United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-96/065 June 1996
EPA Project Summary
Evaluation and Analysis of Gas
Content and Coal Properties of
Major Coal Bearing Regions of the
United States
Sushma Masemore, Stephen Piccot, Eric Ringer, and William P. Diamond
The report presents a compilation of
quality assured data on gas content
and coalbed reservoir properties for 11
major coal bearing regions in the U.S.
The primary source of these data is the
U.S. Bureau of Mines (BOM) gas con-
tent measurements program conducted
during the 1970's and 1980's. In order
to enhance the utility of the BOM data,
an evaluation was conducted to com-
pile and quality assure the original data,
and to adjust the data as needed to
improve quality and representativeness.
The report was compiled to provide
access to these improved data at the
basin level. Under this effort, the origi-
nal raw data records for the core
samples were provided by the BOM.
The raw data were digitized to allow a
computer to accurately and consistently
perform routine quality assurance
checks, consistently determine lost gas
and total gas contents for each sample,
and examine various corrections to the
data. In addition, desorption constants
for each coal sample were determined
from time series desorption curves gen-
erated from the original data. Additional
data presented include the results of
equilibrium adsorption isotherm tests
performed by the Department of En-
ergy (DOE) in 1983 for approximately
100 of the BOM coal samples. These
results give important, basin level in-
formation on the capacity of various
coalbeds to store and release meth-
ane. In order to provide context and
background, the report also character-
izes the geology and coal and coalbed
methane resources in each major U.S.
coal basin.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory's Air Pollution
Prevention and Control Division, Re-
search Triangle Park, NC, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
The U.S. Environmental Protection
Agency's (EPA) Air Pollution Preven-
tion and Control Division (APPCD) has
undertaken a study to identify the most
practical and cost effective means to
use coalbed methane control and utili-
zation technologies to reduce methane
emissions into the atmosphere. An ini-
tial phase of this study was to charac-
terize methane gas content in minable
coalbeds of active coal producing re-
gions across the country. A joint evalu-
ation with the U.S. Bureau of Mines
(BOM) was conducted to compile and
quality assure the data gathered from
the BOM's gas content measurements
program executed during the 1970's
and 1980's. The result of this multi-
tiered quality assurance and computa-
tional effort is a database titled the
Refined Gas Content (RGC) database.
This database and the results of gas
content trend analysis are reported in
the full report. The report presents a
compilation of the gas content data and
coalbed reservoir properties for major
coal bearing regions of the U.S. The
following paragraphs summarize the
sources of data, data management and
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quality assurance activities, and key find-
ings on a national level.
In 1972, the BOM developed the "Direct
Method" (DM) for determining the gas con-
tent of coal cores. The DM is based on
measurements of the gas volume evolved
from coal cores sealed in airtight canis-
ters, where the total gas content is ob-
tained as the sum of three gas volume
determinations identified as desorbed gas,
residual gas, and lost gas. The desorbed
gas volume is determined from cumula-
tive volume measurements made overtime
as gas desorbs from the coal surfaces
and microporous structure. A desorption
rate curve may be developed from the
cumulative desorbed gas volume versus
time. This curve initially increases rapidly
and eventually flattens out as gas sorbed
onto the coal reaches equilibrium with at-
mospheric pressure and methane concen-
tration. The residual gas content is de-
fined as the gas volume that is released
when the core is crushed to a fine pow-
der. Lost gas is defined as the gas that
evolves from the core samples from the
time that the coalbed is first encountered
by the drill to the time the sample is sealed
in the desorption canister.
Since the development of the DM, sev-
eral shortcomings have been identified that
have raised questions about the quality of
the original data. Examples of items not
included in the DM are: accounting for
oxygen sorption onto coal, accounting for
liberation of other hydrocarbons in addi-
tion to methane, correcting the gas vol-
umes for standard conditions, and cor-
recting for ash and moisture content. The
original DM data were compiled and cal-
culated manually. This introduced some
quality and consistency problems. In par-
ticular, the lost gas derivations were con-
ducted somewhat inconsistently by differ-
ent investigators and technicians over the
years.
