Reservoir simulation-based modeling for characterizing longwall 

methane emissions and gob gas venthole production 

C.O. Karacan , G.S. Esterhuizen, S.J. Schatzel, W.P. Diamond 

National Institute for Occupational Safety and Health (NIOSH), PiPinsburgh Research Laboratory, United States 

Abstract 

Longwall mining alters the fluid-flow-related reservoir properties of the rocks overlying and underlying an extracted panel due to 

fracturing and relaxation of the strata. These mining-related disturbances create new pressure depletion zones and new flow paths for 

gas migration and may cause unexpected or uncontrolled migration ofgas into the underground workplace. One common technique to 

control methane emissions in longwall mines is to drill vertical gob gas ventholes into each longwall panel to capture the methane 

within the overlying fractured strata before it enters the work environment. Thus, it is important to optimize the well parameters, 

e.g., the borehole diameter, and the length and position of the slotted casing interval relative to the hctured gas-bearing zones. 

This paper presents the development and results of a comprehensive, "dynamic," three-dimensional reservoir model of a typical 

multipanel Pittsburgh coalbed longwall mine. The alteration of permeability fields in and above the panels as a result of the mininginduced disturbances has been estimated from mechanical modeling of the overlying rock mass. Model calibration was performed 

through history matching the gas production &om gob gas ventholes in the study area. Results presented in this paper include a simulation 

of gas flow patterns from the gas-bearing zones in the overlying strata to the mine environment, as well as the influence of completion 

practices on optimizing gas production from gob gas ventholes. 

Keywod: Longwall mining; Reservoir modeling; Ventilation; Gob gas ventholes; Reservoir simulation 

1. Introduction 

Longwall mining is an underground mining method 

that can maximize coal production in coalbeds that are 

continuous with few geological discontinuities. In these 

operations, a mechanical shearer progressively mines a 

large block of coal, called a panel, which is outlined with 

development entries or gate roads. This is a continuous 

pmess in an extensive area, where the roof is supported 

only temporarily during mining with hydraulic supports 

that protect the workers and the equipment on the face. As 

the coal is extracted, the supports automatically advance 

with the rate of mining, and the roof strata are allowed to 

cave behind the supports. 

It has been suggested (Singh and Kendorski, 1981; 

Palchik, 2003) that the caved zone (Fig. 1) created by 

longwall mining is highly fragmented, and generally extends upwards three to six times the thickness of the mined 

coalbed. In this zone, the overlying rock layers fall into the 

mine void and are broken into irregular shapes of various 

sizes. It has been found that the height of the caved zone 

could reach four to eleven times the thickness of the mining 
height where overburden rocks are weak and porous

(Palchik, 2003).

Above the caved zone is a fractured zone (Fig. 1)

characterized by mining-induced vertical fractures, and

horizontal fractures caused by separations along bedding

planes. The caving of the mine roof causes an area of

relieved stress in this zone. The blocks in each of the broken

rock layers are contacted either fully or partially across the

vertical fractures, with the number and extent of the fractures diminishing with increasing height above the caved

zone. The thickness of the fractured zone can vary up to 100

times the height of the mined coalbed, depending on the

characteristics of the associated rock strata, thickness of the

overburden, and the size of the longwall panel (Palchik,

2003).

The occurrence of such an extensive area of mininginduced stress relief and resultant rock damage changes the

gas flow-related properties in the overlying (and in some

cases the underlying) strata, particularly the permeability.

Any gas that is contained within the coalbeds in this area of

relieved stress will be released slowly over time, while free

gas in other porous formations, such as sandstones, will be

released more quickly, as the permeability of these zones is

dramatically increased and new permeability pathways are

created. Relaxation of the roof and floor rocks and the

associated fracture connectivity allows gas to flow from all

surrounding gas sources toward the low-pressure sink of

the underground workings, including the caved zone. Fig. 2

shows a super-critical longwall panel, where the panel

width is larger than overburden thickness, and flow paths

for gas migration to the active mine workings and gob gas

ventholes after subsidence. Gas-bearing strata, particularly

overlying gas-bearing coalbeds and sandstones, can be

directly exposed to the caved zone or connected to it by

fractures. In the absence of methane drainage boreholes

such as gob gas ventholes, this released gas, commonly

Fig. 1. Schematic of strata response to longwall mining (modified from Singh and Kendorski, 1981).

Fig. 2. Schematic of a supercritical panel situation and the gas migration paths.
ggy p

from above.

In general, experience suggests that it is difficult to

make accurate gas production estimates for gob wells and

that these predictions are underestimated relative to actual

gob gas venthole production by a factor of two or more.

The key factors to this underestimation of gas production

capability are the difficulty in predicting the degree of

stress relief in gas-bearing strata associated with longwall

mining and, consequently, the drainage radius for each

well (Zuber, 1998). To better understand the flow of gas in

the longwall mining environment, a reservoir modeling

study was initiated for a mining area in the Pittsburgh

coalbed in southwestern Pennsylvania for which the

National Institute for Occupational Safety and Health

(NIOSH) had a substantial data set for gob gas production.

1.1. Longwall gob gas ventholes

Gob gas ventholes are drilled into the overburden

above longwall panels to capture the gas released from the

subsided and relaxed strata before it enters the mining

environment, where it can be an explosion hazard. Most

gob gas ventholes are drilled within a short distance, 10–

30 m (30 ft–100 ft), of the coalbed being mined and cased

with steel pipe. Commonly, the bottom section of the

casing [generally about 60 m (200 ft)] is slotted and placed

adjacent to the expected gas production zone in the

overburden strata. The usual practice is to drill the gob gas

ventholes from the surface prior to mining. As mining

advances under the venthole, the gas-bearing strata that

surround the well will fracture and establish preferential

pathways for the released gas to flow towards the

ventholes (Diamond et al., 1994).

Exhausters are placed on gob gas ventholes to maintain

a vacuum on the wellbore, so that they operate at the

minimum possible flowing pressure and create a pressure

sink in the overburden strata to induce gas flow towards

the venthole. Gas production may exhibit variable gas

quality. In the early stages of production, the gas quality is

generally high (N80%), and there is very little contamination by mine ventilation air. Maximum daily methane

production generally occurs within the first several days

after a hole is intercepted by the longwall. Relatively high

production rates are usually sustained for only a few

weeks or in some cases for a few months (Diamond et al.,

1994). Later in time, gob gas production may exhibit

decreased methane levels as ventilation air is drawn from

the active mine workings.

