Development and application of reservoir models for the evaluation and 

optimization of longwall methane control systems 

C.Ö. Karacan, W.P. Diamond, S.J. Schatzel & F. Garcia 

U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National 

Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, Pennsylvania, 

USA. 

1 INTRODUCTION 

Comprehensive assessments of the need for additional methane control capacity beyond ventilation 

often require both an empirical and theoretical approach for an adequate or timely control of increased 

methane emission levels. Thus, the prediction of 

methane emissions and optimization of methane 

control systems prior to starting a new mining operation will be a major improvement towards eliminating the explosions in the underground workplace. 

During longwall mining, the caving of immediate 

strata and stress relief create horizontal fractures 

along bedding planes and vertical fractures in the 

strata overlying the caved zone. These fractures provide an extensive pathway for gas migration from 

the surrounding coalbeds and other gas bearing 

strata into the longwall mining environment (Fig. 1). 

The thickness of the fractured zone can vary up to 

100 times the height of the mined coalbed (Palchik 

2003). The fractured and caved rock mass left behind the advancing longwall face is generally referred to collectively as “gob” (Fig. 1). The methane 

that originates and accumulates in the gob above the 

mined-out longwall panel is the main source of potential gas emissions during longwall mining. 

Gob gas extraction in the Northeastern U.S. is

almost exclusively accomplished using ventholes 

that are drilled from the surface to within a short distance [typically 10-15 m (30-45 ft)] of the coalbed 

being mined (Diamond 1994). Commonly, the bottom section of the well casing [generally about 60 m 

(200 ft)] is slotted. The gob gas ventholes generally 

become productive only when the mining-induced 

fractures are created as mining advances under the 

venthole (Diamond 1994). 

Figure 1 Schematic cross-section view of subsided strata 

zones and methane control system influence zones above

mined-out longwall panel (Mucho et al. 2000). 

Numerical models offer effective capabilities for 

predicting methane emissions, and designing drainage systems accordingly. There have been reported

studies using computational fluid dynamics (CFD), 

boundary element and finite element modeling 

(FEM) techniques to better characterize the parameters for gas emission prediction [Ren & Edwards 

(2002), Lunarzewski (1998), Tomita et al. (2003)]. 

Reservoir simulators developed over the years 

can represent the complex reservoir flow mechanisms in coalbeds (King & Ertekin 1991). However, 

a comprehensive reservoir model capable of realistiABSTRACT: Methane explosions have historically been one of the major causes of fatalities and injuries in 

underground coal mining operations. Advanced numerical models and predictive modeling approaches have 

the potential to offer optimized methane control solutions for general mine planning purposes and to address 

specific methane-related operational problems. This paper describes the development of reservoir models for 

the longwall mining environment and their application for investigating the influence of various completion 

design parameters on the methane drainage effectiveness of gob gas ventholes. The influence of increasing 

longwall panel width on the effectiveness of current gob gas venthole completion and placement strategies in 

the Pittsburgh Coalbed were evaluated and optimized designs developed to capture the expected increase in 

methane emissions on the larger panel. 
cally representing various aspects of mining operations and production from gob gas ventholes has not 

previously existed. In one study, Zuber (1997) modeled the face and rib emissions during development 

mining. Karacan et al. (in prep., 2005) and Esterhuizen & Karacan (2005) have developed “dynamic” reservoir models that include the subsided 

strata above the mined panel during longwall mining 

to evaluate methane emissions and various gob gas 

venthole design factors for their impact on gas 

drainage efficiencies. 

2 MODEL DEVELOPMENT 

The models summarized in this paper were developed for mine sites operating in the Pittsburgh Coalbed in the Southwestern Pennsylvania section of the 

Northern Appalachian Basin. The reservoir models 

were constructed using Computer Modeling Group’s 

(2003) GEM compositional reservoir simulator. 

2.1 General description of the mine sites 

Overburden depths in the area range between 152 

and 274 m (500 and 900 ft). Longwall panels in the 

old mining districts of this mine were around 253 m

(830 ft) wide and were increased to 305 m (1000 ft). 

In the new districts, the panel widths were originally

430 m (1250 ft); however, the first 480 m (1450 ft) 

wide panel is currently being mined. Thus, all the 

panels, particularly the recent ones, are supercritical, which results in a more complete caving of

the overburden strata into the mine void. 

