Coal Bed Discontinuity Effects on the Production of Degasification 

Boreholes and on Emissions during Longwall Mining 

C.Ö.Karacan and G.V.R.Goodman, NIOSH Pittsburgh Research Laboratory 

Abstract 

Geological discontinues in the coalbed, such as faults, sandstone channels, permeability facies, lithotype changes, and large 

scale partings in the coal seam, can cause intermittent production problems or produce unexpected amounts of water or gas 

from degasification boreholes. These discontinuities not only can impact methane emissions into the mine workings, 

especially if they hinder proper and effective degasification of the coalbed but may act as conduits for methane flow from 

gassy strata into advancing mine workings. The effects of these discontinuities are still under debate by mining and gas 

production experts. This study presents a numerical investigation using reservoir simulations of the effects of partings and 

permeability facies and lithotype changes on the production performances of vertical and horizontal degasification boreholes, 

as well as the emissions during longwall operations. 

In this work, the grid block was designed in a multi-layer 3-D structure which enabled spatial descriptions of the 

geometries and properties of the discontinuities within the coal seam. The studied coal seam discontinuities and their 

properties were distributed arbitrarily within the model. Production was simulated by vertical and horizontal boreholes. 

Degasification boreholes operated two years prior to start of the mining. 

The results of this numerical study show that in coal seams with major heterogeneities and discontinuties, the geometry 

and location of the boreholes are important for improved gas production. These considerations are also important for 

controlling methane and water inflow into the working environment during longwall mining. 

Introduction 

Occasionally, unanticipated and unusually high emissions, despite normal ventilation controls in coal mines, may result in an 

explosive mixture at the working face that can be easily ignited during mining. Abnormal, unanticipated mine gas emissions 

in quantities sufficient to create hazardous conditions have often been attributed to various geologic features. During burial

and diagenesis of the organic matter that ultimately forms mineable coal beds, dispersed organic matter many occur also in 

adjacent strata that is not being directly exposed by the mining process. This material can produce methane in quantities far 

exceeding the storage capacity of the coal and surrounding rock (Juntgen and Klein, 1975). The flow of this gas at high rates 

into the mine workings may be either facilitated or temporarily impeded by the presence of geologic structures or anomalies 

during mining of the nearby coal seams. 

One of the most effective approaches to alleviate the gas emissions from mined coal seams before mining starts is to drill 

stimulated vertical or horizontal boreholes to drain excessive gas from the coalbed (Noack, 1998; Diamond, 1994; Thakur, 

1997). This approach has been proven to be very effective in various field applications to degasify fairly continuous and 

uniform coal seams (Perry et al., 1978; Prosser et al., 1981; Diamond, 1994; Aul and Ray, 1991; Zuber, 1998; Kelafant et al., 

1988; Ertekin et al., 1988; Young et al., 1993; Cameron et al., 2007; Diamond et al., 1989; Young et al., 1991). 

Under continuous and uniform coalbed circumstances, the performance of the boreholes and their effectiveness on reducing 

emissions can be predicted by modeling techniques (Karacan, 2007a, 2007b; King and Ertekin, 1988; Sung and Ertekin, 

1987). However, a uniform and continuous coalbed is seldom the case in nature. The existence of various geologic 

anomalies and discontinuities within and surrounding the coalbed, such as partings, faults, sand channels, and changes in coal 

bed permeability and lithotypes, can create serious problems in the drilling, completion, and production of degasification 

boreholes. Thus, site-specific considerations are required for these challenging situations (Diamond, 1982). The emission of 

methane into the mine via geologic discontinuities are often distinctly associated with features of varying scale are 

occasionally immediate and catastrophic; but, more often, these emissions are subtler and not easily detected without field 

reconnaissance and instrumentation. Such emissions may also lead to hazardous accumulations of methane if not recognized 
and remedied. Thus, special site-specific considerations should include the evaluation of the gas emission potential of the 

coal seam and the associated geological factors by assessing the in-place gas content and the reservoir properties of the 

surrounding rock. These assessments can then be integrated into a predictive model of the gas emission into a working mine 

using numerical simulation techniques. 

This study presents a numerical investigation on the effects of a number of geological features, namely lithotype changes, 

permeability facies and partings, on the pre-mining production performance of vertical and horizontal degasification 

boreholes and on the potential methane emissions during mining. Potential emissions due to coalbed discontinuities were 

modeled by assigning sequential regions in the grid. This work evaluated water and gas productions from each borehole and 

the impacts of geological discontinuities on methane emissions and water inflows into mine workings. 

Methodology - Reservoir Modeling to Simulate Effects of Coal Bed Discontinuities 

Coal Bed Reservoir and Modeling Parameters. A base coalbed reservoir model was developed to numerically evaluate the 

effects of various discontinuities within the coalbed on the production performance of vertical and horizontal boreholes and 

on gas emissions into a longwall operation. The base model was designed as a uniform and continuous coalbed reservoir, 

using Computer Modeling Group’s GEM (CMG, 2003) simulator in the dual-porosity formulation. 

The grid block was designed in a multi-layer (17-layers) 3-D structure which enabled spatial descriptions of the 

geometries and properties of the discontinuities within the coal seam. A 2.6 m (8.5-ft) thick coalbed was modeled by 

assigning a 0.15 m (0.5 ft) grid thickness for each of the layers. Square grids were used with dimensions of 23 m × 23 m (75 

ft × 75 ft), in the middle portion of the reservoir, but gradually increasing up to 91.4 m (300 ft) at the boundaries. The 

modeled reservoir area for degasification and mining was approximately 4.9 km2

 (1200 acres). 

