A numerical evaluation on the effects of impermeable faults on degasification

efficiency and methane emissions during underground coal mining

C.Ö. Karacan ⁎, J.P. Ulery, G.V.R. Goodman

NIOSH, Pittsburgh Research Laboratory, United States

⁎ Corresponding author.

E-mail address: cok6@cdc.gov (C.Ö. Karacan).

ABSTRACT

Impermeable geologic faults in the coal seam 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 coal bed. They may also act as barriers for methane flow in the coal seam. 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.

This study uses reservoir simulations to illustrate the effects of impermeable faults, with and without throws,

on the production performance of vertical and horizontal degasification boreholes. These boreholes were

drilled from the surface to intercept the coal layer and fault and were assumed to produce methane two years

prior to mining of the coal seam. Longwall advances and face position related emissions and reservoir

properties were characterized using sector definitions along the path of the panel grids. This work

numerically evaluates water and gas productions of each borehole and presents the effects and the impacts of

impermeable faults on methane emissions due to an advancing coal face. The results can be used to

understand the effects of impermeable faults on borehole productions and face emissions due to these coal

seam anomalies.

1. Introduction

In underground coal mines, the required amount of ventilation air

is based on estimates of gas release under normal conditions.

Occasionally, unanticipated and unusually high emissions are

encountered. These emissions, despite normal ventilation controls,

may result in an explosive mixture at the face that may be ignited

during mining. Investigations have shown that such emissions can

often be associated with anomalous geologic features or conditions.

While most operators are aware that certain geologic features may

adversely affect productivity by increasing mining difficulties, few are

aware of their potential as a gas emission hazard (Ulery, 2006).

Abnormal, unanticipated mine gas emissions in quantities suffi

cient to create hazardous conditions have often been attributed to

various geologic features since the first recorded documentations of

methane explosions in mines. For example, faults have long been

recognized either as conduits for gas flow from strata adjacent to

mined coal seams or as barriers for accumulation of large quantities of

methane that could enter mine workings when intercepted.

Faults can be associated with many problems that threaten the

safety of workers. For example, groundwater entering mine workings

from fault zones can lead to a disaster for that operation. Fault zones

are important since displacements can greatly reduce the strength of

the seam bedrocks due to the creation of weakness zones and the

presence of gauge materials that may be dissolved and become

permeable to gas and water (Wu et al., 2004). McCulloch et al. (1975)

noted that abnormal accumulations of water can be related to

fracturing of the roof rock or to a fault zone.

Faults and the associated weakness zones may also be responsible for

high gas concentrations encountered during mining (McCulloch et al.,

1975). This is especially likely when there is stress redistribution as

mining approaches a large scale fault. In Germany, Thielemann et al.

(2001) showed that in unmined regions, normal faults regularly act as gas

conduits for surface emissions into the atmosphere from deep forma

tions such as coal formations. They further demonstrated that distinctly

higher rates of surface gas emissions occurred from normal faults in

mined areas. This was presumably caused by the increased permeability

of the fault and associated strata in response to mining. Therefore, it

would seem likely that such faults could easily become pathways for gas

emissions into mine workings from adjacent source beds. A shear zone in

the coal seam, associated with faults, contains a large number of fractures

and thus much gas, and may also be a major factor for a catastrophic

failure or “outburst” of gas and coal during mining (Li et al., 2003).

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

(Diamond, 1994; Thakur, 1997; Noack, 1998). Vertical open hole wells

work best for high permeability coals by employing them in close

spacing. These boreholes can be fractured particularly in medium

permeability coals, thin beds and where multiple coal seams are

penetrated. However, the problem is that most vertical wells are usually

unsatisfactory due to low gas recovery rates, long term dewatering, the

large number of wells needed to depressurize and limitations of surface

access. Horizontal open hole wells can be used for thick coal seams, low

permeability coals, and in areas where good lateral continuity is

present. Other alternatives are to use pinnate drainage pattern

established by drilling multiple side laterals off a main horizontal

lateral or a Z pinnate drilling and completion system using one well site

with pinnate development from four main horizontal laterals (PTTC,

2004). Horizontal wells and pinnate drilling systems have been

successfully applied in several basins, including the Appalachian, San

Juan and Arkoma basins. As drilling and completion strategies rapidly

increase, complex horizontal drilling systems can become the most

economically efficient way to produce coal seam methane.

