CHAPTER 4 Gas C ontent Measurement in Coalbed: Field Desorption Test …shodhganga.inflibnet.ac.in/bitstream/10603/32892/14/14... · · 2015-01-16Gas C ontent Measurement in Coalbed:

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CHAPTER 4

Gas Content Measurement in Coalbed:

Field Desorption Test and Isotherm Study

4.1 Introduction

Gas content data are vital for determination of the commercial potential of a field, and core

analyses provide that information. Economic production of coalbed methane is dependent

on the amount of gas content. Thus gas content measurement is an essential parameter for

efficient recovery of methane for CBM field. As mentioned earlier, gas remains in adsorbed

conditions on the coal surface. It is a well-known fact that during coalification stage, large

amounts of gases are produced, and a portion of them is held both in the coal seams and

adjacent rocks [Kim et al., 1998 and Patching, 1970]. Methane is the principal gas in this

mixture. The retention of methane in the coalbed by desorption depends upon the rank of

coal and burial depth which is described in previous chapter (chapter 3).

The measurement of gas content in conventional sandstone or carbonate reservoirs by logs

is a benefit but it is yet to be available in coal seams without extensive calibration of the

logs from previous core analyses. Therefore, gas contents of coals are determined in the

laboratory from cores taken from the field.

Total gas content of the core is the sum of residual gas, desorbed gas in the canister, and

lost gas. The relationship is given below

G V V VR D L

Where

G = gas content of the coal in the formation, scf/ton

VR = residual gas of core, scf/ton

VD

= gas released (desorbed) by the core in the canister, scf/ton

VL = gas lost from the core in the coring process, scf/ton

CHAPTER-4 Gas Content measurement in Coalbed and Isotherm Study

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The amount of methane gas retained may be determined by direct and indirect techniques.

Direct methods measure the volume of gas released from a coal sample as soon as sample is

retrieved from well whereas indirect methods determines gas storage capacity of coal. The

desorbed gas volume is determined by desorption canister test and the gas adsorption

capacity of coal is experimentally measured from derived sorption isotherm data in

laboratory. The amount of gas adsorbed or gas storage capacity of coal may not be the same

as the measured gas content value, thus a gas adsorption test is essential for coal to evaluate

the gas storage capacity [Busch and Gensterblum, 2011 and Rice, 1993].

The direct gas content measurement methods was first developed by U.S. Bureau of Mines

(USBM) in the 1970s and 1980s [e.g. Diamond 1979; Diamond and Levine, 1981; Kim,

1973, 1977; Kissell et al., 1973; Kim and Douglas, 1973; McCulloch et al., 1975].

According to USBM method, the gas desorption measurement was carried out by collecting

coal samples from core barrel and immediately placing it in a leak proof canister. Though

essential steps for measurement of gas have remained same as described by USBM method

but some modifications have been made from time to time [Yee et al., 1993; McLennan et

al., 1995; Diamond et al., 1998; Barker et al., 2002, 2005; Moore and Butland, 2005;

Crosdale et al., 2005; Stricker et al., 2006]. The measured gas content data are used both in

the mining industry and Exploration-Production industry, to control the level of gas

emissions in underground coal mines, to minimize the mining hazards and understand the

amount of gas present in the coalbed respectively. The essential aspect of coalbed methane

research is to determine the amount of methane gas adsorbed in the coal seams to assess the

total reserve and the mechanism of the adsorption-desorption of the methane for

development of efficient exploitation technology.

Coalbed methane remains within the coalbed reservoir in an adsorbed phase and for clear

understanding of methane occurrences on the coal reservoir adsorption study is the most

important. CBM exists mostly as monomolecular layer on surface of coal along with some

free gas and gas in solution. Coal has a large internal surface area which may retain huge

volume of gas by adsorption. Adsorption process is of two types: physical adsorption and

chemical adsorption. Van der Waals force is the main acting force for physical adsorption.

Scientists like [Crosdale et al., 1998, Danies, 1968, Mavor et al., 1990, Yang and Saunders,


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1985] have shown that physical adsorption is the dominant mechanism governing the

storage of methane gas in the coal seam. Physical gas adsorption process is believed to be

reversible in nature, but small to significant hysteresis in sorption isotherm curves has been

reported for many coals. Adsorption isotherm study of coal is generally performed under

the actual reservoir temperature condition. For Indian coalfields, in most of the areas, the

reservoir temperature falls within the range of 30˚C to 60˚C which is much higher than the

critical temperature of methane (-82.57˚C). Gray 1987, Mavor and Owen, 1990, and Ojha et

al. (2013) have shown that under reservoir condition the adsorption of gas phase into the

coal follows Langmuir single molecule adsorption theory. Maximum gas holding capacity

of coal sample at a particular depth can be determined from an adsorption isotherm study

for particular reservoir pressure and temperature condition. Bustin and Bustin, [2008];

Busche and Gensterblum, [2011]; Harpalani [2006], Crosdale [2005], and Mavor et al.,

[2004] have shown that adsorption tests are particularly sensitive to the temperature at

which the experiments are conducted. Thus, clear understanding of the complex process of

gas flow through coal beds or cleats [Paul and Chatterjee, 2011 a, b] as well as the

adsorption-desorption process involved in it are the primary requirements while venturing

into CBM exploration. The growing Indian CBM industry requires detailed understanding

of the reservoir and other technical parameters that determines methane producibility to

develop any CBM exploration project. The gas sorption properties of coal seams depend on

coal type, rank, moisture content, temperature, in-situ stress condition, mineral matter or

nature of mineralization and fracture (particularly cleats or joints) development

[Laxminarayana and Crosdale, 2002]. Reservoir conditions and coal characteristics are

unique for each and every CBM reservoirs; and hence efficient and economic exploitation

of methane requires detailed investigations of each and every reservoir. In present

investigation gas measurement study has been carried on a number of samples collected

from various coalfields of eastern India.

In the present chapter both Canister desorption test methods and isotherm studies are

described and gas content in coal samples were determined.

Indian CBM business is still far behind the targeted commercial production. Scarcity of

data for detailed study is one of the most important reasons behind this. So far, only a


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handful of information is available on the detailed study of gas content of coal with

variation in depth, of coal characteristics, various reservoir conditions and their

correlations. Hence, this work will help in further study or development of CBM fields and

enlighten better in venturing of CBM field in Eastern India.

The main objectives of this chapter are:

i. Determination of In-situ gas content of the recovered coal samples by canister

desorption test.

ii. Correlation of gas content with depth

iii. Identification of CBM potential zone in the study area

iv. Adsorption isotherm study of methane and CO2 on collected coal samples in

Laboratories

v. Identification of best-fit isotherm model.

vi. Correlation of gas storage capacity of coal by adsorption with coal-seam depth, ash

content, fixed carbon content, percentage of moisture content and its vitrinite

reflectance

4.2 Experimental methods

In-situ gas content measurements have been carried out in the field for 3 wells of Jharia

coalfield and 2 wells of Raniganj coalfield.

4.2.1 Canister desorption test

Field desorption test i.e. Canister desorption test gives the direct measurement of gas

content of a coal sample which is just retrieved from the coal-seam during drilling. This test

determines the actual in-situ gas content in coal seam which was adsorbed by it at present

in-situ condition by adopting modified USBM Direct Method. Total 42 coal samples were

collected from different wells located at Jharia and Raniganj coalfields which are the most

promising CBM fields in India.

Apparatus Desorption canister used for the present purpose is shown in part „a‟ of Fig 4.1.

This apparatus consists of two concentric cylinders. Outer cylinder of this apparatus is


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made up of stainless steel (SS) with 190 mm dia and 560 mm length. Inner cylinder of core

barrel is made by stainless steel with insulated lid. Length of this inner cylinder is 534 mm,

dia is 101mm (4") and thickness is 2mm.

The heating element is made up of 80/20 microme wire to be fitted with outer dia for

heating the core upto ~100˚C. A perforated SS cylinder has been placed for easy handling

of samples in between two cylinders of the canister. One Bourdon tube pressure gauge,

range 0 to 15 psi is connected on the upper cap of the canister to measure the pressure of

the core barrel. A thermocouple is attached to this for measuring and controlling the

temperature which is maintained at the reservoir temperature.

Fig. 4.1: Canister and Desorption apparatus set-up

Desorbed Gas (VD) measuring System

Volumetric displacement apparatus for gas measurement consist of two interconnected

measuring burettes of 200 cc and 1000 cc capacity which were clamped to the stand with

stop cocks at their extremities along with fluid reservoir (dropping funnel) of 1500 cc

capacity as shown in part „b‟ of Fig 4.1. The dropping funnel was kept on the fluid reservoir

holder fitted to the other stand. Both the lower open ends of the burettes were connected

a

b


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with fluid reservoir by a rubber tube with the help of “Y” shaped glass connector and stop

cocks. Both the upper open ends of the burettes were connected with the forked arms of one

Y shaped glass connector by a short piece of rubber tube. Rubber tubing connections were

made between the open end of the Y- shaped connector and one end of a copper spiral tube

via a two way stop cock. The other end of the copper tube was connected to desorption

canister by another rubber tube. The fluid reservoir was repositioned to allow the 1000cc

and 200cc burette to fill to the 50 cc or 10 cc graduation mark.

