RESERVOIR ENGINEERING CONSIDERATIONS FOR COAL 


SEAM DEGASIFICATION AND METHANE CONTROL IN 


UNDERGROUND COAL MINES 


C. Ozgen Karacan 

The National Institute for Occupational Safety and Health, P.O. Box 18070 

626 Cochrans Mill Rd., Pittsburgh, PA 15236, Email: cok6@cdc.gov 

ABSTRACT 

Degasification and methane control in underground coal mining is an important area 

augmenting conventional ventilation in order to prevent possible fires and explosions 

due to excessive methane emissions. Coal bed reservoir engineering has been applied 

for many years ~o coalbed methane production. In recent years, modeling the flow 

and draining methane, not only from coal bed itself but also from the overlying strata, 

have been recognized as important areas of expertise for supplementing underground 

ventilation. This paper demonstrates the applications of coal bed reservoir engineering 

and modeling techniques for optimizing and controlling methane emissions in 

longwall and continuous miner sections. Practical applications and some 

consideration~ are presented for reservoir modeling, borehole performances, and face 

emission predictions for various coal bed and borehole configurations. A section on 

field reservoir studies and well bore log analyses for methane control are also 

presented as a complementary way of controlling methane and optimizing ventilation. 

1. INTRODUCTION 

High methane liberation rates experienced during mining can result from reservoir 

properties of the coal bed and mining conditions. For instance, in the case of 

development mining, the emissions can result from the intrinsic properties of the 

coal bed reservoir or from properties of the mining operation such as mining rate, 

mining height, and entry length, to name a few. In longwall mining, the properties of 

the mined coal seam, as well as the properties of overlying strata, can be important 

since this method can create large-scale disturbances in the overlying strata. These 

disturbances manifest themselves as cavings of immediate roof, fracturings and 

separations of bedding planes and bending zones, each of which may have different 

properties depending on the strata characteristics and the height of the mined seam. In 

longwall mines, the "gob" may be the greatest source of methane emissions. During 

mining, high methane emissions and subsequent increases in methane levels must be 

diluted and removed from the active workings by the ventilation airflow. 
Unfortunately, the emissions may not be controllable by ventilation alone or by 

changes in mining parameters. Generally, it is economically feasible to handle 

specific emissions (total gas emission per unit amount of coal mined) up to 1000 

felton with a well-designed ventilation system. At higher specific emission rates, 

however, it is difficult to stay within statutory methane limits using ventilation alone 

(Thakur, 2006). Thus, supplementary methane control measures are needed including 

degasification of the coalbed prior to mining, use of vertical or horizontal boreholes, 

or use of gob gas ventholes. Figure 1 shows a schematic representation of the mining 

environment and the different types of supplementary controls commonly used in 

U.S. 

The recognition of methane in coal beds as a valuable resource has led to the 

development of different numerical approaches for modeling flow mechanisms of gas 

and water in coal seams (King and Ertekin, 1991). The finite-difference-based models 

described by King and Ertekin (1991) have continued to be the most widely accepted 

and applied techniques for simulating the complex storage and transport processes in 

coal beds. These models have also been used to model methane emissions and 

borehole productions with special site-specific considerations including gas emission 

potential of the coal seam, its reservoir characteristics and geologic factors that may 

affect the flow of gas during degasification and mining. This paper is an overview of 

possible reservoir engineering considerations, including numerical modeling and field 

measurements, for controlling methane in coal seams. 

Vertical borehole Gob gas venthole 

To Surface 

Overburden .....~~ ~~~~~ 

1~15~'"?#'17"~,"~~ C:;::·~~ Seam 1 

height '. ~,;".', ",<,/\'. '\" 

Hydraulic fractures 

Hydraulic supports Mining height (shields) A 
Figure 1: Schematic representation of the mining environment and the different types 

of degasification boreholes commonly used in u.S. (Karacan, 2009) 

2. CHANGES IN RESERVOIR PROPERTIES OF OVERBURDEN DURING 

LONGWALL MINING AND SLUG TEST APPLICATIONS 

Methane inflow from overburden strata during longwall mining is affected by the 

magnitude of fracturing and the time that the fractures stay open as the panel is 

extracted. The reservoir characteristics and their changes during mining can affect 

production potential of the gob gas ventholes that are commonly used to control the 

methane emissions from the fractured zone. These holes are typically drilled from the 

surface to a depth that places them above the caved zone. Thus, the ability to predict 

changes in formation characteristics based on various factors at a given location 

increases the understanding of gob gas venthole production and can optimize the 

placement of these holes to improve their productivity. 

