United States National Risk Management

Environmental Protection Research Laboratory

Agency Research Triangle Park, NC 27711

Research and Development EPA/600/SR-96/065 June 1996

E AP

Evaluation and Analysis of Gas

Content and Coal Properties of

Major Coal Bearing Regions of the

United States

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 content 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 compile 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 original 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 generated from the original data. Additional

data presented include the results of

equilibrium adsorption isotherm tests

performed by the Department of Energy (DOE) in 1983 for approximately

100 of the BOM coal samples. These

results give important, basin level information on the capacity of various

coalbeds to store and release methane. In order to provide context and

background, the report also characterizes the geology and coal and coalbed

methane resources in each major U.S.

coal basin.

Project Summary

Sushma Masemore, Stephen Piccot, Eric Ringer, and William P. Diamond

This Project Summary was developed

by EPA's National Risk Management

Research Laboratory's Air Pollution

Prevention and Control Division, Research 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 Prevention and Control Division (APPCD) has

undertaken a study to identify the most

practical and cost effective means to

use coalbed methane control and utilization technologies to reduce methane

emissions into the atmosphere. An initial phase of this study was to characterize methane gas content in minable

coalbeds of active coal producing regions across the country. A joint evaluation 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 multitiered quality assurance and computational 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
2

quality assurance activities, and key findings on a national level.

In 1972, the BOM developed the “Direct

Method” (DM) for determining the gas content of coal cores. The DM is based on

measurements of the gas volume evolved

from coal cores sealed in airtight canisters, where the total gas content is obtained as the sum of three gas volume

determinations identified as desorbed gas,

residual gas, and lost gas. The desorbed

gas volume is determined from cumulative volume measurements made over time

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 atmospheric pressure and methane concentration. The residual gas content is defined as the gas volume that is released

when the core is crushed to a fine powder. 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, several 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 addition to methane, correcting the gas volumes for standard conditions, and correcting for ash and moisture content. The

original DM data were compiled and calculated manually. This introduced some

quality and consistency problems. In particular, the lost gas derivations were conducted somewhat inconsistently by different 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 composition of the desorbed and residual gas is

determined and used to quantify the actual volume of methane released, and to

correct for the oxygen sorption effect. This

change accounts for most of the difference between DM and MDM results. Corrections are also made for ambient atmospheric conditions during sampling and

testing, and for the ash and moisture content of the samples. The currently published 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 geographically representative data set compiled using 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 performed with confidence on 423 of the

samples. That is, complete data necessary 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 assure the quality of the data, make necessary corrections and modifications, and

develop total gas content and sorption

time information for the samples. Examples

of some of the exercises conducted include: digitization of the data which automated 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 graphical data to identify outliers that might indicate errors in the original data; calculating

lost gas values, using a standardized, computer assisted routine to ensure consistent results; and determining sorption times

from desorption curves and Langmuir isotherm 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 substantial subset of the samples.

Desorbed gas volume versus time plots

(defined as desorption curves) can be used

to determine rate constants such as sorption time. Time constants are important in

defining coalbeds which have the potential 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 atmospheric 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 recovery operations.

Adsorption isotherm data were generated 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 adsorption isotherms (a plot of total gas adsorbed

as a function of pressure) according to

the Langmuir adsorption model. The isotherm curve relates the gas storage capacity as a function of coalbed reservoir

pressure and is used to predict gas production 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 Content Data: Some errors and inconsistencies were found in the original desorbed

and lost gas values. Residual gas results

were determined directly from BOM laboratory 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 errors, and failure to include all of the incremental desorbed gas volumes in the cumulative result. Errors in original lost gas

values resulted mainly from imprecise application of DM procedures used to determine 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 exposed, the desorbed volume is a linear

function of the square root of time. Some

errors in lost time calculations in the original data were found and corrected. In the

RGC analysis, lost time calculations were

automated, and a computerized linear regression was executed on the first 10

desorbed gas volumes to determine lost

gas.