These approaches have 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; Kelafant et al., 1988; Ertekin

et al., 1988; Diamond et al., 1989; Aul and Ray, 1991; Young et al., 1993;

Diamond, 1994; PTTC, 2004; Cameron et al., 2007). Numerous studies

have demonstrated that under continuous and uniform coal seam

circumstances, the performance of the boreholes and their effective

ness on reducing emissions can be predicted by modeling techniques

(Sung and Ertekin, 1987; King and Ertekin, 1988; Karacan et al.,

2007b). However, a uniform and continuous coal seam is seldom the

case. The existence of various geologic anomalies and coal disconti

nuities, such as faults, can create serious problems in the drilling,

completion, and production of degasification boreholes.

It is known that faults affect production characteristics of nearby

coalbed methane boreholes. They create boundaries that can limit the

drainage radius of the boreholes. This situation can be favorable or

unfavorable in terms of gas production, depending on the size of the

bounded reservoir. If the bounded volume is small, a limited amount

of reservoir will be drained and thus the production potential of such a

reservoir will be low. If the borehole is drilled into a large but bounded

reservoir, this will potentially lead to a faster pressure drawdown

when dewatering is initiated, causing higher gas saturations and

production rates.

The effects of faults on coalbed gas productions have been

experienced in various regions. For instance, in southeastern Deerlick

Creek Field (Alabama), fault blocks have variable production char

acteristics. Exceptionally productive wells drilled near the eastern

margin of the Strip Mine graben have penetrated a fault and are

actually completed in the Holt Lake half graben (Sparks et al., 1993).

Although production within the half grabens is variable, large scale

production patterns in southeastern Deerlick Creek Field indicate that

normal faults can compartmentalize coalbed methane reservoirs.

Similarly, production patterns in the Cedar Cove Field of Alabama

(Sparks et al., 1993) indicate that being adjacent to a major fault

system has enhanced gas and water production. Pashin (1998) also

reported major production experiences in complexly faulted regions

in the coalbed methane fields of Alabama. Evidence suggests that

faults in the Oak Grove field of the Black Warrior Basin affect gas and

water production as well as the thicknesses of the seams, especially in

the upthrown fault blocks. However, the significance of these

structures in terms of coalbed methane exploration and production

has yet to be fully appreciated.

Effective mapping of faults in coal mining is critical for productivity

and safety of the workers. This is especially true where the coal seams

are impacted by large displacements (Molinda and Ingram, 1989;

Kecojevic et al., 2005). For instance, underground room and pillar

mines in the Coalsburg seam north and south of the Warfield Fault in

Mingo County, West Virginia, have been greatly impacted by faulting

and related structures. Due to the combined effects of the folding and

faulting, the northern mines are about 15 m (400 ft) higher in

elevation than the southern ones. Overland conveyor belts connect

mining blocks separated by the fault. The fault poses an extra

challenge to mine development in this part of the Appalachian Basin

(Coolen, 2003). The location of faults is important to Western

Kentucky No. 4 seam mining also, since much of the mining occurs

along the Pennyrile fault system and many mines are bordered with

faults. In this region, these mines are sometimes forced to ramp up

and down to reach a relatively flat reserve (Greb et al., 2001).