In the base model, a 3:1 permeability anisotropy was created to include the effects of face and butt cleats on flow as 

observed in most coalbed reservoirs. Face and butt cleats were assumed perpendicular to each other and face cleats were 

oriented in the E-W direction and butt cleats in the N-S direction of the model. Mining direction and horizontal borehole 

drilling direction was modeled in the face-cleat direction, which is not uncommon for ground and methane control objectives 

(Karacan et al., 2007a; Karacan et al., 2007b). 

The base model also captured the porosity and permeability changes in the reservoir due to pressure depletion in the 

cleats and gas desorption from the coal matrix during primary gas production operations. During primary methane 

production, two distinct phenomena are associated with reservoir pressure depletion with opposing effects on coal porosity 

and permeability. The first is an increase in effective stress under uniaxial strain conditions. The second is methane 

desorption from the coal matrix resulting in matrix shrinkage and a decrease of effective stress (Harpalani and Chen, 1995; 

Palmer and Mansoori, 1996; Shi and Durucan, 2003). 

The effects of matrix shrinkage and the accompanying increases in permeability and flow rate have been observed in 

field tests. A vertical degasification borehole with seven horizontal extensions from the bottom of a borehole in an 8-ft thick 

section of the Pittsburgh Coalbed produced from 2200 m3

/day (80 Mscfd) to 7300 m3/day (260 Mscfd) per borehole. During 

the first 80 days of monitoring from each of the boreholes, gas production rates decreased for approximately the first 50 days. 

Then, they started to increase (Fields et al., 1973). At another field production test in the Mary Lee Coalbed in Jefferson 

County, Alabama, the effectiveness of vertical boreholes on gas drainage was investigated by stimulating the well with foam 

and sand. In the first 60 days of production, the flow rate decreased from 4000 m3

/day (140 Mscfd) to 1600 m3/day (55 

Mscfd). After this period, the flow rate gradually increased to 90 Mscfd at the end of seven months of monitoring (Lambert 

and Trevits, 1978). 

In this study, Palmer and Mansoori (1996) model built into the GEM’s code system to simulate porosity and 

permeability changes in the reservoir was used and associated effects on flow rate were modeled. General reservoir 

properties and their average values used in building the base coalbed model is given in Table 1. 

Methane Drainage Boreholes. In this study, the effects of discontinuities on degasification performances of vertical and 

horizontal drainage boreholes were evaluated. In all of the models documented in this paper, boreholes were modeled as a 

15.2 cm (6-inch) diameter wells drilled from the surface into the coal seam. The total lengths of the simulated boreholes 

were 2.6 m (8.5 ft) and 1097 m (3600 ft) for vertical and horizontal boreholes, respectively. The horizontal borehole was 

modeled as a long-radius borehole with 46 cm (1.5-ft) vertical section in the upper layers of the coalbed at the drill location. 

This short vertical leg was continued with a 91 m (300 ft) slanted interval (deviation angle is ~ 0.6o

) before the borehole 

trajectory was horizontal. The horizontal section was modeled in the face cleat direction (E-W direction) in the middle layer 

of the coalbed. 

For the numerical simulations, the vertical well skin factor was assumed to be -3, representing that of a well with a 

stimulation treatment, such as hydraulic fracturing, whereas the horizontal borehole was modeled as an undamaged and 

unstimulated well with a skin factor of 0. The roof and floor of the coal seams are very important because they facilitate 

hydrologic isolation of the coalbed, if they are impermeable, which ensures that water and gas will not be produced from 

adjacent permeable beds (Su et al., 2005). In the Powder River basin, it has been found that the stimulation activities resulted 

in hydraulic fracturing of the coal and possibly the adjacent strata, resulting in excess water production and inefficient 

depressurization of coals (Colmenares and Zoback, 2007). All of the wells with exceptionally high water production are 
always associated with vertical fracture growth. In these same wells, there are significant delays in gas production most 

likely due to inefficient depressurization of the coals. In all of the numerical simulations reported in this paper, floor and roof 

rocks are assumed undamaged to hydraulically confine the coalbed. In this study, both of the wells were operated with 20 psi 

bottom-hole-pressure constraints for a total duration of two years prior to mining. During this period, both gas and water 

productions were modeled. 

Simulating Face Advance During Longwall Mining and Associated Emissions. This study modeled the advance of a 

1200-ft wide, flat longwall panel whose start-up end was 1400 m (4500 ft) away from the borehole drill locations (Fig. 1). In 

this figure, colors represent different sector numbers, and thus sequential mining steps. Mining progressed and from east to 

west (right to left) in Fig. 1 towards the borehole location. Every face advance was characterized by sequential “sectors” of 

69 m (225 ft or 3 grids in the model) in length within the grid model. These sectors represented the coal blocks to be mined 

during mining process. Separate “sectors” covering the same grid addresses were specified along the longwall advance 

direction for matrix and fracture elements of the dual-porosity grid model. This “proxy” approach to the implementation of 

moving boundary problem (Karacan et al., 2006a; 2007b) enabled the separate determination of the “remaining adsorbed gas 

volume” in the coal matrix and the “free gas and water volumes” in the fractures (cleats). These volumes could be 

determined at any time after a certain operational period of the boreholes or they could be evaluated as a function of distance

from either the start of the panel or from any of the modeled discontinuities. The gas and water quantities monitored and 

recorded by the simulator in each “sector” were used to calculate potential emissions for a 9.1 m/day (30 ft/day) average face 

advance rate in the presence of discontinuities in the coal bed. 

