study of cbm (coalbed methane) production process
TRANSCRIPT
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Mohammad Asif ( BT09MIN008)
Advisor: Dr. Mallikarjun Rao
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Introduction
CBM formation
Physical Structure of Coal
CBM production controls
CBM production process
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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
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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
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COAL
CHEMICALREACTION
BACTERIALACTION
THERMOGENICMETHANE
BIOGENICMETHANE
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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
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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
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Desorption from coal Surface
Diffusion from matrix to cleats
Movement within the cleats
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Total gas-in-place in coal reservoir
Methane sorption capacity of coal
Diffusion in coal
Permeability of coal
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Gas in place is measured by taking core from drilling , placing it in a canister , and measuring the gas it desorbed
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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
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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
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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
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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
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Coalbed methane production passes through three phases during the life-time of the reservoir
Dewatering phase
Stable production phase
Declining phase
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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
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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
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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
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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
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Visit one of CBM production sites
Study the CBM production process in field
Study the gas and water production characteristics
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CBM in India http://www.fekete.com/software/cbm/media/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
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Estimating methane content of bituminous coalbeds from adsorption data, http://www.cdc.gov/niosh/mining/UserFiles/works/pdfs/ri8245.pdf
Rudy E. Rogers, Coalbed methane : Principles and practices, Prentice Hall, 1994
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