Measuring the gas content of coal: A review 

William P. Diamond @ , Steven J. Schatzel 

Abstract 

Coalbed gas content measurements are commonly used in mine safety as well as coalbed 

methane resource assessment and recovery applications. Gas content determination techniques 

generally fall into two categories: (1) direct methods which actually measure the volume of gas 

released from a coal sample sealed into a desorption canister and (2) indirect methods based on 

empirical correlations, or laboratory derived sorption isotherm gas storage capacity data. Direct 

gas content determination techniques may be fbrther,subdivided into quick-crushing and extended 

desorption methods. The quick-crushing methods are primarily used in mine safety applications 

outside the United States, but have also been used for resource recovery applications. Quick-crushing methods rely on crushing the coal sample soon after collection to release all the desorbable 

gas, thus significantly shortening the amount of time required for desorption measurements. 

However, some data useful for resource recovery applications are lost. Extended desorption 

techniques are most commonly used for resource assessment and recovery applications where 

information on desorption rates is useful for reservoir modeling, and for fundamental coalbed 

methane research. Extended desorption methods allow the gases in the coal sample to desorb 

under controlled laboratory conditions until a defined low desorption rate cutoff point is reached. 

The sample may then be crushed to measure the amount of gas remaining within the sample. 

Direct method techniques for gas content measurement are the focus of this paper. 

Keywords: coal; coalbed methane; gas content; coal mine safety; reservoir engineering; unconventional gas 

reservoirs 
volumes cannot be directly measured and therefore must be estimated fiom the 

subsequently measured gas volume data. Since the lost gas is an estimated quantity, it is 

generally considered to be the least reliable component of the total gas content. 

Considerable research effort has been expended to evolve a more versatile and accurate 

lost gas estimation procedure from that developed by the former USBM (US Bureau of 

Mines) (Kissell et al., 1973) fiom the work of Bertard et al. (1970). 

The volume of gas lost prior to sealing a coal sample into a desorption canister 

generally depends on sample retrieval time, physical character of the sample, the type of 

drilling fluid, and water saturation/relative amount of free gas. Virtually all methods for 

estimating lost gas benefit fiom minimizing the lost-gas time over which measured 

desorption data must be extrapolated. Wireline core retrieval fiom coalbed depths of less 

than 610 m (2,000 ft), common in most mining areas in the United States, is relatively 

quick and generally requires only a few minutes to reach the surface. The retrieval time 

plus the time required to remove the sample from the core barrel, determine the depth 

interval of the coalbed and individual samples, inspect and describe the coal sample(s) to 

be tested, and finally sealing the sample(s) in the canisterts) can generally be accomplished in under one hour, and in many instances in less than 30 minutes. Under these 

conditions, the lost-gas time is relatively short, thus reducing the lost-gas volume and 

therefore, the potential error inherent in the estimation techniques. 

Lost gas estimation becomes a more significant issue under several circumstances 

most commonly associated with commercial production operations. Lost-gas times are 

generally longer (> 1 h) in many of the commercial coalbed methane applications where 

depths may be greater than those associated with mining, and/or conventional coring 

methods are utilized. Extended lost-gas times can increase any associated error by 

increasing the fraction of lost gas relative to the total gas content. 

Kissell et al. (1973) and McCulloch et al. (1975) demonstrated that the physical 

character of the retrieved coal sample can influence the desorption rate and hence the 

volume of lost gas that must be estimated. Blocky coals which remain intact during 

coring and subsequent retrieval emit their adsorbed gas at a relatively slow rate. 

However, friable coals which tend to break apart into smaller fkagments, release their 

adsorbed gas faster because of the shorter diffusion distances. The density of the drilling 

fluid influences, the time during sample retrieval when desorption is assumed to begin, 

i.e., when the hydrostatic pressure of the drilling fluid equals the reservoir pressure of 

the coalbed. 

The frnal factor influencing the accuracy of the lost gas estimation is the water 

saturation and associated level of fiee gas in the virgin coalbed fiom which the sample is 

obtained. Most estimation methods are based on the assumption that the lost gas is 

desorbed gas (behaving according Fick's law of diffusion) and all the measured gas 

volumes used in extrapolation are also desorbed gas. Many coalbeds are essentially 

water-saturated and contain ody a relatively small volume of free gas. However, there 

The US Congress in 1996 directed that the US Bureau of Mines be abolished. USBM health and safety 

research was subsequently transferred to the National Institute for Occupational Safety and Health. 
1. Introduction 

The accumulation of methane in underground coal mines continues to be a safety 

problem in the United States and around the world. Measuring the quantity of methane 

contained within a coalbed can be an important step in evaluating the potential severity 

of gas problems in new mines or in unmined areas of existing mines. Gas content testing 

can be included in the exploratory core drilling program usually conducted during the 

planning phase of mine development (Diamond, 1979). An early assessment of the 

potential for methane emission problems provides the greatest amount of lead time to 

incorporate longer term gas drainage techniques into the mine development plan 

(Diamond, 1994). Once mining is underway, gas content testing can be used periodically 

to assess gas content conditions ahead of mining. 

Whereas mine safety was initially the primary purpose for measuring the gas content 

of coalbeds, the potential for commercial coalbed methane production has resulted in an 

increased interest in this technology. Coalbed gas content data are used for the 

calculation of gas resources and as input data for reservoir models and production 

simulators to forecast gas production and to evaluate methane control options for 

underground coal mining. 

2. Gas content component parts 

Gas is stored in coalbeds in a manner different from conventional reservoirs. Yee et 

al. (1993) described the gas in coal as existing primarily in a "condensed, near 

liquid-like state because of physical sorption", unlike the free gas existing in the pores 

of conventional reservoir rock. Due to the different gas storage mechanism associated 

with coal, formation evaluation methods common in the conventional oil and gas 

industry can not be applied to coalbed reservoirs to determine the volume of gas in 

place. Thus gas content determination methods have been developed to measure the 

volume of gas sorbed within the micropores of coalbed gas reservoirs. 