Site specific considerations, through evaluations of in place gas

content and the minor and major geological faults that may affect the

flow of gas during degasification and mining, are required for these

challenging situations (Diamond, 1982). To help in these evaluations,

numerical simulation techniques may be useful for understanding

borehole production problems during degasification and emission

problems during mining caused by geological faults. This study

presents a numerical investigation of the effects of impermeable faults

with and without throws on the pre mining production performance

of vertical and horizontal degasification boreholes and on the

potential methane emissions during mining. Potential emissions due

to coal seam discontinuities were modeled by assigning sequential

regions in the grid. This work evaluated water and gas productions

from each borehole and the impacts of coal seam discontinuities on

methane emissions and water inflow into mine workings.

2. Methodology — reservoir modeling to simulate effects of

impermeable faults in the coal seam on degasification and on

emissions during mining

2.1. Coalbed methane reservoir and modeling parameters

A base coalbed methane reservoir model was developed to

numerically evaluate the effects of various stratigraphic and geologic

anomalies within the coal seam on production performance of vertical

and horizontal boreholes and on gas emissions into a longwall

operation. Computer Modeling Group's GEM (CMG, 2003) simulator

was used in dual porosity formulation. The base model was designed

as a uniform and continuous coal seam reservoir. The grid block was

designed in a multilayer (17 layers) 3 D structure which enabled

spatial descriptions of the geometries and properties of the anomalies

and discontinuities within the seam. A 2.6 m (8.5 ft) thick coal seam

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, 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 methane reservoirs. Face and butt cleats were assumed to be

perpendicular to each other, with face cleats oriented in the E W

direction and butt cleats in the N S direction of the model. Mining

direction and horizontal borehole drilling direction were modeled in

the face cleat direction, which is not uncommon for ground and

methane control objectives in the southwestern Pennsylvania section

of the Northern Appalachian Basin (Karacan et al., 2007a,b). Although,

from only methane production point of view, the horizontal boreholes

are preferred vertical to the face cleats, the ones drilled parallel to the

mining direction (parallel to face cleats as in this study) are used both

for degasification of the panel area in advance of mining and also for

shielding the longwall entries against methane inflow during

development mining and afterwards.

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 phenom

ena 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 in effective stress (Harpalani and Chen, 1995; Palmer and

Mansoori, 1996; Harpalani and Chen, 1997; 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 2.4 m (8 ft) thick section of the Pittsburgh

seam 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 before starting to increase (Fields et al.,

1973). At another field production test in Mary Lee coal seam in

Jefferson County, Alabama, the effectiveness of vertical boreholes on

gas drainage was investigated by stimulating the well with foam and

420 840 μm size (20 40 mesh) 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, a model integrated into the

GEM's code system by Palmer and Mansoori (1996) modeled flow

rates and porosity and permeability changes in the reservoir. General

reservoir properties and their average values used in building the base

reservoir model are given in Table 1.

Values of important reservoir parameters used in modeling the coal seam

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. (cm3

/g)/(scf/ton) 17.3/556

Desorption time constant (days) 50

Initial water saturation (%) 95

Coal density (g/cm3

)/(lb/ft3) 1.35/84.7

Coal bed pressure (MPa)/(psi) 2.1/300

Poisson's ratio 0.3

Young's modulus (MPa/psi) 3740/550000

Strain at P∞ (infinite pressure) 0.01

Pore compressibility (1/kPa, 1/psi) 0.9/0.0006

P-M constant 3

Table 1

2.2. Methane drainage boreholes

Vertical and horizontal drainage boreholes were used to evaluate

the effects of discontinuities on their degasification performance. In all

of the models documented in this paper, boreholes were modeled as

15.2 cm (6 in.) 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. Thus, the vertical borehole completely penetrated into

the coal seam. The horizontal borehole was modeled as a long radius

borehole with a 46 cm (1.5 ft) vertical section in the upper layers of

the coal seam at the drill location. This short vertical leg was

continued with a 91 m (300 ft) slanted interval (deviation angle is

~0.6°) before the borehole segment was completely horizontal. The

horizontal section was modeled in face cleat direction (E W direc

tion) in the middle layer of the coal seam, due to reasons explained in

the previous section.

