SME Annual Meeting 

Feb. 25-Feb. 28, 2007, Denver, CO 

Copyright © 2007 by SME 

A METHODOLOGY FOR DETERMINING GOB PERMEABILITY DISTRIBUTIONS AND ITS APPLICATION TO RESERVOIR 

MODELING OF COAL MINE LONGWALLS 

G. Esterhuizen, NIOSH, Pittsburgh, PA 

C. Karacan, NIOSH, Pittsburgh, PA 

Abstract 

Methane can be a significant hazard in coal mine longwalling 

operations and extensive methane mitigation techniques are employed 

by coal mine operators. Reservoir modeling techniques are used to 

better understand the liberation and migration of methane from the 

surrounding rocks towards the mine ventilation system. The caved 

rock behind the advancing longwall face, known as the gob, can 

contain high void ratios, providing high permeability flow paths to the 

methane. The gob is progressively compacted by the weight of the 

overburden, resulting in a reduction in the void ratio and associated 

permeability. Estimating the permeability distribution within the gob 

poses challenges due to its complexity. The authors have developed a 

new methodology to determine both horizontal and vertical variations 

in the permeability of the gob. Variations of the permeability in the 

vertical direction are based on a model of caving and block rotation, 

which considers the effect of block dimensions and fall height on the 

void ratio. Gob compaction by the overburden and associated 

permeability changes are determined from a three-dimensional geomechanical model which simulates the gob as a strain hardening 

granular material. The resulting three-dimensional permeability 

distribution in the gob is then transferred to a reservoir model. The 

paper demonstrates the application of the method and shows that 

reasonable results are obtained when compared to empirical 

experience and measurements. 

Introduction 

Underground longwall mining of coal causes large scale

disturbances of the surrounding rock mass such as changes in the 

ground stress and fracturing of the rock. These changes increase the 

permeability of the rock mass and can liberate methane from the 

surrounding strata. Methane can be a significant hazard in longwalling 

operations and extensive methane mitigation techniques are employed 

by coal mine operators. These can include pre-drainage of methane 

ahead of the advancing longwall face by horizontal drainage holes and 

methane extraction by vertical holes drilled from surface, known as gob 

vent boreholes. 

The caved rock behind the advancing longwall face, known as the 

gob, has a significant effect on methane flow patterns. The gob can 

contain high void ratios, providing high permeability flow paths that can 

provide flow paths from the working face to the methane liberated from 

the disturbed strata. As mining progresses, the gob is gradually 

compacted by the weight of the overburden, resulting in a reduction in 

the void ratio and associated permeability. 

The complex processes of methane liberation, flow through

fractures and migration towards the operating longwall face are being 

investigated at the National Institute for Occupational Safety and 

Health (NIOSH), Pittsburgh Research Laboratory, through combined 

geomechanical and reservoir modeling. A geomechanical model is 

used to characterize the stress changes and rock fracturing that 

accompanies the advancing longwall face, as well as gob formation 

and compaction behind the face. Output of the geomechanical model 

is used to develop permeability distributions in the rock mass around 

the longwall panel for input into the reservoir model, as described in 

Esterhuizen and Karacan (2005). The accuracy of the models in 

predicting methane flow in the vicinity of the longwall face is dependent 

on an accurate representation of the gob permeability distribution. 

This paper describes a methodology for calculating permeability 

variations in the gob suitable for reservoir modeling or other flow 

models. The method considers both horizontal and vertical variations 

of the void ratio during gob formation and subsequent compaction. 

The permeability distribution is calculated using equations for flow 

through granular materials. The resulting three-dimensional 

permeability distribution in the gob were transferred to a reservoir 

model of a typical longwall panel in the Pittsburgh Coalbed. The 

leakage flow into the gob, methane distribution within and effects of 

gob vent boreholes on flow patterns are simulated. 

Computational fluid dynamics (CFD) models have been utilized 

before for gob (caved zone) modeling (Balusu et al., 1996; Yuan et al., 

2006). However, to the best of our knowledge, a detailed caved zone 

model investigating the flow in the mine and gob areas has never been 

developed using a reservoir modeling approach. 

Gob Characteristics 

The gob is formed by rock fragments that fall from the roof strata 

into the void created by the removal of the coal bed. The bulking of the 

gob is affected by the fall height of the rock fragments as well as the 

size and shape of the fragments. When the fall height is larger than 

the lateral dimension of the rock fragments, they are more likely to 

rotate and come to rest in a jumble, which produces relatively large 

void spaces, shown in Figure 1(a). This is known as fully caved rock. 