The most commonly used gas content determination methods subdivide the total gas 

content of a coal sample into three parts: lost, desorbed and residual gas. Each of these 

parts is generally measured or estimated by a different procedure, and then combined to 

'yield the total gas content of the sample. Even though the various gas content 

determination methods generally use similar terminology for the individual components 

of the total gas content, the individual components of the various methods may not be 

directly comparable due to variations of procedural conditions between some methods. 

Mavor and Pratt (1996), McLennan et al. (1995) and Mavor et al. (1994) present 

comprehensive analyses of the possible errors associated with measurement of these gas 

content component parts by commonly used testing methodologies. 

2.1. Lost gas 

The lost gas is that portion of the total gas that escapes from the sample during its 

collection and retrieval prior to being sealed into an airtight desorption canister. Lost gas 
are coalbed gas reservoirs that are not water-saturated, which contain larger volumes of 

fiee gas. Some portion of this fiee gas will escape during sample retrieval and will not 

be accounted for by lost gas estimation methodologies based on diffusion of desorbed 

gas. 

2.2. Desorbed gas 

Once a coal sample is sealed in the desorption canister, the desorbing gas accumulates and can be measured directly, commonly by some variation of the water displacement method first described by Bertard et al. (1970) and later refined by Kissell et al. 

(1973). A more accurate method developed by the USBM to measure the low volumes 

of gas desorbed from samples of oil shale (Schatzel et al., 1987), is based on measuring 

pressure differentials in the canister as gas is released, then utilizing the ideal gas law to 

determine the desorbed gas volume. 

2.3. Residual gas 

The volume of gas desorbing from a coal sample gradually declines with time. 

Desorption measurements for the extended desorption techniques are terminated at some 

point when an arbitrary low desorption rate is reached. This rate may be reached in a 

matter of days for very Mable samples or can take months for some bloclcy coals. 

Generally, when the desorption rate reaches an established termination point, some 

volume of gas remains in the sample. Traditionally, this residual gas has been thought of 

as gas that is 'trapped' within the coal structure due to slow di&sion rates. Bertard et 

al. (1970) and Levine (1992) suggest that the residual gas may not be difision 

dependent, but in part, represents gas remaining in equilibrium under approximately 1 

atm of methane pressure in the desorption canister. 

The residual gas volume can be determined by crushing the sample in an airtight 

container and measuring the volume of gas released by the same method as that used for 

the desorbed gas (Diamond et al., 1986). The volume of residual gas measured in the 

laboratory for samples subjected to elevated temperatures to approximate actual reservoir conditions will probably be less than would have been measured if the sample had 

equilibrated to ambient laboratory temperature during desorption monitoring. 

Analysis of the gas content component parts for 1,500 coal samples from 250 

coalbeds in the United States (Diamond et al., 1986), shows that residual gas can 

comprise 40 to 50% of the total gas content, in particular for relatively low-rank (high 

volatile-A bituminous) blocky coalbeds. In contrast, Mable, high rank (medium to low 

volatile) bituminous coalbeds typically had less than 10% residual gas. Caution must be 

used in evaluating the resource recovery potential of coalbeds with a high residual gas 

content, since it may represent methane that will not readily flow to a methane drainage 

borehole under field conditions. Residual gas may also represent a portion of the total 

gas content that will not be admitted into a mine atmosphere either from migration to the 

mine openings from the surrounding coal or from the mining of the coal at the face. 
3. Direct versus indirect gas content determination methods 

3.1. Direct methods 

The gas content of a coal sample can be determined by several available direct 

measurement techniques. These techniques can generally be divided into quick-crushing 

and extended desorption determination methods. The quick-crushing techniques are most 

commonly used outside the United States for assessment of gas conditions ahead of the 

active production face. The coal sbples are usually obtained fiom underground drill 

holes. Instead of waiting for months to obtain results, total gas yield is available within a 

matter of days. Thus quick-crushing methods are appropriate for the mining applications 

for which they were intended. However, by crushing the sample early in the desorption 

process, it is impossible to determine the relative amounts of desorbed and residual 

gases, and the sorption time (time to release 63% of total sorbed gas) commonly used in 

coalbed gas reservoir models (Sawyer et al., 1987). 

The extended desorption gas content measurement techniques allow for a sample to 

desorb gas until a low desorption rate cutoff point is reached. The sample may then be 

crushed to a powder to quickly release any gas remaining within the coal. These 

techniques are most common in the United States and are used for both mining, coalbed 

methane resource recovery applications, and basic research. The coal samples to be 

tested are generally fiesh core or drill cuttings obtained fiom boreholes drilled from the 

surface. 

Pressure coring is an alternative direct measurement method used to obtain a coal 

sample at reservoir pressure, thus eliminating the need for determining lost gas (Owen 

and Sharer, 1992). Pressure cores are generally considered the standard against which 

other gas content determination methods are evaluated. However, pressure cores are not 

without their own problems, including high cost. 

3.2. Indirect methods 

Gas contents can be estimated indirectly based on sorption isotherm data (Kim, 1977) 

or empirical estimation curves of measured gas content results plotted against other 

measurable variables such as coalbed depth and coal rank (Diamond et al., 1976; McFall 

et al., 1986). Laboratory derived sorption isotherms describe the quantitative relationship 

between adsorbed methane at varying pressures and a constant temperature, and provide 

a measure of the maximum methane sorption or storage capacity of the coal sample. 

Thus, gas content estimates based on sorption isotherm data do not necessarily result in 

accurate in situ gas content values. Since all coalbeds are not fully saturated with 

methane, especially those at shallow depths, the isotherm-based methods may over 

estimate the actual gas content. 

