Numerical analysis of the influence of in-seam horizontal methane 


drainage boreholes on longwall face emission rates 

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

National Institute for Occupational Safety and Health (NIOSH), Pittsburgh Research Laboratory. Pittrburgh, PA 15236, USA 

Abstract 

High methane emissions originating from the active face areas and from the fractured formations overlying and underlying the 

mined coalbed can adversely affect both safety and productivity in underground coal mines. Since ventilation alone may not be 

sufficient to control the methane levels in the longwall mining environment, gob gas ventholes have become a standard 

supplementary methane control option in many mines. As mines progress into deeper and gassier coalbeds, or as longwall panel 

size increases, ventilation and gob gas ventholes together may not be sufficient to maintain methane levels within statutory limits. 

To decrease the risk associated with methane emissions under these circumstances, in-seam horizontal methane drainage is often 

used to reduce the gas content of the coalbed prior to mining. Horizontal methane drainage borehole completion designs, drilling 

strategies, and degasification lead times may need to be adjusted for site-specific conditions due to mine design, geology, and the 

gas content of the coalbed. 

This study investigates different horizontal methane drainage borehole patterns, borehole lengths, and degasification times prior 

to and during panel extraction to evaluate their effectiveness in reducing methane emissions using a "dynamic" 3D reservoir 

modeling of a 38 1-m wide longwall panel operating in the Pittsburgh coalbed. Results of this study showed that dual and tri-lateral 

boreholes are more effective in decreasing emissions and in shielding the entries compared to fewer shorter, cross-panel, horizontal 

boreholes parallel to the longwall face. Modeling results showed that after 12 months of pre-mining methane drainage, the average 

longwall face emission rates can be reduced by as much as 10.3 m3/min and 6.8 m3/min using tri- and dual-lateral boreholes, 

respectively. It was also shown that if pre-mining methane drainage time is short, it is important to continue methane drainage 

during the panel extraction to maximize reductions in longwall face emissions since additional face emission reductions achieved 

during this period can be comparable to pre-mining degasification. 

Keywords: Longwall mining; Methane drainage; In-seam horizontal boreholes; Reservoir simulation 

1. Introduction 

I. I. Methane emission sources during longwall mining 

Methane emissions can adversely affect both the safety 

and the productivity of underground coal mines. During 

longwall mining, methane emissions can originate from 

three major sources. These sources can be summarized in 
the following manner: (1) gas emissions from the ribs 

surrounding the bleeder ventilation system, (2) gas 

emissions from the active longwall face and mined coal 

on the conveyor belts, and (3) gas emissions from 

subsided strata (Mucho et al., 2000). 

The first gas source originates from the unmined 

coalbed adjacent to the development entries of the bleeder 

system and from the solid coal ribs. Although this ernission tends to decrease over time, it may become a significant contributor of gas to the bleeder ventilation system 

(Mucho et al., 2000). The second source is the combination of the gas content from the mined coal itself and 

the methane being emitted from the fresh face on the 

longwall. The third source is the fractured and caved rock 

in the subsided strata (gob) overlying the extracted panel 

as the longwall face advances (Fig. 1). The caved zone is 

characterized as a fragmented rock mass, whose height is 

about 3 to 6 times the thickness of the mined coalbed 

(Singh and Kendorski, 198 1; Palchk, 2003). The stress 

relief due to caving causes the overburden strata above the 

caved zone, including gas bearing coalbeds, to fracture 

vertically and horizontally. The thickness of the fractured 

zone (Fig. 1) can vary up to 100 times the height of the 

mined coalbed, depending on the size of the panel, the 

geology and geomechanical properties of the layers 

(Palchik, 2003). The source of the gas emissions determines the selection of an effective methane control 

system. 

1.2. Typical longwall methane control measures 

Ventilation has always been the primary means of 

controlling methane in the mining environment. Mucho 

et al. (2000) demonstrated that methane emissions 

within the caved zone are directly influenced by the 

ventilation system. Therefore, it is desirable to capture 

the methane released in the fractured zone before it can 

migrate to the pressure sink of the caved zone. 

Gob gas ventholes are commonly used to control the 

methane emissions from the fractured zone and are drilled 

from the surface to a depth that places them above the 

caved zone so they do not directly interact with the 

ventilation system (Fig. 1). The bottom section of the 

pipe, generally about 60 m in length, is slotted and placed 

adjacent to the expected gas production zone. These 

ventholes generally become productive after the mininginduced fractures propagate under the well (Diamond, 

1994) and the gas flow is mainly controlled by the permeability of the fractures. Exhaustem are placed on gob gas 

ventholes to maintain a pressure sink around them to 

capture the gas from large distances and reduce the 

migration of methane from the hctured zone to the caved 

zone. 

Methane emissions from the face, ribs, and conveyor 

belt are directly discharged into the mining environment; 

therefore, the ventilation system must have sufficient 

capacity to dilute and render harmless any unexpected 

increases in methane emission levels. As mines move 

into deeper coalbeds with potentially higher gas contents, and as longwall panel widths increase resulting in 

additional gas emissions in the face area, ventilation 

systems may not be sufficient to maintain methane levels 

within statutory limits (Diamond and Garcia, 1999; 

Schatzel et al., 2006). The most common supplemental 

gas management practice is to degasify the coalbed 

prior to mining to reduce the volume and rate of these 

emissions (Thalur, 1997). In addition to reducing the inplace gas content of the coalbed, it has been shown that 

methane drainage is also effective for reducing the risk of 

gas outbursts by decreasing the pressure of the coalbed in 

I 

Bending 

zone Gob gas venthole 

: 


I

Fractured 


1


zone 


Caved Partial 

Fig. 1. Schematic of strata response to longwall mining (modified from Singh and Kendorski, 1981) 
the vicinity of the mine workings (Hungerford, 1995; 

Noack. 1998). 

1.2.1. Degasification of coalbeds using horizontal 

boreholes for reducing emissions 

The most commonly applied methane control solution, 

especially in high in-place gas content coalbeds, is drilling 

methane drainage boreholes into the panel area prior to 

longwall mining to reduce the methane content of the 

coalbed (Diamond and Garcia, 1999). These boreholes 

can be vertical boreholes drilled from the surface or inseam horizontal boreholes drilled from the underground 

entries (Diamond, 1994). Although in-mine drilling can 

be challenging because of the logistics of working in the 

restrictive underground environment, one advantage is 

that virtually the entire drilled lengths of the boreholes are 

in the gas producing horizon of the mined coalbed. In 

recent years, a new pinnate-drillingTM technique has been 

successfully applied in coalbeds in the Appalachian Basin 

and in the Western states of the U.S. using horizontal 

methane drainage boreholes directionally drilled from the 

surface (PTTC, 2004). This technique combines the 

logistically less complicated process of drilling from the 

surface with the high productivity of horizontal methane 

drainage boreholes. 

The design considerations for horizontal degasification 

boreholes may change depending on the mining conditions and on the coalbed to be degasified. Generally, inmine horizontal boreholes are used to perform two basic 

mine safety functions: reducing the in-place gas volume 

within the panel prior to mining and shieldmg active 

workings, especially development sections, from gas 

migration from the surrounding virgin gas reservoir. Inseam horizontal methane drainage boreholes are usually 

completed open hole, with a short segment (9-15 m) 

grouted to seal the end of the borehole near the mine 

workings (Zuber, 1998). Long (>305 m) horizontal 

boreholes drilled parallel to the gateroads in advance of 

entry development can be utilized to drain methane from 

the panel area (Fig. 2A) and they can be used to shield the 

advancing development entries from the flow of gas from 

the surrounding coal reserves (Fig. 2B). Shorter, crossFig. 2. Schematic plan vlew of long horizontal boreholes formethane drainage in longwall mining in advance of mining (A)and for shielding (B)(from 

Diamond, 1994). 
Main entries 

Fig. 3. Schematic plan view of short horizontal boreholes for methane drainage from longwall panels on active panel in advance of face (A), on 

developing panel adjacent to active panel (B), and from advancing development entries (C) (from Diamond, 1994). 

panel, horizontal boreholes drilled fiom the advancing 

entries (Fig. 3A-C) can also be used to drain gas in 

advance of longwall mining from the panel. 