The quality of the gas from gob wells can be controlled

to some extent by varying the vacuum on the well to

correspond with the profile of expected methane release.

yg 

centration in the mine within statutory limits is always the

overriding factor for controlling the vacuum on the gob gas

ventholes, as it is for all other mine-related methane

drainage systems (Zuber, 1998; Mucho et al., 2000).

Commonly, when the methane concentration in the

produced gas reaches 25%, the exhausters are de-energized

as a safety measure, and the holes may be allowed to free

flow.

The location of the ventholes on the panel is important,

as Diamond et al. (1994) showed in a study of a Lower

Kittanning coalbed longwall mine where the holes on the

ends of the panels were generally the highest-quantity and

longest-duration producers. This was attributed to

enhanced mining-induced fractures on the ends of the

panels where the overburden strata are in tension on three

sides due to the support of the surrounding pillars. This

observation led to the experimental placement of gob gas

ventholes in the zone of tension along the margin of a

panel, instead of in the traditional centerline location,

which is in compression due to subsidence and recompaction of the longwall gob. Analysis of seven

months of gas production data indicated that the

experimental near-margin holes produced 77% more gas

than did centerline holes on the same panel (Diamond

et al., 1994).

Improvements in venthole gas drainage evaluation and

prediction capabilities for site-specific mining conditions

and circumstances can address longwall gas emission

issues, resulting in ventholes designed for optimum

production and mine safety. To improve gas capture at a

reasonable cost, it is important to understand the behavior

of the entire gob gas venthole system, including the

venthole placement and completion strategies, the

reservoir properties of the gob in the caved zone behind

the face, the fractured rock mass around the workings,

and, finally, the ventilation system. A theoretical reservoir

modeling approach is the best, if not the only means to

predict methane emissions in advance of mining (Noack,

1998) under varying conditions and to design drainage

systems accordingly for either reducing the chance of

unexpected methane emissions or for responding more

quickly and effectively to unknown conditions encountered during longwall mining.

1.2. Modeling approach for optimizing gob gas venthole performance

A detailed model to realistically represent the multiple

variables associated with underground coal mining

operations and their interaction and influence on the

performance of gob gas ventholes does not currently exist.
y() p

tional fluid dynamics (CFD) modeling approach to

investigate gas capture from surface gob gas ventholes.

That paper introduced how this approach could be used to

improve the design of surface gob wells for methane

recovery while minimizing the leakage of air into the gob.

The results indicated that the position of the wells plays an

important role in determining gas production rate. Wells

drilled into the areas where fractures extend towards the

methane source bed were likely to achieve higher capture

efficiency. Lunarzewski (1998) used boundary element

and sequential bed separation methods for floor and roof

strata relaxation and immediate roof bending separation,

as well as gas emission rate calculations to develop

“Floorgas” and “Roofgas” simulation programs to

characterize the strata relaxation zones, gas emission

boundaries, and parameters for gas emission prediction.

Tomita et al. (2003) developed a three-dimensional (3-D)

finite element model (FEM) to predict the volume of

methane gas emitted from surrounding coal and rock

layers based on stress distribution and permeability

change.

Reservoir modeling methods and simulators have been

developed over the years that can realistically represent the

complex physics of reservoir flow mechanisms in

unconventional reservoirs, such as coalbeds, and gas

production operations with diverse well completions (King

and Ertekin, 1991). These simulators have been successfully applied in various coal basins for gas recovery from

coalbeds using both vertical and horizontal boreholes

(Ertekin et al., 1988; Young et al., 1991; Young et al., 1993;

Zuber, 1998). However, modeling of the reservoir behavior

and prediction of gob gas venthole performances during

active longwall mining requires that additional variables be

considered that are not encountered in a static reservoir

environment. These considerations are due to the moving

boundary conditions and geomechanical response of rock

units to the stresses and strains imposed by an advancing

longwall face, and by the changing reservoir properties

above a panel as a response to mining.

In one study, Zuber (1997) coupled the moving

boundary condition due to mining and modeled the face

and rib emissions during development mining. However,

this study was mainly conducted on a single grid layer to

analyze the gas emissions from ribs and newly exposed

coal face during development mining, and thus did not

consider the longwall mining environment and methane

control issues associated with gobs. In a more recent

study, Karacan et al. (2005) developed a 3-D model of a

new longwall mining district to simulate the effects

longwall panel width would have on methane emissions

and the performance of gob gas ventholes. The focus of

p

methane emissions to be expected due to increasing panel

widths and optimizing gob gas venthole completion and

placement practices to capture more of this gas to prevent

it from entering the underground workplace.

2. Objective and description of this study

This paper presents the development and results of a

comprehensive, “dynamic,” 3-D reservoir model of a

typical multi-panel Pittsburgh coalbed mine using Computer Modeling Group's (2003) compositional reservoir

simulator (GEM). The alteration of permeability fields in

and above the panels as a result of the mining-induced

disturbances has been estimated from mechanical modeling

of the overlying rock mass using FLAC-2D (Itasca Consulting Group Inc., 2000), a finite difference model for

simulating the mechanical behavior of rocks in response to

mining-induced stresses, as described in more detail in

Esterhuizen and Karacan (2005). The moving boundary

problem imposed by the advancing longwall face has been

addressed with many “restart” models, where the output of

the previous run is saved in a file to be used by the succeeding run as the input, within the approach. A pseudoventilation system has been incorporated into the model to

investigate its interactions with the gob gas reservoir.

Model calibration was performed through history matching

of the gas production from the existing ventholes in previously mined longwall panels at the mine site. Results

presented in this paper include a simulation of possible gas

emission sources, as well as the influence of gob gas venthole completion practices on optimizing gas production.

3. General description of the study area and the

mine

The study mine is located in the Appalachian Basin

in southwestern Pennsylvania, which contains one of the

largest mineable coal deposits in the world and is a

promising region for coalbed methane production by

either conventional coalbed methane (CBM) wells or

from the gob gas ventholes associated with the extensive

coal mining operations in the basin.

The multi-panel mining area covered by panels G

through K (Fig. 3), was selected for this 3-D modeling

study because of data availability and previous NIOSH

tracer gas studies that were conducted to characterize gas

flows in the longwall gob in this area. Overburden depths

ranged between 150 and 270 m (500 and 900 ft). Longwall

panels in the primary study area were initially 253 m

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

with F Panel (just above G panel in Fig. 3). Thus, the
panels are super-critical, i.e., the panel width is greater than

the height of the overburden, which results in a more

complete caving of the overburden strata into the mine

void. A generalized stratigraphic section of the strata above

the Pittsburgh coalbed in the study area is shown in Fig. 4.