Efforts to estimate the gas content of coalbeds utilizing wireline geophysical logging 

tools have also been attempted (Mullen, 1989). The wireline logging method does not 

directly measure the gas content of the coalbeds exposed in the wellbore. It is essentially 

an empirical estimation curve technique. Algorithms relating gas content to apparent 

coal rank are derived fiom actual gas content and coal proximate analysis data fiom 
cores obtained in the geographical area of interest. Log-derived coal property data are 

then used to estimate gas content as a function of coal rank and depth. 

Indirect methods can serve as a preliminary assessment tool for mine planning 

purposes or targeting potential areas for commercial coalbed methane exploration. 

Whenever possible, however, engineering and/or economic decisions should include 

data from actual direct measurement tests. 

4. Sampling strategies 

Coal samples for gas content testing are commonly obtained from wireline or 

conventional cores and drill cuttings fiom production wells or test boreholes. Core 

samples or drill cuttings may also be obtained from boreholes drilled underground. In 

general, coal cores are preferable to cuttings. Due to their larger size and lower ratio of 

surface area to volume, cores typically have slower initial desorption rates, effectively 

reducing the amount of gas lost during sample retrieval. Gas content values obtained 

fiom drill cuttings should generally be considered minimum values because of the 

difficultly in applying lost gas estimation techniques to this type of sample. 

An issue that fiequently arises at the drill site is how to select samples for gas content 

testing. It is not uncommon for gas contents to vary over the length of a coal core due to 

coal compositional changes, primarily varying mineral matter content. If samples are 

obtained from cores, it is preferable to test the entire recovered coal section. However, it 

is generally good practice to divide the coal core into several samples instead of placing 

the entire coal section into a single desorption canister. 

When multiple samples are tested fiom the same coal core, it is recommended that 

the samples be selected relative to naturally occurring breaks in the coalbed, such as 

shale partings, which can be left out of the test sample. However, this should be noted so 

that the untested material can be accounted for in resource calculations. 

5. Selected gas content determination methods 

The gas content testing methods summarized in this paper were selected for their 

historical value, innovation or uniqueness, and common usage. Most of the methods are 

based on the work of Bertard et al. (1970) and the USBM (Kissell et al., 1973), with the 

primary differences being the equipment used to measure desorbed gas volumes, and the 

technique for estimating the volume of lost gas. Both the nomenclature and units of 

measure used by the originators of the various described methods have been utilized, 

hence the occasional variability of terminology for similar concepts. 

5.1. Bertard's direct method 

The original direct method was developed in France by Bertard et al. (1970). Similar 

methods are still in general use for gas content testing in high gas emission underground 

coal mines in many European mining operations (C. Tauziede, pers. commun., 1996). 
Bertard's methodology was primarily designed for the testing of drill cuttings collected 

from horizontal holes drilled in underground coal mines. The total gas content of a coal 

sample was separated into three component parts, but the sample had to be transferred to 

a different container for the measurement of each component by a different apparatus. 

The methodology described by Bertard et al. (1970) was based on experimental and 

theoretical studies on the sorption and diffUsion of methane in coal. These studies 

included the fmding that early in the desorption process the volume of gas released was 

proportional to the square root of time. This finding is the basis for determining the 

quantity of gas lost by the sample fiom the moment when it was extracted fiom the 

coalbed to when it was transferred into a transport container. A 10 g coal sample is 

collected fiom the borehole underground and immediately sealed in a glass flask 

connected by tubing to a U-tube manometer (collectively referred to as a desorption 

meter). Plaizier and Hucka (199 1) investigated the application of similar methodology 

using a bubble desorbometer for use in mining operations in the western United States. 

Bertard et al. (1970) estimated the lost gas volume using desorption meter data and a 

derived formula that they considered valid, as long as the lost gas volume represented 

less than 20% of the total gas content, necessitating the speedy recovery and sealing of 

the drill cuttings into the desorption meter. At the end of the lost gas determination 

procedure, the sample is transferred to a second container for transport out of the mine 

to the laboratory, where the gas volurne desorbed during transport is measured. 

Finally, the sample was sealed into a third container for crushing and determination 

of the volume of gas remaining in the coal. This gas volume was measured in a 

graduated cylinder suspended over a pan of water into which the gas released by 

crushing was collected. The total gas content of the sample was then the total of the 

three component parts, expressed in cm3/g. 

Bertard et al. (1970) noted that the final results could be presented either in terms of 

the temperature and pressure conditions of the underground sampling site for the 

site-specific mining application, or at STP conditions for a "stricter comparison of the 

desorbable concentrations of different seams." It was also noted that the "desorbable 

concentration is always referred to pure coal" necessitating a correction for contained 

mineral matter content. Bertard et al. (1970) also recognized the potential for using this 

methodology with core samples, but emphasized the importance of short sample 

recovery time to ensure accurate lost gas estimates. 

5.2. US Bureau of Mines direct method 

The coalbed gas content testing procedure commonly referred to as the USBM direct 

method (Kissell et al., 1973) was an adaptation and simplification of the method 

developed by Bertard et al. (1970). The initial purpose of the USBM method was to 

determine the gas content of virgin coal core samples for use in estimating ventilation 

requirements for new mines. Laboratory experiments confirmed that lost gas assumptions for coal cuttings proposed by Bertard et al. (1970) could also be applied to the 

larger sized coal core samples that would have a longer sample recovery (lost gas) time. 