As caving proceeds upwards, the caved rock occupies an ever 

increasing proportion of the free space, thus reducing the fall height of 

the subsequent fragments. As the fall height reduces, the potential for 

fragments to rotate diminishes and the amount of bulking is reduced, 

shown in Figure 1(b). This is known as partially caved rock. 

h b b h

Falling blocks 

rotate Falling blocks 

cannot rotate 

(a) (b)

Figure 1. Effect of fall height on void space between gob 

fragments. Voids between fragments are large when the fall height 

(h) is large relative to block width (b). Voids reduce as the fall 

height reduces and rotation is inhibited. 

The variation of the bulking of the gob in the vertical direction was 

estimated using a procedure suggested by Munson and Benzley 

(1980). The procedure assumes that maximum bulking of the caved 

rock will occur when the fall height exceeds about twice the block 

width. The maximum bulking factor was assumed to be 75%, after 

tests on simulated gob materials (Pappas and Mark, 1993). The 

bulking factor (S) is expressed as: 
SME Annual Meeting 

Vr +V S = v	 

Vr

Vr is the rock volume and Vv is the void volume. It was further 

assumed that a smooth transition will occur from the maximum bulking 

factor to zero when the fall height is zero. Figure 2 shows the 

relationship between the ratio of fall height to block width (h:b) and 

bulking factor used in this paper. 

Feb. 25-Feb. 28, 2007, Denver, CO 

0.7 

0.8 

0.9 

Fully Partially 

d 

b=15 in 

b=30 in 

b=48 in 

Bulking factor .0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

0 

0 10 20 30 40 50 60

Height above floor (ft) 

Figure 2. Calculated variation of the gob bulking factor above the coal 

bed floor for a mining height of 6ft and various block widths (b). The 

fully caved and partially caved zones are indicated. 

3

 

The resulting variation in the bulking factor with vertical distance 

above the floor of a 6ft high coal bed was calculated using this 

relationship for various block widths. The results are shown in Figure 

3. It can be seen that near the floor of the mined coal bed, the bulking 

factor remains at about 75%, but there is a rapid drop in the bulking 

factor at about 9ft to 15ft above the floor, depending on the block 

width. This lower zone is known as the fully caved zone and the upper 

is the partially caved zone. The results show that the overall variation 

of the bulking factor is not particularly sensitive to the selected block 

width. 

 

Bulking 

0.8 

0.7 

0.6 

0.3 

F a c to r 0.5 

0.4 

0.2 

0.1 

0 

0 0	 .5 1 1.5 2 2.5

Ratio of fall height to block width (h:b) 

Figure 3. Relationship between bulking factor and ratio of fall height 

to block width of caved fragments. 

The effect of gob compaction by the weight of the overlying strata 

was introduced by assuming the bulking factor will reduce in direct 

proportion to the amount of compaction. For example, 10%

compaction will reduce the bulking factor by 10% of the current value 

at all points, regardless of the distance above the coal bed floor. This 

is a simplifying assumption, which does not significantly affect the 

permeability results. Further work is planned in which variable 

compaction will be considered. 

Permeability Calculation 

The Carman-Kozeny equation for flow through porous media was 

used to estimate the permeability of the gob (K) as follows: 

K ⎛ n3 ⎞ K = 0 ⎜

⎜ ⎟⎟

0.241⎝ 2 ⎠ (1− n) 

Where Ko is the base permeability of the broken rock at the 

maximum porosity, and n is the porosity. The value of Ko was taken as 

1x106

 md which places it the “open jointed rock” range according to 

Hoek and Bray (1981). This permeability value also falls within the 

same range of permeabilities calculated from experimental friction 

factors for flow through crushed stone, compiled by Stephenson 

(1979). The Ko value of 1x106 results in realistic gob gas venthole 

production when used in reservoir modeling of longwalls (Karacan et 

al., 2006). Using the above equation, the calculated permeability 

variation in the gob is shown in Figure 4 for various block widths. 

1.00E 01

0 10

Hei

 .


 

1.00E+06
 

b=15 in 

b=30 in 

b=48 in 

20 30 40 50 60

ght above floor (ft) 

1.00E+05 

1.00E+04 

Permeability (md) 

1.00E+03 

1.00E+02 

1.00E+01 

1.00E+00 

-

Figure 4. Variation of permeability in the gob above the coalbed 

floor for a mining height of 6 ft. 