For the characterization of both the fracturing of underground strata and the resultant 

changes in hydraulic conductivity in the gob gas ventholes (GGV's) during long wall 

mining, seven boreholes were instrumented with submersible pressure transducers 

equipped with self-contained data loggers at a mine site in SW Pennsylvania. The 

change in water head was monitored during longwall mining. The data were used to 

calculate hydraulic conductivities and were plotted as a function of different 

parameters (Figure 2). 
0.0009 --,-,-----------,--,-------------­ • MBH-1 

D MBH-2 0.0008 

~ • B-3

~ 0,0007 • B-4 

~ 0,0006 • C-1 • .s; • C-2 =fi 0.0005 :J • D-1 "C8 0.0004 

" .!:! 0,0003 ----!------~--------------------~- ; :; o . . -t;III 0,0002 . I i ..

• • 0 

~ . -~-9--0-r-- II 0,0001 

: 9 i . 0+-~~-,----_,------,_----~~~_.----_4 

600 650 700 750 800 850 900 

Overburden on CB (ft) 

0, 0009 --,----,----,-,-,-----,-------,------,-,------,--------,-----------,--­ • MBH-1 

0.0008 

~ ~ 0.0007 

~ 0,0006 .s; 

~ 0,0005 :J 

"C8 0,0004 

.!:! 0,0003

:; 

I!! 0,0002 "C 

:>.

:z:: 0,0001 

0 

• 

• 

••

• • 

.. 

:<t

. 

61 

• MBH-2 

• B-3 

• B-4 

• C-1 

• C-2 

• D-1 

-1-­ • 

------~-----------

20 30 40 50 60 

Average Face Advance Rate (ft/day) 

Figure 2: Hydraulic conductivity changes in the overburden plotted as a function 

depth and face advance rate (Karacan and Goodman, 2009). The circles are the 

averages of measurements at each borehole 

Based on the slug tests and other data used in Karacan and Goodman (2009), it was 

concluded that the locations of the boreholes impacted their fracturing time and the 

resultant hydraulic conductivities. Results showed that greater overburden depths led 

to lower hydraulic conductivities and potentially less productive boreholes as opposed 

to shallower GGV's (Figure 2). In addition to overburden depth, mining advance rate 

was found to be effective also on the hydraulic conductivities. It was shown that 

higher face advance rates at the study mine site resulted in generally lower hydraulic 

conductivity in the fractures. This may be due to the rapid change of tensile stresses 

to compressive stresses that reduces fracture conductivity. 

In order to determine where the fractures may be occurring, a similar approach that 

was used by Pa1chik (2005) was adapted for the rock layers in the overburden of the 
Pittsburgh coal. Observations on the presence and absence of horizontal fractures at 

different rock-layer interfaces of the overburden showed that the probability of layer 

separations increased with increasing compressive strength difference of neighboring 

rock layers as well as decreasing distance of the layer from the mined coal bed. 

Palchik (2003) noted that separations at the interfaces and formation of fractures may 

be restrained by thick and strong rock layers, called "bridge layers", in the 

overburden. The "bridge layer" in the study area was the thick limestone close to the 

top of the overburden. Also, the probability of bedding plane separations along the 

rock layer interfaces decreases with increasing height above the coal seam (H), with 

increasing thickness of the bridge layer, and with the increasing ratio of formation 

thickness (h) to the coal seam thickness (m). Therefore, in the study boreholes, it is 

more probable that such deformations will occur within an interval between 40 ft-145 

ft from the top of coal bed and up to him of 4 (Figure 3). 

Bridge layers 

Less probability of 

developing fractures 

="--==--.

• Weak Strata 

• Limestone 

«} Sandstone 

o 2 4 6 8 10 12 14 

him, (ftIft) - thickness of layer/CB thickness 

Figure 3: A plot of the ratio of thickness of each layer to extracted coalbed thickness 

and their heights relative to the top of the extracted coalbed. This graph also shows 

the strata identifications as sandstone, limestone and weak strata (coal, shales of all 

kind and clay stones) 

This interval contains 

thinner limestones, sandstones and weaker strata, such as coal beds an shale, and will 

indicate where the slotted section of gob gas ventholes should be located (Karacan 

and Goodman, 2009a). 