The USBM direct method eliminated the multiple sample containers and associated 

measurement techniques used by Bertard et al. (1970) to determine the component parts 
Pan of water 

Fig. 1. USBM direct method gas volume measurement apparatus. 

of the total gas content. Standard desorption canisters were constructed from - 0.3 m 

(1.0 fi) long sections of = 10.2 cm (4.0 in.) diameter aluminum pipe (~ig. 1). All gas 

volume measurements were made by periodically releasing the accumulated gas into a 

water filled inverted graduated cylinder (Fig. 1). 

Kissell et al. (1973) proposed two alternate approaches to estimate time zero and 

cumulative lost gas time. If the hole was cored with water or drilling mud, desorption 

was assumed to begin when the sample was halfway to the surface. The cumulative lost 

gas time ( tlg ) would then be 

where t2 is the time core retrieval began, t3 the time core reached the surface and t, the 

time core sealed in desorption canister. If the hole was cored by air or mist, pressure 

release and gas desorption were assumed to begin at the first penetration of the coalbed 

by the core barrel. In this case, tlg would be 

where t, is the time the coalbed was first penetrated. 

A graphical technique was proposed to estimate the lost gas volume (Q,) based on 

several early direct measurements of desorbed gas after the sample was sealed into the 

desorption canister. Measurements of the desorbed gas volume are made every 15 to 20 

min for the first several hours. These data are then plotted on a graph of cumulative 

desorbed gas versus the square root of desorption time. The initial linear portion of the 

desorption curve is extrapolated through the point on the x-axis representing the lost-gas 

time (fi min, Fig. 2) to estimate the lost-gas volume. 

The desorbed portion (Q,) of the total gas content is measured periodically by the 

water displacement technique described above. At some point, gas desorption slows to a 

rate at which essentially little gas is emitted. McCulloch et al. (1975) proposed that 

desorption measurements be discontinued when daily emissions were less than 0.05 

cm3/g for five consecutive days, which was subsequently revised by Diamond and 

Levine (1981) to an average of 10 cm3 of gas desorption per day for one week. Both of 

these guidelines are arbitrary; but whatever cutoff rate is used, it should be applied 

uniformly to all samples. 
Fig. 2. USBM direct method lost gas estimation graph. 

At the point when the periodic measurement of desorbed gas is discontinued, the 

residual gas (Q,) is determined by crushing the sample to a powder (- 200 mesh). The 

recommended procedure for determining the residual gas (Q,) requires the transfer of all 

or a portion of the desorbed coal sample to a desorption ball mill canister for crushing 

on a roller mill (~iamond and Levine, 1981). 

The volume of gas released is measured by the water displacement method after the . 

sample cools to ambient temperature. The gas volume released by crushing is attributed 

only to the portion of the sample crushed to a powder. The total gas content of the 

sample is then 

Q, = ( Q, + Q, , /Mt + Qr/M, 7 ( 1) 

where Mt is the total air-dried mass (weight) of the sample and M, the air-dried mass 

(weight) of the sample crushed to a powder in the ball mill. 

The value Q, is calculated in cm3/g, but is commonly converted to ft3/st (= cm3/g 

X 32) in the United States for ease in making gas resource calculations. 

Due to the changing uses of the gas content data in the United States beginning in the 

early 19807s7 several modifications to the USBM direct method were proposed. One of 

these modifications was designed to eliminate volumetric measurement errors due to the 

differential heads between the water level in the graduated cylinder and the pan (Fig. 1) 

by adjusting the water level in a separate reservoir vessel to that in the measurement 

vessel. Relative errors due to these differential pressures were estimated by TRW (1981) 

to range up to about 4%. Two examples of these alternative manometric apparatuses are 

shown in Fig. 3. 

Another potential problem with direct method testing methods is related to the 

volume of head space (fiee space) left in the canister after the sample is inserted. 

Changes in ambient pressure and temperature on the head space volume between 

subsequent desorption readings can influence the resultant volumetric measurement. 

These changes can be either positive or negative depending on the net effects of both 
Flexible hose IpOO-ml 

\ inverted graduated 

Leveling 

reservoir 

- Water supply 

tube 

Fig. 3. Examples of manometric gas volume measurement apparatuses. Modified after: (A) Camp et al. (1992) 

and (B). TRW (1981). 

temperature and pressure changes between readings. Since these changes in gas volume 

due to the head space effect tend to cancel themselves out over time, corrections for this 

problem were not included in the original USBM direct method test procedure. 

However, it was recommended that the volume of head space in the ca'nisters be 

minimized by filling the canisters as full as possible to reduce possible errors associated 

with this problem. An even better solution is to customize the canisters to the diameter 

of the core and length of the sample to be collected. For those concerned with the loss of 

accuracy due to the head space effect, it is relatively easy to add a correction factor 

based on the ideal gas law. Circumstances of extreme variations in atmospheric pressure 

(e.g. elevation difference between collection location and laboratory) or temperature 

may necessitate head space effect corrections. 

It is important to periodically collect samples of the gas desorbed to determine the 

actual constituents of the measured gas volume. In most cases, the gas is predominantly 

methane; however, other constituents are present. Kim (1973) reported that the largest 

constituent in coalbed gas after methane is usually CO, or N,. In the United States, CO, 

has most commonly been found in the Pittsburgh Coalbed in the east (Kim, 1973) and 

several coalbeds in the western interior, including the Fruitland Coalbed in the San Juan 

basis (Rice, 1993). Rice (1993) also reports high concentrations of CO, for coalbeds in 

the Peoples Republic of China, Australia and Poland. Other constituents of coalbed gas 

may include minor amounts of higher hydrocarbons (primarily ethane), H,, and He 

(Kim, 1973). 