A generalized stratigraphic section for the study 

area is shown in Figure 2. Several coalbeds with a 

combined thickness of almost 3 m (10 ft) are present 

in the 26 m (85 ft) of strata immediately above the 

Pittsburgh Coalbed. Within this interval, the thickest 

coalbed is the Sewickley, which is about 25 m (75 

ft) above the Pittsburgh Coalbed. Between these two

major coalbeds, there are comparably thin Pittsburgh

rider coals and the discontinuous Redstone Coalbed, 

which are contained in the caved zone after panel 

extraction (Fig. 1). The gas emissions associated 

with the caved zone report to the bleeder ventilation

system (Mucho et al. 2000). In some parts of the 

study area a sandstone paleochannel replaces the 

shale unit usually present above the Pittsburgh Coalbed. The thickness of the paleochannel varies between 0-13 m (0-40 ft). Regionally, the Sewickley 

Coalbed may split into two separate benches, and its 

height above the Pittsburgh Coalbed may vary. Gas 

contained in the fractured zone (Fig. 1), in particular, 

gas in the Sewickley Coalbed, primarily reports to 

the gob gas ventholes, if they are present and operational (Mucho et al. 2000). 

Figure 2. A generalized stratigraphic section of the strata 

above the Pittsburgh Coalbed in the study area (Mucho et

al. 2000).

2.2 Grid model generation for longwall sites 

In order to model the longwall mining process and 

analyze the associated methane control systems, a 

three-dimensional grid model of the mine site has to

be created. The horizontal dimension of the grid 

models were usually determined based on the problem type and the total area of interest. The number

of vertical layers and their thicknesses were based

on generalized stratigraphic sections for the mine 

site. 

The Pittsburgh Coalbed (mining) layer was constructed differently from the other layers in the grid 

model to host both the mined and unmined Pittsburgh Coalbed, and the entries surrounding the 

longwall panels. An example showing this structure 

for one of the study sites is given in Figure 3. 

Entries

Gob

Unmined

coal

Mining direction 

Figure 3. Pittsburgh Coalbed layer as represented in the

models. 

2.3 Gob gas ventholes and the pseudo-ventilation 

system 

A simplified version of the ventilation system was 

incorporated into the model. For each panel, a set of

wells injecting air into the entries with a rate constraint of 1700 m3

/min (60,000 cfm) represented the 

air intake part of the ventilation system. The ex-
hausting bleeder fan at the top of a 1.8-m (6-ft) diameter air shaft was modeled with a large-diameter 

vertical well on the tailgate side of the panel and operated with a bottom-hole pressure constraint of 1.36 

KPa (0.2 psia) negative pressure. 

The locations of gob gas ventholes in the model 

were determined based on their locations on the 

study panels, and they were configured based on 

their actual reported completion data. The ventholes 

are usually drilled to within about 12-13 m (40-45 ft) 

of the top of the Pittsburgh Coalbed at this mine site, 

and 17.8-cm (7-in) casing, with 61 m (200 ft) of

slotted pipe on the bottom is installed as shown for 

Venthole A, Figure 1. However, in some case, the 

ventholes were drilled closer to the mining horizon 

and into the caved zone (Venthole B, Fig. 1), which 

affects their performance, as will be discussed in the 

following sections. 

2.4 Geomechanical calculations for strata response 

and permeability changes 

The geomechanical, fast lagrangian analysis of continua (FLAC) model (Ithasca Consulting Group, Inc. 

2000) was used to evaluate the effects of longwall 

mining on the surrounding rock mass and to calculate mining-related permeability changes. 

Calculation of permeability changes was accomplished using the final stress and rock failure distributions from the FLAC model runs, and employing 

empirical relations [Ren & Edwards (2002), 

Lowndes et al. (2002)]. Details of the model development, permeability calculations, and geomechanical analysis are given in Esterhuizen & Karacan

(2005). 

2.5 Simulation and model-calibration strategy 

During longwall mining, the strata disturbances and 

the onset of production in successive gob gas ventholes move along with the face, This leads to a 

moving-boundary problem in modeling, which was 

addressed with “restart” models. Each model restart 

run was performed so that it would progress up to either the next venthole location or to a defined location on the panel for the distance and time characterizing the intervening face movement (Karacan et al.

in prep.). The reservoir-parameter changes were incorporated into the model as the face was advanced 

between restarts during calibration and prediction

runs. 

