Cbm gupta

COALBED METHANE

By

Dr. A K Gupta

Ex. Scientist G, IIP, Dehradun

Coalbed Methane

• Coalbed Methane primarily is a hydrocarbon gas, dominantly methane with minor amount of other gases like CO2 and N2.

• Underground it exists in “adsorbed” state in coal / lignite and is released when reservoir pressure is reduced by removal of water using oil field techniques.

• In the coal fields, methane in coal is considered as mining hazard and is vented out wastefully into the atmosphere.

• Technology has been developed for safe recovery of methane.

Coalbed methane….

• Generally a ton of coal can hold 5 to 15 cubic meters of CBM gas.

• Higher gas contents of 30 to 40 m3 have also been reported in US basins.

• For coal basins in India, typically 2 to 3 billion tons of coal will be require to yield about 1 MM m3 /day of CBM gas.

• USA is pioneer in commercial exploitation of coalbed methane since 1980’s. Over 7000 CBD wells have been drilled in various coal basins, produced about 80 MM m3/day of CBM.

• Several coal rich countries like China, UK, Poland, Russia, Australia including India have started activities in this area.

Impact of CBD:

• Commercial exploitation of CBD would have substantial economic, environmental and strategic benefits.

• It would improve the energy security of a country, of developing countries in particular

CBM Process

• CBM technology involves drilling of wells down to target the coalbeds.

• Like oil and gas, desorption, diffusion and production of methane is accomplished with the reduction of reservoir pressure by dewatering of coal seams.

• To achieve efficient release of gas, the reservoir pressure needs to be reduced to nearly atmospheric pressure.

• Initially, the CBM well gives high water production with low gas, which increases with time.

Mechanism of generation, adsorption and retention of methane in coal bed

• During early stages of coalification below 50 oC biogenic methane is produced by microbial decomposition of organic material.

• These are Swamp or Marsh gas.• Methane produced at above 50 oC as coal tank

increases is termed as thermogenic Methane.• Generation volume of methane is related to

volatile matter and Rank of coal.

Genaration of Methane:

• Messner proposed the following correlation for estimating generation of Methane:

VCH4 = - 325.6 log ( VM / 37.8)

Where VCH4 - Volume of methane (cc/g)

VM - weight % volatile matter on dry ash free basis

Genaration of Methane:

According to this equation:• Thermogenic methane is initiated when coal

reaches 37.8 % volatile matter or high volatile

A-B Bituminous boundary.

• As VM decreases, the generation of methane increases.

For inertinite rich coal this equation gives higher quantity of methane generated.

Genaration of Methane:

• Most coals have the capacity to generate more gas than their capacity of retention.

• The adsorptive capacity of coal is dependent on:

- pressure ( Burial depth )

- Coal Rank

- ash

- moisture content

- maceral composition

Storage and adsorption capacity of coal:• Methane is stored in the coal as sorbed on molecular surfaces or

with in molecular space and with in matrix porisity.

Storage / adsorptive capacity of coal can be estimated by the correlation developed by Kim (1977):

G(saf) = 0.75 (1 – a - Wc ) Ko ( 0.095 d) * no - 0,14( 1.8 d / 100)+ 11

Where: G(saf) - Dry ash free gas storage capacity

a - ash content , Wt. fraction

Wc - moisture content, Wt. fraction

d - sample depth, m

Xfc - fixed carbon, wt. fraction

Xvm - volatile matter, wt. fraction

Ko - 0.8 ( Xfc / Xvm ) + 5.6

no - 0.315 – 0.01 ( Xfc / Xvm )

Storage and adsorption capacity of coal:

• Using these two Equations, the amount of gas generated and storage capacity or gas content of a particular coal can be estimated.

Estimates of coal bed methane resources:

• The CBM in place depends directly on:1. Coal thichness

2. Gas content

3. Coal density

4. Ash yield

And indirectly depends on coal rank, structure, hydrogeology,

and Topography.

Estimates of coal bed methane resources:

• The basic Equation used to calculate the gas in place is:

GIP = (h * A) * Dc * Gc

where:

GIC = gas in place

Gc = gas content

Dc = coal density

h = coal thickness

A = Area

Generation of methane

Variation of generated methane with volatile matter

0

5

10

15

20

25

30

35

40

0 50 100 150 200 250 300 350

Volume of generated CH4 (cc/g), 20 oC, 1 atm.

Vol

atile

mat

ter

wt.