5.3. US Bureau of Mines mod$ed direct method 

Early mine development experience in the oil shale industry suggested that significant 

quantities of methane could be released during mining. Preliminary assessment of the 

low volumes of gas sorbed within the organic matter associated with oil shale indicated 

that the existing USBM direct method technique was not sufficiently precise or accurate 

for this application. A new gas content testing methodology was developed to more 
Fig. 4. USBM modified direct method instrumentation unit and field customized PVC desorptioncanister. 

accurately measure the expected low gas volumes of oil shale samples. This new method 

also provided increased accuracy for determining the gas content of coal and other rocks 

where sorption is the primary gas storage mechanism. 

The primary improvement of the modified direct method (MDM) was the abandonment of the water displacement apparatus for measuring desorbed gas volumes. Instead, 

the MDM test procedure relies on the measurement of differential pressures (pressure 

buildup between readings) and the ideal gas law to calculate the volume of gas released 

into the desorption canister at STP conditions (Schatzel et al., 1987). The MDM 

measurement apparatus utilizes pressure transducers with digital readouts that offer 

superior resolution compared with other instrumentation incorporated in conventional 

direct method gas content testing apparatuses. 

The small size and ruggedness of the apparatus (Fig. 4) facilitates its use in the field, 

as well as in the laboratory. The inclusion of two pressure transducers permits a choice 

of overlapping pressure ranges for increased accuracy .within each transducer's optimal 

range. A gas sampling port allows for collection of uncontaminated gas samples. 

An additional innovation developed as part of the MDM is the use of canisters 

constructed of polyinyl chloride (PVC) pipe. The internal diameter of the canister is 

sized as close as possible to the external diameter of the core samples. The canister is 

partially constructed and pressure tested in the laboratory with only one end cap. The 

other end cap (fitted with the quick-disconnect valve assembly, as illustrated in Fig. 4) is 

glued in place after the pipe is cut to size in the field to match the length of the actual 

coal sample. Customizing the desorption canisters to the actual size of the sample 

greatly reduces the head space volume in the canister. 

The application of the MDM to coal samples, as described by Ulery and Hyman 

(1991), includes the determination of lost (Q,), desorbed (Q,) and residual (Q,) gas 

components of a sample. As with the original USBM direct method, the lost gas (Q,) is 

estimated by extrapolating early time periodic measured gas volumes to time zero, as 
shown in Fig. 2. The individual data points of desorbed gas volume (V,,) at STP 

conditions are determined by the formula 

v,= y- Vf, 

where V, is the gas volume in the desorption canister at the end of a desorption volume 

measurement time interval and Vf the volume of gas in the canister afier releasing the 

accumulated desorbed gas volume. 

V, and V, are calculated at STP conditions according to the ideal gas law 

Vf) = ( Pambqtd Kmb ) / ( PstdTamb ) 7 

where Pamb is the ambient pressure of gas in canister, Pstd the standard pressure, T,, 

the ambient temperature of gas in canister, ctd the standard temperature and Vamb the 

free space volume at Tab and Pa,,. 

An advantage of the MDM gas volume measurement and calculation technique is that 

the head space effect correction is inherent in the calculation. Negative bP values, as 

well as positive values, are also easily measured with the MDM technique. This is not 

the case with the water displacement gas volume techniques where a negative pressure 

in the canister can draw water into the tubing connecting the measurement apparatus and 

the canister. A further advantage of eliminating the water displacement method of gas 

volume measurements is the elimination of gas dissolution problems associated with 

gases that have high solubilities, such as CO,. 

Desorption in the canisters is generally allowed to continue until the cutoff point 

previously defined for the USBM direct method test is reached. The desorbed gas 

volume (Q,) is then the sum of the periodic gas measurements (Gp). Prior to removing 

the coal sample from the canister, the head space is measured. However, to calculate 

apparent gas volumes as the desorption process is underway, an estimated head space 

volume .is calculated by subtracting the estimated volume of the coal sample from the 

estimated internal volume of canister. A portion of the sample is crushed and the 

residual gas volume (Q,) measured by the MDM. The final gas content of the sample 

(Qt) is calculated according to Eq. (1). The final results are reported at a defined STP, 

gas species, and coal compositional (as-received versus ash- or mineral- matter-fiee) 

basis. 

A common problem occurring in the desorption canisters are oxygen depletions, 

either through oxidation or perhaps adsorption (Popp et al., 1979; Ulery and Hyman, 

199 1). Gas samples are obtained for each MDM desorption measurement. Without these 

gas compositional analysis, the net change in volume between oxygen depletion and 

methane desorption would underestimate the true desorbed methane volume. 

The oxygen depletion problem can be eliminated by purging the sample containers 

with an inert gas, but this entails added expense, as well as an additional complication to 

the testing procedure in the field. Even if the canisters are purged in the field with an 

inert gas, it is recommended that desorption readings be accompanied by a gas 

compositional analysis to determine volumetric changes in gas species desorbed over the 

duration of testing. 

Ulery and Hyman (1991) compared MDM results with simulated direct method 

results derived from MDM data. They concluded that if the USBM direct method is 
conducted according to the recommended specifications and practices, reasonably 

accurate STP results can be obtained. However, the water displacement methods in 

general have been shown to be unresponsive to pressure differentials below about 5 to 8 

cm H,O. The most substantial loss of accuracy occurs when small, low gas content 

samples are placed into a large container resulting in a large free space volume. Errors 

of up to 100% were reported when the ratio between the desorbed gas volume and the 

free space in the canister is less than 2: 1. 

5.4. Smith and Williams method 

Smith and Williams (1981) developed a variation of the direct method technique to 

address the problem of accurately determining the gas content 'of coal cuttings obtained 

fiom surface rotary-drilled boreholes. The lost gas portion of the total gas content was 

the focus of this water displacement measurement method. It is assumed that little, if 

any, gas remains in the coal cuttings at the end of the desorption period; therefore, no 

provision is made for determining residual gas. Due to the speed of gas desorption from 

the coal cuttings, final results are generally obtainable in one week. 

