal Mines. IEA 

Coal Resources Report ICTIS/TR 04. 

Dawson KJ, Istok JD, 1991. Aquifer testing: Design and 

analysis of pumping and slug tests. Lewis Publishers, 

Inc. Chelsea, Michigan. 344 pp. 

Diamond WP, Ulery JP, Kravits SJ, 1992. Determining the 

Source of Longwall Gob Gas: Lower Kittanning 

Coalbed, Cambria County, PA. U.S. Department of 

the Interior, Bureau of Mines, Report of Investigations 

9430, NTIS No. PB 93-132058:1-15 
 

 

Diamond WP, Jeran PW, Trevits MA, 1994. Evaluation of 

Alternative Placement of Longwall Gob Gas 

Ventholes for Optimum Performance. U.S. 

Department of the Interior, Bureau of Mines. Report 

of Investigations 9500, NTIS No. PB 94-171949:1-14 

Ertekin T, Sung W, Schwerer FC, 1986. Production 

performance analysis of horizontal drainage wells for 

the degasification of coal seams. 61st Annual 

Technical Conference of the Society of Petroleum 

Engineers, October 5-8. New Orleans, LA. 

Ingram, DK, Trevits MA, 1995. Characteristics of 

overburden deformation due to longwall mining. In 

3rd Proceedings of Subsidence Workshop due to 

underground mining. 

Jeran, PW, Adamek V , Trevits, MA, 1986. A subsidence 

prediction model for longwall mine design. SMEAIME Fall Meeting, St. Louis, MO. September 7-10, 

pp. 3-8. 

Karacan CÖ, Diamond WP, Esterhuizen GS, Schatzel SJ, 

2005. Numerical analysis of the impact of longwall 

panel width on methane emissions and performance of 

gob gas ventholes. Proc. 2005 International Coalbed 

Methane Symposium, Paper 0505. Tuscaloosa, AL, 

18-19 May. 

Karacan CÖ, Esterhuizen GS, Schatzel SJ, Diamond WP, 

2007. Reservoir simulation-based modeling for 

characterizing longwall methane emissions and gob 

gas venthole production. International Journal of Coal 

Geology, v. 71(2-3):225-245. 

King, G R and Ertekin, T, 1991. State-of-the-art modeling 

for unconventional gas recovery. SPE, Formation 

Evaluation, v. 6, 1, pp. 63-71. 

Mucho TP, Diamond WP, Garcia F, Byars JD, Cario SL, 

2000. Implications of recent NIOSH tracer gas studies 

on bleeder and gob gas ventilation design. Society of 

Mining Engineers Annual Meeting, Feb. 28-Mar. 1, 

Salt Lake City, UT. 16 pp. 

Schatzel SJ, Garcia F, McCall FE, 1992. Methane Sources 

and Emissions on Two Longwall Panels of a Virginia 

Coal Mine. Proceedings, Ninth Annual Pittsburgh 

Coal Conference, University of Pittsburgh, Pittsburgh, 

pp. 991-998. 

Thakur P, 1997. Methane drainage from gassy mines-a 

global review. In Ramani RV, ed , Proceedings of the 

6th International Mine Ventilation Congress, SME, 

Littleton, CO, Pittsburgh, PA, pp. 415-422. 

Young GBC, McElhiney JE, Paul GW, McBane RA, 1991. 

An analysis of fruitland coalbed methane production, 

SPE Paper 22913, 66th Society of Petroleum Engineers 

Annual Technical Conference and Exhibition, pp. 263­

279. 

Young GBC, Paul GW, Saulsberry JL, Schraufnagel RA, 

1993. A simulation-based analysis of multiseam 

coalbed well completions, SPE Paper 26628, 68th 

Socity of Petroleum Engineers Annual Technical 

Conference and Exhibition, pp. 205-215. 

Zuber MD, 1998. Production characteristics and reservoir 

analysis of coalbed methane reservoirs. International 

Journal of Coal Geology, 38:27-45.