For the numerical simulations, the vertical well skin factor was

assumed to be 3, representing 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 coal, 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 stimulation

activities caused 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 associated with

vertical fracture growth. In these same wells, there are significant

delays in gas production most likely due to inefficient depressuriza

tion of the coals. In all of the numerical simulations reported in this

paper, floor and roof rocks are assumed to be undamaged in order to

hydraulically confine the coal seam. In this study, both of the wells

were operated with 0.136 MPa (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.

2.3. Simulating face advance during longwall mining and associated

emissions

This study modeled the advance of a 365.8 m (1200 ft) wide

longwall panel whose start up end was 1400 m (4500 ft) away from

the borehole drill locations (Fig. 1). Mining progressed from east to

west (right to left) in Fig. 1 towards the borehole location. Every face

advance was characterized by sequential “regions” of 69 m (225 ft), or

three grids in the model, in length within the grid model. These

regions represented the coal blocks to be mined during the mining

process. Separate “regions” 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 moving boundary problem (Karacan et al., 2007a,b) 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 “region” were used to calculate

potential emissions for a 9.1 m/day (30 ft/day) average face advance

rate in the presence of stratigraphic discontinuities or geologic

anomalies in the coal seam.

3. Results and discussion

An ideal coal seam from both a mining and a gas drainage point of

view would possess a uniform thickness with no interruptions.

However, this is rarely the case, as most coal seams exhibit geological

features that disrupt this ideal design. Discontinuities, such as faults,

can be structural in origin if they separate groups of rock.

Discontinuities are an important consideration in evaluating the gas

drainage potential of coal seams using vertical and horizontal gas

drainage boreholes. Discontinuities and geologic anomalies 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 effects of impermeable faults in coalbeds on

degasification using vertical and horizontal boreholes, and their

potential effects on methane emissions into mine developments using

numerical reservoir simulation.

3.1. Primary recovery and emissions due to mining: simulation results of

the “base case” using vertical and horizontal boreholes

Degasification simulations using the base coalbed methane model

were performed to establish a comparison for the later simulations

which evaluated the effects of impermeable faults. Vertical and
horizontal boreholes configured as described in Section 2.2 produced

for two years, which is considered as the pre mining degasification

duration for all the simulations in this study. After two years of

degasification, the remaining gas in the coalbed was evaluated for

potential emissions during a longwall mining operation. During these

simulations, both methane and water flow rates were modeled.

Fig. 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.

Fig. 2 shows methane and water productions from single vertical

and horizontal boreholes for a duration of two years. As expected,

methane production from the horizontal boreholes is 2 3 times

greater than from the vertical boreholes. At the beginning of the

production period, this difference is even greater. This is due to the

combination of differences in the lengths of the boreholes and the

pressure depletion areas that they create during production. As noted

by other researchers (Sung and Ertekin, 1987; King and Ertekin, 1988),

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 and 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, a desorption and diffusion dominated region com

mences 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 coal to pressure depletion during production as explained

in Section 2.1. Water production rate was initially 4 m3

/day but

subsequently decreased to 2 3 m3

/day.

Fig. 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 smaller scale.

In the horizontal borehole case, the initial methane production rate

was ~9000 m3

/day and then decreased to ~5000 m3/day at the end of

two years. Similarly, initial water production was 12 m3

/day and 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, a desorption

and diffusion dominated phase starts characterized by a decreasing

production rate. It is also interesting to note that the horizontal

borehole either does not experience significant rate changes due to

the coal's response to pressure depletion or its rate changes due to

coal response are not as obvious as the vertical well in this study.

After two years of degasification, evaluation of possible methane

emissions due to an advancing longwall in the “base model” was

considered. 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

borehole production. The emission rate 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 after degasification and all of the remaining free gas in the

fractures were assumed to be available for emission during the 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 the 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 horizon. 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 the vertical and slanted portion of the borehole, which was not

as effective as the horizontal section. 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 a minimum

(~22.5 m3

/min) when the face almost intercepted the borehole

location. Water inflow to the mine showed a similar trend, decreasing

to less than 0.025 m3

/min as mining advanced. In the vertical borehole

case, water inflow was higher when compared to the horizontal

borehole.