Application of the Smith and Williams method is based on a calculated surface time 

ratio (STR) and lost time ratio (LTR) defined as 

STR = (Ts - TD)/Ts, 

and 

where TD is the time from coalbed penetration to the sample surfacing, Ts the time fiom 

coalbed penetration to the container sealing, and T2,% the time &om coalbed penetration 

to the time when 25% of the measured gas volume has desorbed. 

A volumetric correction factor (N) is then determined fiom a set of STR curves and 

the calculated LTR. The total gas content (Q,), including lost gas (Q,), is then 

where Q, is the total measured gas volume. The lost gas (Q,) is then 

Several comprehensive studies of gas content measurement techniques indicated that 

problems were associated with using the Smith and Williams method for core samples. 

Mavor et al. (1994) and McLennan et al. (1995) indicated that when compared to 

pressure core gas content data, the USBM direct method provided closer agreement than 

the Smith and Williams method and other selected gas content methods. It should be 

noted that most of the comparisons by Mavor et al. (1994) and McLennan et al. (1995) 

were based on data from relatively deep coalbeds fiom a coal basin in the western 

United States. Under different reservoir conditions, the Smith and Williams method may 

provide more reliable data. 
5.5. Decline curve methods 

The use of oil industry decline curve analysis to estimate gas volumes desorbable 

from exploratory coal core samples or recoverable from coalbed gas reservoirs was first 

proposed by Chase (1979). His method eliminated the need for determining the residual 

gas, since true residual gas would not be expected to impact a mining operation or be 

produced by degasification. Thus only the lost and desorbed gas components of the total 

gas content are estimated. Decline curves applied in this manner are a tool to predict 

future desorption performance of a sample without the need for many months of actual 

desorption measurements. 

Chase's decline curve method consisted of plotting gas release rate (cm3/g per week) 

versus cumulative volume desorbed (cm3) on semilog paper to produce a linear trend 

with least-squares regression analysis (Fig. 5). The periodic STP corrected desorption 

volumes were determined by a water displacement method similar to that used in the 

USBM direct method. The lost-gas volume (Q,) was estimated in the same manner as 

that used for the USBM direct method. 

The volume of desorbable gas is estimated from the decline curve (Fig. 5) by 

choosing an arbitrary desorption rate of 0.001 cm3/g per day (average of 0.007 cm3/g 

per week) to approximate complete desorption. This is a substantially lower desorption 

rate than the cutoff of 0.05 cm3/g per day for five consecutive days proposed by 

McCulloch et al. (1975). Projection of the 0.007 cm3/g per week desorption rate to the 

regression line constructed fkom the measured desorption values (Fig. 5), yields the 

predicted desorbed gas volume (Q,) in cm3 on the x-axis. The total desorbable gas 

volume (Qd) is then 

Qs=Q,+ Qp. 

A variation of the decline curve concept estimates the lost gas of a sample instead of 

the predicted desorbable gas volume described by Chase (1979). The Raven Ridge 

modified decline curve method (J.C. Huddleston, pers. cornmun., 1996) for estimating 

CUMULA~VE DESORBED GAS, lo2 cm3 

Fig. 5. Decline curve graph for estimating the volume of desorbable gas. Modified after Chase (1979). 
15 - - 500 

mq . Key 0 

E Meosured - mz 

-- - 

vi - Estimated - 400 ,- 

a w - a 10 - y = 73.649 In (x) + 254.79 a w - 300 

a a 

s: : 5: a * -200: 

s 5- L 3: - 100 G + 

2: z 

3 

0 - I I I 0 

0.01 0.10 1 .oo 10.00 100.00 

ELAPSED DESORPTION TIME, days 

Fig. 6. Decline curve graph for estimating volume of lost gas. Modified after J.C. Huddleston, pers. commun. 

(1996). 

lost gas utilizes all of the available desorption data, instead of only a few early points, to 

extrapolate back to time zero (initial penetration of the coalbed) when desorption first 

began (Fig. 6). This methodology is used for both cores and drill cuttings. The STP 

corrected desorption volumes are determined by a water displacement technique. 

Desorption measurements are discontinued afier three consecutive daily readings of zero 

desorbed gas. The decline curve is a plot of cumulative desorbed gas versus elapsed 

time, described by a logarithmic least squares regression equation. In the example given 

in Fig. 6, the estimated lost-gas volume for the lost-gas time of x = 0.0153 days (22 

minutes) would be 1.7 A similar nonlinear curve fitting methodology has been 

proposed by Amoco Production Co. to estimate lost gas from measured desorption 

volumes (Yee et al., 1993). Tke zero for the Amoco method is defined as when the 

core barrel is lifted off bottom. 

5.6. Gas Research Institute (GRI) method 

The gas content determination method developed as part of the GRI coalbed methahe 

research effort (Mavor et al., 1994; Mavor et al., 1995; McLennan et al., 1995; Mavor 

and Pratt, 1996) is an enhanced version of the USBM direct method. The GRI Method 

measures desorbed gas volumes by a water displacement method using an apparatus and 

associated equipment significantly more complicated than the examples shown in Fig. 3. 

The primary departure from the USBM direct method is associated with the estimation of lost gas. A comprehensive analysis by Mavor and Pratt (1996) of the errors 

associated with the most widely used gas content measurement techniques indicated that 

the greatest cause of error was due to conducting the tests at ambient temperature, which 

resulted in underestimation of lost gas. This occurs due to reduced desorption rates at 

ambient surface temperatures that are lower than the reservoir conditions at which 

desorption was initiated in the borehole. Thus, the actual volume of gas lost from a 

sample initially at a high reservoir temperature may be underestimated from ambient 

desorption readings, even when the readings are corrected to STP conditions. To 

compensate for lost gas estimation errors associated with extrapolating desorption data 
points measured at ambient surface conditions to time zero at a higher reservoir 

temperature, the GRI method only extrapolates those desorption data points measured 

after the sample is stabilized to reservoir temperature in a heated water bath. 