4.2.1.1 Procedure

Before retrieving the core coal from the core barrel, all the apparatus systems must be ready

for collection of gas. The following steps have been followed for in-situ gas content

measurement by the canister apparatus in the field.

I. Pre-hand preparation before recovery of coal at surface

II. Set-up of volumetric apparatus

III. Recording of coal core data and handling of coal sample

IV. Lost gas measurement

V. Desorbed gas measurement and its frequency

VI. Residual gas measurement

Pre-hand preparation before recovery of coal at surface

Before recovery of coal core samples at surface the following actions were executed.

The desorption canister was checked for leakage.

The heating jacket and heat control unit were checked for their working conditions.

Plastic sleeves were prepared for keeping the recovered coal samples.

The empty canister along with its lid was weighed and the weight was recorded in the

coal core data format.

Saturated salt solution was prepared in a plastic bucket for collection of desorbed gas

sample at scheduled time intervals for compositional analysis.

Thermometer and Aneroid Barometer were kept ready for recording ambient

temperature and pressure respectively.


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Recording of coal core data and handling of core samples

Details of the core were recorded viz. well name, core interval and date of coring on data

format. Both the time when core started out of hole and reached the surface was also

recorded. After recovery of the core, a part of it was selected and 33 cm length of core was

quickly placed in a plastic sleeve and put into the inner cylinder of the desorption canister

which was set at the reservoir temperature. The sample filled canister was weighed by

spring balance and the weight was recorded. The desorption canister was quickly sealed and

the time of sealing was also recorded. As soon as the canister is sealed, a heating jacket is

placed around it to avoid any heat loss and to confirm that reservoir temperature is

maintained within the canister.

4.2.1.2 Measurement of Desorbed Gas

Desorbed gas is the portion of the total sorbed gas volume released from a sample into a

desorption canister. As the pressure of the core is released, gas starts to desorb from the

coal and flows out through the rubber tube. The open end of the rubber tube of the

volumetric apparatus was connected to the exhaust valve of desorption canister and

desorbed gas from canister was allowed to flow into the measuring burette by opening the

exhaust valve by displacement of the liquid. The liquid in the measuring burette was

stabilized at a particular level after displacement. The desorbed gas volume was corrected

for the effect of the hydrostatic head by bringing the reservoir fluid near to the displaced

level of the fluid in the burette. After leveling the fluid in the burette and reservoir the

exhaust valve of canister was closed and the volume of desorbed gas in the burette was

recorded. The date and time of desorbed gas collection, ambient temperature (from

thermometer), ambient pressure (from aneroid barometer) and internal temperature of the

canister (from heat control unit) was recorded along with desorbed gas volume. Desorbed

gas volume was recorded at particular interval. Certain procedures were followed to

determine the measuring interval. It generally depends on the rate of gas released i.e. gas

content of coal. Initially, desorption measurements were made at an interval of every 3 to 5

minutes for the first hour and later at an interval of 10 to 15 minutes for two hours and there


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after once or twice every hour for the first 24 hours. Then continued 3 to 4 times every day

till the desorbed gas volume reaches 0.05 cc/g/day for 2 to 3 consecutive days. This final

desorption rate data is further revised by Diamond and Levin [1981] to an average of 10

cm3 of gas desorption per day for one week.

Desorbed gas volume needs to be corrected for the expansion or contraction of gas in the

canister head space caused by temperature and pressure fluctuations between each

successive measurement in addition to data for lost gas volume.

4.2.1.3 Lost Gas Volume (VL) Measurements

Lost gas, VL, refers to gas desorbed from the core from the time when the core is extracted

from the formation to the time the core is placed in the canister and sealed. The gas lost as

the core is retrieved to the surface is an unknown amount. Compensation for the lost gas is

made by noting the core transfer time and using the initial canister desorption rate as the

same rate of loss during the core transfer time. Consequently, coals with shorter sorption

times will have more lost gas for which to account.The amount of gas lost before sealing a

coal sample into the desorption canister depends on the sample retrieval time, physical

characteristics of the coal, drilling medium, and water saturation/relative amount of free

gas. The shorter the time gap between coring the sample and sealing it into the canister, the

greater the confidence in lost gas calculation. Kissell et al., [1973] proposed two alternative

approaches to estimate time zero and cumulative lost gas time. If the hole was cored with

water or another drilling mud, desorption was assumed to begin when the core is halfway

out of the hole; i.e. when the gas pressure is assumed to exceed that of hydrostatic head.

The cumulative lost gas time (t1g) would then be

( ) ( ) / 21 4 3 3 2t t t t t

g 4.1

Where, t2 is the time when core retrieval began

t3 is the time when core reached the surface and

t4 is the time when core sealed in desorption canister


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If the borehole was cored by air or mist as the drilling fluid, then release of pressure and

desorption of gas were assumed to begin at the first penetration of the coalbed by the core

barrel. In this case, t1g would be

1 4 1t t t

g 4.2

Where t1 is the time when the coalbed was first penetrated by the core barrel.

The volume of lost gas is determined from the relationship that the initial values of

cumulative gas released is proportional to the square root of desorption time; this

proportionality held for the first 20% of emitted gas [Bertard et al., 1970]. Lost gas is

estimated by backward extrapolation of a statistically valid number of measured cumulative

desorbed gas volumes against the square root of elapsed time ( for the initial two

hours of the desorption tests. This relationship is obtained from theory of gas diffusion as

explained below.

Gas diffusion in coal

As soon as the pressure on the coal falls below the critical desorption pressure, gas starts to

desorb from internal surface to the external one. Then diffusion of gas from external surface

starts because of concentration gradient. The rate of diffusion also depends on the

diffusivity of the material along with the concentration gradient. Various techniques have

been applied to determine the gas diffusion parameters of coals, that have been successfully

used in prior studies are the unipore and bi-dispersed models [Thimons and Kissell, 1973;

Smith and Williams, 1984a; Olague and Smith, 1989; Sevenster, 1959; Beamish and

Gamson, 1993; Sevenster, 1959; Walker and Mahajan, 1978]. In the present study, unipore

model is used as most of the simulation models are based on a single step unipore diffusion

model.

As the name suggests, unipore model assumes that all the pores in the coal matrix are of

same size [Crank, 1975]. The basis of this model is the Fick‟s second law of diffusion for

spherically symmetric flow. The model also assumes a constant gas concentration and

isothermal condition at the surface of the sphere throughout the sorption process. For


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homogeneous spherical particle with a constant surface concentration and isothermal

condition, Fick‟s second law is given as [Crank, 1975]

2 26 1

1 exp2 21

M D n tt e

M n n rp

4.3

Where Mt is the total mass of diffusing gas that has desorbed in time„t‟ , M∞ is the total

desorbed mass at infinite time, De is the effective diffusivity and rp is the diffusion path

length. After a step change in the surface concentration, the relationship for the desorbed

gas can be expressed as [Clarkson and Bastin, 1999]:

2 26 1

1 exp2 21

D n tV e

V n n rp

4.4

Where, V is the desorbed gas volume, is the total volume of gas desorbed, „t‟ is the time

(sec). For shorter time (t<600s) and when the fraction of gas desorbed is less than 0.5, the

equation 4.4 can be approximated to [Smith and Williams, 1984a, b]

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D tV e

V rp

4.5

Lost Gas

The lost gas (VL) is calculated by graphical method based on the relationship given in

Equation 4.5 for the first few hours of emission; the volume of gas released is proportional

to square root of the desorption time „t‟. A plot of the cumulative emission after each

reading against the square root of the time is drawn. A straight line is obtained which when

extrapolated to the Y axis gives us an intercept which is the value of calculated lost gas. A

Lost gas calculation graph is shown below in Fig. 4.2.


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Fig. 4.2: Calculation of lost gas by fitting straight line into initial first hour desorption data

4.2.1.4 Residual gas volume (VR)

The residual gas volume was described as the portion of the total sorbed volume that

remains in the sample at the termination of the canister desorption test. About 250 gm to

300 gm of desorbed coal sample was selected for residual gas measurement after the

termination of canister desorption test. With the help of pestle the collected coal sample

was broken into small pieces and placed in a ball mill containing steel rods. The ball mill

was purged with N2 gas prior to placing the coal sample into it to avoid oxidation in contact

with air. After putting the samples inside, it is immediately closed and kept in a horizontal

position on rod mill roller. The grinding was continued for about half an hour to release all

the gas trapped inside the micropores of coal. The crushing vessel was then removed from

the roller and connected to the gas measuring apparatus. The released gas volume in the

vessel was measured with the help of volumetric apparatus by water displacement method.