One of the most important aspects of methane 

drainage using any pattern of boreholes is estimating 

the amount of lead time required to reduce the in-place 

gas volume sufficiently in order to reduce gas emissions 

effectively during longwall mining. The life of the 

horizontal boreholes is dependent on the mine plan. 

Typically, 6 to 12 months of gas production prior to 

mining is required to degasify an outlined longwall panel 

sufficiently (Zuber, 1998). Horizontal boreholes drilled 

near the recovery rooms are usually allowed to produce 

gas even after the mining starts. As the longwall face 

approaches these borehole locations, production is 

generally terminated as a safety measure by injecting 

water or gel into the borehole to block the flow of gas. 

Thus, there are two phases in the production history of a 

horizontal borehole: prior to mining and during mining, 

or panel extraction. The relative gas production contributions of these two phases will vary considerably 

depending on the well configuration and spacing, duration of pre-mining drainage, and rate of mining. 

The importance of drainage time in reducing 

methane content of the coal was reported by Aul and 

Ray (1991). They noted that only 30% of the gas could 

be removed from longwall panels in the Pocahontas 

No.3 coalbed if the drainage time was about 2 months. 

However, horizontal boreholes that produced for about 

10 months were able to drain 80% of the in situ gas. 

Based on these data, they concluded that at least 

6 months is required to drain sufficient quantities of gas 

to have a positive impact on methane emissions during 

mining. As a result of the horizontal borehole methane 

drainage program, ventilation air volume at the longwall face was reduced fiom 3400 m3/min (120,000 cfm) 

to 720 m3/min (25,000 ch). 

Numerical models can evaluate methane drainage 

options for controlling underground emissions. Several 

papers discussing the theoretical basis and the use of 

coalbed methane simulators to analyze the performance 

of vertical or horizontal methane drainage boreholes 

have been published (King et al., 1986; Remner et al., 

1986; Ertekin et al., 1988; King and Ertekin, 1994). 

Other papers have discussed the use of simulators for 

designing methane drainage systems for specific field 

studies (Brunner et al., 1997), estimating the extent of 

gas drainage with time (Diamond et al., 1989), and 

evaluating the production potential and strategies of 

various methane drainage systems (Kelafant et al., 

1988). However, fewer numerical modeling studies 

have investigated the applications and interactions of 

gob gas ventholes, ventilation, alternative horizontal inseam boreholes configurations, and drainage lead times 
on the effectiveness of gas drainage before and during 

mining. This is partly due to the unconventional 

modeling required to simulate a moving boundary type 

reservoir, as is the case with longwall mining, coupled 

with the complex geomechanical considerations associated with the extraction of the coalbed. Nevertheless, a 

few studies used a similar moving boundary type 

reservoir simulation approach (Zuber, 1997) to estimate 

methane emission rates during gateroad development. 

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

(2005) adapted the moving boundary problem to the 

longwall mining environment. Their reservoir models 

evaluated the impacts of various venthole completion 

factors on gob gas production and further optimized 

venthole performances for wider longwall panels. 

2. Objectives of the study 

The objective of this study is to determine the impact 

of in-seam horizontal methane drainage borehole configurations, borehole spacings, and gas drainage lead times 

in advance of mining for reducing the in-place methane 

content of a longwall panel operating in the Pittsburgh 

coalbed. Potential reductions in longwall face emissions 

are also estimated. Due to the complexity of the mining 

environment, a numerical modeling approach was 

required to achieve the study objective. Computer 

Modeling Group's GEM software (CMG, 2003) was 

used to develop the 3D reservoir models necessary to 

evaluate the multiple variables included in the study. The 

mechanical response of the overlying strata to mining was 

modeled using Itasca Consulting Group's (Itasca, 2000) 

FLAC2D (Fast Lagrangian Analysis of Continua2D) and 

the calculated permeability changes were incorporated 

into the longwall mining reservoir model. The model was 

calibrated using history matching techniques to match the 

production histories of gob gas ventholes operating in the 

study area and the average methane production rates of 

horizontal degasification boreholes operating in the area. 

Various pre-mining lead times (3, 6, 9 and 12 months) 

were simulated using multiple in-seam horizontal 

borehole configurations. Additionally, the models with 

3 and 12 months of pre-mining methane drainage were 

extended to include a 268-day longwall mining simulation (the actual mining duration of the study panel) to 

evaluate the additional effects of mining and continued 

degasification on face emissions. 

3. Study area 

The model was developed and calibrated for a 

longwall mining area in the Pittsburgh coalbed located 

in Greene County, Pennsylvania (Karacan et al., 2005). 

The study area at the mine was located in a new mining 

district where panels were initially 38 1 -m wide and were 

increased to 442 m starting with the third panel. Panel 

lengths were generally 3350 to 3960 m. The annual 

production from the mine is approximately 5.9 million 

tones per year, of which 5.0 million tones are produced 

from the longwall mining system. Face emission rates 

are variable making peak towards the end of head-to-tail 

passes, but emissions exceeding 0.11-0.12 m3/s (233- 

255 cfin) trigger gas delays for several minutes during 

which measured methane concentrations decrease. The 

methane production rates of gob gas ventholes vary 

depending on their location on the panel. Initial methane 

production rates of gob gas ventholes are usually 

between 8500 and 14,200 m3/day (0.3-0.5 MMscU 

day) in this area and they decline with time. The 

cumulative production fiom four gob gas ventholes 

located on the study panel was about 2.8 x lo6 m3 (100 

MMscQ during 268 days of panel extraction. 

The overburden depths ranged between 152 and 

274 m. A generalized stratigraphic section of the strata 

above the Pittsburgh coalbed in the study area is shown 

in Fig. 5. The Sewickley coalbed as well as the rider 

coals (not shown in this figure) directly above the 

Pittsburgh coalbed are believed to be the primary source 

of gob gas during longwall mining. Gas released fiom 

the Pittsburgh rider coals located in the caved zone is 

expected to migrate to the mine ventilation system, 

while gas in the Sewickley coalbed, as well as gas fiom 

any other gas-bearing horizons above the caved zone, 

migrates to the pressure sink of the operating gob gas 

ventholes in the fractured zone. 

Gob gas ventholes are generally drilled to within 12 

or 13 m of the top of the Pittsburgh coalbed and are 

completed with a 17.8-cm casing and 61 m of slotted pipe 

at the bottom (Mucho et al., 2000). NIOSH has instrumented the gob gas ventholes (four ventholes on the study 

panel) in the study area to continuously measure gas 

production rates and methane concentrations as they 

become operational, and these holes were monitored as 

mining progressed over time. These data became an 

integral part of the model calibration (Karacan et al.. 

2005). 

A methane drainage program, including in-seam 

horizontal boreholes in the Pittsburgh coalbed, is also 

used at the study site to shield the gateroads during 

development mining and to reduce the in-place methane 

content of the outlined longwall panel. As the development sections advance, two to three sets of two horizontal boreholes (configuration A, Fig. 4) are generally 

dnlled fiom what will eventually be the tailgate entnes 
.- Tailgate entries 

Headgate entries 

Mining direction 

Fig. 4. Horizontal methane drainage borehole patterns modeled for degasification of the longwall panel. The entries represent a three-entry system 

with intervening coal pillars. 

of each panel. The number of sets depends generally from the interior of the panel. In most mining operaon the length of the panels and the length of the tions, the individual horizontal boreholes are connected 

individual boreholes. The holes are drilled towards what to a common underground pipeline for gas flow 

will be the start-up end of each panel, with one hole metering and transmission to the surface. This approach 

paralleling the tailgate side of the panel about 35 m from is logistically easier than monitoring each hole individthe margin of the panel. The second hole of each set arcs ually underground. However, this does not provide 

across the panel parallel to the headgate side of the panel the detailed production histories f?om individual boreand provides both shielding of the development entries holes and panels for evaluation and model calibration 

on that side of the panel and general drainage of gas purposes. 

