This document discusses coalbed methane (CBM) formation and production. CBM is formed during coalification from either chemical reactions or bacterial action. It is stored in the micropores and macropores (cleats) of coal. CBM production involves three phases - an initial dewatering phase to remove water from the cleats, a stable production phase where gas production increases as water production decreases, and a declining phase where production declines over time. Key factors that influence CBM production are total gas content, coal's sorption capacity, permeability, and diffusion properties.
2. Introduction
CBM formation
Physical Structure of Coal
CBM production controls
CBM production process
3. Coalbed methane is an unconventional source of
energy that is formed during the process of
coalification, remains in coal in the form of
adsorbed gas
Along with the methane, small amounts of other
hydrocarbon and non-hydrocarbon gases formed
during the process
Coals have an immense amount of surface area
and can hold enormous quantities of methane
Coal can store on the order of six to seven times
more gas than the equivalent volume of rock in a
conventional gas reservoir
4. CBM in coal is a result of chemical and
physical processes
CBM is generated either through chemical
reactions or bacterial action
Chemical action occurs over time as heat and
pressure are applied to coal in a sedimentary
basin, referred to as thermogenic methane
Bacteria obtain nutrition from coal, and
produce methane as a by-product, is referred
to as biogenic methane
5. C
O
A
L
CHEMICAL
REACTION
BACTERIAL
ACTION
THERMOGENIC
METHANE
BIOGENIC
METHANE
6. Coal has a dual porosity structure
◦ Micropores < 2nm
◦ Macropores > 50nm
Macropores are natural fracture that exists
perpendicular bedding plane also called
cleats
◦ Face cleats
◦ Butt cleats
Micropores are exist between cleats which are
also called coal matrix
7.
8.
9. Movement of methane in coal occurs at three
phase:
◦ phase 1: Desorption of methane form internal
surface of coal
◦ phase 2 : Movement of desorbed methane from
coal matrix to cleats through diffusion following
Fick’s 2nd law of diffusion
◦ phase 3 : Transportation of methane from cleats to
production well following Darcy’s law of fluid
through porous media
10. Desorption from
coal Surface
Diffusion from
matrix to cleats
Movement within
the cleats
11. Total gas-in-place in coal reservoir
Methane sorption capacity of coal
Diffusion in coal
Permeability of coal
12. Gas in place is
measured by taking
core from drilling ,
placing it in a canister
, and measuring the
gas it desorbed
13. Adsorption capacity of coal is defined as the
volume of gas adsorbed per unit mass of coal
usually expressed in SCF (standard cubic feet,
the volume at standard pressure and
temperature conditions) gas/ton of coaI
Important factors that affect methane
sorption capacity of coal are
◦ Pressure
◦ Temperature
◦ Rank
◦ Ash and moisture content
14. The volume of gas adsorbed increases with
increasing pressure
The volume of adsorbed methane decreases
with increasing in temperature
Methane sorption capacity of coal increases
with increase in coal rank from peat to
Anthracite
As some of the pores in coal pre occupied
water (moisture), methane sorption capacity
of coal decreases with increase in moisture
and ash content
15.
16.
17.
18.
19.
20. Diffusion in coal is governed by Fick’s 2nd Law
Sorption time is time taken by drill cuttings
to desorb 63% of total gas in place
It is very important factor in determining the
gas production rate in high permeability
reservoir
The relationship between sorption time and
diffusion coefficient is expressed as
21. It is the most important factor in determining the
methane production rate from a coal seam
It is governed by Darcy’s law
It is depend on the cleat spacing and effective
reservoir pressure
Effective pressure defined as the difference
between confining pressure and pore pressure
Increase/decrease in the permeability with gas
depletion depends on shrinkage/swelling
characteristics of coal
22. Coalbed methane production passes through
three phases during the life-time of the
reservoir
Dewatering phase
Stable production phase
Declining phase
23.
24. During this phase CBM wells experiment a
constant water production with a very low or
negligible gas production
Initially, most CBM wells are naturally water
saturated because water liberation occurs
during the coalification process
The water is occupying the principal cleat
network and there is the need of removing
the water from the major fractures system in
order to produce gas
25. The number of days of this dewatering
process and the amount of produced water
can vary widely
They are very difficult to estimate and their
influence in the economics is very hard to
predict
The major physical properties that affect the
efficiency of the dewatering process are:
◦ Permeability,
◦ Adsorbed gas content,
◦ Relative permeability
26. Phase II is described by a dramatically
decrease in the water production and
increase of the gas production rate
The water relative permeability decrease and
the gas relative permeability increase
The gas production has stabilized and starts
to experiment a typical decline trend
27. During phase III, the well is considered to be
dewatered, so the water production is in the
low level or negligible.
The water and gas relative permeability do
not change extensively
The steady state exists for the rest of
producing life
The limit between phase II and III is
determined by the peak gas rate is reached
28.
29. Visit one of CBM production sites
Study the CBM production process in field
Study the gas and water production
characteristics
30. CBM in India
http://www.fekete.com/software/cbm/med
ia/webhelp/Index.htm#c-te-production.
htm
Coal Bed methane(CBM),
http://www.nuenergygas.com/about-cbm/
Coal Bed Methane(black coal , green
future….),
http://www.sgtk.ch/rkuendig/dokumente/
CBM.pdf
31. Estimating methane content of bituminous
coalbeds from adsorption data,
http://www.cdc.gov/niosh/mining/UserFile
s/works/pdfs/ri8245.pdf
Rudy E. Rogers, Coalbed methane : Principles
and practices, Prentice Hall, 1994