The measured gas volume was recorded at ambient temperature and pressure. The crushing

vessel was again placed on the mill roller and rotated for another 25 to 30 minutes. Any gas

volume released was measured by the same water displacement method. The residual gas


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volume measurement apparatus is shown in Fig. 4.3. From the measured volume of gas

obtained after crushing the coal, the residual gas volume (Q3) was calculated for the total

weight of the coal sample desorbed.

Fig. 4.3: Crusher mill for Residual gas measurement

4.2.1.5 Total gas content

Total volume of gas content by coal is the summation of lost gas (VL), desorbed gas (VD)

and residual gas (VR). It is denoted by cc/g or m3/ton. The total volume of gas obtained by

the addition of VL , VD and VR gave the in-situ methane content of the sample when divided

by the total weight (W) of the sample.

Gas content (cc/g), V V V

L R DVt W

4.6

i. Sorption time

Sorption Time denoted by (τ) provides an indication of the rate at which gas diffuses out of

the coal. It is the time in hours required to desorb 63.2% of initial gas volume. Errors in

sorption time estimation can affect prediction of reserves and production rates. The total

amount of desorbed gas is calculated by adding up all the volumes of the gas observed


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throughout the desorption process at successive time intervals. The sorption time is the

value of the cumulative time at that point when the fraction of volume desorbed has a value

of 0.632.

Sorption time can also be calculated from cleat spacing and diffusion coefficient by the

given formula [Sawyer et al., 1987]:

8

S

De

4.7

Where, „τ‟ is the value of sorption time, „S‟ is the cleat spacing (ft) and Diffusion

coefficient (De) (ft2/day).

Normally it is difficult to determine the cleat spacing (S) and diffusion coefficient (De), so

overall property, sorption time (τ) is determined from gas content measurement which is

directly used as input parameter for reservoir simulation.

4.2.2 Results and discussion

Depending upon the availability of drilling site during the research work, in-situ gas content

measurement of different wells of Jharia and Raniganj coalfields have been carried out by

canister desorption test.

4.2.2a Jharia coalfield

A total of 42 coal samples are collected from a number of seams at 3 different locations

(well J1, well J2 and well J3) of Jharia coalfield and in-situ gas content measurements have

been carried out in the field.

For determination of gas content of the coal, desorption experiments were carried out till

the cumulative desorbed gas volume become constant as shown in the representative Fig.

4.4. In the present study, representative early desorption data are plotted in the Fig. 4.5 and

Fig. 4.6 to determine the lost gas volume. The experimental data were fitted with a straight

line and the intercept of the straight line at t=0; is determined as the volume of lost gas. All

reported gas volumes were measured at ambient condition and corrected to standard

condition, i.e., 60°F and 1 atmospheric pressure.


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Fig. 4.4: Desorption of methane as a function of time for coal sample from Well J1 of

Jharia coalfield

Fig. 4.5: Lost gas calculation for coal sample from Well J1 of Jharia coalfield


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Fig. 4.6: Lost gas calculation for coal sample from Well J2 of Jharia coalfield

Lost gas volume: Calculated lost gas volume varies from 0.76 cc/g to 3.87 cc/g for well J1,

from 0.31 cc/g to 3.31 cc/g for well J2 and from 1.94 cc/g to 3.56 cc/g for well J3.

Desorbed gas volume: The desorbed gas content of the collected samples ranges from 4.59

cc/g to 10.02 cc/g for well J1, from 1.07 cc/g to 7.23 cc/g for well J2 and from 5.19 cc/g to

11.89 cc/g for well J3.

Residual gas volume: The residual gas content of the studied samples from well J1, well J2

and well J3 varies from 0.46 cc/g to 1.18 cc/g, 0.27 cc/g to 1.44 cc/g and 0.32 cc/g to 2.32

cc/g respectively.

In-situ gas content values of the most of the desorbed coal core samples from Well J1 are

found to vary from 4.55 cc/g to 13.47 cc/g (as received basis) and from 5.25 cc/g to 15.88

cc/g (dry ash free basis) whereas for well J2 it veries from 3.37 cc/g to 9.18 cc/g (as

received basis) and from 4.81 cc/g to 12.65 cc/g (dry ash free basis). Measured gas content

of the coalbeds from well J3 varies from 8.03 cc/g to 17.80 cc/g (as received basis) and


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from 10.66 cc/g to 25.43 cc/g (dry ash free basis).The average gas content values of all

these 3 wells located at Jharia coalfield is 10.69 cc/g (as received basis) and is 12.48 cc/g

(dry ash free basis). The measured gas content data indicate that the study areas have good

CBM potential, since the average gas content of the seams are mostly greater than the

commercial value of 8.5 cc/g of methane [Mukherjee et al., 1999]. Gas content of coals of

the upper seams of zone 1 from well J1 (depth ranges from 680.50 m to 769.54 m) and well

J2 (depth ranges from 1042.78 m to 1185.11 m) are relatively higher as compared to the

coals of lower seams. Generally, for a given rank of coal, the gas content increases with

depth. However, in the present study, this thumb rule is not strictly followed. From the data

of the upper seams (zone 1) of all three locations it was observed a linear increase in the gas

content with depth as conventionally observed (vide Fig 4.7-4.9). Similar observations were

made in case of lower seam of location at Well J3. This is mostly because of reduction in

moisture content and volatile matters in the coal, resulting in more sites available for gas

adsorption [Crosdale et al., 2008]. In case of zones 2 of Fig 4.7 & Fig 4.8, large variations

in gas content were observed for small variation in depth. Both the lower and upper coal

seams show linear relationship with depth.

Fig. 4.7: Variation of gas content with depth for well J1 of Jharia coalfield


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Fig. 4.8: Variation of gas content with depth for well J2 of Jharia coalfield

Fig. 4.9: Variation of gas content with depth for well J3 of Jharia coalfied

Zone-1, upper seam

Zone-2, lower seam


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Table 4.1: Measured gas content of coal from Well J1 of Jharia coalfield

However, from the results it could be observed that that gas content of coals of the upper

seams for both the wells (well J1 and well J2) is relatively higher as compared to the coals

of lower seams. This abnormal behavior of lower seams of both the wells (well J1 and well

J2) may be attributed to the higher mineral content/ash yield of the deeper burial depth or

due to the presence of weak zone (faults and fractures) at the vicinity of the coal seams

where gas can escape from the coal reservoirs. Methane gas emission through fracture

network or weak zones has been observed in different areas of the Jharia coalfield and this

may be one of the causes for lower gas content of the deeper coal seams. The upper seams

of all these 3 wells may be suitable for coalbed methane extraction because of presence of

coal seams at shallow depth and above the economic limit of gas content. The gas content

values for both the lower and upper seams of Well J3 shows increasing trend with

increasing depth of burial (Fig. 4.9). The gas content values of 3 different wells namely J1,

J2 and J3 are shown in Table 4.1, Table 4.2 and Table 4.3 respectively.

SAMPLE

INTERVAL

Average

depth

(m)

Sample

weight

(g)

Lost gas

volume

(cc/g)

Desorbed

gas

volume

(cc/g)

Residual

gas

volume

(cc/g)

Total

gas

content

(cc/g) From

(m)

To (m)

680.35 680.66 680.50 3100 3.87 7.78 0.62 12.28

682.38 682.70 682.54 2870 3.48 8.99 0.54 13.01

684.35 684.68 684.51 3230 3.56 9.20 0.46 13.22

769.39 769.69 769.54 3220 3.11 9.47 0.83 13.41

771.03 771.33 771.18 3050 1.48 4.59 0.46 6.52

908.64 908.94 908.79 3440 1.98 5.92 1.18 9.08

973.20 973.50 973.35 3300 1.89 10.02 1.03 12.95

1060.17 1060.47 1060.32 3840 2.24 6.61 1.17 10.03

1061.81 1062.11 1061.96 3300 2.73 8.30 0.90 11.93

1064.07 1064.36 1064.21 3950 0.76 6.00 0.88 7.64

1067.03 1067.33 1067.18 4150 1.45 6.34 0.52 8.31

1068.94 1069.24 1069.09 3860 2.03 6.84 1.18 10.05


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SAMPLE

INTERVAL

Average

depth (m)

Sample

weight

(g)

Lost gas

volume

(cc/g)

Desorbed

gas volume

(cc/g)

Residual

gas volume

(cc/g)

Total gas

content

(cc/g) From (m) To (m)

