study of CBM (Coalbed Methane) production process

Mohammad Asif ( BT09MIN008)

Advisor: Dr. Mallikarjun Rao

Introduction

CBM formation

Physical Structure of Coal

CBM production controls

CBM production process

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

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

COAL

CHEMICALREACTION

BACTERIALACTION

THERMOGENICMETHANE

BIOGENICMETHANE

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

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

Desorption from coal Surface

Diffusion from matrix to cleats

Movement within the cleats

Total gas-in-place in coal reservoir

Methane sorption capacity of coal

Diffusion in coal

Permeability of coal

Gas in place is measured by taking core from drilling , placing it in a canister , and measuring the gas it desorbed

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

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

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

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

Coalbed methane production passes through three phases during the life-time of the reservoir

Dewatering phase

Stable production phase

Declining phase

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

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

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

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

Visit one of CBM production sites

Study the CBM production process in field

Study the gas and water production characteristics

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

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