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Coalbed Methane

Coalbed Methane; its formation, retention and production.

Micheal Moroskat

on 10 May 2011

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Transcript of Coalbed Methane

Coalbed Methane GTN456 What is Coalbed Methane? natural gas CH 4 primarily methane with 0-20% other hydrocarbons CBM aka Coal Seam Gas (CSG); e.g. Australia methane is adsorbed into solid matrix of the core. adhesion of molecules of gas to a surface Formation of CBM methane (CH4) carbon dioxide (CO2) Nitrogen (N) water (H2O) generated in 2 ways 1. Biogenic 2. Thermogenic early stages of coalification low T (<50 C) during breakdown of organic matter biological activity reducing conditions removal of O2 and SO4 Catagenesis organic matter altered by increasing temperature cracking process that turns organic kerogens into hydrocarbons peak generation at 150 C higher volume than biogenic Retention of CBM 3 modes 1. dissolved in groundwater within coal 2. free gas in the porespace or fractures 3. adsorbed as molecules onto the organic surface of the coal primary method excess gas not retained is "free gas" and is free to dissolve into groundwater passing through the system, or move to other formations if methane migrates out of the coal seam into overlying sediments, it becomes conventional gas fracture porosity is mainly in the cleats may also be in the micropore structure of the coal adsorptive capacity increases with rank also VERY important for PERMEABILITY gas is held in place mainly by RESERVOIR PRESSURE CBM Production reducing reservoir pressure allows the methane gas to be released from the coal dewatering the coal is a primary way to do this methane desorbs from coal coal flows to the well through cleats generally, only get 50% recovery using dewatering Reservoir Enhancement injection of inert gas, e.g. nitrogen lowers partial pressure of methane and allows for more methane to be desorbed can also use CO2 hydraulic fracturing can also be used to aid in recovery injects sand with the gas, wedges into cracks and increases permeability known for a long time from coal mining a danger in mines because of the risk if explosion gas recovery depends on several factors chemical compositon of the coal
geological history of the coal
previous depressuring of the coal methane production increases with increased depth increased depth means increased pressure increased pressure = decreased permeability closes cleats therefore, generally get: low methane concentration and high permeability high methane concentration and low permeability Alberta relatively low gas content
favourable cleating
south-central Plains potentially favourable cleating
high gas contents, locally
north-central to central Plains moderate gas content
moderate cleating
Pembina area Foothills CBM e.g.Kootenay coal tend to be higher methane content (higher rank) but poor permeability and continuity because of structural disruption Environmental Issues methane in aquifers saline water e.g. N. Alberta farmers with methane in thier well water may be naturally occurring or from drilling activity from dewatering formations must properly dispose of extracted water re-injected into similar environment generally, salinity increases with depth
but amount of water decreases if non-saline may be used, e.g. agriculture
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