Results and Discussions 

Discontinuities are an important consideration in evaluating the gas drainage potential of the coal seams using vertical and 

horizontal gas drainage boreholes. Discontinuities can also be a serious problem for gas control in mining due to their 

potential effects on the flow of gas to the boreholes and into the working areas. This section discusses the types and origins

of geological discontinuities in the coal beds, their effects on coal seam degasification using vertical and horizontal 

boreholes, and their potential effects on methane emissions into an operating longwall using numerical reservoir simulation. 

Primary Recovery and Emissions due to Mining in the Absence of Coal bed Discontinuities: Simulation Results of the 

“Base Case”. Degasification simulations using the base coalbed-methane model were performed to establish a comparison 

for the later simulations which evaluated the effects of formulated discontinuities. Pre-mining degasification duration for all 

the simulations in this study was simulated as two years. After two years of degasification, remaining gas in the coal was 

evaluated for potential emissions during a longwall mining operation. During these simulations, both methane and water 

flow rates were reported. 

Fig. 2 shows methane and water productions of the boreholes for two-year duration. As expected, methane production 

from the horizontal borehole is 2-3 times more than the vertical borehole. At the beginning of production period, this 

difference is even more. This is due to the combination of difference in the lengths of the boreholes and the pressure 

depletion areas that they create during production. As noted by other researchers (King and Ertekin, 1988; Sung and Ertekin, 

1987), the initial production (early-time period) from the boreholes is mainly dominated by the depletion of fractures or 

cleats. Thus during early times, the initial methane and water drainage rate is faster. This triggers a rapid water production 

rate, decreasing water saturation in the fractures while increasing gas saturation and its mobility through a quick shift in the 

gas relative permeability curve. After this initial period, desorption and diffusion-dominated region commences where 

saturation changes and production rates slowly decrease. In the vertical borehole production case, shown in Fig. 2, the initial 

methane production rate was ~2500 m3

/day, which decreased to ~2000 m3/day followed by an increasing trend as shown in a 

narrower scale in the inset plot. This behavior is due to the response of the coal to pressure depletion during production as 

explained in section 2.1. Water production rate initially was 4 m3

/day, but subsequently decreased to 2-3 m3/day. 

In the horizontal borehole case, the initial methane production rate was ~9000 m3

/day and decreased to ~5000 m3/day at 

the end of two years. Similarly, initial water production was 12 m3

/day but then decreased to ~6 m3/day. Since a long 

horizontal borehole is in direct communication with more fractures, its initial methane and water drainage rate is higher 

compared to a vertical well. This triggers a high water and gas production rate. After this initial period, desorption and 

diffusion-dominated phase starts and the production rate and its variability decreases. It is also interesting to note that the 

horizontal borehole does not experience, or it is not as obvious as the vertical well in this study, the rate changes due to the 

coal’s response to pressure depletion. 

After two years of degasification, evaluation of possible methane emissions into the “base model” was made. This 

evaluation was based on the simulator-reported “free gas” and “adsorbed gas” still remaining in the coal fractures and matrix, 

respectively, in each of the sequential regions (Fig. 1) after degasification. The analysis was based on a 9.1 m/day (30 ft/day) 

average face advance rate. The amount of gas available for emission was predicted from the remaining gas in the coal. 

According to this approach, 40% of the remaining adsorbed gas in the matrix and all of the remaining free gas in the fractures 

were assumed available for emission during mining process. Similarly, water in the fracture system was calculated and was 

assumed to flow into the mine. 

Fig. 3 shows calculated methane emission and water inflow rates into the mine as a function of longwall advance 
towards borehole location. These data show that the horizontal borehole modeled in this study was more effective compared 

to a vertical borehole in reducing methane emissions due to its length along the mining direction. The emission rate started 

decreasing notably as mining continued towards the horizontal borehole location and reduced to less than 20 m3

/min when 

the face location was 400 m away from the drilling location. A slight increase when the face was very close to the drill 

location was due to vertical and slanted portion of the borehole, which was not as effective as the horizontal section. Water 

inflow to the mine showed a similar trend, decreasing to less than 0.025 m3

/min as mining advanced. On the other hand, the 

vertical borehole did not affect the emission rates when mining first started. The emission rate decreased at a slow pace and 

reached minimum (~22.5 m3

/min) when the face almost intercepted the borehole location. In the vertical borehole case, 

water inflow was higher when compared to the horizontal borehole. 

These simulations show that in the base model with no discontinuities, a horizontal borehole produced more gas and 

water from the coalbed and reduced methane emission and water inflow rates into the mine when compared to a vertical 

borehole. 

Effects of Partings and Parting Layers in the Coal Bed. Coalbeds can include various non-coal materials deposited during 

peat accumulation.Such non-coal material may be referred as partings in the coalbed. These non-organic rich horizons are 

composed on mineral mater that is commonly argillaceous (clay), siliciclastic (quartz dominated) and occasionally carbonate 

rocks. Most are the result of changes in the depositional environment within the peat swamp in which the coal was forming. 

Influxes of non-organic material that entered the system disrupted the sequence of terrestrial plant growth, death and burial 

and resulted in these rapid and irregular changes in lithofacies. A special form of parting are composed of altered volcanic 

ash layers (tornstein), which represent volcanic events of short duration and therefore are nearly isochronous horizons, such 

as in Ferron group, may also be found within coal seams (Lamarre, 2003). Rock partings or parting layers creates density 

variations in the coal, which is a function of mineral material, or ash, within the coal seam (Beaton et al., 2005; Manchioni, 

2003). Most importantly in terms of fluid and gas flow, these partings create strong discontinuities in the petrophysical fabric 

(porosity and permeability) within the coalbed methane reservoir. These may also be related to gas concentration within the 

coalbed. 