In response to these shortcomings, the
BOM developed and tested a modified
version of the DM known as the "Modified
Direct Method" (MDM). The primary
change in the MDM is that the composi-
tion of the desorbed and residual gas is
determined and used to quantify the ac-
tual volume of methane released, and to
correct for the oxygen sorption effect. This
change accounts for most of the differ-
ence between DM and MDM results. Cor-
rections are also made for ambient atmo-
spheric conditions during sampling and
testing, and for the ash and moisture con-
tent of the samples. The currently pub-
lished BOM data are based only on the
DM results, and do not include MDM data.
Regardless of the shortcomings of the
original method, the large and geographi-
cally representative data set compiled us-
ing the DM is valuable for conducting
mine ventilation analyses, coal mine
degasification analyses, coalbed methane
development assessments, and methane
emission inventory development studies.
Development of the RGC
Database
The BOM obtained gas content data for
1511 coal samples. Original raw data files
were available for 1100 of these samples.
Many of the available BOM files were
incomplete. Missing data included coring
time parameters required to calculate lost
gas, sample weights required to calculate
gas content, and incomplete or missing
pages from the records of the desorption
experiment. Due to lack of pertinent data
required to determine total gas content,
complete calculations could not be per-
formed with confidence on 423 of the
samples. That is, complete data neces-
sary for RGC analysis were available for
677 of the original 1511 BOM samples
(about 45 %).
Several data processing, screening, and
calculational steps were completed to as-
sure the quality of the data, make neces-
sary corrections and modifications, and
develop total gas content and sorption
time information for the samples. Examples
of some of the exercises conducted in-
clude: digitization of the data which auto-
mated error checking and calculations;
screening the data for completeness of
information necessary for calculation of
total gas content; developing time series
desorption curves and screening graphi-
cal data to identify outliers that might indi-
cate errors in the original data; calculating
lost gas values, using a standardized, com-
puter assisted routine to ensure consis-
tent results; and determining sorption times
from desorption curves and Langmuir iso-
therm constants from DOE adsorption
data. In addition, the gas content on an
ash/moisture free basis was calculated for
samples where coal analysis data were
available, and the effect of corrections for
site barometric pressure and temperature
conditions was examined using available
national weather service data for a sub-
stantial subset of the samples.
Desorbed gas volume versus time plots
(defined as desorption curves) can be used
to determine rate constants such as sorp-
tion time. Time constants are important in
defining coalbeds which have the poten-
tial to quickly outgas the largest quantities
of gas. Sorption time is defined as the
amount of time required to release 63% of
the total gas contained in the coal at at-
mospheric pressure. Sorption time has
been identified as an effective measure of
the diffusion rate and is used in coalbed
methane reservoir simulator modeling.
Sorption rate constants are useful to both
coal mining and coalbed methane recov-
ery operations.
Adsorption isotherm data were gener-
ated by the U.S. Department of Energy
(DOE) for approximately 100 coal samples
collected by the BOM. The volume of gas
adsorbed was reported at five pressure
stages ranging from 5 to 50 atm.* The
DOE data were used to develop adsorp-
tion isotherms (a plot of total gas adsorbed
as a function of pressure) according to
the Langmuir adsorption model. The iso-
therm curve relates the gas storage ca-
pacity as a function of coalbed reservoir
pressure and is used to predict gas pro-
duction potential as reservoir pressure is
reduced. The equation for this isotherm
curve is described by two constants:
Langmuir volume and Langmuir pressure.
Initial Corrections to BOM Gas Con-
tent Data: Some errors and inconsisten-
cies were found in the original desorbed
and lost gas values. Residual gas results
were determined directly from BOM labo-
ratory experiments, and there was no need
for corrections in the RGC data. Errors in
the original desorbed gas values resulted
from incorrect data entry, calculation er-
rors, and failure to include all of the incre-
mental desorbed gas volumes in the cu-
mulative result. Errors in original lost gas
values resulted mainly from imprecise ap-
plication of DM procedures used to deter-
mine lost gas values.