3 MODEL APPLICATIONS 

Figures 4 and 5 show the two grid models constructed for this study. In both figures, only the coalbeds are depicted to improve the visualization. The 

models were calibrated by matching the measured 

gas production rates, methane concentrations in the 

produced gas stream, and the flowing bottom-hole 

pressures. 

Gob gas ventholes 

Pittsburgh Coalbed (mining)

layer

Slotted 

casing

Bleeder fan

Elements of pseudo-ventilation 

system

Figure 4. Study mine Case-1, grid model of a multi-panel

mining site, Pittsburgh Coalbed. 

Sandstone Channel Sewickley Coalbed 

Pittsburgh Coalbed

Gob Gas Ventholes 

Bleeder

Air Intake and Return

1-4 1-3 1-2 

1-1

VH 1-2 VH 1-1 VH 1-3

VH 1-4

Figure 5. Study mine Case-2, grid model of a new mining district, Pittsburgh Coalbed. 

Case-1 (Fig. 4) was developed for a previous, 

multi-panel mining district, and was used to evaluate

the influence of various gob gas venthole completion 

parameters on methane capture. The grid model 

shown in Figure 5 (Case-2) was developed for the 

new mining district that started with 381 m (1250 

ft)-wide panels, but would be switching to 442 m 

(1450 ft)-wide panels. The focus of the Case-2 study 

was to estimate the increase in expected methane

emissions and to investigate alternative gob gas venthole completion and placement scenarios on the 

larger panel. 

3.1 Case-1, Evaluation of gob gas venthole 

completion parameters 

3.1.1 Effect of slotted-casing diameter 

The standard casing diameter for the gob gas ventholes in the study area was 17.8 cm (7 in). The ef-
fects of different slotted-casing diameters [25.4 cm

(10 in) and 10.2 cm (4 in)] on methane production 

were evaluated. The length of the slotted casing and

its setting depth above the top of the Pittsburgh 

Coalbed were held constant at their original design

values, 61 m (200 ft) and 12 m (40 ft), respectively. 

The modeling results given in Table 1 for the 

simulated mining period (910 days) predict that the 

cumulative methane production using the 25.4 cm

(10 in) casing will increase 4.9%, as compared to the 

17.8 cm (7 in) standard diameter casing. The amount 

of methane produced with the 10.2 cm (4 in) casing 

was about 6.7% less than that produced with the 

17.8 cm (7 in) diameter casing. However, the 

amount of mine air produced with the 10 in casing 

was 12.3% more, which resulted in a lower predicted methane concentration. Conversely, the 

amount of mine air produced with the 10.2 cm (4 in) 

diameter casing was 15.2% less, as compared to the 

standard casing, which resulted in higher methane 

concentrations. 

The predicted increase in cumulative methane 

production with the larger diameter wellbore was 

due to the increase in the open-to-flow area of the 

wellbore. Also, with larger diameter wellbores, the 

calculated pressure losses were less compared to 

smaller diameter wellbores. The predicted reduction 

in methane concentration with the 25.4 cm (10 in) 

diameter casing is most likely the result of more

mine air being captured due to an expanded pressure 

sink and associated depletion radius created by the 

production of gas from the larger diameter casing. 

However, since the total predicted gas production 

(methane and air) was higher for the 25.4 cm (10 in) 

diameter casing, it still resulted in higher cumulative 

methane production, even though the methane concentration was less. 

Table 1. Predicted effect of casing diameter on cumulative

methane and total gas production for 910 days of simulated 

mining.

Casing 

diameter 

inch

Cum. 

CH4, 

MMscf 

∆CH4 

compared 

to 7-in

casing, % 

Cum. Gas 

(CH4 + Air) 

MMscf 

∆Air compared to

7-in casing, % 

4 391.8 -6.7 609.4 -15.2

7 419.8 - 676.5 -

10 440.1 +4.9 728.4 +12.3

3.1.2 Effect of slotted casing length 

To evaluate the influence of the length of the completion interval on gob gas venthole performance, 

the length of the slotted casing section was changed

in the model to 30.5 m (100 ft) and to 76.2 m (250 

ft), as compared to the original 61 m (200 ft) length. 