During coalbed degasification, boreholes drilled in an area of extensive partings could encounter irregular flows of gas. 

A horizontal borehole drilled completely above or below an extensive parting, or a collection of partings, that effectively 

separates a coalbed into separate reservoirs may drain methane only from that portion of the coalbed. The other portion of 

the coalbed would remain undrained and the gas still would represent a potential hazard to the future mining or would be 

unavailable for commercial production (Diamond, 1982). In the case of vertical boreholes, encountering partings can also be 

problematic to the flow of gas. Since the length of a vertical borehole is much less then a horizontal borehole, drilling into a 

thick parting interval may render the borehole useless. Also, if the fracturing treatment does not penetrate effectively above

and below the parting, the flow could be reduced. Parting layers that compartmentalize the coalbed hydraulically are also 

important from a mine ventilation standpoint since they can isolate large volume of gas that can suddenly be liberated when 

penetrated by a mine entry. 

Numerical Evaluation of the Effect of Partings on Degasification and Methane Emissions during Mining. In order to 

investigate potential effects of partings and parting layers in the coalbed on borehole production and mining emissions, grids 

that represented the properties of parting layers were randomly distributed in the middle part of the grid model of the coalbed

around the borehole location. These blocks or cells within the model were represented with values of 0.05 md and 2% 

fracture for permeability and porosity, respectively. Matrix properties of partings were assigned “null” values so they were 

effectively non-porous and non-permeable, they did not have gas storage capacity, and they did not add to fluid flow. A 

fracture spacing of 6 m (20 ft) was assigned to the partings and initialized with the same water saturation and pressures in the 

fractures. Figs 4-A and 4-B show the distribution of grids representing parting layers in the cut-away picture of the coalbed 

along the horizontal borehole and their sizes and extensions, respectively.

Fig. 5 compares borehole methane productions in the presence of parting layers with those results of the base coalbed. 

The methane production curves show that the region in the coalbed with partings (Fig. 4) resulted in a decrease in methane 

productions with both vertical and horizontal boreholes. In both cases, the production rates were almost half of that of a 

continuous coalbed. The difference may be due to the decrease in methane content in coalbed in the presence of parting 

layers as well as changes in the coalbed permeability around the boreholes. Since both boreholes were drilled through the 

low permeability parting layers on their trajectory or they were surrounded by those layers, the effective permeability of the 

coalbed that the boreholes intersected was lower than the base coalbed case. The changes in the methane rates are similar to 

observed differences reported in King and Ertekin (1988) that reflected the changes between high hydraulic fracture 

permeabilities versus low ones around the boreholes. 

Figs. 6 and 7 shows how potential methane emissions and water inflows change, respectively, during mining after 

degasification using vertical and horizontal boreholes. In these figures, emission and inflow data are compared with the base 

case. Fig. 6 shows that after mining started following degasification using a vertical borehole, methane emissions calculated 

in each sector started to decrease in the parting-rich section of the coalbed at about 229 m (750 ft) from the borehole location. 

This decrease compared to base case was probably due to the decrease in methane content in parting-rich section of the coal 

seam compared to the rest of the coal bed. Fig. 6 shows a similar methane emission potential after degasification with a 
horizontal borehole. By using a horizontal borehole, potential methane emissions into the mine were reduced by about ~3-4 

m3

/min compared to using a vertical degasification borehole. 

On the other hand, a sharp increase in the water inflow into the mine was observed due to water stored in the fracture 

volume of the parting layers compared to base coal seam case (Fig. 7). For degasification with either the vertical or the 

horizontal borehole, water inflows were calculated to increase from an average rate of 0.025 m3

/min to 0.06 m3/min, when 

there are parting layers in the coal seam. There were also increases and decreases in water inflows based on the position of 

mine face relative to the intensity of partings in the coal seam. Thus, it can be argued that the intensity and the distribution of 

partings are important in mining the areas of high parting concentrations. The distribution of partings relative to face 

advance direction might also be something to consider for controlling unexpected emissions. 

 

Effects of Spatial Changes in Coal Types and Coal Microlithotypes. One of the common features of coalbeds is 

variation of coal types. These variations manifest themselves as changes in petrographic characteristics of the coal on a 

various scales, from megascopically recognizable units – lithotypes to microlithotypes and macerals, as well as changes in 

geomechanical properties of the coal seam. The origin of coal type variations can be due to groundwater level fluctuations in 

the ancient peat-forming environments, due to different peat accumulation rates (Silva et al. 2007) or changes in the type of 

primary vegetation that contributed to the peat (Kulczynski, 1949; Moore, 1989; Calder, 1993; Diessel et al., 2000). From 

duller to the brighter lithotypes, there is a significant decrease of inertinite contents and an increase in vitrinite. Local 

variations in inertinite contents can be linked to an incomplete burning of the organic matter by local fires or aerobic 

biodegradation due to bacterial attack when water level dropped (Silva et al., 2007).