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, and they are believed to be

the primary source of strata gas in the area. Within this

interval, the thickest coalbed is the Sewickley coalbed,

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 sometimes-present Redstone coalbed. The Upper and Lower Waynesburg coalbeds with a total thickness of about 2 m (7 ft) are located

about 95 m (310 ft) above the Pittsburgh coalbed.

Although the Pittsburgh coalbed is the primary coalbed

gas reservoir in this section, the Sewickley and Waynesburg coalbeds have also been reported to be CBM producers in this area (Bruner et al., 1995).

Methane control in the study mine longwall district

includes the bleeder ventilation system, gob gas

Fig. 4. A generalized stratigraphic section of the strata above the Pittsburgh coalbed in the study area.

Fig. 3. Layout of the longwall mining area selected for modeling.
g

age boreholes. The bleeder ventilation system includes

the peripheral bleeder entries surrounding the panels, the

former gateroads between the mined-out panels, and

the associated bleeder fan shaft(s). The bleeder fans in

the primary study area are at the top of 1.8 m (6 ft)

diameter air shafts. Gob gas ventholes at this mine site

are generally drilled to within 12 m (40 ft) of the top of

the Pittsburgh coalbed, and 17.8 cm (7 in.) casing with

61 m (200 ft) of slotted pipe on the bottom is installed,

as shown in Fig. 5 (Mucho et al., 2000).

4. Reservoir model development for longwall mining

The reservoir model was constructed using the GEM

software, which is an efficient, multidimensional, equation-of-state (EOS) compositional simulator that can

simulate the dual-porosity behavior of coal seams for

coalbed/enhanced coalbed methane recovery. This section

describes the approach followed to build a reservoir model

to simulate the dynamic process of longwall mining.

4.1. Grid model of the study area

For the current study, a three-dimensional grid model

of the mine area shown in Fig. 3 was created using

gg

based on the area covered by the five longwall panels

(G, H, I, J, K) in the selected study area, and the

development and bleeder entries between them. At this

mine site, each panel was 305 m (1000 ft) in width and

3500–4000 m (12,000–13,000 ft) in length, except the

K panel which was 2130 m (7000 ft), and the total width

of each set of three entries on either side of the panels,

including the gate roads and the pillars, was determined

from the mine maps to be about 61 m (200 ft).

The number of vertical layers and their thicknesses were

based on the generalized stratigraphic log of the area shown

in Fig. 4. In the grid model, the top-most layer of the model

was assigned to the Waynesburg coalbed, and the bottommost layer was the Pittsburgh coalbed, or mining layer.

Floor strata have not been included into the model in this

study. The other lithologies shown in the log were

represented in the grid model based on their thicknesses

and their sequence as a function of depth. Although this log

presents only major generalized stratigraphic layers in the

study area, it was sufficient to determine the number of

vertical layers and their thicknesses for generating the grids

in the vertical direction. Also, the layers shown in Fig. 4

were assumed to be uniformed in thickness and continuous

throughout the field. Thus, the structure map for the top of

the Pittsburgh coalbed (mining layer) was digitized as the

reference elevation, and the depths of other layers were

determined based on their thicknesses.

In the grid model, the Pittsburgh coalbed layer was

constructed differently from the other layers to include the

details of the longwall mining operation, the related

mining environment, and the lower part of the caved zone.

This layer was constructed in such a way that it would host

both the mined and unmined Pittsburgh coalbed, and the

entries that surrounded the panels. The entries were

represented by a single grid rather than a detailed gridding

for each gate road at the tailgate and headgate sides to

simplify these underground transport and ventilation

airflow pathways. Thus, single grid entries represent the

combined effects of the three-entry system.

Fig. 6 shows the 3-D grid model that was constructed for

this study. The figure shows only the Waynesburg coalbed

(top layer) and Pittsburgh coalbed (bottom layer) layers.

The other layers between the Waynesburg and Pittsburgh

coalbeds have been removed for a better visualization of the

wellbores used in the model. The stratigraphic log was

inserted to the right of Fig. 6 for reference purposes. The

bottom figure shows the grid construction for the mining

layer, including the specific features discussed in the previous paragraphs. Fig. 6 also shows the gob gas ventholes

and the elements of the pseudo-ventilation system

described in the following section.

Fig. 5. Typical gob gas venthole completion scheme used in the study

mine area.
4.2. Gob gas ventholes and the pseudo-ventilation

system

To include a simplified version of the ventilation

system (called pseudo-ventilation system in this paper)

in a longwall mining area, a set of horizontal wells

representing the ventilation air input and a vertical well

representing the bleeder fan were built into the grid

model to represent the ventilation airflows to dilute

methane in the entries and in the gob (caved zone). The

horizontal wells were designed to inject nitrogen into the

entries with a rate constraint to simulate the typical

airflow rate at the mine site. One horizontal well was

dedicated to each panel for ventilation purposes (Fig. 6).

Each of the horizontal wells were operated sequentially

for the time period required for mining of the associated

panel as the changing longwall face location was

simulated during model runs, as will be described in

the following sections.

The bleeder fan was modeled with a vertical well in the

grid model at the tailgate side of the G panel, with the

completion interval being equal to the height of the entries

Fig. 6. 3-D representation of grid model of the study area that shows Waynesburg (top layer) and Pittsburgh (bottom layer) coalbeds. Figure also

shows gob gas ventholes, their placements, completion representation, and elements of pseudo-ventilation system. Lower figure shows the panels in

mining layer (Pittsburgh coalbed) and the grids of certain mining-area regions.
(

g)g

diameter air shaft, the diameter of the well was assigned to

the maximum wellbore diameter allowed in the model of

0.74 m (2.5 ft). During simulations, the bleeder fan was

operated with bottom-hole pressure constraints to control

the mine ventilation pressures. The bleeder fan and the

elements of the pseudo-ventilation system are shown in

Fig. 6. The locations and completions for the gob gas

ventholes (Fig. 6) were determined based on their actual

locations and their individual completion records.

4.3. Reservoir characterization and data sources

The construction of dual-porosity reservoir models in

general requires detailed consideration of the geology

and related reservoir-rock properties. The key concept in

this study was the incorporation of parameters that

realistically represent reservoir properties and their

changes in response to longwall mining within the

model. Most of the data required by the simulator for

this study was gathered from previous NIOSH publications documenting research activities in the surrounding

area, as well as external reports and personal communications with the operating mining company for setting

the initial, pre-mining reservoir properties to the coal

and non-coal units. However, since reservoir properties

were not available for each individual layer in the model

grid, particularly for non-coal layers, the same properties were assigned for all similar lithologies.