3. WELL-TEST ANALYSES METHODS FOR APPRAISAL OF RESERVOIR 


PROPERTIES 


Improvements in gob gas venthole gas drainage evaluation and prediction capabilities 

for site-specific mining conditions and circumstances can address longwall gas 
0.025 ...----,---....,----.,...---...,.---,-----.,..----,---,--,....-,----, 

o Measured 


-Model 


0.OO5+---~-~~~_r~_H~--T--__~~~~~ 

i 

~ 0.01 

1.E+OO 1.E+02 1.E+04 

Superposition Radial Time (lAta), hr 

emission issues, resulting in ventholes designed for optimum production and mine 

safety and also for improved gas capture. It is difficult to' predict production 

performance of gob gas ventholes due to the involvement of multiple influential 

factors, the complex properties of the gob, and the lack of knowledge on interactions 

of the GGV's with the gob reservoir. 

Routine production tests are performed in the oil and gas industry for long-producing 

wells in order to identify their productivity changes and to better manage the 

reservoir. Owing to the importance of the measurement and analysis methods, various 

researchers developed techniques to test conventional oil and gas wells. (Kuchuk and 

Onur, 2003). In this study, multi-rate drawdown gas well test analysis techniques 

were applied to understand the behavior of the gob gas ventholes and the reservoir 

properties of the gob. In order to achieve this objective, the production rate-wellhead 

pressure behavior of six gob gas ventholes drilled over three different panels was 

analyzed for parameters such as skin, permeability, radius of investigation, flow 

efficiency and damage ratio. In these analyses, the venthole production periods during 

panel mining (DM) and after mining (AM) were analyzed separately using infinite 

flow and pseudo-steady state (PSS) flow models in a composite reservoir, 

respectively; since the reservoir capacity, behavior and gas production potentially 

change between these two phases (Karacan, 2009b). Figure 4 shows plots of real gas 

flow potential versus superposition time for a gob gas venthole. Various reservoir 

properties were calculated from the slopes of these plots (Table 1). 
0.025 .---~r---'"---,-....,...----r----..-----~-,---,-----, 

o Measured 

-Model 

0.005 +-.;.-...;---;---,--+--+--+--,...--,......-+-...,---+--,.--+-+-..--,--;-____-;--,---1 

O.E+OO 	 2.E+03 4.E+03 6.E+03 8.E+03 1.E+04 

Superposition PSS Time (I~t,.), hr 

-a 

~ 0.01 11J'i'>;__~ 

Figure 4: Superposition time versus real-gas flow potential for a gob gas venthole's 


DM and AM production phases, using infinite-acting and pseudo-steady state 


boundary conditions, respectively 


Table 1 shows that the near-borehole radius of investigation of the borehole decreased 

after completion of the panel as a result of boundary effect.

Table 1. Reservoir properties calculated for gob using multi-rate drawdown 


Techniques (re l.2: radii of investigation of the tested borehole, 


k1•2: gob permeability, FE: flow efficiency, DR: damage ratio) 


DM AM 

rei (ft) 1198.9 579.8 

krel (md) 1173.0 1937.1 

re2 (ft) inf. 9179.2 

kre2 (md) 1974.4 10788.2 

Total skin (s') -5.5 -3.6 

Mechanical skin -4.5 -2.6 

Final rate (MMscf/day) 0.406 0.073 

Cum. prod. (MMscf) 59.143 31.046 

Final flow pressure (psia) 11.9 12.4 

A ve. error (%) 1.08 0.79 

FE 2.2 1.8 

DR 0.5 0.5 

Permeability, on the other 

hand, showed an increasing value at the AM phase, compared to the DM phase, 

within the boundaries of the gas reservoir. Total skin and FE values decreased due to 

reduced gas production and near-borehole effects. The results specific to this study 

and the mine site showed that although the gob reservoir and venthole data calculated 

using well testing methods generally corroborated the measured productivity of the 
ventholes, there were other factors that should have been considered in evaluations 

(Karacan, 2009). For instance, reservoir heterogeneities during caving and formation 

of the gob were very complex and were highly dependent on the local strata. In such 

an environment, the locations of the ventholes were of critical importance. This study 

showed that ventholes located close to the entries, particularly tailgate entries, 

produced better. It was shown that conventional well test techniques could be a 

valuable tool for reconciling gob gas reservoirs and gob venthole performances since 

there was no other more effective alternative. 