1042.62 1042.94 1042.78 4150 1.78 4.46 0.32 6.57

1116.81 1117.13 1116.97 5100 1.96 5.86 0.37 8.19

1117.97 1118.29 1118.13 3700 3.35 4.76 0.40 8.51

1130.31 1130.63 1130.47 3220 2.53 5.64 0.42 8.59

1131.16 1131.48 1131.32 3450 1.38 7.23 0.30 8.92

1183.76 1184.03 1183.89 4050 2.09 6.81 0.27 9.18

1184.96 1185.26 1185.11 3500 1.95 6.26 0.36 8.58

1409.83 1410.15 1409.99 3700 0.31 1.07 1.98 3.37

1412.58 1412.90 1412.74 3480 0.95 3.61 0.60 5.16

1417.03 1417.35 1417.19 3140 0.99 3.33 1.11 5.43

1418.33 1418.64 1418.48 2950 0.49 5.88 1.08 7.45

1420.68 1420.99 1420.83 3470 0.58 4.20 1.31 6.09

1422.75 1423.07 1422.91 4300 0.75 2.99 1.05 4.79

1459.15 1459.47 1459.31 3320 0.98 2.64 1.44 5.06

1462.02 1462.33 1462.17 3260 0.90 2.36 1.26 4.52

SAMPLE

INTERVAL

Average

depth (m)

Sample

weight

(g)

Lost gas

volume

(Q1)

(cc/g)

Desorbed

gas volume

(Q2)(cc/g)

Residual

gas volume

(Q3) (cc/g)

Total gas

content

(cc/g) From

(m)

To (m)

1073 1073.90 1073.40 6200 1.94 5.19 0.91 8.03

1074.25 1074.80 1074.50 4800 2.41 6.28 0.32 9.00

1127.31 1127.60 1127.50 3100 3.35 9.14 0.82 13.31

1127.95 1128.30 1128.10 3150 2.62 7.92 1.20 11.75

1128.82 1129.10 1129.00 3250 3.49 8.79 1.38 13.66

1186.72 1187.00 1186.90 3100 3.34 8.90 1.21 13.45

1187.49 1187.80 1187.60 3900 3.48 9.35 0.97 13.79

1232.9 1233.20 1233.05 3680 2.95 9.48 0.83 13.25

1233.39 1233.70 1233.50 3500 3.33 10.17 0.60 14.10

1234.1 1234.40 1234.30 3530 3.28 11.01 0.99 15.27

1451.17 1451.50 1451.30 2950 3.35 9.27 1.08 13.70

1451.63 1451.90 1451.80 3000 3.41 10.05 1.18 14.64

1520.73 1521.00 1520.90 3150 3.30 8.78 1.75 13.83

1521.99 1522.30 1522.10 3540 3.56 10.93 1.77 16.26

1524.85 1525.20 1525.00 3850 3.59 11.89 2.32 17.80


84

4.2.2b Raniganj coalfield

Total 7 numbers of coal samples have been collected from 2 different locations (Well R1

and Well R2) of Raniganj coalfield. Gas desorption measurement have been carried out for

two representative coal samples collected from well R1of this coalfield. The residual gas

content of the coal samples have been found to lie in the range from 0.17 cc/g to 0.19 cc/g

at STP. The lost gas of coal samples of this well varied from 0.73 cc/g to 0.88 cc/g at STP.

The moisture equilibrated gas content for coal seam was found to be 3.98 cc/g to 4.23 cc/g

at STP (140.55 SCF/ton to 149.38 SCF/ton). Similarly gas content of samples as dry ash

free basis were determined and the values were found to be 4.42 cc/g to 4.52 cc/g at STP

(156 SCF/ton to 159.66 SCF/ton) [Ojha et al., 2013]. The gas content results are shown in

Table 4.4. As mentioned earlier, the gas content of the samples collected from well R1 is

below the commercial value. The low gas content value may be due to erratic maturity

trend which is due to varied degree of intrusive effect at different level of the coalfield.

Although the depth (depth ranges from 973.22m to 972.83m for sample RC 01 and 973.43

m to 973.90m for sample RC 02) of coal seams encountered in this well is less but due to

presence of lower gas content values and thin coal seams thickness (average thickness

0.50m), the coalbed at the studied location is not suitable target for CBM extraction.

However, other parts of Raniganj coalfields content economical amount of methane and

under commercial production.

Table 4.4: The estimated gas content of the samples (Well R1)

Sample Sample

depth (m)

Wt of

the

sample,

gm

Total

lost gas,

cc, at

STP Q1

Desorbed

gas, cc, at

STP, Q2

Residual

Gas, cc,

at STP,

Q3

Total

gas, cc,

at STP

∑Q

Gas

content,

cc at

STP/gm

RC 01 973.08 1064 777.58 3268.15 186.55 4232.28 3.98

RC 02 973.66 1100 973.87 3465.98 217.32 4657.17 4.23

4.3 Sorption isotherm study for determination of gas storage capacity

Isotherm refers to volume of gas adsorbed on a solid surface as a function of pressure for a

specific temperature, gas concentration, and solid material. Brunaur [1938] categorized the


85

adsorption of gas on a solid into five types of isotherm. According to his classification, a

Type-I curve is applicable to the adsorption of gases in microporous solids. At high

pressures, the amount adsorbed becomes asymptotic with pressure. At higher temperatures,

the adsorbed volume decreases. At low pressures, large volumes of gas adsorb or desorb

with small changes of pressure. Type I isotherms closely describe the adsorption/desorption

behavior of methane on coals, and the model has been applicable without exception.

Gas molecules become adsorbed or attached to the coal surface for adsorption isotherms

and in desorption measurements gas molecules become desorbed or detached from the coal

surface on depletion in pressure as mentioned earlier. The quantity of gas which could be

stored at different pressure can be determined from adsorption isotherm study and amount

of gas produced from coalbeds can be predicted from desorption isotherm study.

Adsorption isotherm experiments have been carried out in the laboratory by taking

representative coal samples to determine the methane holding capacity with reservoir

pressure. Studies by Mavor [1990] and Rice [1993] shows that if the reservoir pressure, the

gas capacity at that pressure and the measured gas content from desorption test are known,

then critical information such as the pressure at which the gas will begin to desorb, i.e.,

Critical Desorption Pressure (CDP), in-situ gas saturation and maximum recoverable gas

can be interpreted (Fig. 4.10). The rank of the coal, the temperature, the moisture content of

the coal matrix and pressure are the most important factors for determination of gas storage

capacity of coal. Isotherm is also important for determining the recovery factor of the CBM

reservoir at abandonment. Adsorption isotherm thus becomes essential information for

venturing CBM potential.

4.3a Determination of sorption isotherms

Sorption isotherm measurement involves two procedures:

A) Selecting and preparing of coal samples according to ASTM standard, and

B) Performing isotherm test.


86

i. Selecting and preparing coal samples

Preparation of sample is the most important step to carry out this experiment. The

preparation of sample consists of following steps:

I. Selection of sample

II. Crushing of sample

III. Proximate analysis and

IV. Determination of equilibrium moisture content

Fig. 4.10: Typical methane desorption isotherm and initial adsorbed gas content at reservoir

temperature (45˚C)

Selection of sample

Sample for adsorption isotherm is generally selected on the basis of gas content, ash content

and depth of burial. Suitable samples which are relatively free of ash (ash content 15% or


87

less) are desirable to avoid the influence of mineral matter on adsorption capacity. Fresh,

non-oxidized samples are important. In order to characterize the average reservoir behavior,

representative samples are prepared by quartering and coning method at room temperature.

Crushing

Representative 300 gms of split samples were crushed to -60 mesh (< 250 µm) in order to

minimize the time required for methane to diffuse through the sample and to obtain a

composite/uniform sample.

Proximate analysis

100 gm of crushed (-60 mesh size) coal samples was taken for proximate analysis and

recalculating the resulting data to a dry ash free basis. Comparison of isotherm data for

several samples from different depths or wells is possible by normalizing the data to a dry

ash free basis.

Equilibration of moisture content

The moisture content of coal significantly affects the adsorption of methane and other

gases, as available sites for adsorption is occupied by moisture. For this reason, samples are

re-equilibrated with moisture to nominally restore the moisture content to the original in-

situ level. The ASTM method [Storer, R.A.1985] is used to determine the equilibrium

moisture of coal samples. About 200 g of representative sample was air dried to constant

weight. The sample was then placed on a tray and sprayed with a volume of de-ionized

water approximately equivalent to the residual moisture content which was determined by

proximate analysis. The weight of the water added to the sample was determined by placing

the sample tray on a balance adding water to increase the total weight of the sample.