Fig. 3. Methane and water emissions into the mine in the base coal bed model after

2 years of degasification with vertical and horizontal boreholes.

These simulations show that a horizontal borehole, when

compared to a vertical borehole, produced more gas and water from

the coalbed, thus reducing these inflows into the mine.
3.2. Effect of “impermeable faults” on degasification efficiency and

emissions during mining

3.2.1. Coalbed degasification efficiency using surface drilled vertical and

horizontal boreholes

In this study, the effects of impermeable faults on coalbed

degasification efficiency using vertical and horizontal boreholes

were investigated. A geological fault was located 340 m (1125 ft)

east of the borehole drill location in the base model. The fault

direction was along the N S (north south) direction. A fault array was

described in the model so that the flow interactions between the grids

on either side of the fault blocks in a multilayer grid system would be

in all directions in formulation. In order to make this fault an

impermeable discontinuity in the model, the horizontal transmissi

bility along the fault block was set to zero.

The fault configurations simulated in this study included no throw,

an upthrown block and a downthrown block. In all simulations, the

block where the borehole was drilled was taken as stationary, and the

block on the opposite side of the fault plane was moved according to

the throw amount and direction. Thus, the fault block on the borehole

drilling side was steady, whereas the vertical positions of the grids in

the other fault block were moved by +/ 1 m (+/ 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.

It should be noted that, after this treatment, the distance between

the borehole section and the top of the reservoir block had increased

or decreased by 1 m (3 ft) based on the direction of throw. In a drilling

operation or during shaped charge jet penetration through fractures,

possible rock damage around the tunnel may create a positive skin

and impede fluid flow (Halleck and Dogulu, 1996). In this study, it was

assumed that such damage was not present around the borehole along

the fault line in the vertical flow direction. Fig. 4 shows cut away

pictures of the reservoir with a horizontal borehole and the three

different fault structures.

Fig. 4. Coal bed models that show the modeling of impermeable faults (A) with no

throw, (B) with downward throw, and (C) with upward throw. The figures also show the

relative positions of horizontal and vertical boreholes in each case.

It should also be mentioned that the fault geometries were

idealized in this study by modeling them as vertical planes separating

the two blocks. In nature, in most cases, fault planes have some kind of

an angle depending on they are normal or reverse faults. The reason

for modeling faults as vertical planes rather than oblique surfaces was

to eliminate the possible effects of different fault angles on the results.

The other reason was due to the necessity of creating extremely small

grid dimensions in horizontal direction around the fault to create an

angle approximately 60°. This would change the uniformity of grid

sizes, increasing computational cost and also possibly would affect the

results especially around the fault.

Fig. 5 A shows the methane production behavior of the vertical

well in a faulted reservoir. It shows that the presence of an

impermeable fault 343 m away from the borehole formed an effective

boundary in the coal layer. Production data and a comparison with the

base case production curve show that at 175 days, pressure transients

created by the vertical borehole reached the fault location. After this

period, since the size of the reservoir was effectively limited, the

reservoir pressure declined more rapidly and promoted more gas

desorption and an increased production from the wellbore side block.

The pressure decline and associated desorption possibly accelerated

the porosity and permeability changes in the reservoir due to

compaction and matrix shrinkage processes in the coal. Thus,

increased production after ~175 days was a combination of pressure

transients in a bounded reservoir and the coal permeability change.