The casing diameter was kept at 17.8 cm (7 in), and

the setting depth of 12 m (40 ft) above the top of the 

Pittsburgh Coalbed was maintained. The modeling

results predict that the cumulative methane production will increase with increases in slotted casing

length (Fig. 6). The methane production with 76.2-m 

(250-ft) of slotted casing was 459.4 MMscf, as compared to 391.8 MMscf with the standard 61-m (200-

ft) of slotted casing. This difference corresponds to a 

9.5% increase in methane capture from the four panels modeled in Case-1 (Fig. 4). However, when the 

slotted casing length was shortened to 30.5 m (100 

ft), the predicted methane production decreased to 

314.7 MMscf, which was about 25% less than what 

was captured with 61 m (200 ft) of slotted casing. 

0

50

100

150

200

250

300

350

400

450

500

0 200 400 600 800 1000

Time from Start of Mining (Days)

Cumulative Methane Production, MMSCF

100-ft slotted casing

200-ft slotted casing

250-ft slotted casing

Figure 6. Simulated cumulative methane production

from ventholes with varying lengths of slotted casing. 

3.1.3 Effect of slotted-casing setting depth 

The effect of casing setting depth (distance from the 

top of the mining layer) on gas production was investigated by modeling alternative completion

depths of 19.8 m (65 ft), 7.6 m (25 ft) and 4.6 m (15 

ft), as compared to the original, 12 m (40 ft) depth. 

In these alternative cases, the 7.6 m (25 ft) completion depth generally corresponded to a close proximity to the caved zone, which was modeled as 7.3 

m (24 ft) above the Pittsburgh Coalbed for the Case1 study site, and the 4.6 m (15 ft) depth corresponded to circumstances where the venthole was 

drilled into the caved zone. The 19.8 m (65 ft) completion depth corresponds stratigraphically to a 

depth slightly below the Sewickley Coalbed (Fig. 2). 

For these scenarios, the casing diameter and slotted

casing lengths were kept at their original design values, 17.8 cm (7 in) and 61 m (200 ft), respectively. 

Raising the slotted casing setting depth to 19.8 m 

(65 ft), as compared to 12 m (40 ft), above the Pittsburgh Coalbed resulted in a 4% predicted cumulative methane production increase. The predicted cumulative methane production declined by about 5% 

and 29% when the casing was set to within 7.3 m 

(25 ft) and 4.6 m (15 ft) of the top of the mining 

layer, respectively. In the 15 ft setting depth scenario, the lower slots of the casing were in the caved 

zone influenced by the mine ventilation system
where flow resistance was small. Therefore, the ventholes pulled 74% more mine air, as compared to the 

operator’s standard ~12 m (~40 ft) setting depth. 

Since most of the produced gas was mine air at the 

4.6 m (15 ft), the average methane concentration in 

the cumulative produced gas at the end of mining 

was about 40%, as opposed to 60-70% average 

methane concentration calculated for other depths 

(Fig. 7). 

A real-world example of the gas quality consequences of completing gob gas ventholes into the 

caved zone is illustrated with measured gas concentration data from two ventholes continuously monitored in the Case-2 study area (Fig. 8). For this site,

the height of the caved zone was estimated to be ~12 

m (~40 ft), higher than the Case-1 site shown in Figure 4, due to the presence of the sandstone paleochannel. The first venthole on the study panel (1-1, 

Fig. 5) was completed to a depth of 14.3 m (47 ft) 

above the top of the Pittsburgh Coalbed, generally

within the standard depth range for the mine site.

However, the second venthole drilled on the study 

panel (1-2, Fig. 5) was inadvertently drilled deeper 

to a depth of 10.6 m (35 ft) above the top of the 

Pittsburgh Coalbed, which is in the caved zone. As

shown in Figure 8, the methane concentration in the

produced gas from the venthole completed into the

caved zone averaged about 30% less than that of the 

standard completion depth above the caved zone due 

to the increased production of mine ventilation air. It 

should be noted that even though the estimated 

caved zone height for which the predictions for the 

influence of different casing setting-depths were 

made (Case-1) is slightly different than that at the

Case-2 study site where these actual field production

data were measured, a similar methane concentration

decrease (25-30%) was predicted for the venthole 

penetrating into caved zone for the Case-1 study 

(Fig. 7). 