Different coal components have different capacities for gas generation and storage capacity (Lamberson and Bustin, 

1993; Gurdal and Yalcin, 2000; Karacan and Mitchell, 2003; Beaton et al., 2005). Most researchers agree that vitrinite-rich, 

bright coals have a greater methane adsorption capacity than inertinite-rich, rank-equivalent coals. However, in some cases, 

inertinite-rich coals have been found to have the greatest methane adsorption capacity (Ettinger et al., 1966; Clarkson and 

Bustin, 1996; Mastalerz et al., 2004) mostly due to the volume created by the fusinites with open-cell lumina. 

Different coal types also have different desorption rates, which can impact gas emissions and borehole productions. 

Studies on coal samples showed that dull coals desorb more rapidly than bright coals (Karacan and Mitchell, 2003; Karacan 

and Okandan, 2001). Both rank and coal type were found to influence effective diffusivity. Dull coals have faster desorption 

rates (2-3 times) than their bright equivalents in most cases (Laxminarayana and Crosdale, 1999) due to predominance of 

large, open cell lumina (Karacan and Mitchell, 2003; Karacan and Okandan, 2001; Crosdale et al., 1998). Bright, vitrinite 

rich coals usually have the slowest desorption rates which is associated with their highly microporous structure. 

The importance of different coal lithotypes and microlithotypes for underground coal mining were summarized in 

different studies. For instance, variable gas desorption rates for different coal types were speculated to impact outbursts in

mines (Beamish and Crosdale, 1998). Effectively, in the mine-face environment as pressure is removed from the coal, 

different desorption rates of the coal lithotypes create a large gas content gradient, namely: (1) vitrinite-rich or inertodetriniterich coal bands do not desorb rapidly, retaining their gas and thus producing a steep gas content gradient and (2) fusinite-rich 

or semi-fusinite-rich coal bands lose their gas more rapidly and thus have a shallow gas content gradient. In addition, when 

the coal strength information is combined with the sorption behavior of coal macerals, it is suggested that outburst proneness 

of coals rich in vitrinite and inertodetrinite is greatly increased (Beamish and Crosdale, 1998). 

Numerical Analysis of the Effects of Coal Type Variations in Coalbed Degasification and Methane Emissions during 

Mining. In this analysis, the properties of the coalbed given in Table 1 are considered as the continuous coal type in the 

coalbed. In order to model the discontinuous coal type, the gas storage (adsorption properties), volume change parameters 

due to primary recovery (Palmer-Mansoori model parameters) and diffusion properties (desorption time constant) of the 

coalbed were changed to values representative of an inertinite-rich coal. For this purpose, the grids representing partings in

Fig. 4 were assigned the properties of inertinite-rich coals reported in the literature.

The properties of the inertinite-rich coals were gathered from different studies. For geomechanical properties of coals, 

Gentzis et al. (2007) reported that the values of Young’s modulus ranged from 1119 MPa to 5070 MPa and Poisson’s ratio 

ranged from 0.26 to 0.48 within the seams but reported generally higher Young’s modulus and lower Poisson’s ratio for 

inertinite-rich seams. These measurements were based on static and dynamic tests performed on six large blocks of 

bituminous coal from active mines in the Foothills and Mountain regions of Western Canada. Gas transport and storage 

properties of dull, inertinite-rich coals were also based on different studies stating dull coals have faster desorption rates (2-3 

times) than their bright equivalents (Laxminarayana and Crosdale, 1999) and documenting data on their gas adsorption 

capacities. The properties used in this study to represent an inertinite-rich coal are given in Table 2. 

Fig. 8 shows the production behavior of vertical and horizontal boreholes in a coal seam that had randomly distributed 

inertinite-rich patches along the borehole trajectories and in the way of mining. The graph shows that the presence of coaltype discontinuities did not change the production performance of vertical and horizontal boreholes dramatically. Production 

values are somewhat better than in the base case, probably due to this type of coal having more gas content and greater 

volumetric changes during pressure depletion. However, this situation may change if the borehole is either partially or 

completely in inertinite. For instance, Fig. 9 shows two different cases where inertinite was not randomly distributed within 

the coal seam, but was in the form of blocks and layers. In these cases, vertical and horizontal boreholes are drilled partially 
and completely into the inertinite-rich coal. 

Fig. 10 shows the comparison of base-case methane production from a vertical borehole with the cases shown in Fig. 9. 

This figure shows that base-case production is better than the other two coal seam scenarios. The base case production is 

followed by the layered and blocky discontinuity situations: a vertical borehole drilled completely into a block of inertiniterich coal has a steeper methane-rate decrease compared to base case. This may be due to a fast decrease in pore pressure, less

gas content in the coal and a lower Langmuir pressure that affects the shape of the isotherm during desorption. The decrease 

of production rate has almost stopped after 100 days and followed by a relatively stable trend and a production increase due 

to permeability rebound. 

The effects of randomly distributed coal type variations, such as in Fig. 4, on emissions during mining were investigated 

in advancing sectors as before. Fig. 11 shows that mining in the base coal seam that was degasified by a horizontal borehole 

resulted in less methane emission into the mine as compared to degasification with a vertical borehole due to producing more 

gas with this type of borehole. When there are random channels of inertinite as a discontinuity in the base case, the potential 

emissions after using both of the borehole patterns are higher due to slightly less amount of gas produced during 

degasification (Fig 8). 

Effects of Permeability Facies and Their Changes in the Coal Bed. Permeability is a critical factor controlling methane 

and water production from coal seam reservoirs and determining the efficiency of a degasification project. However, 

reservoir permeability usually is not constant and not uniform throughout the reservoir. Coal cleats, their spacing and 

permeability are known factors creating preferential flow direction and magnitude in the coal seam. Coal deformation is 

another factor affecting the permeability in coal. For instance, in Qinshui Basin of China, the coalbed is severely deformed 

and contains mylonitic structures where the primary permeability is obscured. Thus permeability of the coalbed methane 

reservoir is controlled by shear fractures creating a significant reservoir heterogeneity making it difficult to describe 

permeability distribution (Su et al., 2005) for degasification studies.