Fig. 5 B shows the methane production history of a horizontal well

in a faulted reservoir. The data show that the horizontal wellbore was

not affected by the presence of an impermeable fault as much as the

vertical borehole. The productions in faulted reservoirs are close and a

little lower compared to the base case and follow a similar decline

trend. This may be because the horizontal borehole was in commu

nication with both sides of the fault and the pressure continuity was

almost maintained. However, the presence of an impermeable

boundary may still be effective around the borehole and may affect

the water drainage in the reservoir and saturation of phases. In the

field, the preferred practice is to drill the horizontal boreholes in the

coal bed in a slightly up dip direction, in order for the water to drain

and be removed more efficiently, which results in the gas rates to

increase.

Since the performance of the vertical borehole was affected more

by the presence of an impermeable fault in the reservoir, the effect of

the proximity of the fault to the borehole location was also
investigated. Impermeable faults 114 m and 572 m away from the

borehole, in addition to the original fault location at 343 m, were

simulated and compared. Fig. 6 shows that, based on the proximity of

fault, the shape of the production rate curve changes. The reservoir

with a fault which was far enough (572 m) from the borehole acted

like the base case, which did not have any discontinuities. However,

the boundary was still felt by the borehole at later times during the

production period. If the borehole produced for one year, this effect

probably would not have been felt, and the production curve would

have been exactly the same as the base case. In this case, the

production period for the vertical borehole dictates whether the coal

seam behaves as a bounded reservoir or an infinite one. In the case of

an impermeable fault at a very close proximity (114 m in this study) to

the borehole, the interference due to the presence of the fault was felt

almost instantly and manifested itself with a fast pressure decline and

with a high gas rate that initially reached a peak production. Due to

the instant interference of the boundary, the reservoir behaved like an

infinite conductivity medium that produced gas at a high rate as in the

case of hydraulic fractures with infinite conductivity (King and

Ertekin, 1986, 1988). However, since the reservoir volume and gas

content were low, the production rate declined as production

continued (Fig. 6).

Fig. 5. Methane production curves with (A) a vertical borehole (B) a horizontal borehole

in a coalbed with different fault geometries.

Fig. 6. Methane production curves with a vertical borehole that is located at different

distances from an impermeable fault.

3.2.2. Reservoir pressures prevailing in the longwall panel after

degasification

In order to evaluate the effects of impermeable faults on average

reservoir pressure remaining after degasification of the panel area, the

average pressures in the fracture sectors were calculated. Fig. 7 shows

the change of the reservoir pressure in the panel area as a function of

the distance to the borehole drill location along the panel length, the

fault characteristics, and the borehole type. The figure shows that the

horizontal borehole was, as expected, capable of influencing reservoir

pressure at larger distances from the drill location due to larger

drainage area, and that it was possible to reduce reservoir pressure

more than was achievable when employing a vertical borehole. With

the horizontal borehole simulated in this study, reservoir pressures

declined from 1.96 MPa (287 psi) at 1140 m (3750) from the borehole

drill location to 1.35 MPa (200 psi) at 23 m (75 ft) along the panel

length towards the borehole drill location. Although reservoir

pressures appeared to be somewhat higher when the displacement

was in the upward direction, especially towards the fault line,

pressures were not greatly disturbed by the impervious zone at the

fault line. Slightly higher pressures at the upthrown block were

possibly due to the borehole being at a lower horizon in the reservoir

and production being affected by water accumulation in this zone

under gravity drainage. At the fault line, pressure decline curves

crossed over and followed a reversed path towards the drill location.

In the case of a vertical borehole, the reservoir pressure distribu

tion after two years of production was logarithmic from the start of the

panel to the drill location in the base model (Fig. 7). However, when

there was an impermeable fault present in the coal seam, the reservoir

pressure of the changed abruptly between the two blocks on either

side of the fault. This was because of the fact that pressure transients

did not reach across the fault line during degasification. For the

modeled case, the coal bed reservoir pressure difference between both

sides of the fault was as much as 0.45 MPa (66 psi), which might also

be associated with a significant methane content difference. Also, in

the wellbore block side of the reservoir, pressures were lower than

the base case due to a smaller reservoir volume to drain. As these

findings demonstrate, a significant pressure difference on both sides

of a fault may be a problem during mining as the fault is mined

through. This will be especially important if the longwall face

approaches the fault from the borehole side.