In the central and northern Appalachian Basin coalbeds,

face and butt cleats are perpendicular and parallel,

respectively, to the fold axis. In the northern Appalachian

Basin, the face cleat of the Pittsburgh coalbed rotates from

N 80° W in northwestern West Virginia to N 57° W in

southwestern Pennsylvania (McCulloch et al., 1974),

where the study mine was located. This rotation of the

cleat direction in the field enabled close orientation of the

cleats in the direction of grids.

Cleat spacing is an important reservoir property for

modeling gas flows in coalbeds. Law (1993) reported

cleat spacings of 0.5–9.7 cm in the northern Appalachian

Basin. In this study, cleat spacings were taken as 3 cm

(1.2 in.), which was a value based on the reported mean

cleat spacing values of 2.4 cm (0.94 in.) and 3.20 cm

(1.26 in.) from outcrops of the Pittsburgh and Sewickley

coalbeds, respectively (Law, 1993). The gas content and

adsorption-related data for the Pittsburgh coalbed and

other major coalbeds in the area were obtained from direct

method gas content determination tests (Diamond et al.,

1986), and the adsorption data were obtained from

available adsorption isotherms. For the coal layers for

which no adsorption isotherms data were found (i.e.,

g)pp

were assumed to be the same as for the Pittsburgh coalbed.

In this study, history matching techniques were used

to estimate the undisturbed or initial permeabilities for

major coalbeds and the permeability of the caved zone

in the longwall gob. The fracture permeabilities for the

Pittsburgh, Sewickley, and Waynesburg coalbeds were

estimated to be 4 md in the face-cleat direction and 1 md

in the butt-cleat direction. In the simulations, these

initial values, like others, were assumed to be uniformed

throughout the individual layers.

The permeability of the caved zone in the longwall gob

is not easily predictable and little data are available in the

literature. For this study, the permeability value was

determined as a result of the overall model calibration

procedure, using geomechanical calculations generated

by the FLAC (Fast Lagrangian Analysis of Continua,

Itasca, 2000) program (to be discussed in greater detail in

the following sections), and by verifying the estimated

values for methane concentrations (b1%) in the caved

zone grids near the face location as calculated by the

simulator. This approach resulted in an estimated

permeability of about 10−9 m2 (106 md) in the horizontal

direction and 10−11 m2 (104 md) in the vertical direction

for the caved zone. These permeability values fall into the

range of open-jointed to heavily fractured rock in the

horizontal direction and into jointed rock classification in

the vertical direction (Hoek and Bray, 1981).

Brunner (1985) investigated the migration of air

through longwall gobs using a ventilation model, and

constructed a laminar resistance grid over the caved zone.

He assigned a resistance value to each resistance element

in the grid. The permeability values he used and tested in

his resistance model for the gob ranged between 10−5 and

10−7 m2 (1010 and 108 md). However, he did not consider

any distinction between vertical and horizontal permeability. Based on fractured porous medium vs. permeability classification, these values fall into the range of

heavily fractured rock (Hoek and Bray, 1981).

In the model, the entries surrounding the longwall

panels serve as main pathways for ventilation airflow.

Thus, the permeabilities for this portion of the Pittsburgh

coalbed grid layer (three entries plus the associated coal

pillars combined) were assigned an average high value

[10−7 m2 (108 md)] within the allowed limits of the

simulator for minimum resistance. The fracture porosity

and fracture spacing required for the entry area were

calculated from the mine maps. For these calculations, the

pillars were conceptualized as solid structures and the

roadways were conceptualized as fracture openings.

Thus, from the area ratios of entries to the total area

(pillars and entries) in a unit length of the section, an
approximate area of 40% (or volume) occupied by

fractures was calculated. The length of one side of a

pillar, which was about 30 m (100 ft), was taken as the

fracture spacing for the entries. Table 1 gives a few of the

representative reservoir parameters that were calculated,

obtained from the literature, or estimated from calibration

runs and used in the model. These values, particularly

permeabilities, represent the values prior to mining.

p

modeling of longwall mining process is the estimation

of permeabilities or change of competent rock properties

in the overlying strata of the panel during and after mining

disturbance has occurred, and including these changes to

the reservoir model at different face positions. These

changes were predicted by geomechanical techniques and

their effects on reservoir permeabilities were estimated as

discussed in the following section.

4.4. Geomechanical modeling of permeability changes

The effect of longwall mining on the permeability of the

surrounding rock was evaluated using the FLAC finite

difference program. The program has the capability to

model the mechanical behavior of rocks in both the preand post-failure modes, can model the caved rock as a

strain hardening material, and simulates the mining process

as a sequence of interrelated modeling steps. Rock failure

is allowed to take place in response to the stress redistribution around the longwall panel. Failure may occur

in the overburden strata either by shearing along bedding

planes or by fracturing through intact rock material.

Rock strength values used in the FLAC model were

selected from the results of extensive mechanical

property testing of coal measure strata, as reported by

Fig. 7. Vertical section of the stratigraphy at the mine site, the caved rock, and the direction of mining as modeled using FLAC.

Representative prior to mining reservoir-rock properties used in the

study

Parameter Pitt.

coal

Sandstone Limestone Shale Entries

Permeability-x (md) 4 20 2 0.2 9 × 107

Permeability-z (md) 0.25 20 2 0.1 9 × 107

Effective porosity

(fraction)

0.04 0.1 0.02 0.01 0.4

Eff. fracture spacing

(m)/(ft)

0.03/

0.01

15/50 60/200 60/

200

30/100

Langmuir P.

(MPa)/(psi)

2.25/

326

– – ––

Langmuir vol.

(cc/g)/(scf/ton)

15.5/

490

– – ––

Desorption time

(days)

20 – – ––

Coal density (g/cc) 1.35 – – ––
() ()

The model calculated the strength of the intact material

as well as the strength of the bedding planes by using a

ubiquitous joint approach in which bedding planes are

assumed to be uniformly distributed throughout the rock

mass. Rock failure was based on the Coulomb failure

criterion, with strain softening and dilation occurring

once the rock has failed. The model runs were taken to

equilibrium at each step of the simulated mining

process, so that rock failure and stress re-distribution

would occur. The modeling steps included a sufficient

lateral extent of mining, so that full subsidence of the

overburden strata would occur over the mined area.

Two-dimensional FLAC models were created to

simulate both longitudinal and cross-sectional profiles

through an active longwall panel. The models included

the rock mass from 100 m (328 ft) below the Pittsburgh

coalbed up to the ground surface. The initial vertical

stress in the model increased with depth according to the

overburden load, while relatively high horizontal

stresses were imposed as described by Dolinar (2003).

Rock mass parameters were included in the model, with

each layer having a specific strength, stiffness, and initial

stress state. The caved rock was simulated to a height of

four times the longwall extraction height, 7.3 m (24 ft) in

pp

model in response to longwall mining using failure

criteria and strength properties appropriate for the

individual strata layers. Fig. 7 shows an example of a

longitudinal model.