In the Black Warrior Basin, a strong structural control is present on gas and water production and the relationship of 

production to structure is different in each field analyzed. Variable well performance in all areas suggests that hidden 

interwell heterogeneity related to fractures, high permeability pathways and shear structures influences production (Pashin, 

1998). Thus, the regional permeability and the gas content generalities cannot be applied uniformly across the basin or coal 

seam. 

Generally, high permeability areas in the coalbed are more productive and better suited for methane production 

compared to low permeability zones. For example, the highly productive “fairway zone” of the Fruitland formation in the 

San Juan Basin of Colorado and New Mexico contains individual wells that can produce 28-170×103

 m3/day (1-6 

MMscf/day). Permeability within San Juan Basin ranges from 15-60 md in the high productive fairway zones, and tapers to 

less than 5 md in the least productive areas of the basin (Ayers, 2002). This and similar studies suggest that the highest 

permeability coals have the best reservoir characteristics. However, Bustin and Clarkson (1998) showed that such reservoirs 

consequently may have the lowest gas saturation, due to leakage, and thus may have poorer coalbed methane resource 

potential. On the other hand, if the coalbed is non-permeable, it may have a higher gas content, which may be released only 

during the crushing process as mining progresses. This condition may lead to lower gas production potentials during 

degasification but high methane emissions during mining that may create an unsafe atmosphere. 

Numerical Investigation of the Effects of Permeability Facies in the Coal Bed on Degasification and Mining 

Emissions. The effects of changes in coal seam permeability on degasification and consequently on methane emissions 

during mining were investigated using the same model geometries shown in Fig. 4 (random patches of low permeability-Case 

I) and in Figs. 9A and B (Case II and III). For these analyses, the base permeabilities of coal (30 md in E-W direction, 10 

md in N-S direction) in those regions were changed to low and high permeabilities (5 times) representing myolitic factures 

and calcite filled cleats, and large permeable deformation fractures, respectively. In assignment of the permeabilities, 3:1 

anisotropy was kept constant. Fig. 12 schematically shows the assigned permeabilities in different regions for the three cases 

examined. 

Fig. 13 shows methane productions from simulated vertical and horizontal boreholes. Results show that existence of 

randomly distributed low-permeability patches along the trajectory of the boreholes result in decreased methane production 

compared to the base case. 

Fig. 14 shows the performances for boreholes drilled in the coalbed but intercepting various permeability zones shown as 

Case II (blocky) and Case III (layered) in Fig. 12. The figure shows that the worst performing borehole was the vertical 

borehole drilled in a low permeability zone in the coalbed. Although this is an expected result, it emphasizes the importance 

of finding sweet spots or “fairways” in the coalbed for a successful degasification operation. A vertical borehole drilled in a 

layered-permeability coalbed performed close to the base situation with uniform permeability at later times in the production. 

However, its initial methane production rate was higher due to the high-permeability zone at the bottom. The rate started to 

decline as water accumulated in that zone due to gravity drainage. Thus, the borehole lost its advantage of being drilled into

a high permeability zone at the bottom. Although water was either produced or drained from the low-permeability top layer, 

which might have been a production-enhancing factor for coal seams, this advantage was outweighed by the low permeability 

of this layer. The high production would have been longer lasting if the high permeability zone was located at the top. 
The production of the horizontal borehole was also affected by the location and geometry of the permeability facies. 

Results show that a horizontal borehole drilled in a coal seam such as in Case II, produced as well as the base case, may be 

even better in the long run. However, when the borehole was drilled between a low- and a high-permeability layer, it entered 

a declining trend very quickly due to small amount of production from the low-permeability top layer. Again, having the 

high permeability layer at the top might have improved production behavior. 

Fig. 15 shows the impacts of random permeability variations on methane emissions after degasification of the coal seam 

shown in Case I with vertical and horizontal boreholes. This figure shows that the emissions after degassing with a vertical 

well are very close those of the base case. This is due to the limited number of low-permeability zones intercepted by the 

borehole. However, the emissions that might be measured after degassing with a horizontal borehole were higher since the 

horizontal borehole intercepted more low permeability zones. 

Fig. 16-A shows the methane emissions and water inflow into an operation for Cases II and III after degassing with a 

vertical borehole. This figure shows that the highest methane emissions into the mine occur after the heterogeneous coal bed 

of Case II was degasified with a vertical borehole which was drilled in a low-permeability zone. Since the borehole was not 

as effective in producing gas from the coal bed, most of the gas remained in the seam and entered the mine. A similar 

explanation could be valid for the water inflow as well for this situation. 

The methane emissions after degasification of a layered reservoir (Case III) with a vertical borehole were similar to the 

base case (Fig. 16-A). However, it made a difference in the case of water inflow, where the water inflow rate was lower than 

the base case. This may be due to removing a high amount of water from the reservoir from the lower perforations, which 

were in the highest permeable zone of the borehole. 

Fig. 16-B shows the methane emissions and water inflow into a longwall mine operating in a coal seam of cases II and 

III type after degassing with a horizontal borehole. This figure shows that after degassing the coal seam with a horizontal 

borehole, both water and methane emissions into a mine are less compared to a situation where a vertical borehole is used. 