The same or similar well-test analysis techniques can also be used for vertical and 

horizontal degasification boreholes to determine the relationships between their 

production efficiencies and the decrease of in-mine emissions. The analyses results 

can be used to determine coal seam reservoir parameters and to design degasification 

borehole locations accordingly. 

4. METHODS FOR FURTHER FORECASTING GGV PERFORMANCES 

Despite the improvements in analytical and numerical modeling approaches, 

experience suggests that it is still difficult to accurately predict methane production 

for gob gas ventholes and that these predictions may be underestimated relative to 

actual gob gas venthole production by at least a factor of two or more. The key factor 

to this underestimation is the difficulty in incorporating in any predictive approach the 

many factors that affect venthole performance (Zuber, 1998). In almost all cases, 

these factors are related non-linearly to each other and to the production 

performances. Thus, production predictions may be more successful if these nonlinearities can be understood and unconventional modeling techniques, such as 

artificial neural networks (ANN), can be used to handle these non-linearities. 

In order to predict gob gas venthole production rates and methane concentrations in 

the face of many complicating factors such as venthole location, mining parameters, 

borehole location with respect to panel and surface terrain, and exhauster pressure, the 

production data that were obtained from 10 gob gas ventholes drilled over three 

adjacent panels mined in the Pittsburgh coal bed in the Southwestern Pennsylvania 

section of the Northern Appalachian basin were analyzed (Karacan, 2009c). The 

production data were obtained by monitoring these ventholes for flow rate and 

methane concentration during and after panel extraction. The ANN model was built 

using a multilayer perceptron (MLP) approach and was trained (calibrated) and tested 

using the database to achieve minimum mean square error and high correlations 

between measurements and predictions. After a reasonable prediction with ANN was 

obtained, sensitivity analyses were run about the means of the input variables to better 

understand the effects of each of these inputs on predicted outputs. 
Table 2: Sensitivity (in standard deviations) of gas flow rate and percent methane to 


individual input parameters used in the ANN model and the total sensitivity 


(third column) when both performance parameters are combined. 


Shaded cells are the total sensitivity above the average 


sensitivity value (33.8) calculated using all inputs 


Input Variable Gas Percent Total 

Rate Methane (Std. Dev.) 

Panel Completed (Yes) 13.1 8.1 21.2 

Panel Completed (No) 28.2 3.6 31.9 

Is Face Advancing? (No) 24.3 10.3 34.7 

Is Face Advancing? (Yes) 18.7 0.3 19.0 

% of Panel Mined 45.2 3.1 48.3 

Linear Advance Rate 7.5 1.4 8.9 

Surface Elevation 34.4 6.6 41.0 

Overburden 16.1 4.2 20.3 

Casing Diameter 81.2 4.4 85.6 

Casing Distance to Coalbed 7.7 2.2 10.0 

Distance to Tailgate 67.7 1.7 69.5 

Distance from Panel Start 52.3 12.2 64.5 

Panel Length 37.7 4.3 41.9 

Panel Width 5.1 1.4 6.5 

Barometric Pressure 14.0 0.6 14.6 

Average Vacuum at Wellhead 22.1 1.9 24.0 

Sensitivity analyses about the means of the input variables were conducted using the 

ANN model to identify which input variables had more influence on the performances 

of gob gas ventholes (Karacan, 2009c). The results of these analyses showed that for 

total gas flow rate (a) percentage of the panel that has been mined, (b) surface 

elevation of the ventholes, (c) casing diameter, (d) distance to tailgate, (e) distance of 

the venthole to the start of panel, and panel length were most influential. However, for 

GGV methane percentage (a) distance of the venthole to the start of the panel, (b) 

whether or not panel is completed, (c) whether or not face is advancing were most 

important. When considering the overall performance of gob gas ventholes for both 

flow and methane percentage, (a) whether or not face is advancing, (b) percentage of 

the panel that has been mined, (c) surface elevation of the venthole (above sea level), 

(d) casing diameter, (e) distance of the venthole to the tailgate, (f) distance of 

venthole to panel start, and (g) panel length were important variables. 