The results show that the permeability is 1x106 md in the lower

part of the gob, where bulking is at the maximum, as specified in the 

assumptions. This high permeability zone extends about 1.5 times the 

mining height. The permeability rapidly decreases at points that are 

more than about 10ft above floor. The permeability soon drops to 

100 md at a height of about 20 to 30 ft, which places it the permeability 

range of jointed rock. This height corresponds with the empirical 

observation that cave rock extends about 4 to 6 times the height of the 

mined coal bed (Mucho et al., 2000). 

Distribution of GOB Permeability in a Mined Panel 

The compaction of the gob behind the advancing longwall face 

and the associated permeability c hanges are determined through the 

use of the FLAC3D numerical mo deling program (Itasca, 2005). The

program allows realistic modeling of stress redistribution about a 

longwall panel and is able to mode l rock fracture and gob compaction. 

The gob is modeled using the dou ble yield material type in FLAC3D, 

which represents materials in which there may be significant 

Copyright © 2007 by SME
SME Annual Meeting 

Feb. 25-Feb. 28, 2007, Denver, CO 

irreversible compaction in addition to shear yielding. The material 

parameters were selected to simulate a gob material that has an initial 

bulking factor of 0.75, displaying an exponential increase in load as it is 

compacted by the weight of the overlying strata. Figure 5 shows the 

relationship between gob stress and compaction used in the models. 

Further details of the modeling approach and typical input parameters 

are presented in Esterhuizen and Karacan (2005). 

Copyright © 2007 by SME 

2000
 

1800
 

1600
 

1400
 

1200
 

1000
 

800
 

600
 

400
 

200
 

0
 

Figure 5. Relationship between gob strain and stress used in the 

FLAC3D numerical model. Gob stress (psi) . 

0.00 0.10 0.20 0.30 0.40 

Strain 

The FLAC3D model can be set up to simulate the progressive 

extraction of coal by an advancing longwall panel. The void behind the 

face and the roof rocks up to 4 or 6 times the mining height is filled with 

gob material. The overburden rocks in the model will subside and 

compact the gob until a state of equilibrium is reached. The 

compaction of the gob is obtained by querying the FLAC3D model at 

selected points. Knowing the compaction distribution, it is possible to 

calculate the remaining void space at each point. It is then a simple 

matter to calculate the porosity and associated permeability values 

using the Carman-Kozeny equation. It was assumed that the 

horizontal permeability is twice the vertical permeability, owing to the 

shape of rock fragments. 

A simple algorithm was written using the internal programming 

language in FLAC3D to write a text file with x,y,z coordinates, porosity 

and permeability values in an appropriate format for import into the 

reservoir model. Figure 6 shows the resulting distribution of 

permeability in a horizontal slice taken 3 ft above the floor of the 

coalbed, as determined for a longwall panel at a depth of 600 ft. It can 

be seen that permeability varies considerably within the gob. Near the 

edges of the gob, the permeability is relatively high, with a maximum of 

1x106

 md near the corners of the mined panel. The permeability in the 

central part of the gob is relatively constant at about 1x105

 md. 

Unmined coal bed 

1250 ft 

Gob in mined 

longwall panel 

Permeability 

(md) 

Face area 

Figure 6. Plan view of a longwall panel showing the distribution of 

permeability in the fully caved gob as determined from the results of 

FLAC3D modeling. 

GOB Representation in Reservoir Model 

Model and gob geometry 

A reservoir model was created based on the characteristics of a 

typical Eastern U.S. longwall mine operating in the Pittsburgh Coalbed. 

The model was created using the GEM (General Equation of State 

Model) reservoir simulator (Computer Modeling Group, 2003). In 

building the model, the geology above the Pittsburgh Coalbed that 

would form the caved zone after mining was considered. The 

thickness of the caved zone in this area is estimated to be 4-6 times 

the thickness of the mined height (Mucho et al., 2000). Within that 

zone, thin sandstone-shale sequences, limestone and Redstone 

Coalbed are the major layers. Their representative reservoir properties 

were assigned the same as reported in earlier publications 

(Esterhuizen and Karacan, 2005; Karacan et al., 2006). 

