coal bed methane[1]

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HAMMAD ATTAULLAH NED UNIVERSITY OF ENGG. & TECHNOLOGY

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COAL BED METHANE: AN UNCONVENTIONAL

SOURCE OF NATURAL GAS & ITS PROSPECTS IN

PAKISTAN.

ABSTRACT The concept of this paper arose from the

Pakistan’s rising energy needs. Pakistan has the

world’s second largest coal reserves of the

world which is about 185 billion tons of coal[1]

,

with energy equivalent to 450 billion barrels of

oil which is the reserves of Saudi-Arabia and

Iran put together. An Unconventional natural

gas resource which is “Coal-Bed Methane” or

commonly known as CBM exists in the coal beds

or coal seams. Because of its large internal

surface area, coal stores 6 to 7 times more gas

than the equivalent rock volume of a

conventional gas reservoir[2]

.

Its presence in Pakistan is quite promising

because of the lithology of lignite coal present

in the coal fields of Thar (175 billion tons),

Lakhra (1.382 billion tons) and Sonda, distt.

Thatta (7.112 billion tons); few activities were

carried out in Pakistan’s coal fields of in order to

recover the gas from the coal seams; but their

results were not encouraging for the companies

to drill for CBM furthermore. This may be due

to an unpredictable nature of the coal seams in

which the CBM exists.

This paper will discuss “the prospects of CBM in

Pakistan” with emphasis on Thar Coal field

which is understood to be of same age as that is

of India’s Cambay Coal field, presently

producing CBM. Its present scenario in the

world, application of GTL technology for its

easier transportation, the environmental

concerns regarding the water produced during

CBM production and also a comparative study

with the US CBM producing coal fields.

INTRODUCTION:Increasing fuel prices and shortage compel us to

search for more hydrocarbon resources to meet

the rising energy needs of our country.

Presently, Pakistan’s natural gas reserves stands

up to 28 TCF, the need of hour is to search for

new reserves of natural gas. For this purpose

many exploration activities are being carried

out throughout the country in the promising

fields.

Natural gas produced in Pakistan is from

conventional reservoirs i.e., Sandstone or

Limestone for example Sui, Adhi, Zamzama etc.

But the unconventional ways include

Coal-bed Methane,

Tight Gas Sands,

Shale Gas &

Gas Hydrates.

They are regarded unconventional because it

technically more challenging to produce and the

reservoir rock is normally not a conventional

reservoir rock.




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WHAT IS COAL BED METHANE

AND HOW IS IT FORMED?

Coal-bed methane (CBM) is a form of natural

gas occurs in coal seams. It is formed during the

conversion of organic material to coal and

becomes trapped in cleats in the coal seam. The

coal bed methane is trapped in the coal seam in

part by water pressure and in part by weak

covalent bonding forces known as Van de Waals

forces.[3]

In recent years it has become an important

source of energy in United States, Canada and

other countries. Pakistan has one of the world’s

largest coal reserves. There are chances of

presence of CoalBed Methane in the coal seams

of Pakistani coal fields like Thar, Lakhra etc.

CBM is thought to be generated due to three

stages

BIOGENIC: In this stage, the gas is produced due to

bacterial action especially, during the

conversion of peat to coal.

THERMOGENIC: Due to increasing overburden pressure and

hence the temperature of coal, which tends

to increase the coal rank, the gas is also

produced.

LATE BIOGENIC Gas produced due to bacterial action after

maturity of coal.

CBM & CMMThe methane gas found in the minable zones of

coal is known as the Coal-Mine Methane-CMM

generally it is for coal seams up to 300 ft.

Whereas, the gas found in coal seams with

depth greater than 300 ft is called as the

CoalBed Methane-CBM.

EXISTENCE OF CBM & COAL

AS AN UNCONVENTIONAL

RESERVOIR It exists in a coal reservoir as adsorbed gas or

the process of adsorption, a phenomena in

which each individual molecule of gas is bound

by weak dipole forces to solid organic molecules

of coal.

The natural fracture system present in the coal

is known as the cleats. Coal cleats are of two

types: butt cleats and face cleats, which occur

at nearly right angles. This is the main factor

when permeability of a coal is considered.

In conventional hydrocarbon reservoirs, gas lies

above oil, both of which lie over the adjacent

water aquifers. Perforations are selectively

placed to maximize production of hydrocarbons

and reduce the production of water. In contrast,water penetrates and spread through coal

seams, and water pressure traps any CBM

present.

Therefore during production of CBM it is

necessary to remove water in order to decrease

pressure on the coal matrix, allowing free gas to

flow into the well bore (Figure 1- At the bottom

of the paper). The water is usually saline. Water

production is especially a problem in the earlystages of production, when large amounts of

water are produced to decrease trapping

pressure on the methane.




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PRESENT SCENARIO IN

THE WORLDCountries with large coal resources have

invested in their coal fields In order to search

for the presence of CBM. For example,

countries like US, Canada, and Australia have

started the production of CBM.