The other suggested improvement in estimating lost gas is in the calculation of time 

zero. McLennan et al. (1995) defined time zero as the actual time during sample 

retrieval when the hydrostatic pressure of the drilling fluid was equal to the reservoir 

pressure. The USBM direct method estimates time zero to be the point when, in a water 

filled hole, the sample is half way to the surface. The GRI time zero assumption is a 

more accurate representation of the desorption history of coal sample during retrieval, 

and will generally result in a larger lost gas volume estimate. It does, however, require a 

measure of the reservoir pressure, and an approximation of the pressure history of the 

sample as it is retrieved fiom the borehole. McLennan et al. (1995) present several 

practical alternatives for obtaining a pressure history during sample retrieval. 

The error analysis by Mavor and Pratt (1996) was based on several suites of samples 

collected £ram the San Juan basin of Colorado and New Mexico and the Piceance basin 

of Colorado in the United States. Benchmark values for error analysis were obtained 

fiom pressure core gas content data and isotherm data. It should be noted that most of 

the samples were fiom a single coal interval (Fruitland Formation) and were generally 

from depths greater than 600 m (2,000 ft), with extended lost gas times. Lost gas errors 

of the magnitude reported by Mavor and Pratt (1996) may not be typical for other coal 

intervals. They also would be less for coalbeds occurring at shallower depths or at lower 

geothermal gradient, where the difference between reservoir temperature and surface 

temperature would be less than that of the studied sample suite. 

5.7. Australian standardQ3 method 

The Standards Association of Australia (1991) has recommended a gas content test 

procedure based on the USBM direct method. The Australian standard" addresses 

equipment construction, sampling and testing procedures, and methods of calculating the 

fmal gas content results. The procedure is recommended for core or lump coal samples 

with a mass of at least 500 g. Two water displacement apparatuses are recommended for 

determining desorbed gas volumes. The preferred apparatus (~ig. 7) consists of one or 

more connected graduated cylinders into which the desorbing gas is allowed to fkee 

flow. Since this apparatus does not have a separate water reservoir for equalizing water 

levels to directly compensate for the differential head effect, a data correction procedure 

is provided. 

One unique recommendation provided as part of the Australian Standard" is that if 

CO, is expected to be present in the desorbed gas, an acidified brine solution should be 

used in the gas volume measurement apparatus to prevent the CO, from going into 

solution in the water. However, Saghafi et al. (1995) believe that the acidified brine 

solution only prevents the formation of the carbonate ion and has little impact on CO, 

solubility. 

The lost gas (Q,) is determined in the same manner as that for the USBM direct 

method (~ig. 2). Time zero is defined as the point when the sample is halfway out of the 

hole. The measurable gas (Q,) is determined in the laboratory using the apparatus 
To vacuum --L 

source 

 as sarn~lina 

tube 

To next cylinder --- (if required 1- --- I.. L- overflow 

66 'water pan 

U Desorption 

canister - 

Fig. 7. Australian standarda method gas volume measurement apparatus (Standards Association of Australia, 

1991). 

shown in Fig. 7. If the desorption rate is sufficiently low, the desorbing gas is allowed to 

free flow into the graduated cylinder. A series of water filled cylinders can be connected 

so that as one cylinder is filled with gas, additional gas is transferred to the next 

cylinder. Provisions are made in the apparatus to obtain gas samples for compositional 

analysis. The measurement of desorbed gas volumes is discontinued when there is no 

observed change in the gas quantity for one week or when the desorption curve 

approaches an asymptotic value. A portion of the coal sample is analyzed for ash content 

and relative density by standard laboratory procedures. 

If desired, a portion of the coal sample is crushed in a ball mill to determine the 

residual gas content (Q,). A minimum of three representative small samples (10 to 15 g) 

are crushed and the results averaged to determine the residual gas content. 

The desorbable gas content (Q,) is defined as 

QD=Q,+Q,, 

and the total desorbable gas content (Q,) is defmed as 

e, = average Q3 + (QI + Q2) - 

5.8. CSIRO-CET quick desorption method 

CSIRO-CET has proposed a quick-crushing gas content determination technique, 

which Saghafi et al. (1995) reported is commonly used in Australia in addition to the 

Australian Standardo. This methodology is applied primarily to core samples collected 

from underground boreholes. One advantage that Saghafi et al. (1995) point out is that 

the quick-crushing method minimizes the volume of water-soluble gases, such as C02, 

that are lost to the water column when a water displacement technique is used to 

measure a large number of desorbed gas volumes over an extended period. 

The coal sample is placed in a desorption canister containing stainless steel balls for 

subsequent crushing of the sample in the laboratory without having to transfer the 

sample to another container. The lost gas (Q,) is determined by the USBM method (Fig. 
2) utilizing fiee flow desorption data points measured by the apparatus previously 

described for the Australian standardB. However, to minimize problems with the loss of 

CO, to the water column, these early desorption measurements are limited to 30 min of 

elapsed time. 

The quantity Q, is determined by dividing the lost gas volume (Q,) and.the desorbed 

gas measured in the field by the air-dried mass of the sample in the desorption canister. 

The volume of gas desorbed during transport (Q,) and the subsequent volume of gas 

released by crushing (Q,) are determined in the laboratory using the water displacement 

apparatus. The total gas content (Q~) is then 

6. Reporting of results 

Historically, gas content data have been reported on various bases, and in many 

cases, without any defined basis. All volumetric readings should be corrected to standard 

temperature and pressure (STP) conditions. Gas industry convention for STP conditions, 

15.6"C and 760 mm Hg (60" F and 14.70 psia), are most commonly used for reporting 

coalbed gas content data. However, chemistry STP conditions, 0°C and 760 mm Hg 

(32°F and 14.70 psia), are occasionally used. The STP conditions applied to a particular 

data set should be clearly defined. The methodology and type of coal sample tested (core 

versus drill cuttings) should also be specified. 