5. MODELING OF BOREHOLES IN COAL BEDS WITH FAULTS 

Impermeable geologic faults in the coalbed can cause intermittent production 

problems or can cause unexpected amounts of water or gas to issue from 

degasification boreholes. These faults also can impact methane emissions into the 

mine workings, especially if they hinder proper and effective degasification of the 

coalbed. They may also act as barriers for methane flow in the coalbed. Although 

this might seem beneficial for advancing mine workings, faulting may also cause gas 
pressure buildups and result in compartmentalization of the gassy regions from which 

large quantities of water and methane may rush into mine workings. 

The effects of impermeable faults on coalbed degasification efficiency using vertical 

and horizontal boreholes were assessed using numerical reservoir simulation. Three 

different fault configurations were simulated: no throw, up throw, and down throw. 

To create throws, the fault block on the borehole drilling side was kept stationary, 

whereas the vertical positions of the grids in the other fault block were moved by +/­

3 ft up and down based on the throw direction. The geometry of the vertical borehole 

was not affected by these changes. However, in the case of the horizontal borehole, 

the borehole trajectory was adjusted after it penetrated the moving block, based on the 

throw amount and direction so that the borehole was always horizontal in both blocks. 

Figure 5 shows the grid models that represent these situations. 

Up throw 

rection of m~ 

Figure 5: Coalbed models that show the modeling of impermeable faults with 

downward throw and upward throw. The figures also show the relative positions of 

horizontal and vertical boreholes in each case 

The advance of a 1200-ft wide longwall panel whose start-up end was 1400 m (4500 

ft) away from the borehole drill locations (Figure 5) was modeled. Mining progressed 

in the direction shown in Figure 5. Face advances were characterized by sequential 

"regions" with lengths of 225 ft within the grid model. These regions represented the 

coal blocks to be mined during the mining process. This "proxy" approach to the 

moving boundary problem (Karacan et aI., 2005; 2007a; 2007b) enabled the 

determination of "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 "region" were used to calculate potential emissions for a JO-ft/day 

average face advance rate in the presence of stratigraphic discontinuities or geologic 

anomalies in the coalbed (Karacan et aI., 2008). 

The results given in Karacan et ai. (2008) showed that for a coal bed de gasified using 

a horizontal borehole, the maximum changes in methane emission and water inflow to 
the mine occurred around the fault location, particularly while the mining face passed 

through the fault with a down-throw. This might be due to the fact that the bulk of the 

reservoir volume on the right side of the fault line remained underneath the borehole 

trajectory. The methane at the top of the reservoir was produced effectively, while 

the remaining gas stayed in the coal bed and was released into the mine during mining 

operations. The next highest emission occurred when there was an up throw along the 

fault line. This might be because the borehole was positioned close to the bottom of 

the coal bed on the right-side reservoir block and possibly flooded by water entering 

the borehole by gravity drainage (Karacan et ai., 2008). 

On the other hand, the effects of fault geometries on emissions after degasification 

with a vertical well showed that there would be a sharp decrease in methane 

emissions and water inflow after the mining face passed through the fault into the 

borehole block which was degassed effectively. Therefore, it is important to locate 

the fault and determine the presence and extent of any throw in order to plan more 

efficient borehole drilling and placement strategies. 

6. SUMMARY AND CONCLUDING REMARKS 

Degasification of coal seams and methane control in mines using vertical, horizontal 

and gob gas ventholes are very important for the safety and productivity of any 

underground coal mining operation. Despite the importance of de gasification and 

methane control to supplement conventional ventilation for eliminating high methane 

levels, there are no standard methods or dedicated techniques to determine coal and 

gob properties or to optimize borehole locations. In this regard, it has been shown that 

reservoir engineering techniques are very suitable and promising. Coalbed methane 

reservoir modeling, estimation of fractures to better locate slotted casings, well test 

analyses techniques and intelligent computing methods can be used to determine coal 

and gob reservoir properties to optimize well locations. These techniques give a better 

understanding of wellbore productivities and help to determine the relations between 

various parameters and the efficiency of controlling methane in mines. 

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analysis of the impact of longwall panel width on methane emissions and 

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