Interval Thicknesses Major Layers with Thicknesses 

Shaly Sandstone-6.1 m 

Sewickley-1.8 m 

Shale-1.2 m 

LimestoneS.5 m 

Shale-1 0.4 m 

Sandstone Chan.-0112.2 m 

Fig. 5. Generalized stratigraphic section of the study area (not to scale) showing major rock units and their thicknesses used in the reservoir model. 
4. Development and application of a reservoir model 

for evaluating longwall mining methane drainage 

4.1. Grid model of the area 

The 3D grid model of the area was created using 

Cartesian grids. Mine maps and existing panel dimensions in the study area were used to structure the grids for 

associated mine-related features. The number of vertical 

layers and their thicknesses were based on the generalized stratigraphic section representing the major layers in 

the mine area (Fig. 5). In generating the grids, each of the 

individual layers shown in Fig. 5 was assumed to be 

uniform in thickness and continuous throughout the 

simulated mine area, except for the sandstone paleochannel complex and associated shale unit overlying the 

Pittsburgh coalbed. For these two layers, non-uniform 

grids were used based on the spatial thicknesses 

determined ii-om isopach contour maps. 

Fig. 6 shows a cut-away of the 3D reservoir model 

that was constructed for this study. In this figure, some 

of the intervening rock layers have been removed for 

visualization purposes. The figure also shows the gob 

gas ventholes on the panel, wellbores that represent the 

basic elements of the ventilation system in the model, 

and the type of horizontal methane drainage wellbores 

typically used in this study area (configuration A, Fig. 4). 

The grids of the Pittsburgh coalbed layer were 

structured differently from the other layers in the area 

to include the details of the longwall mining environment, the lower part ofthe caved zone, and the horizontal 

methane drainage boreholes. This layer was constructed 

in such a way that it would host both the mined and 

unrnined Pittsburgh coalbed and the gateroads that surround the panel (Fig. 7). 

4.2. Representation of elements of the methane control 

system 

4.2.1. Gob gar ventholes 

Gob gas ventholes are used as an aid to the ventilation 

system by capturing released strata (gob) gas as the rock 

layers are fractured during longwall mining. At this site, 

four gob gas ventholes were present on the study panel, 

each having a 17.8-cm (7-in.) casing and 6 1 m of slotted 

pipe at the bottom. However, the ventholes were drilled 

Gob gas ventholes 

Bleeder 

Return 

Degasification boreholes Mining direction 

Fig. 6. Transparent,3D,cut-away grid model ofthe study area (inner layers removed) showing the major coalbeds and the sandstone paleochannel. This 

figure also shows elements of the methane control system used in the model. The entries represent a three-entry system with intervening coal pillars. The 

heavy lines in the well trajectories represent the open-to-flow sections of the wells and lighter traces represent the cased sections of the wells. 
Fig. 7. Pittsburgh coalbed model layer grid showing simulated longa [all panel, development entries, and ventilation system elements. The entries 

represent a three-entry system with intervening coal pillars. 

to varying proximities, 14, 11,9, and 12 m above the top 

of the Pittsburgh coalbed, as opposed to the preferred 

12 m (40 ft) distance for all holes. This placed the 

bottoms of the ventholes above or close to the top of the 

caved zone. The ventholes were located about 100 m 

from the tailgate side of the panel and were 166, 830, 

1635, and 2458 m from the start-up end of the panel. 

Gob gas ventholes instrumented by NOSH yielded 

continuous readings of gas production rates and methane 

concentrations. The production data measured in the 

field during panel extraction were used to calibrate the 

model using a history matching process. Venthole 

configurations were based on actual completion parameters of the study panel, while operations were based 

on the actual production histories during 268 days of 

mining. The history matching results for gob gas 

venthole productions in this panel area and details of 

model calibration are given in Karacan et al. (2005). 

4.2.2. Simplified ventilation system 

A simplified version of the ventilation system was 

incorporated into the model to simulate ventilation 

airflows in the gateroads during pre-mining degasification and during longwall panel extraction. For this 

component of the model, wells were used to simulate 

the injection (intake) and removal (return) of ventilation airflows, including the bleeder fan shaft (Fig. 7). 

The air intake part of the ventilation system was 

modeled by a single well injecting air at constant flow 

rate conditions. 

The bleeder fan was modeled by a large-diameter 

vertical well on the tailgate side of the panel with the 

completion interval being equal to the height of the 

entries (thickness of the Pittsburgh coalbed). During 

simulations, the bleeder fan was operated with a bottomhole pressure of -13.6 kPa (-2.0 psia). Another vertical 

well modeled the return air side of the ventilation 

system. The vertical wells representing the ventilation 

intake, return, and bleeder alrf3ow also were used to 

control the ventilation pressures at the mining layer. 

4.2.3. In-seam methane drainage boreholes 

Five different horizontal, in-seam, methane drainage 

borehole patterns were evaluated in this study (Fig. 4, 

patterns A-E). The boreholes in patterns C and D were 

spaced equally. In all of the patterns, each borehole was 

modeled as a 3-in. (7.5-cm) diameter, unstimulated well 

drilled fiom the tailgate entries into the Pittsburgh 

coalbed. The total lengths of the simulated boreholes 

were 5606 m, 8476 m, 1158 m, 2316 m, and 1845 m 

(18,388 ft, 27,810 ft, 3800 ft, 7600 ft, and 6052 ft) for 

patterns A, B, C, D, and E, respectively. The boreholes 

operated at bottom-hole pressures equal to atmospheric 

pressure to represent the absence of any exhausters used 

to aid gas flow. Gas production was simulated for 3,6,9, 

and 12 months in advance of longwall mining. To 

evaluate gas production potential during mining, the 3- 

and 12-month pre-mining methane drainage models 

for each borehole pattern were extended by 268 days 

to include the production of gas during the longwall 

mining phase. To simulate methane drainage during 

mining, boreholes were shut-in as the face reached their 

locations, a practice employed at the study mine site. 

Pattern A (dual-lateral borehole) horizontal methane drainage boreholes, commonly used at the study 

mine, were also used in calibrating the model by 

matching the average reported methane productivity 

(methane volume/production timeilength of borehole) 
-- - 

of the actual boreholes. The gas average gas production rate from the simulated boreholes in the model 

was matched to the reported average methane production rates of three horizontal degasification boreholes 

in the mining area [0.782 m3/day/m (8.42 scfldayift) 

vs. 0.807 m3/day/m (8.70 scfldaylft)]. A similar fielddata-based adjustment could not be made for water 

production because no data were available. Thus, the 

boreholes were assumed to produce minimal amounts 

of water. 

4.3. Reservoir description 

4.3.1. Basic reservoir description of the mining area 

Table 1 gives the representative reservoir parameters 

for both coal and non-coal lithologies that were obtained 

by calculations, the reports about other districts in the 

area, personal communications with the operating 

mining company, and estimates from history matching 

techniques. These values, particularly the permeabilities, represent the reservoir values prior to mining and 

thus prior to any disturbance of the strata. 

Some of the most important coal-related reservoir 

parameters are the cleat characteristics (spacing, direction, etc.), gas content, and adsorption data. In this study, 

cleat spacing was estimated at 3.0 cm, a value based on 

the reported mean cleat spacing values of 2.4 cm and 

3.2 cm from outcrops of the Pittsburgh and Sewickley 

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

adsorption data for the Pittsburgh coalbed and for the 

other major coalbeds in the area were obtained fiom the 

results of laboratory methane adsorption isotherm tests 

and direct method gas content determination tests 

Table i 


Examples of pre-mining basic reservoir-rock properties (for fractures) 


used in the study (x and z are horizontal, face-cleat. and vertical 


permeabilities, respectively) 


Parameter 	 Pittsburgh Sandstone Limestone Shale Entries 

coalbed 

Permeability-x, md 4.0 10 2.0 0.2 9x lo7 

Permeability-z, md 0.1 10 2.0 0.1 9x lo7 

Effective porosity, % 4.0 10 2.0 1.0 40 

Effective fracture 0.031 lSl(50) 601(200) 601 301 

spacing, d(A) (0.01) (200) (100) 

Langmuir pressure, 2.251 - - - -

MPa/@si) (326) 

Langmuu volume, 15.41 - - - -

cm31g (scffton) (490) 

Desorption time, 20 - - - -

days 

Coal density, gicml 1.35 - - - -

on various coal samples (Diamond et a]., 1986). The 

hcture permeabilities for the coalbeds were estimated 

using history matching to be 4 md in the face cleat 

direction (approximately an East-West direction at the 

study site), and 1 md in the butt cleat direction (NorthSouth direction). 