Since the lost gas volume cannot be
quantified directly, it is inferred from an
empirical relationship between time and
desorbed volume. This relationship is that,
within the first hours after a core is ex-
posed, the desorbed volume is a linear
function of the square root of time. Some
errors in lost time calculations in the origi-
nal data were found and corrected. In the
RGC analysis, lost time calculations were
automated, and a computerized linear re-
gression was executed on the first 10
desorbed gas volumes to determine lost
gas.
The impact of the corrections and re-
finements that were made is most signifi-
cant for the lost gas values. On average,
(*) 1 atm = 98 kPa.
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RGC lost gas values are about 24% less
than the published BOM values. RGC lost
gas values are lower in each of the ba-
sins, ranging from -37 to -4% at the basin
level. The largest differences were for the
Arkoma and Raton Mesa Basins (-35 and
-37%, respectively); however, the overall
average is driven by the large number of
samples from the Northern Appalachian
Basin where the average difference was
-22%. RGC desorbed gas values range
from -9 to +19% at the basin level, but the
overall average difference is zero. The
relatively large increase evident for
samples from the Greater Green River
Basin (+19%) is notable. Except for
samples from the Greater Green River
Basin, there is little overall difference in
total gas content between the published
BOM and the RGC samples. Since lost
gas values typically make up less than
5% of the total gas, relatively small in-
creases in the corrected desorbed gas
values tend to offset the larger decreases
in lost gas values.
Corrections for Ash and Moisture
Content: The BOM conducted coal analy-
ses on a selected number of the original
coal samples. The BOM's original publi-
cation presented ash content and appar-
ent coal rank for each sample, and the
remaining data were not published. These
data are presented in the full report. Since
it is generally true that methane is not
adsorbed onto non-coal material, ash and
moisture values can be used to make
appropriate corrections on the total mea-
sured gas contents. Total gas values for
ash and moisture content were corrected
for RGC samples. The ash and moisture
weight percent distribution for the RGC
samples is used to determine changes in
total gas content when ash and moisture
are corrected. Ash/moisture free gas con-
tents are higher than the as received gas
content by +9 to +34% at the basin level
with an overall average of +21%.
Corrections for Ambient Temperature
and Pressure: It has been recommended
that the volume of gas measured at ambi-
ent or atmospheric conditions in the field
be corrected for standard temperature and
pressure. In the original BOM data, only
the volume of gas at ambient conditions
was used to determine total gas content
of coal cores. Unfortunately, temperature
and pressure at the sampling sites and
during desorption analysis were not re-
corded, so the correction to standard con-
ditions could not be applied.
In order to examine the effect of ambi-
ent pressure and temperature variations
on gas content data, temperature and pres-
sure for the BOM sampling sites and sam-
pling dates were obtained from National
Weather Service data. Temperature and
pressure data for 395 of the RGC coal
samples were obtained, and conversion
from actual to standard conditions (de-
fined as 60°F* and 1 atm) was applied to
these samples. The analysis may be lim-
ited somewhat by the fact that ambient
conditions during desorption testing may
not be reflected if the samples were trans-
ported away from the site for testing. How-
ever, during the BOM sampling program,
samples often remained at or near the
site during much of the desorption testing.
Thus, these corrections may be consid-
ered reasonably representative overall.The
corrections to total gas content values
ranged from -3 to -21% at the basin level
with an overall average of -6%.
Data Summary and Trends
The BOM/RGC database was reviewed
and analyzed to examine (1) representa-
tiveness of the data in terms of U.S. coal
production, and (2) basin level trends in
terms of gas content, gas content rela-
tionships (e.g., with depth, coal rank), and
reservoir properties (sorption time, and gas
carrying capacity).
The BOM/RGC database provides broad
geographic coverage of major coal pro-
ducing regions in the U.S. Nearly 90% of
1992 U.S. coal production is in the 11
major coal basins that are the focus of the
full report. The BOM samples represent
coals from 81 counties in 17 states. Some
of the counties represented in the original
data are no longer producing coal. In 1992,
only 59 of the 81 counties represented in
the original data were still producing coal.