Equilibrium moisture was then established by placing the sample in a vacuum desiccator

over a saturated solution of K2SO4. The temperature of the chamber should be maintained

at 30˚C. Samples were periodically re-weighed until the change in weight does not exceed


88

0.001g. Thus the coal sample obtained in equilibrium moisture condition is ready for

methane adsorption studies. The equilibrium moisture content is expressed as:

wceq = 1 - (msad/mseq)

Where, wceq is the fractional equilibrium moisture content, msad is the dry sample weight,

gm and mseq is the sample weight at equilibrium moisture, gm

ii. Performing isotherm test

Methane adsorption isotherm analyses were conducted according to procedures outlined by

Moore and Crossdale [2006]. The principle underlying operation in an adsorption isotherm

unit is a known quantity of adsorbent (methane) is introduced to an adsorbate (coal sample)

and the amount of gas adsorbed by the coal can be calculated by measuring the drop in

pressure of the system. The steps for performing isotherm study are given below:

1. The sample cell is packed with 100 gm of crushed (-60 mesh) and moisture equilibrated

coal samples.

2. Interconnected reference cell and sample cells are put into a temperature controlled

system.

3. Volume of the reference cell and the sample cells are determined using Boyel‟s law by

Helium gas expansion.

4. Helium is charged again to determine the void volume and the coal density.

5. Valve between the reference cell and sample cell is closed.

6. The reference cell is charged to a pressure greater than the estimated stabilized pressure

to achieve equilibrium.

7. Valve between sample cell and reference cell is opened and let the pressure to stabilize

8. The steps 5 to 7 are repeated for 6-8 times till reservoir pressure is reached. The

pressure steps are equally divided.

9. In each step pressure variation with time is noted and final pressure of each step is

noted.

10. Volume of gas adsorbed in each step is then calculated using the Boyel‟s equation


89

4.3b Adsorption isotherm equipment

Adsorption capacity of coal samples was determined in the laboratory by adsorption

apparatus.

The schematic diagram of experimental set-up for determination of adsorption isotherm is

shown in Fig. 4.11.

Fig. 4.11: Schematic diagram of experimental set-up

The experimental setup of adsorption isotherm consists of air bath, control panel, sample

cells, reference cells, pressure gauge and vacuum pump. The air bath was used to maintain

isothermal conditions. The control panel regulates flow of gas to the sample and reference

cell and monitors gas pressure. The gas outlet is connected to gas chromatography to

analyse gas composition.

4.3c Determination of gas storage capacity

Gas storage capacity of coal was determined at equilibrium for each pressure increments

and sum of the each individual end point gas capacities is the total gas storage capacity for

each coal samples collected from different seams at different depth.


90

Change in gas storage capacity at each end points of each pressure step was given by Mavor

et al.,[1990]. The relationship is given below:

1 1 1 132.0682 1 2 1

VV rvrG B B B Bs gr gr grv grvm mc c

4.8

Where,

Change in gas storage capacity, scf/ton

Reference cell volume, cm3

mc= Mass of coal, grams

Bgr2 = Gas formation volume factor at the final reference cell pressure

Bgr1 = Gas formation volume factor at the initial reference cell pressure

Vrv = Sample cell void volume, cm3

Bgrv2 = Gas formation volume factor at the final sample cell pressure

Bgrv1 = Gas formation volume factor at the initial sample cell pressure

The gas formation volume factor (Bg) can be evaluated by the following equation

459.69

459.69

P Z TscBg

PZ Tsc sc

4.9

Where,

Bg = Gas formation volume factor

Psc = Pressure at standard condition, psia

z = Real gas deviation factor

T = Temperature, F

P = Pressure, psia

Zsc = Real gas deviation factor at standard conditions

Tsc = Temperature at standard conditions, F


91

4.3d Various isotherm models

A number of isotherm models are available for describing the isotherms for coal. A few of

them are described below.

i. Langmuir isotherm

The most commonly used equation to describe the adsorption of gases on a solid is that of

Langmuir, who developed the theory in 1918 [Langmuir 1981]. The major assumptions in

deriving the equation are as follows [Gregg et al., 1982]:

• One gas molecule is adsorbed on a single adsorption site.

• An adsorbed molecule does not affect the molecule on the neighboring site.

• Sites are indistinguishable by the gas molecules.

• Adsorption is on an open surface, and there is no resistance to gas access to adsorption

sites.

The Langmuir model has frequently been applied to the description of Type I isotherms

obtained from microporous solids such as activated carbons. Several studies of methane

adsorption on coal showed that for the range of temperatures and pressures Langmuir

equation is the best fit equation. Further, its simplicity is appealing. A form of the Langmuir

isotherm that can be used for single component is given by the equation below.

11

BPG V f fa mLS BP

4.10

Where, „Gs’ is the Gas storage capacity, scf/ton, „VL’ is the dry ash-free Langmuir storage

capacity, scf/ton, „fq’ is the ash content, weight fraction, „fm’ is the moisture content, weight

fraction, „B’ is the Langmuir constant, psi-1

, and „P’ is the pressure, psia or KPa.

Another form of this equation uses the constant PL, referred to as the Langmuir pressure,

with b =PL-1

. At low pressures, the relationship between storage capacity and pressure is

linear and is referred to as Henry‟s Law isotherm. At very high pressures, all storage sites

will be occupied if sufficient molecules are available, and the storage capacity will reach its

maximum value equal to the Langmuir storage capacity. If sufficient number of molecules


92

is not available, some storage sites will remain unoccupied. When the coal has the capacity

to adsorb more gas than is available, the coal is considered to be under saturated. Langmuir

pressure (PL) is defined as the pressure that gives a gas content equal to one-half of the

monolayer capacity. The equation can be derived thermodynamically or from the kinetic

theory of gases. At low pressures attainable in the laboratory but difficult to attain in a coal

seam in the field, (1 + BP)≈1, VL, the Langmuir pressure is defined as the maximum

volume of gas that can be adsorbed per unit mass of coal at infinite pressure.

To determine, the PL and VL, the Langmuir equation can be written in the form of following

equation:

1

max max

P P

V V B V 4.11

Thus a plot of P/V versus P gives a straight line with an intercept of 1/ maxV B and a slope

of 1/ maxV .

Extended Langmuir Isotherm:

The Langmuir model has been extended to account for adsorption of multiple gas

components in a mixture [Harpalani & Pariti, 1993]. The extended Langmuir isotherm is

represented by Eq. 4.10.

max,

11

V B Pi i iV ni

B Pj jj

4.12

Where,

Vi = gas volume of component „i’ adsorbed per unit weight of solid at partial pressure, Pi

Vmax,i = monolayer volumetric capacity of component „i’ per unit weight of solid, scf/ton

n = total number of „j’ gas components in mixture

Bj = reciprocal of Langmuir pressure of j component


93

ii. BET (multilayer) theory

Brunauer, Emmet, and Teller or BET isotherm model is applicable for multilayer

adsorption [Brunauer et al., 1938]. This model assumes that the surface of the adsorbent

(coal) is energetically homogeneous with no interaction between the adsorbed (here CH4)

molecules. This model was developed for description of type II isotherms and reversible

part of type IV isotherms. The BET isotherm equation is given below:

1 1 1

1

C P

P V C V C Po m m oVP

4.13

Where, „V‟ is the quantity of gas adsorbed at equilibrium pressure on coal matrix, „Po‟ is

the saturation vapor pressure, (psi), „P‟ is the equilibrium pressure, psi, „Vm‟ is the

monolayer volume and „C‟ is the constant. According to Yang and Saunders 1985 the

application of the BET equation to supercritical fluid adsorption cannot be justified

physically as multilayer formation is considered unlikely.

iii. Freundlich isotherm

Freundlich adsorption isotherm is a curve relating the concentration of a solute on the

surface of an adsorbent, to the concentration of the solute in which it is in contact. In 1906,

Freundlich gave an empirical expression representing the isothermal variation of adsorption

of a quantity of gas adsorbed and plotted against the temperature which gives an idea about

the variation of adsorption with temperature. The first mathematical fit to an isotherm was

published by Freundlich and Küster (1894) and is a purely empirical formula for gaseous

adsorbates which is given below:

1x nKPm 4.14

Where, „x‟ is the mass of adsorbate, „m‟ is the mass of adsorbent, „P‟ is the equilibrium

pressure of adsorbate, „K‟ and „n‟ are the constant for given adsorbate and adsorbent at a

particular temperature.


94

iv. Dubinin theory of adsorption

In 1967, Dubinin described adsorption on microporous adsorbents and proposed a new

theory known as the „Theory of Volume Filling of Micropore‟ (TVFM). Pore size within

the coal ranges below 20 nanometer, so coal being a microporous adsorbent. Thus the

mechanism by which adsorption of microporous solid occurs is restricted to volume rather

than their surface. Dubinin [1975] thus introduced a new theory called the theory of volume

filling of micropore. TVFM postulates that in micropores, the adsorbate occupies the pore

volume by the mechanism of volume filling and does not form discrete layers in the pores.