The model dimensions were based on the width of a typical 

longwall panel, the surrounding entries and the thickness of the 

expected caved zone. According to this, the width of the model was 

1,650 ft, 1,250 ft of which was due to the panel width and 200 ft on 

each side of the panel, for the gate entries. The height was 35 ft, 

based on the expected thickness of the caved zone. Thus, the gob in 

this study represents only the fully and partially caved zone. The 

dimensions of the entries and pillars were based on representative 

dimensions from mine maps. The tailgate and headgate consisted of 

three entries each, spaced 100 ft apart. The length of the model was 

2,300 ft. 

In the development of the reservoir model, non-uniform grids and 

local grid refinement were used in order to be able to create the rock 

layers and the details of the mining environment. In the model, 12 

layers ranging 1-6 ft in thickness represented the rock layers 

immediately above the Pittsburgh Coalbed (bottom-most layer). Figure 

7 shows the 3-D grid model. The layers above the mining layer were 

cropped in order to better visualize the details. A face area (15 ft in 

width) was created between the unmined coalbed and the gob to 

represent the working face and enable face ventilation. 

Coal, gob and rock properties 

Due to the nature of the environment repres ented, the model 

includes three different reservoir sections for modeling purposes: 

The first section includes the “undisturbed” rock units outside the 

gob,which consist of the unmined coalbed and the rock layers above 

the coalbed. The initial, pre-mining reservoir properties to the coal and 

non-coal units were based on data given in earlier publications 

(Esterhuizen and Karacan, 2005; Karacan et al., 2006). 
SME Annual Meeting 

Feb. 25-Feb. 28, 2007, Denver, CO

Copyright © 2007 by SME 

The second, and most important section, is the inner part of the 

model representing the gob. Changes in flow and fluid storage-related 

properties (mainly permeability and porosity) were imported directly 

into the reservoir simulator to specific grid addresses from the FLAC3D 

results by the techniques described in previous sections of this paper. 

The third section comprises of the entries and longwall face in the 

mining layer. Since these are high permeability, low resistance and 

high porosity areas, the property assignments were made to reflect 

those characteristics. In the model, the entries surrounding the 

longwall panels serve as main pathways for ventilation airflow. Thus, 

the permeability for this portion of the mining layer were assigned an 

average high value [10-7 m2

 (108 md)] within the allowed limits of the 

simulator for minimum resistance. A fracture porosity of 99%, on the 

other hand, was designated to these areas. Since matrix-to-fracture 

mass transfer is not important in these areas, the matrix was assigned 

null properties. 

Rock Layers 

Gob (Caved Zone) 

Pillars 

Entry 

Coalbed Unmined Layer Coalbed Longwall 

Face 

Figure 7. A three dimensional grid model of the gob and the mining

layer. The top layers were cropped to visualize the modeled mining 

layer.

Ventilation scheme and ventilation controls 

Ventilation is one of the most important elements of underground 

longwall mining to dilute and render harmless toxic or explosive gas 

and dust in the mine. In the Northern Appalachian Basin, the longwall 

mines operate with a bleeder ventilation system. This scheme 

ventilates the entries, face and gob through leakage and carries away 

the contaminated air along return entries and the bleeder. A typical 

bleeder ventilation scheme was modeled in the mining layer to 

simulate those effects. The flow rates and pressure differentials were 

created using injection and producer wells to represent intakes, returns 

and the bleeder fan, which dictate the ventilation pressures as well as 

the direction of the flow in the model. Auxiliary ventilation structures 

such as curtains, stoppings and regulators were also modeled to divert 

or stop the flow and to control the pressures, shown in Figure 8. 

Ventilation air intakes 1 and 2 (I1 and I2) shown on the headgate 

side (Figure 8) were represented by injection wells providing air into 

the entries with 15 psi pressure and 30,000 and 50,000 cfm air flow 

rate, respectively. The injection wells also controlled the intake 

ventilation pressure in the entries. Some of the intake flow admitted 

into the headgate entries is diverted into the face area using ventilation 

curtains. The curtains were represented by permeable grids that had 

higher resistance compared to the entries themselves. A small portion 

of the face air was also diverted to the conveyor belt entry before it 

enters the face area. This was achieved utilizing a producer well 

operating at atmospheric bottom-hole conditions to create a pressure 

sink. 

The amount of air leaving the face was removed by returns (R1

and R2) which were modeled as vertical producer wells operating 14.5 

bottom hole pressure. The bleeder fan was similarly modeled with a 

bottom-hole-pressure of 12.5 psi. The air flowing in the headgate 

entries was wrapped around the gob by the influence of the bleeder 

fan. Regulators, represented by higher resistance grids, were also 

employed at the usual locations to control the flow around the gob 

area. 