In 1992, there were 247 counties produc-
ing coal in the U.S. Approximately 55% of
1992 production is in counties represented
by BOM/RGC samples. Within each of
the basins, gas content data for many
different coalbeds are available.
Gas Content Summary: The aggre-
gated RGC and "as-published" BOM data
were used to develop a range of com-
monly encountered gas content values for
each of the 11 basins. The data were
grouped into five categories: 500 - 709,
300 - 499, 100 - 299, 50 - 99, and less
than 50 ft3/ton.* The full report presents
the number of coal samples within each
of these ranges in each basin and the
average sample depth.
Total gas content can vary widely across
a given basin and within a given coalbed
depending on depth, rank, and other fac-
tors; however, some trends are evident.
Coalbeds in the Arkoma, Black Warrior,
Central Appalachian, Northern Appala-
chian, Greater Green River, and Raton
Basins can contain very high levels of
gas. Within these basins, the Mary Lee
coalbed in Black Warrior, the Pocahontas
No. 3 coalbed in Central Appalachian, the
Williams Fork coalbed in Greater Green
River, and the Peach Mountain coalbed in
Northern Appalachian have coals which
contain the highest gas contents; over 600
ft3/ton. The Arkoma and Black Warrior Ba-
sins exhibit the most consistently high gas
contents. Samples from the Central Ap-
palachian Basin are evenly distributed
across the full range of gas contents. Most
Northern Appalachian Basin samples have
gas contents less than 300 ft3/ton, with
most between 100 and 300 ft3/ton. Most
Illinois Basin coal samples have gas con-
tents less than 100 ft3/ton. Samples from
the Greater Green River Basin also ex-
hibit the full range of gas contents, includ-
ing some of the highest values repre-
sented; however, many samples have low
gas content and are associated with rela-
tively shallow coal. San Juan and Raton
Basin samples generally contain less than
300 ft3/ton, and are more concentrated at
the lower gas content levels (less than
100 ft3/ton). Piceance Basin samples cover
a broad range, but with most samples
containing less than 100 ft3/ton. Powder
River Basin coalbeds contain the least
gas (generally less than 50 ft3/ton).
The relationship of total gas content to
coalbed depth and coal rank was exam-
ined based on the complete RGC/BOM
data set. In general, gas content is thought
to increase with both depth and coal rank.
Higher rank coals are associated with in-
creased gas generation, and deeper
coalbeds are associated with increased
methane adsorption (due to higher pres-
sures) and a higher probability of gas con-
tainment. Gas content has been observed
to increase more rapidly at shallower
depths, then level off with increasing depth.
A logarithm curve (i.e., Gas Content = a*
In (depth) + b, where a and b are con-
stants) provides a simple mathematical
description of this general relationship. This
functional form is used to represent the
relationship of gas content with depth
throughout the report.
The relationship of increasing gas con-
tent with coalbed depth and rank is evi-
dent in the RGC/BOM data. Gas content
O °C = 5/9 (°F - 32).
O 1 ft=/ton = 3.1218 x ia5 m3/kg.
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is seen to increase with depth, and low
volatile coals are associated with the high-
est gas contents, followed by medium and
high volatile coals. In addition, the loga-
rithm model appears to describe the data
reasonably. There are also clear differ-
ences at the basin level in the relationship
of gas content and depth, both in overall
gas content and in the rate of increase of
gas content with depth.
While the increasing trend of gas con-
tent with depth and coal rank is evident in
the data, this simple relationship does not
fully explain the variability observed in the
RGC/BOM gas content data. Regression
(using the logarithm model) of total gas
content versus depth is generally weak (r2
< 0.5), even when the data are segre-
gated by rank. More robust relationships
are sometimes observed locally within spe-
cific coalbeds or counties, indicating that
changes in local coal properties are im-
portant. For example, it has been found
that the microporous structure of coal is
related to methane storage capacity. This
is discussed in more detail below. Local
relationships of gas content with depth
are examined within basin level discus-
sion of the full report.
Sorption Time: Sorption time was com-
puted from the time series desorption
curves for each RGC coal sample. Sorp-
tion time can vary significantly within a
given basin; however, there are clear dif-
ferences in sorption time among basins.