20

30

40

50

60

70

80

90

0 200 400 600 800 1000

Time from the Start of Mining (Days)

Methane Concentration,

%

65-ft from the coalbed

45-ft from the coalbed

25-ft from the coalbed

15-ft from the coalbed

Figure 7. Simulated methane concentrations from ventholes completed to varying depths above the Pittsburgh

Coalbed.

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300

Time from the Start of Mining (Days)

Methane Concentration, %

Depth: 47 ft

Depth: 35 ft

Figure 8. Actual methane concentrations measured

from two gob gas ventholes completed to different depths. 

3.2 Case-2, Evaluation of longwall panel width on 

gob gas venthole performance 

The first two longwall panels in the new mining district at the study mine site were 381 m (1250 ft) 

wide with a length of approximately 3350 m (11,000 

ft). However, starting with the third panel, the panel 

widths were to be increased by 61 m (200 ft) to 442 

m (1450 ft). Due to the uncertainty of the methane 

emission consequences associated with mining of 

the larger panel, the area in question (Fig. 5) was 

modeled to estimate the expected increase in gas

flow and to investigate methane control options. The 

question to be answered was whether the current 

number and configurations of gob gas ventholes 

would adequately control the projected increase in 

gob gas on the larger longwall panels (Karacan et al. 

2005). 

3.2.1 Evaluation of the increase in methane emissions due to the mining of a wider longwall 

panel 

Modeling the increase in panel width from 381 m 

(1250 ft) to 442 m (1450 ft) results in about 47 

MMscf of additional methane liberation from the 

coal mined on the longwall face and 137 MMscf

from the overlying disturbed strata over the 268 days 

of mining simulated for this study. Depending on the 

availability of additional gob gas drainage capacity, 

some of the additional 137 MMscf of methane originating in the overlying strata may report to the ventilation system. This would represent the potential of 

up to about 355 cfm of additional methane entering 

the underground workplace. 

3.2.2 Analysis of expanding panel width from 381 m 

(1250 ft) to 442 m (1450 ft) with the four actual ventholes in operation 

The four actual gob gas ventholes on the first panel 

in this new mining district at the study mine site 
were drilled to varying depths [14, 11, 9, and 12 m 

(47, 35, 30, and 40 ft)] above the top of the Pittsburgh Coalbed, and, therefore, two of the ventholes 

were completed into or at the top of the caved zone,

as opposed to the preferred ~12 m (~40 ft) distance. 

For this analysis, the results of simulation runs for 

the 381 m (1250 ft) wide panel with the four actual 

gob gas ventholes were compared to the results 

when the panel width was increased to 442 m (1450 

ft), using the same venthole placement [91 m (300 

ft) from the tailgate side of the panel], completion, 

and production histories (Fig. 9). 

Tailgate 

Headgate

1250-ft

1450-ft

Scenario A 

1450-ft

Scenario B

Scenario C

1450-ft

Scenario D

Scenario E

1450-ft

Scenario F

Base Case Base Case

Optimal wells

Actual wells Mining direction

Figure 9. A schematic representation of gob gas venthole

configurations for the simulation of methane control options on wider longwall panels. 

The performance of the four individual gob gas 

ventholes on the simulated wider panel was very 

similar to those on the original panels since the

wellbore flow model is not dependent on panel 

width and due to the similar reservoir permeability, 

irrespective of the panel width as confirmed by the 

FLAC computations (Karacan et al. 2005). Since the 

performance (gas production potential) of individual 

gob gas ventholes is not influenced significantly 

with an increased panel width, the additional 137 

MMscf (355 cfm) of methane released from the 

overlying strata as a result of mining the larger panel 

will potentially enter the underground workplace, if 

additional methane drainage capacity is not provided. Thus, the constructed model (Fig. 5) was used 

to evaluate multiple scenarios to optimize the number and locations of the gob gas ventholes on the 

wider panel and to minimize the volume of additional methane entering the ventilation system. 