The fracture permeability of rock strata is highly

dependent on the in situ stress, and most importantly in a

mining environment, mining-induced changes in stress

with time (Hoek and Bray, 1981; Lowndes et al., 2002).

An exponential relationship was used to compute the

effect of stress changes on the rock mass permeability

after the work of Ren and Edwards (2002) and Lowndes

et al. (2002). Permeabilities were calculated independently for the horizontal and vertical directions:

kh 1⁄4 khoe−0:25ðryy−ryyoÞ ð1Þ

and

km 1⁄4 kmoe−0:25ðrxx−rxxoÞ ð2Þ

where kho and kvo are initial permeabilities, σxx and

σyy are the prevailing stresses and σxxo and σyyo are the

initial stresses in horizontal and vertical directions,

respectively. Figs. 8 and 9 show the results of a FLAC

model for the horizontal and the vertical permeabilities,

Fig. 8. Vertical section showing vertical stress contours (in MPa) and the horizontal permeability distribution as a result of longwall mining

determined by FLAC modeling.
respectively. In these figures, the vertical stress contours

are also shown. A low permeability zone is indicated

ahead of the advancing longwall face in the unmined

coalbed, and higher permeability is present in the caved

zone behind the face in the gob, and decreasing further

away from the face as the subsided strata is recompacted over time. Further details of FLAC model

development for permeability calculations are given in

Esterhuizen and Karacan (2005).

4.5. Scheduling of runs for “dynamic” reservoir

modeling for longwall mining

Longwall mining and the response of overlying strata

to mining disturbances are dynamic processes. As the

mining face advances, the reservoir properties, especially the fracture permeabilities of individual layers,

change based on their strength and based on how much

stress they are exposed to. Production from each gob gas

venthole usually starts soon after the location is

undermined by the longwall and can continue for a

significant length of time, even after the panel is

completed (Diamond, 1994) because of the protracted

influence of the mining-induced strata disturbances.

This situation leads to a moving-boundary-value

problem. Two methods can be used to handle this

problem: one is at the differential equation level, and the

other is at the coefficient matrix level in which

transmissibilities are treated in such a way that fully

mined cells are assigned the mine pressure and partially

mined cells are treated as constant-pressure boundaries

(King and Ertekin, 1991). However, this situation seems

more applicable to simulation of single layers, where

only the production from completions in unmined

coalbeds is considered.

In this study, the moving boundary problem in 3-D

was handled with 15 different “restart” model runs in the

simulation. These restarts were run sequentially, each

characterizing a stage in the mining/face advance and

associated strata disturbances. Each restart run (mining

step) was performed in such a way that mining would

progress up to the next venthole location for the distance

and time characterizing the face movement between

successive ventholes. In this process, the simulation

outputs from the previous model run are written to a

“restart” file, then used by the next model run as updated

longwall face and reservoir parameters for the next face

position. Fig. 10 schematically presents the scheduling

Fig. 9. Vertical section showing vertical stress contours (in MPa) and the vertical permeability distribution as a result of longwall mining determined

by FLAC modeling.
of face positions and the actual time (in days) when each

gob gas venthole started production.

5. Model calibration through history matching

Model calibration through history matching can be

used with accurate field and laboratory data for

determining the remaining unknown reservoir properties and for predicting future well performance (Zuber

et al., 1987). In this study, the known gas flow rates and

gas composition from existing gob gas ventholes were

history matched, while constricting the predicted

bottom-hole pressures of gob gas ventholes around or

below atmospheric pressures, as observed in the field, to

estimate the disturbed and undisturbed permeabilities of

fractured strata as a result of mining, a coal desorption

time constant, and the permeability of the caved zone.

The fractured rock permeabilities, or permeability

changes, calculated by FLAC computations, were

incorporated into the model to update the properties of

the regions affected by mining as the longwall face was

advancing. Advancement of the longwall face was

simulated during history match runs and became an

integral part of the matching procedure. This was

basically a trial-and-error approach, where a “dynamic”

longwall mining simulation run was made using all

known and fixed values and the “initial estimates” for the

unknown values. Then, the simulated gas production rate

and concentrations from each gob gas venthole were

compared with the actual data.

Based on the comparison between simulated and

actual productions for the entire period of simulation,

adjustments were made to the unknown parameters, and

additional runs were completed until an acceptable

agreement was achieved. While adjustments were being

made to the unknown parameters during the history match

runs, the gob gas ventholes were operated with the

targeted gas rates (the observed daily rates given on a

weekly basis) as the well control constraint, and wellbore

bottom-hole pressures and produced gas compositions

were computed by the simulator. The acceptance criteria

for the calculated pressures were that they should be lower

than, but close to atmospheric pressures, as they normally

should be in the longwall gob producing reservoir. This

alternative approach was taken due to the lack of reported

well bottom-hole pressure data for the gob gas ventholes,

since these parameters are not commonly measured as

part of the collected production data at most mine sites.

Although the model had been set up for two-phase

flow, the water phase was mostly treated as immobile,

except in the caved section, where the available water in

the fractured strata was allowed to drain into gob during

caving, increasing the water saturation in the caved zone

by 5%. This was an estimated value, but the cooperating

Fig. 10. Representation of panels in Pittsburgh coalbed layer and face position and time scheduling for “dynamic” modeling of longwall mining in the

panels. The numbers show the time (in days) that passed since mining start in G panel. The distance and time interval between each solid bar represent

the face advance and the duration it takes for each “restart” model run. The scheduling was made based on mine reports and the start of gas production

from each gob gas venthole.
g py y 

production from any of the ventholes, and confirmed a

general lack of water influx into the entries from the roof

and the gob. Although there might be regional variations, these observations in the underground mining

environment suggest that this region has been desaturated and depressurized during its geological past, as

well as from the extensive coal mining and oil and gas

production activities, as further inferred by Hunt and

Steele (1991).

Figs. 11 and 12 show the actual and simulated gas

production rates and methane concentrations for one

venthole on each longwall panel in the study area. As can

be seen from these figures, the gas production rate data

match well for most of the data points, and the methane

concentrations can be matched for average values and

general trends. The shifts in time scale for each venthole to

become productive are caused by, and depend on, when the

venthole location was intercepted by the longwall face.