In the model constructed for this study, the open 

entries surrounding the longwall panel served as main 

paths for ventilation airflow. The permeabilities for the 

area including the three entries and intervening coal 

pillars were assigned high values [9 x lo-' m2 (lo8 md)] 

within the allowed limits of the simulator for minimum 

resistance. The associated fracture porosity and fracture 

spacing for the entries were calculated from mine maps 

as 40% and 30 m, respectively. 

Horizontal methane drainage boreholes drilled either 

into virgin coalbeds or into areas far from active mining 

may contain significant amounts of water that must be 

drained to initiate gas production. In this case, these 

boreholes show the characteristic production behavior of 

vertical coalbed methane wells (Ertekin et a].: 1988). 

However, the Central and Northern Appalachian Basins 

have generally been depressurized, leaving the coalbeds 

undersaturated because of their geologic histories, extensive coal mining, and the presence of numerous oil 

and gas wells (Hunt and Steele, 199 1). Also, horizontal 

methane drainage boreholes in this region are generally 

drilled from active mining operations that have themselves partially dewatered the coalbeds. Thus, these 

drainage boreholes generally do not experience significant amounts of water production and generally exhibit 

a declining methane production profile (Zuber, 1998). 

Although, water production data were not available for 

matching the model predictions for the study panel, the 

operating mining company reported no major water 

influx into the caved zone and no significant water 

production from either horizontal boreholes or gob gas 

ventholes. Therefore, it was assumed that there was 

minimal mobile water in the area. 

4.4. Simulation of longwall mining methane drainage 

Simulation of longwall panel methane drainage was 

conducted in two phases. In the first or pre-mining phase, 

methane drainage was simulated for periods of 3, 6, 9, 

and 12 months using the horizontal borehole configurations shown in Fig. 4. In the second simulation phase, the 

models with minimum and maximum pre-mining 

methane drainage intervals (3 and 12 months) were 

extended to include gas drainage during longwall 

mining. This defined the effects of the additional gas 

drainage time on total gas recovery and subsequent face 
emissions. However, incorporating longwall panel 

extraction required additional reservoir modeling considerations since longwall face advance and related strata 

disturbances had to be represented in the simulations. 

The moving boundary problem imposed by panel 

extraction was addressed with "restart" models, in 

which the simulation outputs from the previous model 

run were written in a "restart" file. This file was then 

used by the next model run as the initial conditions for 

that run. The mechanical response of the overlying strata 

to mining was modeled using Itasca Consulting Group's 

(Itasca, 2000) FLAC2D (Fast Lagrangian Analysis of 

Continua 2D) and the calculated permeability changes 

were incorporated into the reservoir model between 

"restart" runs to simulate reservoir changes during panel 

extraction (Karacan et al., 2005). To extend the premining methane drainage into the panel extraction 

phase, the restart files saved at the end of the 3- and 12- 

month pre-mining intervals were used as the initial 

conditions for the start of longwall mining. While the 

longwall face was advancing, the horizontal boreholes 

were allowed to produce methane from the unmined 

portions of the panel until they were intercepted by the 

longwall face at predefined datesltimes in the data set. 

For simulations during panel extraction with concurrent methane drainage, the gob gas ventholes became 

operational when their locations were intercepted by the 

Table 2 

advancing face. Methane drainage through the horizontal 

boreholes continued with the same constraints as during 

the pre-mining phase, although gas production terminated 

as the longwall face intercepted each borehole. 

5. Results and discussion 

5.1. Efects of pre-mining degasification time and 

horizontal borehole pattem configuration on methane 

drainage performance 

The effects of pre-mining methane drainage lead time 

and configuration of the horizontal borehole pattem on 

reductions of gas-in-place within the longwall panel 

were assessed for 3, 6, 9, and 12 months of production 

periods prior to mining (Table 2). The original gas-inplace within the outlined longwall panel was calculated 

to be 10.3 x 1o6 m3 (364 MMscf) based on an average 

methane content of 2.26 m3/ton (80 scf7ton) for the 

Pittsburgh coalbed (Karacan et al., 2005). Table 2 gives 

a summary of the general performance of each borehole 

pattern. Within the study area, the dual-lateral pattern 

(A) was the most commonly used borehole configuration. The data show that the cumulative methane 

production from the panel increases, as expected, with 

increasing time interval. Cumulative methane productionusingpattern Ais 1.1 x106, 1.8~10~,2.3~ lo6 and 

General performance comparison of different horizontal methane drainage borehole patterns shown in Fig. 4 after various pre-mining degasification 

time intervals 

Time (months) Parameter 

OGIP ": 10.3 x lo6 m3 

3 Cumulative methane production (x lo6 ml) 

6 

9 

12 

3 Percent of initial gas in panel area (%) 

6 

9 

12 

3 Average production rate (x lo3 m3/day) 

6 

9 

12 

3 Average production per unit borehole length (m3/m) 

6 

9 

12 

3 Average specific production rate (m3/day/m) 

6 

9 

12 

Wellbore patterns (Fig. 4) 

A B C D E 

1.1 1.7 0.5 1.O 0.8 

1.8 2.8 0.8 1.6 1.3 

2.3 3.5 1.1 2.0 1.6 

2.6 4.0 1.2 2.4 1.9 

11.0 16.6 5.1 9.6 7.5 

17.6 26.7 8.2 15.5 12.4 

22.1 33.5 10.4 19.8 15.9 

25.4 38.6 12.0 23.0 18.5 

12.3 18.7 5.7 10.7 8.4 

10.1 15.3 4.7 8.9 7.1 

8.4 12.8 4.0 7.5 6.1 

7.2 10.9 3.4 6.5 5.2 

202.5 202.5 451.5 426.4 4 19.9 

323.3 325.2 727.4 692.1 693.1 

406.0 407.8 920.7 879.8 887.2 

468.2 469.2 1068.4 1021.9 103 1.2 

2.20 2.20 4.92 4.63 4.56 

1.79 1.81 4.03 3.84 3.85 

1.51 1.5 1 3.42 3.25 3.28 

1.28 1.28 2.93 2.80 2.84 

a Original gas in place within the panel area calculated based on an average methane content of 2.26 m3/ton (80 scffton). 
2.6 x lo6 m3 for 3, 6, 9, and 12 months of drainage from 

the panel, respectively. This corresponds to reductions 

of 11.0%, 17.6%, 22.1%, and 25.4% of the original 

10.3x lo6 m3 gas-in-place, respectively for the same 

drainage times. Fig. 8 shows the cumulative methane 

production for each simulated horizontal borehole 

pattern at the end of each degasification time interval. 

Among all horizontal borehole patterns simulated, 

pattern B (tri-lateral pattern), with the longest total 

borehole length, provided the maximum reduction of 

gas-in-place within the longwall panel. Its total methane 

productions were 1.7 x lo6 m3 and 4.0 x lo6 m3 of 

methane after 3 and 12 months of methane drainage, 

respectively. Gas reductions in the panel area after these 

periods were 16.6% and 38.6% (Table 2). The difference 

in cumulative methane production fiom patterns A and 

B was due to the enhanced methane drainage in the 

middle part of the panel from the additional horizontal 

borehole placed in that area with pattern B. 