Also, both water and methane inflow rates do not differ much between cases with different heterogeneities. This may be due 

to the fact that horizontal borehole production influences an extensive area and the effect of local heterogeneities are less 

pronounced on emissions afterwards during mining. 

Conclusions 

This work presents the impacts of geologic discontinuities such as the presence of partings within the seam and tabular 

variations in coal type, microlithotypes, and permeability facies on the performances of horizontal and vertical boreholes and 

the inflow rates of water and methane into an active longwall operation. Compared to a continuous coalbed, the presence of 

partings resulted in a reduced methane production from both horizontal and vertical degasification boreholes. This was likely 

due to the lower methane content in the presence of these partings and changes in permeability around the boreholes. 

Methane inflows to the longwall face were generally lower in the presence of a parting. Water inflow rates were higher in the 

presence of a coal seam parting due to the amount of water stored in the fracture volume of the parting. These inflows 

increased in intensity as the longwall face approached either the horizontal or vertical borehole. 

Lithotype variations modeled as randomly distributed inertinite-rich regions along horizontal and vertical borehole 

trajectories did not change the degasification performances of the horizontal or vertical boreholes. With the inertinite then 

modeled as blocks and layers, methane production from the vertical borehole showed a significant decrease followed by a 

relatively stable increase. 

Subsequent analyses assuming random permeability variations revealed that methane productions from horizontal and 

vertical boreholes were less than those obtained from the original continuous coalbed model. Zones of increased 

permeability were then modeled as either blocky or layered regions. A horizontal degasification borehole drilled into a 

blocky zone could produce as good as the continuous model. When drilled between low and high permeability layers, 

methane output quickly decreased due to limited production from the low-permeability top layer. This work also showed that 

a vertical degasification borehole drilled into a blocky zone had the worst performance and permitted increased methane and 

water inflows to the mining face. Borehole production improved when drilled into a layered zone which produced lower 

methane and water inflows. Methane emissions and water inflow into a longwall mine operating in a coal seam with blocky 

and layered permeability facies show that after degassing the coal seam with a horizontal borehole, both water and methane 

emissions into a mine are less compared to a situation where a vertical borehole is used. Also, both water and methane 

inflow rates do not differ much between cases with different heterogeneities. This may be due to the fact that horizontal 

borehole production influences an extensive area and the effect of local heterogeneities are less pronounced on emissions 

afterwards during mining. 

The results of this numerical study show that in coal seams with major heterogeneities and discontinuties, the type and 

location of the boreholes are important for improved gas production. These considerations are also important for controlling 

methane and water inflow into the working environment during longwall mining. 
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 Table 1. Values of some of the reservoir parameters of used in modeling the coalbed. 

Parameter Value 

Permeability-face cleat (md) 30 

Permeability-butt cleat (md) 10 

Effective porosity (%) 0.005 

Effective fracture (cleat) spacing (m)/(ft) 0.03/0.1 

Langmuir p. (MPa) / (psi) 2.25/326 

Langmuir vol. (cc/g) / (scf/ton) 17.3/556 

Desorption time constant (days) 50 

Initial water saturation (%) 95 

Coal density (g/cc) / (lb/ft3) 1.35/84.7 

Coalbed pressure (MPa) / (psi) 2.1/300 

Poisson’s ratio 0.3 

Young’s modulus (MPa/psi) 3740/5.5×105

Strain at P∞ (infinite pressure) 0.01 

Pore compressibility (1/KPa, 1/psi) 0.9 / 6.0×10-4

P-M constant 3 

Table 2. Parameters used to represent an inertinite-rich coal material for geomechanical 

changes, adsorption and gas emission properties. 

Parameter Value 

Langmuir p. (MPa) / (psi) 1.25/182 

Langmuir vol. (cc/g) / (scf/ton) 12.4/400 

Desorption time constant (days) 15 

Coal density (g/cc) / (lb/ft3) 1.44/90.2 

Poisson’s ratio 0.2 

Young’s modulus (MPa/psi) 4830/7.1×105

Strain at P∞ (infinite pressure) 0.005 

Pore compressibility (1/KPa, 1/psi) 0.03 / 2.0×10-5

P-M constant 3 
Sector 36: Matrix 

Sector 37: Fracture 

Sector 3: Matrix

Sector 4: Fracture 

Mining steps 

(sectors) 

Mining direction 2.6 m 

 

Figure 1. Grid model of the coalbed that shows horizontal 

borehole and the sequential mining steps as described by 

different “sectors” for matrix and fracture elements. s

 

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 200 400 600 800

Time (days)

Methane Production Rate 

(m3/day)

2

4

6

8

10

12

14

Water Production Rate 

(m3/day)

Horizontal-CH4

Vertical-CH4

Horizontal-Water

Vertical-Water

1800

3000

0 800

Figure 2. Methane and water production curves for 

vertical and horizontal boreholes in the base model. Inset 

figure is the production rate from the vertical well at a 

maller scale.

15

20

25

30

35

0 200 400 600 800 1000 1200

Distance of Longwall Face from Borehole Location (m)

CH4 Emission during Mining 

(m3/min)

0

5

10

15

20

25

30

35

Water Inflow during Mining 

(m3/min x 10-3)

Hoizontal-CH4

Vertical-CH4

Horizontal-Water

Vertical-Water

Face advance direction 

 Figure 3. Methane emission and water inflow into a mine advancing towards the borehole (Figure 1) in the base coalbed 

 model after degasification with vertical and horizontal boreholes. 