US started its production of CBM in mid 80’s

and presently its in-place CBM reserves stands

700 TCF with the technology available today

100 TCF of which is recoverable. CBM’s

production in US is 1.5 TCF/annum which is

actually 10% of its total annual Natural Gas

Production.[4]

Whereas, Canada has started producing CBM

quite recently with its in place CBM reserves

upto 1258 TCF. And Australia, which actually

started its CBM production in 1998, its present

in-place CBM reserves stands upto 300-500

TCF.[5]

Whereas, India which has also recently started

its CBM project, but the commercial productionis expected to begin in 2009. India estimates its

in-place CBM reserves to be 2 Trillion m3 (equal

to 70 TCF), which is spread over 56 coal basin

with cumulative area of 64,000 km2.[6]

Whereas, the worldwide CBM reserves are

estimated to be around 4000-7500 TCF in 69

coal-bearing countries, which have investigated

for the CBM development.[4]

(Figure 2)

CBM-AVAILABILITY AND

COAL-RANK.The presence of CBM in coal seam becomes

more convincing as the coal-rank is nearer to

Bituminous, the farther it is the lesser are the

chances of presence CBM. Coal rank increases

as the overburden over it increases, which

results in an increase in pressure and hence the

temperature which tends to increase the coal’s

rank. So deeper we go into a coal formation

higher is the rank of coal and thus its quality,

thereby increasing chances of CBM’s presence.

But if rank is further more increased then the

probability of finding gas increases as well, but

only till bituminous.

The figure(Figure 3) shows a curve which

signifies that how the gas volume increases with

the rank of coal getting nearer to bituminous,

and how it decreases as the rank get farther.

THAR COAL-FIELD IN A BROAD

VIEW.Thar, located in the south-eastern side of Sindh

province, is the largest coal-field of Pakistan.

This field covers an area of 9000 km2 and its

estimated coal reserves are 175 billion tons.

The Formation is named as Bara Formationwhich is producing coal and it belongs to

Paleocene to Early Eocene age with average

thickness of 52 meters.[7]

The rank of coal found in the Thar Coal-Field is

mostly between Lignite-B and Sub-Bituminous.

That is the coal of Thar is categorized as low-

rank coal with samples collected so far. Since

the above figure shows that the gas volume and

presence of gas is max. at biltuminous and soon. So looking at the figure one would say that

there is less probability of finding gas formed at

stage 2. But there probability of finding CBM at

stage-1 (Biogenic stage) is more in low-rank

coal.[8]




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The litholoy of the thar field is shown by this

figure. This figure clearly shows that as we

move towards the eastern side that is towards

the Pak-Indian Border the formation remain

shallow, but as we move towards west that is

more towards the interior Sindh side, the there

is a dip and the formation gets deepen, also it

gets a bit thinner as it deepens. (Figure 4)

Since in past the activities were mostly done in

the areas of eastern side of the Thar area. That

was because the interest of government was

more likely to mine for coal rather than to drill

for CBM. Since the deeper the coal, the lesser it

is reachable to mine therefore lesser is the

interest. But the wells drilled in those areas in1994 desorbed very little gas. The reason could

be that no proper water removal was done and

also the equipments were primtive, this may be

because CBM was not the expected guest.[8]

Following figure is a conceptual model of Thar

presented by John SanFilipo which clearly

shows the dip of Thar, which extends as we

move westwards. So as the western part is

deeper hence the coal must be of higher rank,that have higher probability of having CBM.

COMPARISON OF THAR WITH

GULF COAST BASIN

It is important to note that the lithology of coal

bearing formation of the Gulf Coast Basin near

Texas is the same as that of Bara Formation

(coal bearing formation of Thar). The coal of thisfield is also lignite which is low-rank coal.In this

areas methane was desorbed, but in lesser

quantity. Experts predict that in this area if

deeper wells were drilled more amount of gas

could be desorbed.

So is the case with Thar deeper wells could lead

to the discovery of CBM.

RECOMMENDATIONS At the end of the paper followings could be the

recommendations in order to initiate a CBM

development program in Pakistan.

1. Government bodies like GSP, HDIP etc.

must initiate the CBM projects in order

to meet the natural gas needs of

Pakistan.

2. More wells must be drilled in the

western areas of Thar in order todiscover any possible CBM.

3. Interest should be shown not only in

mining for coal rather to drill for CBM.

ACKNOWLEDMENTS I am very thankful to Mr. Ateeq from GSP, who

has been a Field-Geologist in Thar, for guiding

me referring me a couple of papers. I m alsograteful to Mr. John R. SanFilipo, from USGS,

who explained to me concepts regarding CBM

via e-mail. At last, my friends from NED

University who were a constant source of

motivation for me, especially Muhammad Ibad

Siddiqui.

REFERENCES

[1] Harnessing of Indigenous Coal ResourcesGeneration: GOVERNMENT OF SINDH, MINES

& MINERAL DEVELOPMENT DEPARTMENT

(SINDH COAL AUTHORITY).

[2] U.S. Geological Survey : “Energy Resource

Surveys Program”; Fact Sheet No. FS-019-97.




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This USGS Fact Sheet can be found at

<URL:http://energy.usgs.gov/factsheets/Coal

bed/coalmeth.html>

[3] Coal Bed Methane: the Past, Present and

Future of Alberta’s Most Abundant Non-

traditional Source of Natural Gas ----By

Casendra brown-University of Alberta.

[4] Vito Nuccio USGS-“Coal-Bed Methane:

Potential and Concerns” USGS Fact Sheet No.

FS–123–00.

[5] “Producing Natural Gas From Coal”-

OilField Review-Autumn 2003

[6] “Progress of Coalbed Methane in India”

Singh, U.P., Coal India, Ltd. Presentation to

North American Coalbed Methane Forum,

November, 2002.

[7] “THAR COAL RESOURCES IN THAR

DESERT, SINDH– PAKISTAN”- Thar

Whitepaper

[8] John R. SanFilipo, - USGS-Open-File Report

00-293.

FIGURES

coal bed methane[1]

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