The other patterns (C-E) drilled shorter, cross-panel, 

horizontal holes parallel to the longwall face. These 

produced lesser amounts of methane for the simulated 

periods than pattcrns A md B, mainly because of the 

shorter borehole lengths. After 12 months of methane 

drainage, patterns C, D, and E produced 1.2, 2.4, and 

1.9 x lo6 m3 of methane, respectively, corresponding to 

12.0%, 23.0%, and 18.5% reductions in the calculated 

methane content of the panel area. However, as the 

number of short cross-panel boreholes increases, their 

total production can match or exceed the performances 

of A and B. For instance, drilling short cross-panel 

boreholes that will give the same total borehole length as 

3 months 6 months 

pattern B will require thlrty of these boreholes along the 

panel. Although it is not simulated, present data with 

only eight of them @attern D) suggest that the total 

production fiom 30 short boreholes will exceed the 

productions of both A and B. However, it should be 

noted that drilling costs may be more than that of A and 

B and they may not be as effective in shielding the 

entries, as will be discussed later. 

The sirnulat~on results presented in Table 2 show that 

the average gas production rates for all patterns are 

highest for the initial 3 months of methane drainage. The 

average production rates for the first three months are 

12.3, 18.7, 5.7, 10.7, and 8.4~ lo3 m3/day for patterns 

A, B, C, D, and E, respectively. As methane drainage 

time increases, average production rates decrease. This 

is an expected consequence of reducing the gas-in-place 

over time within the boundaries of the outlined longwall 

panel and producing lesser amounts of gas during the 

late stages of degasification. For 12 months of methane 

drainage, average methane production rates from 

patterns A, B, C, D, and E are calculated to decline to 

7.2, 10.9, 3.4, 6.5, and 5.2 x lo3 m3/day, respectively. 

Normalizing cllmulative methane production and 

average flow rates to borehole length gives a better 

productivity comparison between drilling patterns 

(Table 2). This comparison highlights the importance 

of borehole orientation relative to face cleat and butt 

cleat direction, which are in the East-West and NorthSouth directions, respectively, and associated permeability anisotropy, which is 4: 1. Total methane production per unit borehole length and average flow rate per 

unit borehole length are higher for patterns C, D, and E, 

9 months 12 months 

Fig. 8. Cumulative methane production and possible reductions in longwall face emission rates (100% emission degree basis) for different methane 

drainage time intervals and different horizontal borehole patterns (A-E) shown in Fig. 4. 
compared to patterns A and B for the first 3 months of 

degasification. For instance, the total production and 

average flow rate per unit borehole length for 3 months 

of simulated methane drainage from pattern C, where 

the boreholes are drilled perpendicular to higher 

permeability face cleats, are 451.5 m3/m and 4.92 m3/ 

day/m, respectively. However, the total production and 

average flow rate per unit borehole length for pattern A, 

where the boreholes are drilled perpendicular to the 

lower permeability butt cleats, are only 202.5 m3/m and 

2.20 m3/day/m, or less than half that of pattern C. 

A similar evaluation for pattern D, where eight 

shorter, cross-panel methane drainage boreholes are 

drilled parallel to the longwall face (perpendicular to the 

face cleats) with a total length of only 2316 m, shows 

that this pattern drains methane quantities comparable to 

that of pattern A, which has a total borehole length of 

5606 m. The cumulative methane production of pattern 

D is 2.4~ lo6 m3, as compared to 2.6~lo6 m3 for 

pattern A after 12 months of degasification. The 

productivities of pattern D, based on total methane 

production (102 1.9 m3/m) and average flow rate per unit 

borehole length (2.80 m3/day/m) are higher than both A 

and B and are similar to those of pattern C (1068.4 m3/m 

and 2.93 m3/day/m, respectively) based on 12 months of 

methane production. These data show that, although the 

cumulative production and thus reduction in initial gas 

content of the panel area is higher with an increased 

number of wells in pattern D compared to C, the 

production and production rate per unit borehole length 

are similar because of equal operating and completion 

conditions of the boreholes and uniform reservoir 

behavior. 

Table 3 

The performance of pattern E for pre-mining degasification is bemeen C and D in terms of productivity per 

unit borehole length. It produces 419.9,693.1,887.2, and 

103 1.2 m3 methane per meter of drilled boreholes after 3, 

6, 9, and 12 months, respectively. 

5.2. Effect of pre-mining degaszfication time and 

horizontal borehole pattern conJiguration on reducing 

potential face emissions 

Methane drainage prior to panel extraction reduces 

the potential for face emissions during mining as it 

reduces the in-place gas content of the coalbed to be 

mined. The horizontal borehole production data were 

evaluated to estimate the potential average reductions 

in face emission rates as a result of pre-mining methane 

drainage. The calculated average face emission reductions are presented in Table 3. For this calculation, 

cumulative methane production given in Table 2 was 

divided by the duration (268 days) of mining for the 

study panel. The assumption in this approach was that 

either the majority (75%) or the total (100%) of the 

produced methane would be released as face emissions 

during mining, if it was not removed prior to mining. 

This assumption is based on the work of Noack (1 998), 

who proposed that, in the absence of empirical data, 

the degree of gas emission (percent of gas-in-place) 

could be assumed to be 100% in stratigraphic zones 

within 20 m to - 11 m of the top and bottom of the 

mined coalbed, respectively, and 75% in the mined 

coalbed. 

The calculated average face emission rates that can 

be expected if methane drainage is not utilized are 

Potential reductions in average longwall face emission rates after degasifying the coalbed for different horizontal methane drainage borehole panems 

and pre-mining degasification time intervals 

Time (months) Parameter 	 Borehole patterns 

(Fig. 4) 

Average face emission 


reduction (m3/min) 


(degree of emission: 100%) 


1L 

Average face emission (100% basis) without 

degasification (m3/min) 

3 Average face emission 

6 reduction (m3/min) 

9 (degree of emission: 75?4) 

I L 

Average face emission (75% basis) without 

degasification (m3/min) 
26.7 m3/min (943 cfm) for the 100% gas emission basis 

and 20.0 m3/min (707 cfm) for the 75% emission basis 

(Table 3). These predictions are based on the calculated 

total in-place methane content of 10.3 x lo6 m3 (364 

MMscf) in the outlined panel and 268 days of longwall 

mining. The importance of methane drainage using inseam horizontal boreholes for reducing average face 

emission rates is demonstrated by converting the 

production volumes in Table 2 to face emission rates. 

The potential average face emission reductions calculated for the 100% and 75% degree of gas emission are 

given in Table 3. These data show that using horizontal 

borehole pattern B in the Pittsburgh coalbed reduces the 

average face emission rates by as much as 10.3 m3/min 

(364 ch) for the 100% emission basis and by 7.7 m3/min 

(273 cfin) for the 75% basis after 12 months ofpre-mining 

degasification. The methane emission reduction values 

for pattem A are as much as 6.8 m3/min (240 ch) for 

the 100% basis and 5.1 m3/min (180 ch) for the 75% 

basis. The possible face emission reduction values 

calculated for D are 6.1 and 4.6 m3/min (215 and 

162 ch) for the 100% and 75% basis respectively for 

12 months degasification period. Lesser amounts of face 

emission reductions are calculated for patterns C and E. 

Fig. 8 compares cumulative methane productions, 

different horizontal borehole patterns, and the reductions 

Gob gas ventholes 

1-4 

Return 

Air intake 

boreholes 


(Pattern A in Figure 4) 


in face emission rates on the 100% degree of gas 

emission basis. 

5.3. Contribution of continued degasification during 

mining and individual horizontal borehole pattern to 

overall degasification performance 

The effects of continued methane drainage on 

face emissions during active mining were evaluated by 

switching on the panel extraction phase of the model and 

extending the modeling of the degasification process 

beyond the previously simulated 3 and 12 months of premining methane drainage. During simulation of longwall 

degasification, gob gas ventholes were put into production as the longwall face advanced to their location. Gas 

production £rom the horizontal methane drainage boreholes continued until the longwall face reached the 

location of each borehole. This section analyzes the 

impact of continued degasification on gas production 

and the resultant additional reductions in face emissions. 