% d

ry a

sh f

ree

basi

s

Series1

Adsorptive/storage capacity of coal vs Depth

0

5

10

15

20

25

0 500 1000

Depth, m

Ad

sorb

ed v

ol.

CH

4,

cc/g

Anthracite

LVBitumeneous

MVBitumienous

HVBitumeneous

Series5

Outlook for CBD:

• India has large natural gas deficit.• Against the present demand of about 150 MM

m3/day, the production of natural gas is about 100 MMm3/day.

• The demand is expected to increase upto 285 MM m3/day by the year 2009 – 10.

• This widening demand-supply gap of natural gas cannot be filled up with country’s limited conventional gas resources.

Outlook for CBD….

• To increase the gas supply and meeting the future need, unconventional gas resources like coalbed methane (CBD) need to be aggressively explored and developed in India.

Coalbed methane basins:

• Coal in India occur at two stratigraphic horizons viz. Gondwana and Tertiary.

• India has vast coal resources, estimated to the order of about 460 billion tons (MMMt) from outcropping to 1500 meter depth.

• About 68% of total i.e. 315 MMMt is beyond the general mining depth of 300 meters and is thus available for CBD.

• Indian coals exhibit favourable geologic setting, suitable depth, thick multiple seams, optimum rank and maturity, good gas content.

CBM activities in India:

• An information well drilled during 1986 – 87 for Under Ground Coal Gasification under National coal gasification programme had shown good gas contents in deeper lignito-bituminous coals in Western Gujarat.

• A few core holes drilled by Modi Mckenzie Methane td, in Western part of Raniganj coalfield have shown good gas content and other CBM related parameters.

• Three R & D wells were drilled during 1993 -94 in Cambay basin by Essar Oil Ltd., which have shown favourable gas content and permeability.

• Recently ONGC has drilled few exploratory wells in West Bengal and Bihar which showed good gas content in the range of 6 -515 m3/ ton

CBM Production Potential in India Table 1

Basin/Area CBM productionPotential(MM m3/day)

EnergyEquivalent(MW)

EnergyEquivalent(LNG, MM tpa)

Cambay Basin North GujaratBarmer Basin South RajasthanDamodar Basin Raniganj Jharia East Bokaro North KaranpuraRajmahal Basin Rajmahal Birbhum

30

19

123.52.56.0

4.56.0

5500

3500

2200650450

1100

8001100

7.50

4.75

3.001.000.601.50

1.201.50

Table 1 continued…..

Basin/Area CBM productionPotential

EnergyEquivalent

EnergyEquivalent

(MM m3/day) (MW) LNG, MM tpa)

Others Singarauli Sohagpur Satpura IB-river Talchir Wardha valley Godavari valley Cauvery Basin

1.04.01.55.02.51.54.02.5

180720270900450270720450

0.251.000.401.250.600.401.000.60

All India 105.5 19260 26.55

Recoverable CBM Reserves in India:

Basin/Area Recoverable reserves, MMM m3

% share

Barmer 140 18

Damodar 180 23

Rajmahal 80 10

Cambay 225 28

Others 168.5 21

Total Approx. 800 MMM m3

Economics and market outlook for CBM:

• In India CBM is expected to be priced similar to natural gas in terms of heating value.

• The viability of CBM projects would be govened differently as these are upfront capital intensive and characterized with relatively late cash inflows.

• In normal circumstances 10 CBM wells may be required to match the production of one natural gas well.

• CBM requires better fiscal and commercial terms than natural gas.

• US operators were given large Tax credits as incentives to invest in CBM projects in the initial years.

Economics and market outlook for CBM….

• CBM economics is particularly senstive to:

1. Coal permeability

2. Gas pricing

3. Project location

Economics and market outlook for CBM…..

• At net wellhead price of $ 2.5 per MMBtu with gas reserve of about 2 tcf, a CBM project will be viable in India

Economics and market outlook for CBM….

• Major market for CBM is for power generation.• Under certain conditions CBM can also be

competitive to LPG and piped coke oven gas in urban areas like Kolkatta, if free gas marketing is considered.

• CBM has a competitive edge over imported LNG or natural gas via the cross border pipelines as the transportation costs for these imports are likely to be higher.l

Economics and market outlook for CBM….

• CBM prospective areas in Gondwana coalfields are located away from natural gas pipelines or active gas markets. For CBM project in these locations adequate markets need to be developed.

• Cambay and Barmer basins in Western India are exceptions, where gas marketing and pipeline network are well developed.