3.2.3 Alternative gob gas venthole placement scenarios and performance analysis for a 442 m 

(1450 ft) wide panel 

The simulated alternative gob gas venthole placement and completion scenarios investigated for this

study, as shown in Figure 9, were: (A) moving the 

four actual ventholes (as drilled and completed on 

the first panel in the Case-2 study area) to locations 

152 m (500 ft) from tailgate side, i.e. 61 m (200 ft) 

closer to the centerline of the panel than on the first 

panel; (B) adding four optimal (continuously operating with 2.7 psi suction pressure and completed to

12 m (40 ft) above the Pittsburgh Coalbed) infill 

ventholes located between each actual venthole; (C) 

adding four optimal ventholes located 90 m (300 ft) 

from the gateroads on the headgate side of the panel, 

positioned diagonally to the actual ventholes; and

(D) adding four optimal ventholes located 90 m (300 

ft) from the gateroads on the headgate side and positioned directly opposite the actual ventholes on the 

tailgate. 

Simulation runs showed that the optimal gob gas 

ventholes produced gas that is 85-90% methane 

through the entire mining period, as compared to the 

65-70% range for the actual wells during most of

their production history (due to close proximity of 

two of the holes to the caved zone). Optimal wells 

were also predicted to produce about 50% more 

methane than the actual wells. 

The data presented in Table 2 compare the cumulative methane production volumes from each of the 

gob gas venthole configuration scenarios shown in 

Figure 9. The lowest predicted cumulative methane 

production (Case A) is obtained when the four ventholes with the actual completions are located 152 m

(500 ft) from the tailgate entry of the 442 m (1450 

ft) panel. This configuration produces 0.37 × 106

 m3

(13 MMscf) less methane compared to the base case 

production from the 381 m (1250 ft) panel, thus illustrating the importance of near-margin venthole 

placement (Diamond et al. 1994). 

The highest predicted cumulative methane production is achieved when additional optimal infill 

ventholes are used (Table 2). Case D, with four actual ventholes on the tailgate side of the panel and 

four additional optimal headgate ventholes located 

directly across from the tailgate holes produces 3.91 

× 106

 m3 (138 MMscf) more methane compared to

the four actual ventholes on the 381 m (1250 ft) 

panel base case. Case C, with four actual ventholes 

on the tailgate side of the panel and four additional 

optimal ventholes located on the headgate side of the 

panel (but diagonal to the tailgate ventholes) is the 

next highest incremental producer of methane at 

3.37 × 106

 m3 (119 MMscf). While Cases C and D 

are similar, Case D is probably the higher producer 

because the tailgate and headgate ventholes are 

closer to each other (since they are directly opposite 

each other on the panel) and are intercepted by min-
ing at the same time. This scenario results in a 

quicker overlap of the venthole drainage radiuses, 

which enhances gas desorption from the overlying 

coalbeds associated with the subsided strata. Also, 

when headgate and tailgate ventholes are intercepted 

at the same time, the headgate ventholes start producing earlier and stay on production longer, as 

compared to the diagonal location in Case C, resulting in more methane production. 

One of the main considerations in gob gas venthole design and operation is to locate and drill the 

ventholes optimally and operate them continuously 

to minimize the number of ventholes while maximizing the production. Thus, Cases E and F were 

simulated to determine the minimum number of optimal ventholes that will produce the same maximum 

amount of methane as in the highest-producer configuration (Case D). In Case E, six optimal ventholes 

were placed along the tailgate side of the panel, and 

in Case F, two of the optimal ventholes were placed 

on the headgate side directly opposite the first two 

ventholes on the tailgate side (Fig. 9). 

Table 2. Cumulative predicted methane production differences from gob gas ventholes on a 442 m (1450 ft) wide 

panel.

Venthole placement 

scenario ID 

Cummulative methane production 

difference compared to 381 m 

(1250 ft) base case, MMscf 

442m (1450-ft) 

Base Case 

- 0.9 

A - 13 

B 113 

C 119 

D 138 

E 86 

F 134 

The predicted methane production performance 

of venthole configurations E and F using six optimal 

gob gas ventholes were compared to the performance of Case D, the highest predicted methane producer [3.91 × 106

 m3 (138 MMscf)] in the previous 

simulations. Table 2 shows that Case E, where six 

optimal ventholes were located along the tailgate 

side of the panel, produced about 2.44 × 106

 m3 (86 

MMscf) more methane than the 381 m (1250 ft) base 

case, whereas Case F, with two of the optimal wells 

on the headgate side, produced 3.79 × 106

 m3 (134 

MMscf) more methane. Thus, the six optimal ventholes of Case F produced almost as much methane 

as the eight (four actual and four optimal) ventholes 

of Case D. The predicted performance differences 

between Case E and F are due to the location of the 

ventholes and to the length of time they stay on production. 