Fig. 9 shows, as an example, a snapshot of the pressure 

distributions in the primary coalbed gas reservoir layers 

associated with longwall mining of the Pittsburgh 

coalbed utilizing both gob gas ventholes and horizontal 

borehole methane drainage pattern A. This figure shows 

the position of the face at the second gob gas venthole 

-

Pressure 

Fig. 9. A snapshot of coalbed pressure distribution at a defined longwall face position in mining layer and two other coalbed horizons during longwall 

mining and continued degasification using pattern .A in Fig. 4. The boundaries of the model and the panel area are represented by dashed lines. The 

open-to-flow sections of the gob gas ventholes are depicted as heavy lines and the cased sections are shown as lighter lines. 
location during panel extraction phase of the model. It 

should be noted that the pressures in the entries and 

around the degasification boreholes are close to atmospheric pressure levels. Also, reservoir pressures in and 

above the mined-out sections, including overlying 

fractured zones, have decreased from initial reservoir 

pressure to atmospheric pressure ranges due to the pressure sink created by the ventilation boundary conditions 

and the effect of imposed permeability fields during 

panel extraction calculated by FLAC modeling. 

Fig. 10 shows methane production from the simulated horizontal methane drainage borehole patterns shown 

in Fig. 4 before and during panel extraction. The results 

show that methane production during panel extraction is 

substantially less than that of the pre-mining phase. 

Additionally, as the pre-mining degasification time 

increases from 3 to 12 months, the amount of methane 

produced by the horizontal boreholes during panel 

extraction decreases even more. The reduced methane 

production is probably due to a combination of two 

factors. First, the methane content of the coalbed is 

reduced during the pre-mining methane drainage phase, 

leaving less methane available for production at a slower 

rate during mining, especially after a longer pre-mining 

degasification. Second, the methane production from 

each horizontal borehole is progressively terminated as 

the longwall face reaches its location. Therefore, their 

production lives are significantly shortened during the 

panel extraction phase of the simulations. However, it 

should be noted that after only a short period of premining methane drainage (3 months in these simulations), continued degasification during panel extraction 

becomes more important since more gas remains in 

place to be produced during this phase. The simulation 

results shown in Fig. 10 indicate that the highest 

cumulative methane production is achieved with long 

horizontal borehole patterns A and B. However, as the 

number of shorter, cross-panel, horizontal boreholes 

perpendicular to the higher permeability face cleats 

increases, as in pattern D, their cumulative production 

approaches that of pattern A. 

5.4. Effect of continued horizontal borehole methane 

drainage 'during mining on reducingface emissions 

The overall reduction in average face emission rates 

due to methane drainage can be evaluated by analyzing 

the panel extraction phase and combining it with premining methane drainage performance. The average 

reductions in longwall face emission rates due to methane drainage before and during mining were calculated 

based on the 75% and 100% degrees of emission bases 

(Fig. 10 and Table 4). The data show that longwall face 

emissions were reduced by 10.3 m3/min (364 ch) for the 

100% basis and by 7.7 m3/min (273 ch) for the 75% 

basis with 12 months of pre-mining methane drainage 

using borehole pattern B. Reductions of 6.8 m3/min 

(240 ch) for the 100% basis and 5.1 m3/rnin (180 ch) 

for the 75% basis were obtained using pattern A prior to 

mining. The face emission reductions achieved with 

borehole patterns B and A after 3 months of pre-mining 

degasification were 4.4 m3/min (1 57 ch) and 2.9 m3/min 

(104 ch), respectively, for the 100% emission basis. As 

would be expected, the simulations show that 3 months of 

" I 	 Solid Fill Colors: &months Pre-rnine.Degas. I 

Patterned Fill Colors: 12-months Pre-mine.Degas. 

(D0 

Before Mining During Mining Cumulative 

A-E A-E A-E 

Fig. 10. Performances of different horizontal methane drainage borehole patterns shown in Fig. 4 during different phases of degasification, after 3 and 

12 months of pre-mining degasification. 
Table 4 

Simulated reductions in face emission rates after different phases of methane drainage using horizontal boreholes from the longwall panel 

Pre-mining degasification Quantity 

(months) 

Degree of emission (%) 

Pre-mining degasification (m3/min) 

During-mining degasification (m3/min) 

Total reduction (m3/min) 

pre-mining methane drainage is less effective in reducing 

face emission rates compared to 12 months. 

After 12 months of pre-mining degasification, the 

additional reductions in face emissions with continued 

degasification during mining are 0.9 m3/min and 1.2 m3/ 

min (30.1 and 43.7 ch) for the 100% basis with 

patterns A and B, respectively. The additional face 

emission reduction that can be achieved with patterns D 

and E due to continued degasification is similar to that 

of A (0.8 and 0.7 m3/min, respectively), and it is less 

with C after 12 months of pre-mining degasification. 

The data in Table 4 show that after only 3 months ofpremining degasification, continued degasification during 

panel extraction becomes more important. 

The data presented in Table 4 show that after 3 months 

of pre-mining degasification, continued methane drainage during mining using borehole pattern A reduces face 

emission rates by 1.8 m3/min (63.6 cfm) for the 100% 

degrees of emission basis and 1.4 m3/min (47.7 cfm) for 

the 75% basis. These reductions are more than 50% of 

what can be achieved in addtion to 3 months of premining degasification. Similar impacts are achieved with 

other borehole patterns as well, as a result of continued 

degasification during panel extraction after a short (3- 

month) pre-mining degasification period. On the other 

hand, the incremental reduction obtained with continued 

degasification during panel extraction in average face 

emission rates after 12 months of pre-mining degasification using the same borehole pattern is about 15% of 

what can be achieved by 12 months pre-mining degasification. Again, similar reductions in average face 

emissions were observed for the other borehole patterns 

after continued degasification. This analysis shows that 

continuation of degasification during panel extraction 

becomes more important if the pre-mining degasification 

duration is short. 

After 3 months of pre-mining degasification, additional drainage during mining using borehole pattern B 

reduces face emission rates by 2.7 m3/min (95.3 ch) for 

Wellbore patterns (Fig. 4) 

A B C D E 

100 75 100 75 100 75 100 75 100 75 

2.9 2.2 4.4 3.3 1.4 1.0 2.6 1.9 2.0 1.5 

6.8 5.1 10.3 7.7 3.2 2.4 6.1 4.6 5.0 3.7 

1.8 1.4 2.7 2.1 0.7 0.5 1.6 1.2 1.4 1.0 

0.9 0.6 1.2 0.9 0.4 0.3 0.8 0.6 0.7 0.5 

4.7 3.6 7.2 5.4 2.1 1.5 4.2 3.1 3.4 2.6 

7.6 5.7 11.5 8.7 3.6 2.7 6.9 5.2 5.7 4.2 

the 100% emission basis and 2.1 m3/min (74.2 ch) for 

the 75% basis. These numbers show (Table 4) that 

additional degasification after a 3-month pre-mining 

production period with B is almost as effective as using 

pattern A (2.9 m3/min) for pre-mining degasification for 

3 months. This comparison emphasizes the importance 

of wellbore pattern design, continued degasification 

during panel extraction, and the duration of degasification before and during mining. 

The total (including degasification before and during 

panel extraction) reductions in average face emissions after 

12 months of methane drainage before and during mining 

are significant, particularly with horizontal borehole 

patters A, B, and D. The total reduction that can be 

achieved with pattern B is between 8.7 and 11.5 m3/min 

(307-408 ch). This reduction can decrease the average 

maximum emission rate that may be expected fiom the 

Pittsburgh coalbed from 20.0-26.7 m3/min (707- 

943 cfm) to less than 11.3-15.2 m3/min (400-500 ch). 