The median sorption time across all ba-
sins is about 30 days, and this value can
be used as a benchmark for identifying
regions with high, low, and average sorp-
tion times. By this measure, Northern Ap-
palachian and San Juan Basin coals can
be characterized as slow desorbers, while
Powder River and Raton Mesa coals des-
orb most rapidly. Coals in the remaining
basins can be considered "average"
desorbers. Several figures in the full re-
port compare the average sorption time of
the basins.
Sorption time indicates how rapidly ini-
tial desorption from coal takes place, but
does not describe diffusion through the
coal matrix. Thus, sorption time alone can-
not be used as an indicator of coalbed
gas production. However, sorption time is
a useful indicator of direct gas emissions
from a coal mining operation, and from
post-mining coal handling operations.
Mines producing coal with high gas con-
tent and low sorption time will likely pro-
duce significant quantities of gas as coal
is continually exposed by mining opera-
tions. Run-of-mine coal emerging from
such mines will continue to emit signifi-
cant quantities of gas to the atmosphere
during storage, handling, and transport op-
erations prior to consumption.
Gas Storage Capacity: The Langmuir
adsorption isotherm is a model describing
the gas storage capacity of coal as a
function of pressure at a constant tem-
perature. Generally, gas storage capacity
increases with increasing pressure; in-
creasing rapidly at first, and then leveling
off to a maximum. For the full report,
Langmuir adsorption coefficients (Langmuir
volume and pressure) were calculated from
the DOE results. These data are summa-
rized in the full report.
Langmuir volume varies considerably
across the samples analyzed (ranging from
300 to over 3000 ft3/ton), but is symmetri-
cally distributed with an average of about
1075 ft3/ton. There is some variability
among basins; however, variability within
basins is equally significant. That is, there
seems to be no clear trend for coals from
one basin to have significantly more or
less adsorptive capacity than coals from
another basin. Langmuir volume also
seems largely independent of coal rank
and depth.
Gas content and desorption data are
available for 36 of the 100 samples ana-
lyzed by DOE. DOE did not specifically
examine the relationship of gas content
and sorption rate to Langmuir adsorption.
Since the Langmuir curve represents the
maximum gas storage capacity at a given
pressure, the measured gas content should
be less than or equal to the volume indi-
cated by the Langmuir curve. Based on
generalized information on the hydrostatic
gradient (psi/foot) at the basin level and
sample depth, it is possible to obtain a
rough idea of the fraction of the volume
indicated by the Langmuir curve repre-
sented by the measured gas content. The
gas volume predicted by the Langmuir
model appears to be about a factor of 2 to
3 higher than the measured gas volume
(corrected for ash/moisture content). This
is based on calculations for a limited num-
ber of samples at pressures exceeding
200 psi* from the Black Warrior, Central
Appalachian, Northern Appalachian, and
Raton Mesa Basins, and appears to be
fairly consistent across basins. For some
shallow samples from the Northern Appa-
lachian Basin (less than about 200 psi
bed pressure, or about 500 ft** deep), the
measured gas content is near, and some-
times exceeds the Langmuir model pre-
diction. This may be an artifact of the
pressure gradient's being non-representa-
tive at shallow depths, or may be related
to the rapid increase of Langmuir model
predictions at low pressures.
O 1 psi = 6.89 kPa.
(**) 1 ft = 0.30 m.
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Sushma Masemore, Stephen Piccot, and Eric Ringer are with Southern Research
Institute, Chapel Hill, NC27514; and William Diamond is with the U.S. Bureau of
Mines, Pittsburg Research Center, Pittsburgh, PA 15236
David A. Kirchgessner is the EPA Project Officer (see below).
The complete report, entitled "Evaluation and Analysis of Gas Content and Coal
Properties of Major Coal Bearing Regions of the United States, "(Order No. PB96-
185 491; Cost: $38.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air Pollution Prevention and Control Division
National Risk Management Research Laboratory
U. S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
National Risk Management Research Laboratory (G-72)
Cincinnati, OH 45268
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