Coalbed by various method. It can be recovered from

Coalbed methane is an unconventional
natural gas and a very good source of energy. It is extracted from the coal
beds or coal seams. The formation take place during the process of coalification
and the transformation of plant into the coal. Eventually it is one of the most
important energy resource with reserves and production. CBM stable source of
energy because we can get it by various method. It can be recovered from
underground coal before, during and after mining operations. For extraction and
development of CBM horizontal and vertical 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.

Why
CBM?

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 CBM can fulfil the national goals in India:

Provide a clean burning fuel. Improve
safety of coal mining. Decrease methane vented to the atmosphere that might
cause global warming.  Most importantly
provides a means to use an abundant coal resource that is too deep to mine. It
minimizes the environmental impacts due to coal mining as methane is not lost
to the environment anymore. 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 prognosticated CBM resources are around
4.6TCM. 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.

Progressive cavity pump:

The Progressive Cavity
Pump (PCP) was invented by Rene Moineau in 1932. PCP’s became widely used as
surface pumps, especially for the pumping of viscous mixtures. The PCP is able
to produce highly viscous sand layden fluids, making it the primary artificial
lift system for conventional heavy oil production around the world. KUDU along
with PCM, have led the development of high capacity, high head lift pumps as
well as elastomers that are resistant to medium and light crudes. PCP’s are
also utilized for dewatering coal bed methane and conventional gas wells.

Principles of operation:

 

PCP’s
are positive displacement pumps, which consists of two key parts, the stator
and the rotor. The stator remains stationary and the rotor rotates. The PCP is
described as a gear pump with a single helical rotor, which rotates inside a
double internal helical elastomer lined stator. The stator is run into the well
on the bottom of the production tubing, while the rotor is connected to the end
of the rod string. When the rotor is inserted into the stator, it creates a
continuous seal line that extends from the pump suction to discharge. This
results in a series of identical but separate cavities that progress from the
pump suction to the discharge at the end of the pump, as the rotor turns
eccentrically in the stator. One cavity opens as the other closes, creating a
non-pulsating pumping action.

PCP
delivers a constant flow that is proportional to the size of the cavity and
rotation speed of the rotor. Rotation of the rod string at surface is powered
by a hydraulic or direct drive head.