3.2.4 Evaluation of the impact of gob gas venthole 

configurations on controlling ventilation system methane on a wider face 

Table 3 presents the predicted amount of uncaptured 

methane that will be available for flow to the ventilation system on a 442 m (1450 ft) wide panel with 

the various gob gas venthole configurations that 

have been simulated relative to the gob gas produced 

volume for the 381 m (1250 ft) base case panel. The 

greatest amount of uncaptured methane available for 

potential flow from the overlying disturbed strata to 

the ventilation system [4.19 × 106

 m3 or 0.179 m3/s 

(148 MMscf or 380 cfm)] is Case A, where the gob 

gas ventholes are placed an additional 61 m (200 ft) 

towards the center line of the panel. The minimum 

amount of uncaptured methane on the wider panel 

occurs with Case D (-1 MMscf) when the four actual 

tailgate ventholes are supplemented with four optimal headgate ventholes. The minimal uncaptured 

methane flow into the ventilation system for Case D 

(eight ventholes) is comparable to the 0.85 × 106

 m3

(3 MMscf) of uncaptured methane for Case F, where 

only six optimal ventholes are used. 

Table 3. Predicted uncaptured methane volumes potentially 

available for flow for simulated gob gas venthole configurations on the 442 m (1450 ft) wide panel. 

Venthole placement 

scenario ID 

Cummulative uncaptured methane 

volume compared to the 1250 ft 

base case, MMscf (cfm) 

442 m (1450-ft) 

Base Case 

137 (355) 

A 148 (380) 

B 25 (65) 

C 17 (44) 

D -1 (-) 

E 50 (130) 

F 3 (10) 

4 SUMMARY 

Comprehensive “dynamic” reservoir models for 

simulating gas flows associated with the longwall 

mining environment have been developed. The 

models were constructed based on reservoir parameter data available for the study site and through history matching techniques for those parameters that 

were not easily obtainable. Simulating the mining 

component of the model, and updating the changing 

reservoir parameters due to strata disturbance, were 

addressed with the use of “restart” models, which 

were configured based on a schedule built from the 

actual longwall face advance and gob gas venthole 

interception times. 

Theoretical results indicated that, while keeping 

the other completion parameters constant, increasing 

the casing diameter increased cumulative methane 

production from subsided strata. However, a mar-
ginal decrease in the produced methane concentration was evident, possibly due to increased mine-air 

extraction. It was demonstrated that longer slotted 

casing lengths produced more gob gas, and thus 

more methane, depending on the geology and the 

presence of gas bearing strata in the horizons adjacent to the extra slotted casing length. Most importantly, it was demonstrated that casing setting depth 

played an important role relative to the methane 

concentration in produced gas stream and the volume of methane captured. Modeling results showed 

that when the setting depth was within the estimated 

caved zone, the methane concentration in the produced gas decreased by about 30%. Conversely, 

raising the casing setting depth into the fractured 

zone above the caved zone increased methane production. 

The models were also used to predict the methane 

emission consequences of mining a wider longwall 

panel in the Pittsburgh Coalbed. It is predicted that 

increasing the panel width from 381 m (1250 ft) to 

442 m (1450 ft) would result in about 3.88 × 106

 m3

(137 MMscf) from the overlying disturbed strata, resulting in an average of about 0.167 m3

/s (355 cfm) 

of additional methane potentially entering the ventilation system over the 268 days of simulated mining 

for this study. The importance of completing and 

operating the ventholes optimally to increase their 

methane control capability was also demonstrated. 

The model simulations predicted that six optimally 

completed, placed, and operated ventholes could 

produce the same amount of methane on the wider 

panel as eight non-optimal wells would produce. 

Reservoir modeling has been shown to be a viable approach for evaluating methane emission and 

control issues in the longwall mining environment. 

This approach is far superior to the traditional trialand-error methods, and has the capability of addressing unexpected methane emission problems as they 

evolve. 

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 for Occupational 

Safety and Health. 

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