The calculated bottom-hole pressures for the same ventholes are given in Fig. 13. This figure shows that before the

ventholes are intercepted by the mining disturbances, the

bottom-hole pressures (equal to the reservoir pressure

when the well is shut-in) are slowly declining, basically

because of the effect of the approaching longwall face and

the gobs of nearby longwall panels. However, when the

venthole location is intercepted by the longwall face, the

bottom-hole pressure sharply drops to below-atmospheric

pressures because of the venthole connection with mine

pressures through the mining-induced fractures. Thus,

depending on the completion depth and amount of applied

vacuum, the venthole competes to varying degrees with the

ppg

strata. Even during the periods when the ventholes are not

actively being produced (i.e., no vacuum pressure is being

applied), they stay close to or under atmospheric pressure

conditions. The bottom-hole pressures calculated in this

step were used as the operating constraint for gob gas

venthole completion scenarios that are presented in the

following sections.

6. Results and discussion

6.1. Characterization of methane emission sources in

relation to longwall mining

A better understanding of how the reservoir characteristics and subsided strata mechanics associated with longwall mining influence the release and migration of

methane in the underground environment will help

operators increase the efficiency of their methane control

systems. Investigations of the source of longwall gob gas

by Diamond et al. (1992) showed that overlying coalbeds

were the primary contributors of gas to the gob. Previous

studies by NIOSH at this current site suggested that the

overlying Pittsburgh Rider coals, the Redstone coalbed

(when present), and the Sewickley coalbed were most

likely the primary sources of gob gas associated with the

Pittsburgh coalbed (Mucho et al., 2000).

The longwall subsided strata reservoir model described

previously was used to further characterize the methane

sources that contribute to the gob gas at the study mine site.

Fig. 14 shows a vertical cross section of the subsided strata

reservoir over the headgate side of H panel and the

Fig. 11. Comparison of observed and simulated gas productions from one gob gas venthole at each panel mined. Values were obtained using the

“dynamic” modeling approach described in this study (arrow showing the axis for H-3 data points).
Pittsburgh coalbed layer 630 days after the start of mining

on the adjacent G panel. The methane mole percent data

show that the methane concentration is high in gas emissions from Sewickley coalbed, and this coalbed seems the

primary source of longwall gob gas in this region.

Modeling results show that there is some methane emission

from Waynesburg coalbed also due to a general pressure

decline in the rock layers because of mining. These two

coalbeds have been reported as producing horizons in

coalbed gas wells in the Northern Appalachian Basin

(Bruner et al., 1995). However, the Waynesburg coalbed is

not thought to be a major contributor to the Pittsburgh gob

gas, since its gas storage capacity is low (0.1–0.3 cm3

/g)

according to Diamond et al. (1992), geomechanical modeling of the area showed that vertical fracturing does not

normally extend up to the Waynesburg coalbed's elevation,

and the slotted casings of the gob gas ventholes are set

about 100 ft below the Waynesburg coalbed elevation). The

Redstone coalbed (if present) and the Pittsburgh Rider coals

are within the caved zone, and thus, gas from these

coalbeds may report to the ventilation system because it is

the closest pressure sink to this gas source.

Fig. 15 illustrates the model predictions across another

vertical section of the subsided strata reservoir over the I

panel tailgate. This figure shows two I panel gob gas

ventholes and their effect in reducing methane concentrations within their effective radius and in the sheared zone

below the Sewickley coalbed at the 630th day of mining

of these panel series. From the modeling results shown in

this figure, gas released from the Sewickley coalbed is

Fig. 13. Bottom-hole pressures calculated by the model for the shown gob gas ventholes of each panel (the opposing arrows show the times before and

after the interception of each of the gob gas ventholes).

Fig. 12. Comparison of observed and simulated methane concentrations from one gob gas venthole at each panel mined based on current longwall

mining modeling approach.
expected to be the primary source of gas captured by the

gob gas ventholes, since they are the closest pressure sink

acting upon this gas source. This also suggests that the

methane emissions from Sewickley coalbed may enter the

ventilation system if these wells do not operate continuously and effectively.

6.2. Effects of gob gas venthole completion practices on

production

At the study mine site, the general completion practice

for the gob gas ventholes was to drill to within 12 m (40 ft)

of the top of the Pittsburgh coalbed, then install 17.8 cm

Fig. 15. A vertical section of the subsided strata reservoir over the I panel tailgate after 630th day since the start of mining. This figure shows two I

panel gob gas ventholes and their effect in reducing methane concentrations within their effective radius and the sheared zone below the Sewickley

coalbed at that specific time. It is also shown that gas released from the Sewickley coalbed is the primary source of gas captured by the gob gas

ventholes, since they are the closest pressure sink acting upon this gas source. This observation also suggests that the methane emissions from

Sewickley coalbed may enter directly to the ventilation system if these wells do not operate continuously, and effectively.

Fig. 14. A vertical cross section of the subsided strata reservoir over the headgate side of H panel and the Pittsburgh coalbed layer 630 days after the

start of mining on the adjacent G panel. The methane mole percent data show that the methane concentration is high in gas emission from Sewickley

coalbed and this coalbed seems the primary source of longwall gob gas in this region.
(7 in.) casing with a 61 m (200 ft) slotted section at the

bottom, as shown in Figs. 5 and 6. This completion

strategy places the slotted casing adjacent to the gasbearing strata in the fractured zone, and high enough

above the caved zone so that the venthole does not extract

excessive amounts of mine air. Thus, this completion

strategy maximizes the capture of methane originating in

the fractured zone, in particular gas from the Sewickley

coalbed at the study mine site, before it can migrate to the

caved zone and into the mine environment.

In this modeling study, a series of gob gas venthole

completion scenarios were run using well pressures

calculated in previous steps as the operating constraints

to investigate how methane capture characteristics would

be affected if the venthole diameter, slotted casing length,

and slotted casing setting depth were changed. However,

it should be kept in mind that, in such a complex

environment, the permeability changes in the subsided

strata are directly related to the strength of the associated

rock units and to their response to a given stress level.

Thus, the lengths of slotted casing located in different rock

layers may make a difference in methane capture

efficiency. Therefore, some of the results from this

study may be specific only to the study mine site, other

mines operating in Pittsburgh coalbed, and perhaps to

other mining areas with similar mining and geologic

conditions. However, it is reasonable to assume that the

results may offer some degree of generalization in

understanding and predicting the influence of various

changes in completion design on gas production.

6.2.1. Effect of slotted casing diameter

The casing diameter of the gob gas ventholes at the

study mine site was 17.8 cm (7 in.). To investigate the

possible effects of different slotted casing diameters on

methane production and concentration in the produced

gas, alternative diameters were tested with the model.

For this study, the casing diameter was increased to

25.4 cm (10 in.) and decreased to 10.2 cm (4 in.). The

other completion parameters, such as length of slotted

casing and setting depth, were held constant at their

original design values, 61 m (200 ft) and 12 m (40 ft),

respectively.