I1 I2 CB1 R1 R2

Face area

Stopping

Pillar

Regulator

Ventilation 

curtain

Regulator

GOB

Shields

Boundaries of gob 

Bleeder fan

Headgate Tailgate 

1250 ft 

Ventilation direction 

Figure 8. Mining layer in the model that shows the ventilation paths,

elements of ventilation system and various modeled features. 

It is known that air leakage from the face area to the gob is 

resisted by the shield supports. The shields were modeled by 

decreasing the transmissibility of the first layer of gob grids in the 

direction perpendicular to the face by 50%, an estimated value in the 

absence of in-mine measurements of this resistance. It was not 

possible to model the presence of each shield individually. Therefore, 

the total number of grids were divided into groups so that every 5 

shields were represented in one grid for this purpose. 

Applications of Reservoir Model-Example Results 

The developed reservoir model was used to investigate flow in the 

caved zone section of the gob and to compare the results to empirical 

experience and field measurements. 

Air leakage into gob 

Air leakage into the caved zone in longwall operations affects the 

efficiency of the ventilation system. In gassy mines, air leakage may 

be used as a means of ventilating the gob so that the methane being 

released in the caved zone close to the face line will not enter to the 

working area (Brunner, 1985). Also, in longwall mines, the velocity of 

air leaking into the gob is important to control spontaneous 

combustion. The availability of air and its velocity may affect oxidation 

of coal in the caved zone. Thus, it is important to evaluate and 

quantify the leakage pattern and quantities. Numerical models, CFD or 
reservoir, may help understanding and controlling leakage into the 

caved zone. 

The reservoir model was used to analyze the air flow within the 

caved zone imposed by the ventilation pressures in the entries. Figure 

9 shows the flow patterns within the gob close to the face. The arrows 

indicate the direction of flow without any magnitude. The arrows 

indicate that most of the leakage into the gob occurs around the 

headgate entry where the flow is forced into work area. Some of the 

flow is distributed within the gob, and some migrates along the face 

line behind the shields and then converges into the return entries at the 

split point in tailgate. 

Headgate Tailgate 

Longwall face 

1250 ft 

gure 9. The air flow lines within the caved zone in the vicinity of the 

ce area. 

Fi

fa

Figure 10 shows the cross-directional velocity contours within the 

caved zone. The highest velocities, in the face and entries, were 

eliminated from the map in order to better visualize the flow in the gob. 

The contour map shows that the highest velocities occur behind the 

shields where the caved zone is rather loose and permeable. The 

velocities in this region are 60-80 ft/min and converge towards the 

return at the end of the shields. In deeper regions behind the shields, 

both the flow broadens and starts to flow towards the tailgate entries 

and attenuates in velocity. A higher-velocity leakage zone is observed 

at the back of the panel along barrier pillars also, where the values 

may reach 40-50 ft/min. 

Previous studies performed to investigate the leakage flow into 

gob revealed flow paths similar to those reported in this study. 

Michaylov and Vlasseva (1995) showed that velocities are higher 

alongside the porous and permeable sections of the gob. They have 

calculated that the velocities behind the shields can be around 30 

ft/min. They have also calculated increased velocities at the back of 

the panel as observed in this study. The differences in calculated 

velocities between the two studies may be due to differences in the 

imposed boundary conditions and how the gob is modeled. 

The contours of the x-component of the velocity of the air entering 

to the tailgate cross cuts (Figure 10) show that the velocity decreases 

from a maximum value in the cross cut in the immediate vicinity of the 

face line to a minimum close to back of the panel. The value ranges 

from 50-90 ft/min to 0-10 ft/min based on the location of the cross cut 

along side the gob. The measured data presented by Brunner (1985) 

shows a similar behavior for the change of magnitude of the air flow 

path. The velocity is highest in the first cross cut outby the face line 

and lowest in the cross cut closest to the back of the panel. The 

measured data, reported as volumetric rate, shows that the maximum 

flow velocity is around 30-40 ft/min and the mini mum is around 0-10 

ft/min. 

Boundary of 

Headgate Longwall face Tailgate 

X 

Y 

1250 ft 

caved zone 

Figure 10. Contours of the x-component of the velocity in the fully 

caved zone of the gob. 