Fig. 7. The change of the coalbed pressure in the panel area as a function of distance to

the borehole-drill location, fault characteristics, and the borehole type.
Fig. 9. Methane and water emissions into the longwall face after degasifying a coalbed

with a vertical borehole located at various distances to an impermeable fault.

3.2.3. Methane emissions during longwall mining after degasification of

the panel area

The effects of impermeable faults on methane and water inflows

on an advancing face after degasification using vertical and horizontal

boreholes were evaluated by using the technique described in Section

3.2.1. Fig. 8 A shows methane emissions and water inflow to an

advancing face based on its distance from the horizontal borehole

location. This figure compares the behavior of faults with different

throws to the base case. The data show that, for a coal bed degasified

using a horizontal borehole, the maximum changes in methane

emission and water inflow to the mine occurred around the fault

(343 m). Fig. 8 A also shows that the maximum methane emission

into the mine occurred while the mining face passed through the

upthrown fault block. This might be due to the fact that the bulk of the

reservoir volume on the right side (upthrown block) of the fault line

remained above the borehole trajectory. This might have caused the

borehole segment in upthrown block be flooded by water entering the

borehole by gravity drainage. Thus, the remaining gas that could not

be produced effectively due to high water saturation was released into

the mine. The next highest emission occurred when there was a right

hand fault block was downthrown along the fault line. In this case, the

methane at the top of the reservoir was produced effectively by the

borehole, while the remaining gas and water stayed in the coal seam

underneath the borehole trajectory and was released into the mine

during mining operations. Water inflow data shown in Fig. 8 A

support these arguments for methane emissions.

In the water inflow case, mining through a fault from downthrown

block direction releases more water into the mine since water cannot

be removed effectively by a borehole located close to the top of the

reservoir. The least amount of water in the mine was found when

there was a throw in the upward direction. These data suggest that the

presence of a fault and the direction of throw (if any) make a

difference in methane and water emissions during mining. Gravity

drainage plays a role in this process when there is a throw that creates

a large difference in the relative height of the borehole in the coal

seam. This situation may be improved by redirecting the borehole

based on the throw direction after drilling through the fault line.

The effects of fault geometries on emissions after degasification

with a vertical well are demonstrated in Fig. 8 B. In this case, one of

the fault blocks was not drained at all and it did not make any

difference whether there was a throw between two blocks across the

fault line. The data show, however, 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 degasified

effectively. For the model used in this study, this difference in methane

emission was as much as 4 m3

/min. The same argument would also be

true if the mine approached from the other direction. In that case,

there would be a jump in the methane and water emissions into the

mine, which could create gas control difficulties. Therefore, it is

important to locate the fault and determine the presence and extent of

any throw in order to plan better borehole drilling and placement

strategies.

3.3. Effect of proximity of vertical boreholes to impermeable faults on

methane emissions during mining

Results in the previous sections show that vertical boreholes are

not as effective as horizontal boreholes for draining methane from

both sides of impermeable faults. In that case, the reservoir block that

was not associated with the borehole was not drained. The data show,

however, that there would be a sharp decrease (or increase,

depending on the mining direction) in methane emissions and

water inflow rates after the mine face passed through the fault. This

raises the question of whether the proximity of the fault line to the

vertical borehole location could change the degasification perfor

mance prior to mining and thus the methane emissions during

mining. Thus, simulation runs were conducted with the vertical

borehole drilled in a coal seam with an impermeable fault without any

displacements. The models were constructed so that the borehole

would be at different proximities to the fault and be operated two

years prior to mining, as before.

Fig. 8. Methane and water inflow into a mine advancing in a faulted coalbed. Figure

(A) is after degasification with a horizontal borehole. Figure (B) is after degasification

with a vertical borehole. In Figure B some of the data overlap.