5.5. Effect of pre-mining degasijication and in-seam 

horizontal boreholes on shielding the development 

entries from methane migration from the panel area 

and surrounding coalbed 

In-seam methane drainage boreholes not only degas 

the longwall panel itself, but can also shield the advancing 

development entries from methane emissions fiom the 

surrounding virgin coalbed gas reservoir (Diamond, 

1994). The impacts of borehole pattern on the effectiveness of shielding gateroads fiom methane migration were 

evaluated by simulating the cumulative methane emissions into the modeled ventilation system. In this analysis, 

two approaches were used. In the first approach, methane 

emissions were predicted in conjunction with operating 

in-seam horizontal methane drainage boreholes, configured as shown in Fig. 4. In the second approach, three 

horizontal boreholes were placed in the virgin coalbed 

along the gateroads on both sides of the outlined panel, as 
Tailgate entries 

q- Headgate entries 

Fig. 1 1. An example layout showing additional horizontal, in-seam boreholes on either side of the gateroads in addition to pattern A. The entries 

represent a three-entry system with intervening coal pillars. 

shown in Fig. 1 1 for pattern A. It should be noted that this 

configuration of additional boreholes is only completely 

applicable to the first longwall panel developed in a new 

mining district, since the tailgate entries of each 

subsequent panel will be shielded by the holes drilled 

on the headgate side of the previous panel. These 

additional boreholes were modeled with the same completion and operating parameters as discussed previously 

for the in-seam horizontal boreholes of Fig. I.The first 

approach is intended to analyze the amount of methane 

migrating into the gateroads from the panel area and the 

surrounding coalbed only in the presence of in-panel 

horizontal methane drainage boreholes. The second 

approach analyzes the effects of additional boreholes on 

shielding the entnes fiom methane emissions from the 

surrounding coalbed during pre-mining degasification. 

Table 5 gives the predicted methane inflow into the 

ventilation system from the surrounding virgin coalbed 

at the end of various methane drainage time intervals 

based on the presence of in-panel horizontal methane 

drainage boreholes (A to E in Fig. 4) along with methane 

inflow into the ventilation in the absence of any degasification borehole. These comparative results show that 

borehole patterns A and B, where near-margin horizontal 

Table 5 

Predicted methane emissions into the ventilation system from the 

surrounding coalbed (within the panel and virgin coal) during different 

pre-mining degasification time intervals using different borehole 

pattems 

Well pattern Methane lnflow into mine ventilation system 

(x lo6 m3) 

3 months 6 months 9 months 12 months 

No degasification 2.23 3.56 4.5 1 

A 2.11 3.28 4.07 

B 2.11 3.28 4.06 

C 2.2 1 3.5 1 4.43 

D 2.19 3.48 4.38 

E 2.2 1 3.51 4.41 

A+ 1.89 2.82 3.38 

B+ 1.89 2.82 3.38 

C+ 2.04 3.05 3.75 

D+ 1.97 3.02 3.70 

E+ 1.99 3.04 3.72 

boreholes are drilled from the tailgate, are more effective 

against methane inflow into the gateroads during premining degasification than the other patterns because of 

their extended length along the entries. The simulation 

indicates that by using either pattern A or B, the potential 

methane inflow into the entries can be reduced by as 

much as 62 x 10" m3 over 12 months, which corresponds 

to a decrease in methane inflow by - 1.2 m3/min 

(-40 ch), just by shielding against methane inflow 

originating fiom the coalbed within the panel area. It 

should also be noted that methane emissions into the 

mine ventilation system can be reduced by similar 

amounts using both patterns A and B. As would be 

expected, this suggests that the middle horizontal 

borehole segments in pattern B do not contribute much 

to gateroad shielding. The emissions into the gateroad 

entries originate mostly from the gas desorbing fiom the 

margins of the panel where the near-margin borehole 

segments are located and these boreholes are more 

effective in capturing this gas. Patterns C, D, and E, 

which use short, cross-panel methane drainage horizontal boreholes parallel to longwall face, are not as effective 

in shielding the entries as patterns A and B since they 

cannot effectively block all of the methane migration 

pathways. 

Table 5 also shows the predicted methane emissions 

into the mine ventilation system fiom the surrounding 

coalbed using six additional horizontal boreholes (A+ 

configuration shown as an example in Fig. 11) with a total 

length of 6340 m (20,800 ft) added to the base set of 

boreholes drilled in the panel (Fig. 4). These six additional 

boreholes were estimated to produce 1.10, 1.72,2.13, and 

2.44 x lo6 m3 of additional methane during 3, 6, 9, and 

12 months of pre-mining methane drainage, respectively. 

The model simulations predict that these six additional 

methane drainage boreholes would reduce methane 

inflow to the gateroad entries fiom the surrounding virgin 

coalbed by 0.22,0.46,0.69, and 0.89 x 10h3 of methane 

between A and A+ (Table 5) for the same drainage times. 

Again, horizontal methane drainage borehole patterns A 

and B with six additional boreholes (designated as A+ and 

B+ in Table 5) were the most effective in reducing 

methane inflow into the gateroads. Compared to a case 

where no boreholes were present to shield the entries, 
operating A+ and B+ for 12 months resulted in 29% 

decrease in methane emissions into the ventilation entries 

(Table 5). 

The results of a borehole drilling study for shielding 

the entries from methane inflow were reported by 

DuBois et al. (2006). The study was conducted in 

Pittsburgh coalbed at the same mining district modeled 

in this study. The horizontal boreholes of pattern A were 

drilled to maximize the shielding for both belt and 

return entries during headgate and tailgate development. This approach resulted in a pattern similar to the 

one designated as A+ in this study. The drilling strategy 

described in their paper permitted horizontal boreholes 

to be active for 6-24 months prior to any mining. They 

reported that as a result of employing horizontal 

boreholes, methane concentration decreased by 41% 

and methane emission into the entries decreased 

between 30 and 35%, close to the predicted values in 

this study for patterns A+ and B+. 

6. Summary and conclusions 

A 3D reservoir model was constructed for a 381-111 

wide longwall panel operating in the Pittsburgh coalbed. 

Multiple horizontal borehole patterns and degasification 

durations prior to and during panel extraction were 

simulated to evaluate their relative effectiveness in 

reducing in-place gas volumes and longwall face 

emission rates. The basic conclusions of this numerical 

modeling effort can be summarized as follows: 

1. ~umulative methane drainage from the Pittsburgh 

coalbed increases with increasing in-seam methane 

drainage time. Among the horizontal well patterns 

modeled, pattern B produces the highest amount of 

methane: 1.7 x lo6 m3 after 3 months and4.0 x lo6 m3 

after 12 months of methane drainage. Patterns A and 

D produce similar amounts (-2.5 x lo6 m3 after 

12 months). The other simulated horizontal borehole 

patterns (C and E), use fewer shorter, cross-panel, 

horizontal boreholes parallel to the longwall face and 

produce lesser amounts. 

2. Normalization 	 of horizontal borehole gas production by their productive length in the coalbed gas 

reservoir shows the importance of cleat direction 

and associated permeability anisotropy on borehole 

performance. Average methane production rates per 

unit length of borehole for patterns C, D, and E, 

drilled perpendicular or at angles to the higher 

permeability face cleat, are greater when compared 

to patterns A and B, which are drilled perpendicular 

to the lower permeability butt cleat. The average 

methane production and rate (3-month production 

basis) for the horizontal boreholes represented by 

pattern C are 451.5 m3/m and 4.92 m3/day/m of 

hole length, respectively. For pattern A, these 

values are 202.5 m3/m and 2.20 m3/day/m of hole 

length. 

3. For the Pittsburgh coalbed with 12 months of premining methane drainage, the average longwall face 

emission rates can be reduced by as much as 

10.3 m3/min (364 ch) for the 100% emission basis 

and 7.7 m3/min (273 ch) for the 75% basis, using 

horizontal borehole pattern B. Using pattern A, 

reductions were 6.8 m3/min (240 ch) for the 

100% basis 'and 5.1 m3/s (180 ch) for the 75% 

basis. Similar emission reductions can be achieved 

using pattern D, and lesser amounts with patterns C 

and E. 