Dubinin theory commonly describe the type I isotherms [Dubinin, 1966]. Langmuir

isotherm also described the type I isotherm but main difference between Langmuir and

Dubinin theories is the mechanism of pore filling. In the Langmuir theory monolayer on the

adsorbent surface is occupied by the sorbed phase and monolayer is assumed to be

homogeneous. Dubinin, [1966, 1975] theory assumes that the adsorption space in

micropores filled with adsorbate and hence does not form discrete monolayers in the pores.

Dubinin-Radushkevich (D-R) and Dubinin-Astahov (D-A) are the two equations developed

by Dubinin [1966], Kapoor et al., [1989] and Yang [1987]. The Dubinin- Astakhov (D-A)

equation, is expressed as follows:

exp ln

nPoV V DoP

4.15

Where, „V‟ is the amount of adsorbed gas, „Vo‟ is the micropore volume, n is the structural

heterogeneity parameters that varies between 1 and 4,

RTD

E

„D‟ is a constant for a particular adsorbent-adsorbate system, and is determined

experimentally, „R‟ is the universal gas constant, „T‟ is the absolute temperature, „‟ is the

adsorbate affinity coefficient, „Po‟ is the saturation vapor pressure of the adsorbate at

temperature T, and „P‟ is the equilibrium free gas pressure.


95

4.4 Results and discussion

4.4.1 Selection of best fit model

Sorption data of the studied samples from 3 different wells (Well J1, Well J2 and Well J3)

of Jharia coalfield are fitted with various isotherm models as mentioned above and their

absolute error and regression values are compared to determine the best fitting model.

Various isotherms for adsorption of gases on solids have been analyzed using different

approaches such as Langmuir theory, BET theory, Freundlich theory and D-A theory. The

plots of different isotherm models for all the samples are shown in Fig. 4.12 to Fig. 4.23.

From the results, it is seen that the regression coefficient (R2) varies between 0.994-0.999

and the value of absolute least error is within the range of 0.04%-1.40% for Langmuir

Isotherm. For BET isotherm model the regression values ranges from 0.81% to 0.95% and

absolute error varies from 5.61% to 10.77%, for Freundlich isotherm model the regression

values varies from 0.96 to 0.97 and absolute error varies from 7.08% to 9.94%. The

regression values for D-A isotherm models varies from 0.95 to 0.96 and absolute error

varies from 7.33% to 11.77%. So, from the result it may be concluded that Langmuir

isotherm model is fitting best with the present CBM sorption data. Hence the Langmuir

isotherm should be the best choice for fitting of coalbed methane sorption data. The results

of different isotherm fitting parameters are shown in Table 4.5, Table 4.6 and Table 4.7

respectively.


96

Well J1

Fig. 4.12: Modeling of sorption isotherm of coal sample (A-1) from 682.38 m to 682.70 m

depth (at 50˚C)


depth (at 60˚C)


97


depth (at 60˚C)

Well J2

Fig. 4.15: Modeling of sorption isotherm of coal sample (B-1) from 1130.31 m to 1130.36

m depth (at 65˚C)


98


m depth (at 65˚C)


m depth (at 68˚C)


99


m depth (at 65˚C)

Well J3

Fig. 4.19: Modeling of sorption isotherm of coal sample (C-1) from 1073.36 m to 1073.86

m depth (at 64˚C)


100

Fig. 4.20: Modeling of sorption isotherm of coal sample (C-2) from 1127.31 m to

1127.61 m depth (at 65˚C)

Fig. 4.21: Modeling of sorption isotherm of coal sample (C-3) from 1232.9 m to 1233.2 m

depth (at 70˚C)


101


m depth (at 75˚C)


m depth (at 75˚C)


102

Table 4.5: Isotherm parameters determined from experimental data and various isotherm

models for samples from Well J1 of Jharia coalfield

Sample

No. Langmuir isotherm

daf basis Equilibrium moisture basis

PL

(KPa)

VL

(cc/g)

R2 % error PL (KPa) VL (cc/g) R2 % error

A-1 2520.00 19.21 0.9983 1.23 2720.43 21.25 0.9958 1.94

A-2 2521.00 24.11 0.9985 1.35 2475.40 21.37 0.9987 1.42

A-3 2685.70 25.85 0.9938 1.40 2680.40 22.23 0.9944 1.43

Sample

No. BET isotherm

Vm(cc/g) C R2 % error Vm(cc/g) C R2 % error

A-1 1.94 0.00074 0.9000 13.36 2.23 0.00726 0.9000 14.90

A-2 2.35 0.00057 0.8589 11.84 2.62 0.00571 0.8544 15.70

A-3 2.19 0.00060 0.8750 15.05 2.47 0.00619 0.8744 15.07

Sample

No. Freundlich isotherm

K n R2 % error K n R2 % error

A-1 0.0027 2.07 0.98 4.36 0.0026 2.12 0.98 3.48

A-2 0.0034 2.17 0.96 3.65 0.0030 2.19 0.96 4.47

A-3 0.0032 2.14 0.97 4.12 0.0028 2.14 0.97 4.12

Sample

No.

D-A isotherm (n=3)


V0 (cc/g) D R2 % error V0 (cc/g) D R2 % error

A-1 15.34 0.042 0.950 6.10 13.54 0.041 0.948 5.92

A-2 16.01 0.037 0.974 5.32 14.23 0.037 0.973 5.16

A-3 16.23 0.038 0.974 5.78 13.96 0.038 0.958 5.83


103



Sample



PL (KPa) VL

(cc/g)

R2 %

error

PL (KPa) VL

(cc/g)

R2 % error

B-1 3025 22.68 0.99 0.17 3023.40 19.11 0.99 2.34

B-2 3171 22.92 0.99 0.10 3147.60 18.39 0.99 0.07

B-3 4501 19.23 0.99 0.22 4456.00 16.26 0.99 0.16

B-4 2796 21.97 0.99 0.04 2796.70 19.51 0.99 0.09

Sample

No. BET isotherm

Vm(cc/g) C R2 %

error

Vm(cc/g) C R2 % error

B-1 1.90 0.00038 0.940 7.33 1.90 0.00038 0.95 8.16

B-2 2.33 0.00066 0.947 9.66 1.09 0.00176 0.94 9.16

B-3 3.25 0.00065 0.957 5.61 0.28 0.00954 0.97 8.24

B-4 0.82 0.00190 0.920 10.77 0.97 0.00183 0.92 10.77

Sample


K n R2 %

error

K n R2 % error

B-1 0.00028 2.15 0.97 7.08 0.000239 2.15 0.970 7.08

B-2 0.00029 2.17 0.97 7.37 0.000239 2.18 0.969 7.66

B-3 0.00030 1.93 0.97 9.94 0.000131 1.94 0.978 9.20

B-4 0.00038 2.33 0.96 8.12 0.000338 2.33 0.962 8.03

Sample

No. D-A isotherm (n=3)


V0 (cc/g) D R2 %

error

V0 (cc/g) D R2 % error

B-1 14.64 0.00506 0.960 7.35 12.34 0.00506 0.968 7.33

B-2 14.72 0.00534 0.960 8.03 11.83 0.00531 0.963 7.94

B-3 11.20 0.00702 0.956 11.80 9.51 0.00699 0.956 11.77

B-4 14.60 0.00491 0.963 7.52 12.97 0.00492 0.963 7.55


104



Sample



PL (KPa) VL

(cc/g)

R2 %

error

PL

(KPa)

VL

(cc/g)

R2 %

error

C-1 3037.70 25.79 0.99 0.04 3028.40 18.74 0.99 0.12

C-2 2934.10 23.60 0.99 0.02 2924.20 19.71 0.99 0.09

C-3 3086.70 24.18 0.99 0.08 3067.80 19.86 0.99 0.08

C-4 2099.20 24.36 0.99 0.05 2056.90 20.03 0.99 0.07

Sample

No. BET isotherm

Vm(cc/g) C R2 %

error

Vm(cc/g) C R2 %

error

C-1 1.79 0.000781 0.940 9.18 0.33 0.00542 0.940 8.10

C-2 2.02 0.000743 0.936 10.22 2.39 0.00075 0.930 10.18

C-3 0.42 0.00360 0.934 9.67 0.55 0.00334 0.934 9.55

C-4 0.11 0.01255 0.818 12.19 0.15 0.01140 0.811 12.27

Sample


K n R2 %

error

K n R2 %

error

C-1 0.000361 2.21 0.970 6.09 0.000260 2.21 0.970 6.31

C-2 0.000346 2.23 0.968 6.04 0.000290 2.23 0.968 6.97

C-3 0.000358 2.25 0.968 7.12 0.000297 2.26 0.967 7.43

C-4 0.000884 2.84 0.943 6.40 0.000750 2.86 0.942 6.33

Sample

No. D-A isotherm (n=3)


V0 (cc/g) D R2 %

error

V0

(cc/g)

D R2 %

error

C-1 16.86 0.00522 0.960 6.73 12.28 0.00518 0.962 6.59

C-2 15.44 0.0499 0.964 6.90 12.91 0.00497 0.964 6.83

C-3 15.83 0.00537 0.960 7.52 13.02 0.00535 0.962 7.44

C-4 17.58 0.00412 0.963 4.26 14.50 0.00405 0.964 4.10


105

4.4.2 Adsorption isotherm study on coal samples from Raniganj block

The representative coal samples from well R1 of Raniganj coalfield have been performed

for isotherm study. The isotherms of high pressure methane sorption experiments

performed on the dry and moisture equilibrated coal samples are shown in Fig.4.24 and

4.25 respectively. The adsorption isotherm gives the adsorption-desorption characteristics

of a coal with variation of pressure at a particular temperature. This is of utmost importance

in characterization of coalbed for estimation and production of methane from CBM

reservoir. From the Fig. 4.24 and Fig. 4.25, it is observed that the reservoir under study is

under-saturated and needs dewatering upto the desorption pressure to start production of

methane from the coal bed. Production of methane from this field requires further depletion

in pressure below this limit for availability of sufficient free gas.