Effect of introducing a gob gas venthole 

In the Eastern U.S., gob gas ventholes ideally are drilled to within 

40 or 45ft of the top of the coalbed (into the fractured zone, but not into 

the caved zone) and are completed with 7-in casing and 200ft of 

slotted pipe on the bottom. Usually, a vacuum is applied to these 

ventholes by installing exhausters on the surface well head. The 

purpose of this completion strategy is to create a pressure sink across 

several potential gas-bearing horizons and to capture the released gas 

before it can enter the underground workplace. For these gob gas 

ventholes to work effectively, they must be drilled close enough to the 

mine to capture methane from the fractured zone but far enough above 

the caved zone to minimize the amount of ventilation air that is drawn 

into the ventholes. However, sometimes gob gas boreholes are 

inadvertently drilled into the caved zone. This results in the production 

of excessive mine air from the gob gas venthole and decreases its 

effectiveness (Karacan et al., 2005 and 2006). 

The influence of a gob gas venthole accidentally drilled into the 

caved zone was investigated using the reservoir model. For this 

purpose, a gob gas venthole was placed on the tailgate side of the 

panel (Figure 11). It has been modeled as 7-inch-diameter and 

operated with 2 psi of suction, to simulate the exhauster. However, it 

was modeled as drilled half way into the caved zone, to the boundary 

between fully caved and partially caved zones in the gob. 

Figure 11-A shows the disturbance in the air flow lines created by 

an over-drilled gob gas venthole in the fully caved zone of the gob. 

The flow lines show that the gob gas venthole attracts the air towards 

its drainage area, although it does not penetrate that deep. Of course, 

this situation both impedes the ventilation of the caved zone and 

reduces the ventilation system efficiency. Figure 11B presents the 

impact of the gob gas venthole on a vertical cross section across the 

venthole and shows that, besides draining gob gas in horizontal 

direction, it produces excessive amounts of air from the lower levels of 

the caved zone. This observation is substantiated by the field 

measurements of methane production from gob gas ventholes 

unintentionally drilled too deep into the caved zone, presented in 

Karacan et al. (2006). The methane concentration measurement from 

such a venthole showed that the methane concentrations are 30-40% 

lower than its counterparts drilled above the caved zone (Figure 10). 

SME Annual Meeting 

Feb. 25-Feb. 28, 2007, Denver, CO 

Copyright © 2007 by SME 
SME Annual Meeting 

Feb. 25-Feb. 28, 2007, Denver, CO 

Copyright © 2007 by SME 

(A) 

(B)

Gob gas venthole 

Partial caving 

Fully caved 

Location of 

gob gas 

venthole 

(A)

Figure 11. Influence of a gob venthole penetrating to the caved zone. 

(A) A plan view of the lower part of the caved zone near tailgate entries 

and (B) A vertical cross section through the gob gas venthole location.

Discussion and Conclusion 

The paper presents a new methodology for calculating 

permeability variations within the gob of a coal mine longwall panel. 

The method accounts for permeability variations in the vertical 

direction as the bulking factor changes as well as the lateral direction 

as the gob is compacted by the weight of the overburden. 

The results show that gob permeability is a maximum in the fully 

caved, lower part of the gob. The fully caved zone extends to about 

1.5 times the mining height. 

The permeability rapidly drops by several orders of magnitude in 

the partially caved zone which is located above the fully caved zone. 

The permeability in this zone drops to that of fractured rock at a height 

of about 4 to 6 times the mining height. This is consistent with 

empirical observations of the height of caving. 

Gob compaction by the weight of the overburden rocks is shown 

to cause further lateral variations of permeability of about one order of 

magnitude. 

Application of the methodology to a reservoir model of the gob 

and surrounding entries in a longwall operation shows that realistic 

results of flow within the gob and surrounding excavations are 

obtained. 

A model investigating leakage flow in the gob behind the longwall 

shields predicts leakage penetration near the headgate part of the face 

and enhanced flow in the loosely compacted gob near the face. The 

model results are consistent with general observations and 

calculations for flow paths and velocities reported in the literature. 

A second example shows the unfavorable effect of a gob gas 

venthole intersecting the gob area. The model results indicate that the 

venthole will attract mine air, resulting in reduced methane production. 

These results are consistent with empirical observations and 

measurements reported in the literature. 

It is concluded that the methodology presented in this paper 

provides an improved method for estimating the permeability 

distribution in the gob behind a longwall face. The methodology allows 

realistic numerical experiments to be conducted for site specific cases 

to enhance the safety and effectiveness of methane mitigation 

techniques. 

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