4. Methane production from horizontal methane drainage boreholes is less during the mining phase of 

degasification than during the pre-mining phase. Premining degasification reduces in-place gas volume so 

that there is less gas to produce later. Also, production 

from each horizontal borehole is progressively 

terminated as the longwall face reaches its location 

on the panel. The impact of pre-mining degasification 

is particularly evident as the degasification time 

interval increases from 3 to 12 months. If the premining methane drainage time is short, it is important 

to continue methane drainage during the mining 

phase to maximize reductions in longwall face 

methane emissions. In fact, it has been shown that if 

the panel gas volume is drained for shorter time 

periods, the contribution of methane produced during 

panel extraction may be a significant portion of the 

total gas production (more than 50%). This suggests 

that an additional average face emission reduction 

comparable to pre-mining degasification can be 

achieved during this period. Conversely, if the premining degasification time is longer, then the 

additional methane production during panel extraction may be only 10-20% of the pre-mining 

degasification. 

5. Methane migration into the mine ventilation airflow 

in the gateroad entries surrounding the outlined 

longwall panel can be reduced effectively by using 

horizontal methane drainage holes paralleling those 

gateroads (patterns A and B). Patterns C, D, and E that 

use short horizontal cross-panel boreholes drilled 

parallel to the longwall face are not as effective in 

shielding the development entries. They are not 

oriented to drain large volumes of gas along the 

margins of the panel, and they cannot block methane 
migrating towards the entries beyond their effective 

radius. This suggests that emissions into the gateroad 

entries are originating primarily from the margins of the 

panel. Additional reductions in methane ernissions into 

the gateroads can be achieved by employing horizontal 

methane drainage boreholes in the surrounding virgin 

coalbed gas reservoir on either side of the gateroads. 

Appendix A. Supplementary data 

Figure 6. Transparent, 3-D, cut-away grid model of the study area (inner layers removed) showing the 

major coalbeds and the sandstone paleochannel. This figure also shows elements of the methane control 

system used in the model. The entries represent a three-entry system with intervening coal pillars. The 

heavy lines in the well trajectories represent the open-to-flow sections of the wells and lighter traces 

represent t he cased sections of the wells
References 

Aul, G.N., Ray, R., 1991. Optimizing methane drainage systems to reduce 

mine ventilation requirements. Proc. 5th US Mine Ventilation Symposium, 

Morgantown, WV. 

Bmnner, D.J., Schwoebel, J.J., Li, J., 1997. Simulation based degasification 

system design for the Shihao mine of the Songzao Coal Mining 

Administration in Sichuan China. Proc. 6th International Mine Ventilation

Congress, Pittsburgh, PA. 

King, G.R., Ertekin, T., 1994. A survey of mathematical models related to 

methane production from coal seam: 111. Recent developments (1989-

1993). The International Coalbed Methane Extraction Conference, 

London, UK. Computer Modeling Group Ltd., 2003. Generalized equation of state modelGEM. User's Guide, Calgary, Alberta, Canada. 

Diamond, W.P., 1994. Methane control for underground coal mines.

Information Circular No. 9395, US Dept. of Interior, US Bureau of Mines, 

Pittsburgh, PA. 

King, G.R., Ertekin, T., Schwerer, F.C., 1986. Numerical simulation of the 

transient behavior of coal-seam degasification wells. Society of 

Petroleum Engineers (SPE) Formation Evaluation, pp. 165-183. April. 

Diamond, w.P.; Garcia, F., 1999. Prediction of longwall methane emissions:

an evaluation of the influence of mining practices on gas emissions and 

methane control systems. Report of Investiga- tions No. 9649, National

Institute for Occupational Safety and Health, Pittsburgh, PA. 

Law, B.E., 1993. The relationship between coal rank and cleat spacing: 

implications for the prediction of permeability in coal. Proc. Coalbed 

Methane Symposium, Tuscaloosa, AL. 

Mucho, T.P., Diamond, W.P., Garcia, F.,Byars, J.D., Cario, S.L., 2000. 

Implications of recent NlOSH tracer gas studies on bleeder and gob gas 

ventilation design. Proc. Annual Meeting of Society for Mining, 

Metallurgy and Exploration, Salt Lake City, UT. 

Diamond, W.P., La Scola, J.C., Hyman, D.M., 1986. Results of direct- method 

determination of the gas content of the US coalbeds. Information Circular 

No. 9067, US Dept. of Interior, US Bureau of Mines, Pinsburgh, PA. 

Diamond, W.P, Bodden, W.R., Zuber, M.D., Schraufnagel, R.A., 1989. 

Measuring the extent of coalbed gas drainage after 10 years of production 

at the Oak Grove Panem, Alabama. Paper 896 1, Proc., Coalbed Methane 

Symposium, Tuscaloosa, AL. 

DuBois, G.M., Kravitz, S.J., Reilly, J.M., Mucho, T.P., 2006. Target drilling's

long boreholes maximize longwall dimensions. Proc. 11th U.S. Mine 

Ventilation Symposium. Penn State University, University Park, PA. 

PTTC, 2004. Horizontal drilling, a technology update for the Appalachian 

Basin. Available at http:llwww.pnc.orglsolutionsi soU0041535.pdf. 

Ertekin, T., Sung, W., Schwerer, F.C., 1988. Production performance analysis 

of horizontal drainage wells for the degasification of coal seams. Journal 

of Petroleum Technology 625-632 (May). 

Esterhuizen, G., Karacan, c.O., 2005. Development of numerical models to 

investigate permeability changes and gas emission around longwall mining 

panels. Proc., AlaskaRocks, Proc. 40th US Symposium on Rock 

Mechanics, Anchorage, Alaska. 

Hungerford, F., 1995. Status of underground drilling technology. Intl. Symp.

Workshop on Management and Control of High Gas Emission and 

Outbursts. Wollongong, Australia. 

Singh, M.M., Kendorski, F.S., 1981. Strata disturbance prediction for 

mining beneath surface water and waste impoundments. Proc. 1st 

Conference on Ground Control in Mining, Morgantown, WV. 

Hunt, A.M., Steele, D.J., 1991. Coalbed methane development in the

Appalachian Basin. Quarterly Review of the Methane from Coal Seams 

Technology 1-4, 10- 19. 

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

6th International Mine Ventilation Congress, Pittsburgh, PA, pp. 415-

422. 

ltasca Consulting Group, 2000. Fast Lagrangian Analysis of Continua, 2nd

Edition. Minnesota. 

Zuber, M.D., 1997. Application of coalbed methane reservoir simulators for 

estimation of niethane emissions in longwall mining. 6th International 

Mine Ventilation Congress, Pinsburgh, PA, pp. 435-440. Karacan, C.O., Diamond, W.P., Esterhuizen, G.S., Schatzel, S.J., 2005. 

Numerical analysis of the impact of longwall panel width on methane

emissions and performance of gob gw ventholes. International Coalbed 

Methane Symposium, Paper 0505, Tuscaloosa, Alabama. 

Zuber, M.D., 1998. Production characteristics and reservoir analysis of 

coalbed methane reservoirs. International Journal of Coal Geology 38, 

27-45. 

Karacan, c.O., Diamond, W.P., Schatzel, S.J., Garcia, F., 2006. 

Development and application of reservoir models for the evaluation and 

optimization of longwall methane control systems. Proc. 1 Ith u.S. Mine 

Ventilation Symposium, University Park, PA, pp. 425-432. 

Kelafant, J.R, Boyer, C.M., Zuber, M.D., 1988. Production potential and 

strategies in the Central Appalachian Basin. SPE Eastern Regional 

Meeting, Paper no. 18550, Charleston, WV, pp. 305-312. 

Noack, K., 1998. Control of gas emissions in underground coal 

mines. International Journal of Coal Geology 35, 57-82. 

Palchik, V., 2003. Formation of fractured zones in overburden 

due to longwall mining. Environmental Geology 44, 28-38. 

Rernner, D.J., Ertekin, T., Sung, W., King, G.R., 1986. A parametric study 

of the effects of coal seam properties on gas drainage efficiency. SPE 

Reservoir Engineering, pp. 633-645. November. 

Schatzel, S., Krog, R.B., Garcia, F., Marshall, J., Trackemas, J., 2006. 

Prediction of longwall methane emissions and the associated 

consequences of increasing longwall face lengths: a case study in the 

Pittsburgh coalbed. Proc. I lth U.S. Mine Ventilation Sympo- sium, 

University Park, PA, pp. 375-382.