Fig. 16 shows cumulative methane production from all

gob gas ventholes on the four mined panels in the study

area as a function of mining time. The modeling results

predict that methane production will increase with the use

of the larger diameter casing. The cumulative methane

production was 4.9% more with the 25.4 cm (10 in.)

casing, as compared to the 17.8 cm (7 in.) diameter casing.

Conversely, the amount of methane produced with the

smaller, 10.2 cm (4 in.) diameter casing was about 7% less

than that produced from the 17.8 cm (7 in.) diameter

casing in the base case.

However, compared to the 17.8 cm (7 in.) casing base

case, the amount of air produced from the 25.4 cm (10 in.)

Fig. 17. Average methane concentration based on cumulative

productions from all ventholes in four panels for different casing

diameters.

Table 2

Effect of casing diameter on cumulative methane and total gas production and average methane composition of the produced gas

Casing diameter

(inch)

Cum. CH4 (MMscf) CH4 diff. rel. to

7 in. casing (%)

Cum. gas

(CH4+ Air)

(MMscf)

Diff. (CH4+ Air) rel. to

7 in. casing (%)

Cum. air

(MMscf)

Air diff. rel. to

7 in. casing (%)

Ave. CH4

conc. (%)

4 391.8 −6.7 609.4 −9.9 217.6 −15.2 64.3

7 419.8 – 676.5 – 256.7 – 62.1

10 440.1 +4.9 728.4 +7.6 288.3 +12.3 60.4

Fig. 16. Simulated cumulative methane production from all ventholes

in four panels for different casing diameters.
diameter casing was 12.3% higher, and it was 15.2%

lower with the 10.2 cm (4 in.) diameter casing. Thus, the

methane concentrations were higher in the produced gas

from the 10.2 cm (4 in.) diameter casing, and lower from

the 25.4 cm (10 in.) diameter casing. In the case of the

25.4 cm (10 in.) diameter casing, the reduced methane

concentration was most likely caused by more mine air

being captured because of an increased depletion radius

created by the larger diameter pressure sink. Since the

total (CH4+ air) gas production (Table 2) increased with

an increase in casing diameter, the marginal decrease in

methane concentration still resulted in a higher cumulative methane production volume with a larger diameter

venthole. The change of methane concentration in the

cumulative gas production is shown in Fig. 17. The

average predicted methane concentrations in the produced

gas stream from the simulated gob gas ventholes with

25.4 cm (10 in.) and 10.2 cm (4 in.) casing was 60.4% and

64.3%, respectively, as compared to 62.1% for the

17.8 cm (7 in.) casing base case.

6.2.2. Effect of slotted casing length

Another gob gas venthole completion parameter

investigated was the influence of the length of the slotted

casing on gas production. For this set of simulations, the

length of the slotted casing section was changed to 30.5 m

(100 ft) and 76.2 m (250 ft), and the results compared to

g() g 

casing diameter and setting depth were kept the same,

17.8 cm (7 in.) and 12 m (40 ft), respectively, for these

simulations. Fig. 18 shows the cumulative methane

production from the simulated gob gas ventholes as a

function of mining time. The modeling results predict that

the cumulative methane production will increase with an

increase in casing length. The methane production from

the simulated gob gas ventholes with 76.2 m (250 ft) of

slotted casing was 459.4 MMscf (13.0× 106 m3

), as

compared to the 391.8 MMscf (11.1 × 106 m3

) in the

original 61 m (200 ft) slotted casing base case (Table 3).

This represents a 9.5% increase in methane capture from

the ventholes in the four-panel study area. However, when

the slotted casing length was shortened to 30.5 m (100 ft),

the methane production decreased to 314.7 MMscf

(8.9× 106 m3

), an approximate 25% reduction from the

61 m (200 ft) slotted casing base case.

When the length of the slotted casing completion

interval was increased to 76.2 m (250 ft) for the simulated

gob gas ventholes, the cumulative volume of produced air

was 302.8 MMscf (8.6× 106 m3

), or 17.9% more than that

for the 61 m (200 ft) slotted casing base case. This results

in a lower average methane concentration in the produced

gas stream from the ventholes with an increased

completion interval. The cumulative volume of air

produced by the gob gas ventholes simulated with

30.5 m (100 ft) of slotted casing was about 154 MMscf

(4.4× 106 m3

), or 43.5% less than that from the base case

ventholes, resulting in a higher methane concentration in

the produced gas stream from these holes.

The decrease in simulated methane concentrations

resulting from the increase in slotted casing length for

gob gas ventholes at the study site may be due to setting

the extra slotted casing length adjacent to less fractured

(lower permeability) strata, the increased flow resistance

in that zone, which likely to increase the production and

gas flow from the lower section of the hole closer to the

caved zone. Although the flow rate is lower in the less

fractured strata, compared to the deeper, more permeable sections of the strata, extending the slotted casing

length to 76.2 m (250 ft) still resulted in the production

of 12.7% more gas, as compared to the 61 m (200 ft)

Table 3

Effect of casing length on cumulative methane and total gas production and average methane concentration of the produced gas

Casing length

(ft)

Cum. CH4

(MMscf)

CH4 diff. rel. to

200 ft casing (%)

Cum. gas

(CH4+ Air) (MMscf)

Diff. (CH4+ Air) rel.

to 200 ft casing (%)

Cum. air

(MMscf)

Air diff. rel. to

200 ft casing (%)

Ave. CH4

conc. (%)

100 314.7 −25.1 468.7 −30.7 154.0 −43.5 67.1

200 419.8 – 676.5 – 256.7 – 62.1

250 459.4 +9.5 762.2 +12.7 302.8 +17.9 60.3

Fig. 18. Simulated cumulative methane production from all ventholes

in four panels for different casing lengths.
base case. This eventually resulted in the production of

9.5% more methane that otherwise might have migrated

into the mining environment.

Decreasing the casing length to 30.5 m (100 ft)

resulted in the production of gas with a higher average

methane concentration, mostly from the Sewickley

coalbed, where the strata disturbance was more severe

because of its proximity to the mining. However, the

modeling results indicate that the shorter casing length

was not efficient in capturing and producing methane

from the upper strata. Although the average methane

concentration in the produced gas stream at the end of

the simulated mining period was 5–7% higher with the

shorter simulated slotted casing interval, as compared to

that from the ventholes with longer casing intervals, the

total gas and methane production for the venthole with

30.5 m (100 ft) of slotted casing was 30.7% and 25%

less, respectively, as compared to the 61 m (200 ft) base

case length. Table 3 summarizes the production data to

show the effect of casing length on methane capture.