4.2.2a Determination of Langmuir constant from adsorption isotherm

Langmuir constants for samples RC 01 and RC 02, both the moisture equilibrated and dry

ash free basis were determined from the adsorption experiments and the results are shown

in Fig. 4.26 and Fig.4.27. The plot of applied pressure (P) versus P/V (V= volume of

methane adsorbed onto coal surface, cc/g of sample at STP) showed linear relationship

(Fig. 4.26 and Fig. 4.27).

Fig. 4.24: Methane adsorption isotherm curve of sample RC 01


106

Fig. 4.25: Methane adsorption isotherm curve of sample RC 02

Fig. 4.26: Langmuir constant curve for sample RC 01


107

Fig. 4.27: Langmuir constant curve for sample RC 02

The Langmuir constants were determined from the Langmuir equation. The Langmuir

volume and pressure determined from the Langmuir constant curve for the present sample

as shown in Table 4.8.

Table 4.8: Langmuir adsorption isotherm data from experimental result of sample from

Raniganj block

Variable Value

moisture equilibrated dry ash free

RC 01 RC 02 RC 01 RC 02

Langmuir volume (VL) (cc at

STP/gm)

14.08 14.53 15.90 16.26

Langmuir pressure (PL) (KPa) 3899.15 4076.00 3970.60 4178.90


108

From the experiments on adsorption isotherms, it is observed that the coal is having higher

adsorption capacity, i.e., the volume of gas adsorbed at saturated condition at the reservoir

pressure. The values range from 9.90 cc/g to 11.5 cc/g at moisture equilibrated and dry ash

free basis respectively. However, the collected samples showed much lower values of in-

situ gas content, varying from 3.98 cc/g to 4.23cc/g only, which is approximately 40% of

adsorption capacity. This may be due to escape of gas in presence of fracture or lowering of

reservoir pressure [Ojha et al. 2013].

4.4.3 Adsorption isotherm study of coal samples from Jharia coalfield

A total of 12 coal samples are selected from different depth intervals of 3 different wells

(Well J1, well J2 and well J3) located at Jharia coalfield. Adsorption isotherm experiments

have been performed for all these samples for determination of adsorbed gas volume, i.e.,

Langmuir volume, gas saturation and critical desorption pressure at reservoir pressure and

temperature condition.

i. Location 1 (Well J1)

The adsorption isotherms for representative samples collected from different depths of

location 1 (well J1) have been constructed at reservoir temperature and shown in Fig.4.28,

Fig.4.29, Fig.4.30 and Fig.4.31 respectively. The details of the samples and different

parameters derived from the isotherm curves are shown in Table 4.6. For all these samples,

difference in the in-situ gas content and adsorption capacity were observed and in-situ gas

content is always less than the maximum adsorption capacity in the present case. In-situ

gas content varies from 12.20 cc/g to 15.88cc/g whereas sorption capacity at reservoir

pressures varies from 18.9 cc/g to 22.4cc/g (daf basis). It shows that the adsorption

isotherm data as analysed when compared with measured in-situ gas content of the samples

was found to vary from 68% to near saturation (99.28%). The extent of gas saturation plays

significant role in gas production. If the gas saturation is high, sorbed gas from the coal will

be produced much faster compared to coals having lesser degree of saturation. Maximum

adsorption capacity of coal depends on its chemical and physical properties like

composition, grade & rank, presence of pores (micro & macro pores), pressure and

temperature etc. However, with time, there may be changes in the depositional


109

environment, variation in pressure and temperature, development of fractures or cleats,

contamination with external elements and so on. These effects cause escape of adsorbed

gases from coal; resulting in the lesser value of in-situ gas content compared to its capacity.

Langmuir adsorption isotherm not only gives the information regarding the storage

capacity, it gives the idea about need of dewatering for gas production from CBM, provided

reservoir pressure and in-situ gas content is known.

The estimated reservoir pressures for studied samples (A-1, A-2, A-3 and A-4) are 6693.23

KPa (970.77 psi), 8912 KPa (1292.57 psi), 9545 KPa (1384.38 psi) and 10532 KPa

(1527.53 psi) respectively. In this situation to achieve the production of CBM, the pressure

should be reduced from reservoir pressure to critical desorption pressure (CDP). For A-1,

the critical desorption pressure and reservoir pressure are same, so CBM can be produced

after minimum release of pressure, i.e., minimum dewatering However, for all others

samples reduction is required to make the gas free to flow as shown in the respective

figures and Table 4.9.

Fig. 4.28: Adsorption isotherm and gas saturation of coal sample (A-1) from 682.38 m to

682.70 m depth (at 50˚C)


110


908.94 m depth (at 60˚C)


973.50 m depth (at 60˚C)


111

Fig. 4.31: Adsorption isotherm and gas saturation of coal sample (A-4) from 1073.92m to

1074.2m depth (at 65˚C)


Well J1 of Jharia coalfield

Sl.

no

Average

depth

(m)

In-situ

gas

content

(cc/g),

daf

Reservoir

pressure

(KPa)

Langmuir volume Langmuir

pressure

(KPa)

CDP

(KPa)

Saturation

(%)

daf

(cc/g)

Moisture

equilibrated

(cc/g)

A-1 682.54 15.88 6693.00 21.90 18.8 2520 6693 99.28

A-2 908.79 10.45 8912.00 21.10 17.6 2935 2990 67.42

A-3 973.35 15.60 9545.00 22.40 18.5 2450 5910 88.10

A-4 1074.06 12.20 10533.00 18.90 16.2 3048 5785 68.00


112

ii. Well J2

Adsorption isotherm curves for representative samples (B-1, B-2, B-3 and B-4) are shown

in Fig.4.32, Fig.4.33, Fig.4.34 and Fig.4.35 respectively. Methane adsorption values at

reservoir pressure for B-1 is 20.2cc/g (daf basis); for B-2 is 20.5cc/g (daf basis); for B-3 is

17.1cc/g (daf basis) and for B-4 is 19.6cc/g (daf basis). These coal samples show a variation

in the degree of gas saturation from 34.90% to 67.73%. The reservoir pressures for sample

B-1 to B-4 varies from 11086 KPa (1607.88 psi) to 14311 KPa (2075.63 psi) and critical

desorption pressure varies from 1200 KPa (174.04 psi) to 4900 KPa (710.68 psi). The In-

situ gas content data, reservoir pressure, Langmuir isotherm data and gas saturation are

shown in Table 4.10. The results show that it is necessary to decline the reservoir pressure

to critical desorption pressure of gas so that gas would start to desorb from the coal seams

for all these well because of larger difference between reservoir pressure and critical

desorption pressure.