The impact of the corrections and refinements that were made is most significant for the lost gas values. On average,

(*) 1 atm = 98 kPa.
3

RGC lost gas values are about 24% less

than the published BOM values. RGC lost

gas values are lower in each of the basins, 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 increases 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 analyses on a selected number of the original

coal samples. The BOM’s original publication presented ash content and apparent 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 measured 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 contents 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 ambient 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 resure for the BOM sampling sites and sampling 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 (defined as 60°F* and 1 atm) was applied to

these samples. The analysis may be limited somewhat by the fact that ambient

conditions during desorption testing may

not be reflected if the samples were transported away from the site for testing. However, during the BOM sampling program,

samples often remained at or near the

site during much of the desorption testing.

Thus, these corrections may be considered 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) representativeness of the data in terms of U.S. coal

production, and (2) basin level trends in

terms of gas content, gas content relationships (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 producing 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 producing 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 aggregated RGC and “as-published” BOM data

were used to develop a range of commonly 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 factors; however, some trends are evident.

Coalbeds in the Arkoma, Black Warrior,

Central Appalachian, Northern Appalachian, 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 Basins exhibit the most consistently high gas

contents. Samples from the Central Appalachian 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 contents less than 100 ft3/ton. Samples from

the Greater Green River Basin also exhibit the full range of gas contents, including some of the highest values represented; however, many samples have low

gas content and are associated with relatively 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 examined 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 increased gas generation, and deeper

coalbeds are associated with increased

methane adsorption (due to higher pressures) and a higher probability of gas containment. 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*

ln (depth) + b, where a and b are constants) 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 content with coalbed depth and rank is evident in the RGC/BOM data. Gas content

corded, so the correction to standard conditions could not be applied.

In order to examine the effect of ambient pressure and temperature variations

on gas content data, temperature and pres-

(*) °C = 5/9 (°F - 32).

(*) 1 ft3/ton = 3.1218 x 10-5 m3/kg.
4

is seen to increase with depth, and low

volatile coals are associated with the highest gas contents, followed by medium and

high volatile coals. In addition, the logarithm model appears to describe the data

reasonably. There are also clear differences 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 content 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 segregated by rank. More robust relationships

are sometimes observed locally within specific coalbeds or counties, indicating that

changes in local coal properties are important. 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 discussion of the full report.

Sorption Time: Sorption time was computed from the time series desorption

curves for each RGC coal sample. Sorption time can vary significantly within a

given basin; however, there are clear differences in sorption time among basins.

The median sorption time across all basins is about 30 days, and this value can

be used as a benchmark for identifying

regions with high, low, and average sorption times. By this measure, Northern Appalachian and San Juan Basin coals can

be characterized as slow desorbers, while

Powder River and Raton Mesa coals desorb most rapidly. Coals in the remaining

basins can be considered “average”

desorbers. Several figures in the full report compare the average sorption time of

the basins.

Sorption time indicates how rapidly initial desorption from coal takes place, but

does not describe diffusion through the

coal matrix. Thus, sorption time alone cannot 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 content and low sorption time will likely produce significant quantities of gas as coal

is continually exposed by mining operations. Run-of-mine coal emerging from

such mines will continue to emit significant quantities of gas to the atmosphere

during storage, handling, and transport operations 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 temperature. Generally, gas storage capacity

increases with increasing pressure; increasing 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 summarized in the full report.

Langmuir volume varies considerably

across the samples analyzed (ranging from

300 to over 3000 ft3/ton), but is symmetrically 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 analyzed 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 indicated 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 represented 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 number 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 Appalachian Basin (less than about 200 psi

bed pressure, or about 500 ft** deep), the

measured gas content is near, and sometimes exceeds the Langmuir model prediction. This may be an artifact of the

pressure gradient's being non-representative at shallow depths, or may be related

to the rapid increase of Langmuir model

predictions at low pressures.

(*) 1 psi = 6.89 kPa.

(**) 1 ft = 0.30 m.
5

Sushma Masemore, Stephen Piccot, and Eric Ringer are with Southern Research

Institute, Chapel Hill, NC 27514; 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

Propertries 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

Official Business

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