Partings 

B

Direction of mining 

A 

 Figure 4. A cut-away of the coalbed reservoir along the horizontal borehole location. Figure (A) shows the distribution 

 of partings in the coalbed. Figure (B) shows the rendered 3-D image of the modeled partings to show their population 

 and distribution. 
1000

2000

3000

4000

5000

6000

7000

8000

9000

0 100 200 300 400 500 600 700 800

Time (days)

Methane Production Rate 

(m3/day)

Base-Horizontal

Base-Vertical

Partings-Horizontal

Partings-Vertical

Figure 5. Methane productions from the boreholes drilled 

in a coalbed with parting layers (shown in Figure 4) 

12

14

16

18

20

22

24

26

28

0 200 400 600 800 1000 1200

Distance of Longwall Face from Borehole Location (m)

CH4 Emission during Mining 

(m3/min)

Direction of mining 

Base-Horizontal BH

Base-Vertical BH

Partings-Horizontal BH

Partings-Vertical BH

Figure 6. Methane emissions into the advancing 

mine in a coalbed with parting layers. 

 

20

30

40

50

60

70

80

0 200 400 600 800 1000 1200

Distance of Longwall Face from Borehole Location (m)

Water Inflow during Mining 

(m3/min x 10-3)

Base-Horizontal BH

Base-Vertical BH

Partings-Horizontal BH

Partings-Vertical BH

Direction of mining 

Figure 7. Water inflows into the advancing longwall 

in a coalbed with parting layers compared to the base 

case. 

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 100 200 300 400 500 600 700 800

Time (days)

Methane Production Rate 

(m3/day)

Base-Horizontal BH

Base-Vertical BH

Lithotype Variation-Horizontal BH

Lithotype Variation-Vertical BH

Figure 8. Methane productions from the vertical and 

horizontal boreholes that were drilled into a coalbed 

with microlithotype variations. 

 

Direction of mining 

Different coal 

type 

A 

Different coal 

type 

Direction of mining 

B 

Base coal 

 Figure 9. Cut-aways of the coal seam reservoir along the horizontal borehole location to show the effect of blocky (A), 

 and layered (B) coal-type variations in the reservoir on methane production. 
1400

1600

1800

2000

2200

2400

2600

0 100 200 300 400 500 600 700 800

Time (days)

Methane Production Rate 

(m3/day)

Base-Vertical

Layered-Lithotype-Vertical

Block-Lithotype-Vertical

Figure 10. Methane productions from a vertical borehole 

drilled into a coalbed with coal type (microlithotype) 

variations. 

Direction of mining 

18

20

22

24

26

0 200 400 600 800 1000 1200

Distance of Longwall Face from Borehole Location (m)

CH4 Emission during Mining 

(m3/min)

Base-Horizontal BH

Base-Vertical BH

Lithotype Variation-Horizontal BH

Lithotype Variation-Vertical BH

Figure 11. Methane emissions into the advancing 

mine in a coalbed with random (shown in Figure 4) 

coal-type variations after degasification with vertical 

and horizontal boreholes. 

6:2 

Case I 

30:10 6:2 30:10 150:500 30:10 

30:10 

6:2 

150:500 

Vertical borehole 

Horizontal borehole 

Case II Case III 

30:10 

 Figure 12. A schematic representation of the permeability cases that were modeled in the coal seam and the 

 assigned permeability values. 

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 100 200 300 400 500 600 700 800

Time (days)

Methane Production Rate 

(m3/day)

Base-Horizontal BH

Base-Vertical BH

Perm. Case I-Horizontal BH

Perm. Case I-Vertical BH

Figure 13. Methane productions from the vertical and 

horizontal boreholes that were drilled into a coalbed with 

random permeability variations (Case I). 

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 100 200 300 400 500 600 700 800

Time (days)

Methane Production Rate 

(m3/day)

Base-Horizontal

Base-Vertical

Layered-Facies-Horizontal

Layered-Facies-Vertical

Block-Facies-Horizontal

Block-Facies-Vertical

Figure 14. Methane productions vertical and 

horizontal boreholes drilled into a coalbed 

with various permeability facies schematically 

shown in Figure 12. 
 

18

20

22

24

26

0 200 400 600 800 1000 1200

Distance of Longwall Face from Borehole Location (m)

CH4 Emission during Mining 

(m3/min)

Base-Horizontal BH

Perm.Case I=Horizontal BH

Base-Vertical BH

Perm.Case I=Vertical BH

 

 

Figure 15. Methane emissions in a coal bed with random permeability variations after degasification 

with vertical and horizontal boreholes. 

20

22

24

26

0 200 400 600 800 1000 1200

Distance of Longwall Face from Borehole Location (m)

CH4 Emission during Mining 

(m3/min)

20

25

30

35

Water Inflow during Mining 

(m3/min x 10-3)

CH4-Base

CH4-Block-Facies

CH4-Layered-Facies

Water-Base

Water-Block-Facies

Water-Layered-Facies

17

19

21

23

25

0 200 400 600 800 1000 1200

Distance of Longwall Face from Borehole Location (m)

CH4 Emission during Mining 

(m3/min)

20

25

30

35

Water Inflow during Mining 

(m3/min x 10-3)

CH4-Base

CH4-Block-Facies

CH4-Layered-Facies

Water-Base

Water-Block-Facies

Water-Layered-Facies

A 

B 

Direction of mining 

 

 

 

Figure 16. Methane and water emissions into a mine operating in a coal bed with permeability variations 

(Case II and Case III) after degasification with a vertical borehole (A) and a horizontal borehole (B).