Fig. 4.32: Adsorption isotherm and gas saturation of coal sample (B-1) from 1130.31m to

1130.36 m depth (at 60˚C)


113

Fig. 4.33: Adsorption isotherm and gas saturation of coal sample (B-2) from 1184.96 m to

1185.26 m depth (at 65˚C)


1418.64 m depth (at 68˚C)


114


1459.47 m depth (at 65˚C)



Sl.

no

Average

depth

(m)

In-situ

gas

content

(cc/g),

daf

Reservoir

pressure

(KPa)


pressure

(KPa)

CDP

(KPa)

Saturation

(%) daf

(cc/g)

Moisture

equilibrated

(cc/g)

B-1 1130.47 10.14 11086 20.20 17.00 3004 3190 63.81

B-2 1185.11 10.28 11622 20.50 16.50 3210 3450 65.90

B-3 1418.48 8.73 13910 17.10 14.50 4470 4900 67.73

B-4 1459.31 5.67 14311 19.60 17.40 2800 1200 34.90


115

iii. Well J3

Total 4 coal samples are designated as C-1, C-2, C-3 and C-4 from this well which have

been selected for isotherm study. The depth intervals and different isotherm parameters of

the samples are shown in Table 4.11. Adsorption isotherm curve for 4 coal samples have

been represented in Fig. 4.36, Fig.4.37, Fig.4.38 and Fig.4.39 respectively. The estimated

reservoir pressure for all these 4 samples varies from 10528 KPa (1526.95 psi) to 14237

KPa (2064.90 psi). From the isotherm study, it has been found that adsorption capacity of

methane or Langmuir volume (VL) of the studied coal samples varies from 21.20 cc/g to

23.00 cc/g (daf basis) and 16.70 cc/g to 17.90cc/g (received basis), whereas Langmuir

pressure (PL) varies from 2080 KPa (301.67 psi) to 3050 KPa (442.36 psi). Measured gas

saturation of sample C-2 and C-3 are 93.98% and 95.51% which is the value of near

saturation. Critical desorption pressure is also determined from the isotherm curves. The

critical desorption pressure for sample C-2 and C-3 are 10025 KPa (1454 psi) and 9290

KPa (1347040 psi) which are vary close to the value of reservoir pressure. Thus, coal from

these depths has enormous CBM production potential as the gas would be released easily

with minimum dewatering and depressurization. Saturation studies of sample C-1 and C-4

shows that the degree of gas saturation varies from 64.85% to 77.05%. The reservoir

pressure of sample C-1 and C-4 are 10528 KPa (1526.95 psi) and 14236 KPa (2064.75 psi),

whereas critical desorption pressures are 2840 KPa (411.90 psi) and 6000 KPa (870.22 psi).

Thus, for these two samples the reduction of pressure would be more prior to any gas being

released.



Sl.

no

Average

depth

(m)

In-situ gas

content

(cc/g), daf

Reservoir

pressure

(KPa)


pressure

(KPa)

CDP

(KPa)

Saturation

(%) daf

(cc/g)

Moisture

equilibrated

(cc/g)

C-1 1073.61 10.96 10528 23.00 16.70 3030 2840 64.85

C-2 1127.46 15.62 11056 21.20 17.60 2935 10025 93.98

C-3 1233.05 15.99 12092 21.50 17.70 3050 9290 93.51

C-4 1451.78 14.64 14237 21.70 17.90 2080 6000 77.05


116

Fig. 4.36: Adsorption isotherm and gas saturation of coal sample (C-1) from 1073.36 m to

1073.86 m depth (at 64˚C)

Fig. 4.37: Adsorption isotherm and gas saturation of coal sample (C-2) from 1127.31 m

to1127.61 m depth (at 65˚C)


117


f1233.2m depth (at 70˚C)


1451.93 m depth (at 75˚C)


118

4.5 CO2 sorption characteristics on different coal samples

The adsorption capacity of CO2 and methane on coal samples from location 1 of Jharia

coalfield were measured from the isotherm study. The representative data are shown in Fig.

4.40. Complete sorption isotherm characteristics of CH4 and CO2 were studied by varying

pressure condition of different coal samples. All isotherms were conducted at 50˚C on

moisture equilibrated coal samples up to a maximum experimental pressure of ~ 6722.38

KPa (975 psia) and 6515.54 KPa (945 psia) for CH4 and CO2 respectively. The CO2 to CH4

adsorption ratio decreases with increasing pressure and the overall variation is between 3:1

and 3.5:1 respectively. Although the 2:1 adsorption ratio of CO2 to methane was widely

reported in the past, more recent works reported much higher ratio in some coals. From the

experiments, it was observed that sorption capacity of CO2 and CH4 to be 22.5 and 13.4

cc/g respectively at 6722.38 KPa (975 psi) and 50˚C. Langmuir volume and Langmuir

pressure of CO2 are 30.71cc/gm and 1994.23 KPa (289.24 psi) respectively.

Fig. 4.40: Sorption of CH4 and CO2 on coal sample (A-1) from 682.54 m depth of

location1 of Jharia coalfield


119

4.6 Ash content effects on adsorption capacity

Methane adsorption capacity or gas holding capacity of coal is strongly correlated with ash

content. Ash content into the coal acts as a simple dilutent and reduces the storage capacity

of gas. Therefore increasing ash content indicates the linear decrease in adsorption capacity

(Fig. 4.41). Studies by Laxminarayana and Crosdale [1999, 2002] also showed that with

increasing ash yield, the Langmuir volume decreased.

Fig. 4.41: Effect of ash percentage on methane adsorption capacity of coal at reservoir

temperature

4.7 Relation between methane adsorption and fixed carbon percentage

Methane adsorption capacity obtained from the methane adsorption isotherm study on the

studied coal samples were compared with the values of fixed carbon of the coals. In Fig.

4.42 the methane adsorption capacities at reservoir pressure and temperature are plotted


120

against fixed carbon (daf) which is a general indicator of coal rank. From the figure, it has

been shown that with increasing fixed carbon percentage, the adsorbed gas capacity into the

coalbed increases. This increasing adsorption capacity may be due to loss in moisture with

increasing rank of coal as well as increase in porosity, so higher adsorption site are

available in the micropores for the methane adsorption site. Studies by Hildenbrand et al.,

2006; Kim, 1977; Leavy et al., 1997; Yee et al., 1993 have found that with increasing rank

of coal, the maturation of coal increases and methane adsorption capacity of coal also

increases continuously.

Fig. 4.42: Effect of fixed carbon percentage on methane adsorption capacity of coal at

reservoir temperature

4.8 Relation between methane adsorption and moisture content

With increasing rank of coal as well as increasing depth of the coal seam, moisture content

decreases [Sivek et al., 2010]. Gas holding capacity of coal depends upon the moisture

content of coal [Joubert et al., 1974; Levin et al., 1993]. Studies by Bustin and Clarkson,


121

1998; Crosdale et al., 2008; Joubert et al., 1974; Ozdemir and Schroeder,2009 shows that

gas holding capacity increase with increasing rank of coal because of less moisture

competing for methane adsorption site. Presence of 1% moisture may reduce the adsorption

capacity by 25%, and 5% moisture results in a loss of adsorption capacity of 65% [Lama

and Bodzionv, 1996]. Lower rank of coal has higher porosity but because of high moisture

content less CH4 adsorption site are available. Fig. 4.43 shows the variation of adsorbed

gas volume with moisture. From the figure it has been observed that as moisture content

decreases, the adsorbed gas volume increases in a non-linear fashion.

Fig. 4.43: Effect of moisture percentage on methane adsorption capacity of coal at reservoir

temperature

4.9 Adsorption isotherm for recovery

After the isotherm has been established, it may be used to follow the progress of the CBM

process and to estimate a percentage recovery. For example, assume the isotherm of Fig.


122

4.10 has been generated in the laboratory from crushed coal samples. If Pi in Fig. 4.10 is

the pressure on the coal seam initially, then an undersaturated state exists. Pressure must be

reduced by dewatering until the saturation pressure, Ps, is reached on the isotherm.

Subsequently, further dewatering proceeds to the abandonment pressure, Pa, where it is no

longer economical to further reduce pressure and produce methane. Unfortunately, Pa falls

on the steep part of the curve where a small incremental pressure decrease involves the

greatest incremental volume of methane production. The percentage recovery is then given

by Eq. 4.16.

/ 100R V V Vs a s 4.16

Where

R = % recovery

Va = methane content of coal at abandonment, scf/ton

Vs = saturated gas content after initial dewatering, scf/ton

4.10 Conclusions

Sorption studies of the samples when fitted to various isotherms reveal that Langmuir

isotherm is the best choice for modeling of adsorption isotherm data. The regression values

for Langmuir isotherm theory range from 0.994 to 0.998 which is better than all other

isotherm theories and percentage of error is least compared to other isotherm models.

Adsorption isotherm studies have been performed in the laboratory, from these studies it is

observed that measured gas saturation of the maximum coal samples under study varies

from 60% to near 100% saturation, so fields need depressurization and dewatering for gas

production.

The proximate analysis of the coal samples reveals that with increasing ash percentage and

moisture content of coal, methane adsorption capacity decreases, i.e., methane adsorption

capacity is inversely related to ash and moisture contents. With enrichment of fixed carbon

percentage of coal seam, methane adsorption capacity increases, i.e., with increasing rank

of coal upto bituminous rank methane adsorption capacity increases.


123

From the study it is observed that all the 3 wells (Well J1, Well J2 and Well J3) of Jharia

coalfield have the potential of producing coalbed methane. The average methane contents

in these 3 wells indicates moderate to high gas content of the coal seams. Gas content of

seams is found to decrease with depth. From the studies, it has been found that studied coal

samples from Raniganj coalfield under Gondwana basin contain low gas compared to

economic limit of methane present in the coalbed. Hence, presently it is not considered for

CBM extraction. However, in future this field may be considered for methane extraction

using advance technology and in emergency conditions.

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