For design purposes, it should be emphasized that sitespecific distribution of gas-bearing strata associated with

the various lengths of slotted casing will have a profound

effect on the methane production, e.g., coalbeds and gasbearing sandstones and shales in the upper strata being in

or out of the slotted casing completion zone. Therefore, it

is important to make an assessment of the geological

layers in the overlying strata and their gas emission

pppgg 

(Diamond and Schatzel, 1998), along with the simulations, in determining the optimum length of slotted casing.

6.2.3. Effect of slotted casing setting depth

This scenario was simulated to investigate the effect

of casing setting depth (distance above the top of the

mining layer) on gas production. In the original venthole

completion design, the casing setting depth was 12 m

(40 ft) based on the caving height value estimated by

Mucho et al. (2000) as 10.7 m (35 ft) above the

Pittsburgh Coalbed. As alternative approaches, 19.8 m

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

were simulated. In these alternative cases, the 7.6 m

(25 ft) depth corresponded to the upper section of the

caved zone, which was simulated in this study as 7.3 m

(24 ft) above the Pittsburgh Coalbed, and the 4.6 m

(15 ft) depth corresponded to circumstances where the

venthole was drilled well into the caved zone. 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.

Fig. 19 shows the cumulative methane production for

the simulated alternative casing setting depths. Raising

the casing setting depth to 19.8 m (65 ft) above the

Pittsburgh coalbed as compared to 12 m (40 ft) resulted

in 4% more cumulative methane production. 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. The total gas production increased

by 4.9% with the 19.8 m (65 ft) slotted casing setting

depth scenario (mostly due to the increase in methane

production), and decreased by 5% for the 7.3 m (25 ft)

setting depth (mostly because of the decrease in methane

production). The total gas production increased by about

10.3% with the 4.6 m (15 ft) slotted casing setting depth.

In the 4.6 m (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 (Table 4). Since most of the produced gas

Table 4

Effect of slotted casing setting depth on cumulative methane capture, total gas production, and average methane composition at the end of mining

Setting depth

(ft)

Cum. CH4

(MMscf)

CH4 diff. rel. to

40 ft depth (%)

Cum. gas

(CH4+Air) (MMscf)

Diff. (CH4+ Air) rel.

to 40 ft depth (%)

Cum. air

(MMscf)

Air diff. rel. to

40 ft depth (%)

Ave. CH4

conc. (%)

65 436.1 +3.9 709.3 +4.9 273.2 +6.4 61.5

40 419.8 – 676.5 – 256.7 – 62.1

25 399.8 −4.8 642.9 −5.0 243.1 −5.3 62.2

15 298.7 −28.8 746.1 +10.3 447.3 +74.0 40.3

Fig. 19. Simulated cumulative methane production from all ventholes

in four panels for different casing setting depths.
was mine air at the 4.6 m (15 ft) setting depth (Figs. 20

and 21), the average methane concentration in the

cumulative produced gas at the end of mining on the

panel was only 40%, as opposed to the 60–70% average

methane concentration calculated for other slotted

casing setting depths.

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 at another site, very

close to the study area simulated in this paper. For this new

site, the height of the caved zone was estimated to be

∼12 m (∼40 ft), higher than at the primary site

investigated for this study, because of the presence of the

sandstone paleochannel with varying thickness above the

Pittsburgh coalbed layer. The first gob gas venthole on one

of the new monitored panels 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 on the same panel was

y pp()

above the top of the Pittsburgh Coalbed, which is in the

caved zone. As shown in Fig. 22, 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

because of 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 study site

described in this paper is slightly different than the new site

where the data shown in Fig. 22 was measured, a similar

methane concentration decrease (25–30%) was predicted

for the ventholes penetrating into caved zone.

The increased production of mine ventilation air from

the gob gas ventholes is a problem for several reasons.

The first concern is that if the venthole is producing at its

maximum capacity, then all of the available methane in

the subsided strata may not be captured, and thus can

migrate to the underground workplace where it is a

potential explosion hazard. In addition, when coalbeds

and associated caved zone strata are prone to spontaneous

combustion, the flow of additional mine air into this zone

may pose an increased risk of a mine fire of spontaneous

combustion origin. There are also economic issues

associated with producing higher levels of mine ventilation air from the gob gas ventholes. Obviously, there are

costs associated with providing ventilation air to the

underground workings, as there are with the drilling and

operation of the ventholes. Economically, it is counterproductive to incur cost to first introduce the ventilation

air to the mine, and then incur additional cost to remove it

from the mine via the gob gas ventholes. Finally, for those

mining operations capturing gob gas for commercial sale,

it is very important to maintain as high a methane

Fig. 22. Methane concentrations measured in the gas production

stream from two gob gas ventholes completed to different depths

above the Pittsburgh coalbed on the same longwall panel at a mine site

close to the study site.

Fig. 21. Average methane concentration based on cumulative

productions from all ventholes in four panels for different casing

setting depths.

Fig. 20. Simulated methane concentration in the instantaneous

productions evaluated based on production from all ventholes in

four panels for different casing setting depths.

Methane Mole 

Fraction 

Figure 14. A vertical cross section of the subsided strata reservoir over the 

headgate side of H panel and the Pittsburgh coalbed layer 630 days after the 

start of mining on the adjacent G panel. The methane mole percent data 

show that the methane concentration is high in gas emission from Sewickley 

coalbed and this coalbed seems the primary source of longwall gob gas in 

this region. 
 

Waynesburg coalbed 

Sewickley coalbed 

Pittsburgh coalbed 

Methane Mole 

Fraction 

Effect of gob gas venthole 

at specific time during its 

operation 

Figure 15. A vertical section of the subsided strata reservoir over the I panel 

tailgate after 630th day since the start of mining. This figure shows two I 

panel gob gas ventholes and their effect in reducing methane concentrations 

within their effective radius and the sheared zone below the Sewickley 

coalbed at that specific time. It is also shown that gas released from the 

Sewickley coalbed is the primary source of gas captured by the gob gas 

ventholes, since they are the closest pressure sink acting upon this gas 

source. This observation also suggests that the methane emissions from 

Sewickley coalbed may enter directly to the ventilation system if these wells 

do not operate continuously, and effectively.) 
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Central West Virginia Topical Report (August 1990–August

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Model-GEM. User's Guide, Calgary, Alberta, Canada.

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coal: a review. International Journal of Coal Geology 35, 311–331.

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alternative placement of longwall gob gas ventholes for optimum

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Dolinar, D., 2003. Variation of horizontal stresses and strains in mines

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Proc. 22nd Int. Conf. Ground Control in Mining, Morgantown,

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