The coal compositional basis for the gas content values is another important 

data-reporting issue. Raw (as-received) gas content results are commonly corrected to an 

ash-fkee value to aid in the comparison of results on a normalized basis. 'While an 

ash-free basis for reporting gas content values is desirable from a standpoint of 

comparing results, it can lead to confusion when calculating gas resource estimates. Gas 

resource calculations should always be made with gas content values and coal resource 

data corrected to the same basis (Levine, 199 1, Scott et al., 1995). Ash-free gas content 

values should not be applied directly to raw tons of coal in place for calculation of gas 

resources because an inflated value for the gas resource results. 

Calculation of gas content on a volume-to-mass (weight) ratio (i.e., cm3/g (fi3/st)) 

is inherited fiom the mining industry where coal reserves are calculated on a weight 

basis. As an alternative to expressing gas content values as a volume-to-mass ratio, they 

can be expressed as a volume-to-volume ratio (i.e., m3 of gas/m3 of sample) (Levine, 

1991). This volumetric expression of gas content values has been used effectively in the 

calculation of gas in place for studies related to longwall gob gas, where the gas is 

contained not only in coalbeds, but also, in other rock types for which a volume-to-mass 

ratio has little meaning, and is awkward for gas in place calculations (Diamond et al., 

1992) 

Several other types of information should be provided to enhance the use and 

interpretation of reported gas content values. The depth of the sample should be included 

for each individual gas content value. If data fiom multiple samples from a single 

coalbed are combined for a single gas content value, they should be normalized by mass 
or volume, as opposed to calculating a simple arithmetic average (Levine, 1991). If any 

shale partings were excluded from the sample(s) sealed into the desorption canister(s) 

that should be indicated on any stratigraphic sections or for stated coalbed thickness 

values associated with the gas content values. It is also helpll to include either an 

inferred coal rank calculated fiom a coal proximate analysis and Btu data (ASTM, 1977) 

or a coal rank estimated from vitrinite reflectance measurements. Finally, any extenuating circumstance in the collection of the sample or its location relative to outcrop or 

mining that could have a bearing on the interpretation of the results should be noted. 

7. Summary 

The coalbed gas content determination methods reviewed in this paper require 

varying levels of equipment sophistication, auxiliary test data and data reduction 

complexity to arrive at a final gas content value, Selection of a method should be 

determined by the particular application of the gas content data and the degree of 

accuracy required. For mining applications where the data are to be used for estimation 

of emission rates, as is common outside the United States, the various quick-crushing 

methods are generally sufficient. 

For general purpose applications, the original USBM direct method and its water 

displacement based variants are a good choice. These methods should include the 

generally accepted data correction factors (STP, differential water heads, and head 

space), as well as periodic gas compositional analyses. One important operational 

practice to increase the accuracy of gas content testing methods, especially for low gas 

content samples, is to fill the canisters as completely as possible with coal to minimize 

problems associated with a large fkee space. Conducting desorption testing at reservoir 

temperature may also be appropriate for samples retrieved fiom coalbeds with reservoir 

temperatures that are judged by those utilizing the data, to be significantly above 

ambient laboratory conditions. 

The USBM modified direct method provides a significant level of increased accuracy 

and sophistication, and eliminates the logistical and technical problems associated with 

the water displacement techniques. Complicated and cumbersome apparatuses with 

fragile glassware are not required for measuring desorbed gas volumes. The MDM 

eliminates the problem of losing water soluble gases to the water column in the 

manometric devices, does not require an apparatus to correct for differential water 

heads, and the head space effect is directly compensated for in the test methodology. 

It is recognized that cost is a consideration when selecting a gas content testing 

methodology. The quick-crushing gas content testing methods are generally at the low 

end of the cost scale; however, a determination of the true residual gas content is lost. 

This may not be a serious limitation for the traditional.mining applications of these 

methods or for coalbeds where the residual gas contents have been shown to be 

relatively low. 

The original USBM direct method and similar water displacement methods are also a 

relatively low-cost choice, even with the inclusion of the basic correction factors. The 

USBM modified direct method has a higher initial cost for equipment than most of the 
traditional water displacement based techniques, and the increased level of accuracy may . 

not be necessary for some data applications. However, for large scale sampling 

programs, the higher initial equipment costs can be amortized over a large number of 

samples and several years of operation. Conducting desorption testing at reservoir 

temperature and utilizing the GRI procedure for estimating time zero based on actual 

reservoir and nuid pressure in the borehole may significantly increase the accuracy of 

the lost gas estimate under some circumstances, but the added cost may not be justified 

or necessary for all sampling situations. 

No single technique is necessarily appropriate for all applications for which coalbed 

gas content data are utilized. It is up to the user to select the methodology that best 

addresses the intended application of the data at an acceptable level of accuracy and 

cost. In some cases, it may be appropriate to select individual components fiom multiple 

methods to meet one's needs (e.g., using the USBM MDM apparatus and general 

methodology, but utilizing the modified decline curve method for estimating the lost gas 

volume and the GRI pressure calculation to define time zero). It should be noted that the 

method of gas content determination can effect the quantity of each of the component 

parts. This can be a highly significant variable in comparing results determined by 

differing methodologies. 

No matter which gas content determination method is used, applying good experimental method and practices is essential to maximize the inherent accuracy of any test 

procedure. Adhering to the reporting guidelines for gas content data will enhance the 

value of the results while minimizing the potential for erroneous conclusions or use of 

the data. 

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