Fig. 9 shows simulated methane and water emissions during

mining after two years of degasification of the coal bed with a vertical

borehole prior to its mining. The data show that as the borehole drill

location approached the fault during degasification, the methane and

water inflow rates across the impermeable fault increased. For

instance, when the fault was 572 m away from the borehole, methane

emissions were constant until the face reached the fault location.
While mining through the fault line, methane emissions decreased by

about 1.5 m3

/min and then continued with a gradual decline as mining

approached the borehole location. On the other hand, when the fault

was 114 m or 343 m away from the borehole, the decrease in emission

rates was 4 and 6 m3

/min, respectively. Water inflows followed a

similar trend based on the fault location. These results suggest that if

there is an impermeable fault in close proximity to a vertical

degasification borehole, abrupt changes in emissions during mining

can be expected. This situation is probably more dangerous when the

mining face approaches from the opposite direction (from left to right)

as compared to the direction (from right to left) modeled in this study.

In that case, a huge increase in emission and water inflow can occur

and endanger miner safety. Thus, it is advisable to pay attention to the

proximity of the faults to the vertical borehole locations in order to

better degasify the coal beds and also to better control methane

emissions and water inflows in the mines.

Fig. 10 shows coal reservoir pressure in the panel to be extracted as

a function of proximity of the vertical borehole to the fault. The figure

shows that as the distance of the borehole to the fault decreased,

reservoir pressures decreased more quickly due to the pressure

transients reaching the impermeable boundary earlier during the

degasification period and draining a smaller reservoir volume. As a

result, the reservoir pressure in the borehole block side was less and

the differences in pressure between each side of the fault were greater,

making gas pressure differentials higher during mine through of the

fault.

Fig. 10. Reservoir pressures prevailing after degasification of a longwall panel in a

faulted coalbed reservoir using a vertical borehole drilled at three different proximities

to the fault.

4. Conclusions

This study presented a numerical investigation using reservoir

simulations of the effects of impermeable faults with and without

throws on the production performances of vertical and horizontal

degasification boreholes. Longwall advances, face position related

emissions, and reservoir properties were also characterized using

sector definitions along the path of the panel grids to evaluate the

impacts of impermeable faults on methane emissions into mine

workings.

Evaluations of a base coalbed methane model showed decreasing

methane and water productivities with time for both vertical and

horizontal boreholes. The effect of permeability changes in the coal

seam was more pronounced with a vertical borehole compared to a

horizontal borehole. The base model simulations showed that, as

expected, methane production from the horizontal borehole was 2 to

3 times greater than from the vertical borehole. At the beginning of

production period, this difference was even greater. This was due to

the differences in the lengths of the boreholes and the pressure

depletion areas that they created during production. These simula

tions also showed that two years of degasification with a horizontal

borehole reduced methane and water emissions into an approaching

longwall face compared to a similar degasification period using a

vertical borehole.

The methane production of a horizontal borehole was not

significantly affected by the presence or displacement direction of a

fault located 343 m distant. This was not the case with a vertical

borehole, which showed, after 175 days of operation, increased

methane production compared to the base model. This increased

methane likely arose from the pressure transients from the vertical

borehole reaching the impermeable fault boundary and causing

increased production from the wellbore side block. Further simula

tions showed that an impermeable fault in close proximity to the

vertical borehole had greater impact on its production. A fault located

farther from this borehole had much less impact.

Location of a vertical borehole is important to control the methane

emissions into a mine in a faulted coal seam. Simulations were

conducted of methane and water inflows in the presence of a fault

located 343 m distant from a longwall face. These results showed

dramatic reductions in gas and water inflows once the face passed

through the fault into the borehole block. These data suggest thatmining

into the borehole block is preferable to mining in the other direction,

which would lead to dramatic increases in methane and water inflows.

Similar results were found when the impermeable fault was located

114 m and 572 m distant from a vertical borehole. Methane emissions

and water inflows again decreased once the face passed through the

fault. Average reservoir pressures also decreased at this point.

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