Coal are depressurized by pumping of water for regular

Coal bed methane is an
unconventional natural gas and a very good source of energy, which are
extracted from the coal beds or coal seams. It is formed during the process of coalification
and the transformation of plant into the coal. CBM is considered as important
energy resource with reserves and production. Various method of recovery makes
CBM stable source of energy. It can be recovered from underground coal before,
during and after mining operations. For extraction and development of CBM or
CBM resources vertical and horizontal wells are used, which require drilling
into the coal seam or coal beds and the water present in the seam is removed to
reduce hydrostatics pressure and released adsorbed gas out of the coal.

CBM production behaviour is
complex and difficult to predict in the early stages of recovery. Coal bed
methane (CBM) recovery is associated with production of large quantity of groundwater.
The coal seams are depressurized by pumping of water for regular and consistent
gas production. Usually, CBM operators need to pump >10 m3 of water per day
from one well, which depends on the aquifer characteristics, drainage and recharge
pattern.

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Why
CBM?

 CBM can fulfil the national goals in India:

(1) Provide a clean burning
fuel.

 (2) Increase substantially the natural gas
reserve base.

(3) Improve safety of coal
mining.

 (4) Decrease methane vented to the atmosphere
that might cause global warming.

(5) Most importantly provides
a means to use an abundant coal resource that is too deep to mine.

 (6) It minimizes the environmental impacts due
to coal mining as methane is not lost to the environment anymore.

 (7) It is used for other purposes like steel
manufacturing, fuel for industries, fertilizer manufacture, ceramics glass
textiles etc.

 

Indian
scenario of CBM

India is a coal rich nation
and has significant volumes of coal bed methane reserves. India has the 3rd
largest proven coal reserves and is the 4th producer in the world. In India,
CBM is being produced from the Gondwana coalfields of Raniganj, Jharia and
Sohagpur. Total coal resources of India are approximately 300B. The mining
industry is over two centuries old. The prognosticated CBM resources are around
4.6TCM. Moreover, the estimated CBM resources hold significant prospects for
commercial recovery of natural gas. Deeper reserves from which recovery of CMB
seems less feasible, technologically as well as economically, may be the prime
targets for enhanced recovery techniques, for example the coal seams which are
found at greater depths and are below the limit of mining are considered
suitable for these techniques. India stepped into CBM exploration and
production decades after the US success. Methane is native to coal and forms
during the coalification process. It has traditionally been recognized as an
evil companion of coal bringing in major safety problems in underground mining
until recently when it changed from a mining hazard to an effective energy
resource. In recent times CBM is receiving increased attention as a valuable
fuel source.

Coal is the source as well as
reservoir to significant quantities of methane, a potentially economic
resource. The huge reserves of coal in India may be categorized in two types,
based on their period of formation namely Gondwana coal and Tertiary coal.
Nearly 99% of the total coal reserves in India belong to Gondwana basins and
are characteristically different from the rest of the tertiary coal. The major
focus of research and development in CBM sector has been based mainly on
Gondwana coals.

PROBLEMS
ASSOCIATED IN CBM WELLS

Drilling operation:

Underbalanced
Drilling (UBD) is a technique in which oil, gas or geothermal wells are drilled
using pressures lower than the reservoir pressure. The result is an increase in
rate of penetration (ROP), reduced formation damage and reduced drilling costs.
Air drilling provides an efficient system in terms of operations costs and
environmental safety benefits.

Two major problems occur at the time air drilling i.e. caving of the
well and the waterinflux comes into the wellbore from the formation because

Ø  Formation pressure
control is minimal and, therefore, drilling is limited to geological regions
where reservoir pore pressures are low.

 

Ø  There is little or no
fluid pressure to support the borehole wall and prevent sloughing.

 

Ø  There is limited
ability to cope with significant volumes of water entering the annulus from
water producing formations.

 

In most of the wells (about 80%) air drilling method is not successful
and we switch to the mud drilling.

In few of the wells the shale section could not be air drilled and in
such cased drilling was carried out by KCl (shale inhibitor), PHPA (polymer)
mixed in the mud and the top section completed. The coal seams have been
drilled with air/mist to avoid any formation damage.