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Coal Micro-Fabric as a Trigger for Coal Bursts

Underground » Coal Burst

Published: June 25Project Number: C28012

Get ReportAuthor: Rhys Pitchers, Yvette Heritage, Winton Gale | SCT Operations

This project was undertaken in two stages:

  • Stage 1 consisted of optical assessment of porosity, with a focus on the assessment microfractures using newly available and cost accessible CT scanning to render a full 3D volume of the samples for a better evaluation of microfracture characteristics.
  • Stage 2 aimed to define the zones around dykes and faults in which the coal is structurally modified and capable of generating sufficient gas related energy to initiate a coal burst.

The project was successful in defining gas related coal burst risk zones adjacent to the assessed geological structures at the study mine and helped define the characteristics associated with the burst mechanisms. The computed tomography (CT) scanning outcomes have also provided supporting data for the theory of microfracture propagation to increase the microfracture frequency and gas desorption rates by imaging the 3D microfracture networks.

Research conducted within earlier project C26060 Mechanics of gas related coal bursts indicated that areas of structurally generated microfractures have the potential to allow significantly greater gas energy to be liberated from the coal than in nonstructured coal. Under the appropriate stress and gas pressure such microfractures can propagate to form closely spaced macro fractures commonly associated with bursts.

The case study mine used in this project has experienced coal bursts and outbursts in relation to various structures consisting of dykes and a thrust fault.

The CT scanning showed that microfracture spacing of 0.25mm is possible and therefore has the potential to desorb 10-25% of the total gas in 1 second. This is substantial enough to provide the energy required to cause a coal burst. The research also highlighted that variability in microfracture networks was a key feature adjacent to structure, where low microfracture frequency or low permeability zones were observed that could act to restrict gas drainage into roadways.

Based on the CT scanning of coal microstructure and the optical porosity assessment of the original study, the coal burst risk zones about structure were observed to be 3m to 5m for dykes and 13m to 15m for the thrust fault. For dykes, this risk zone consists of a high porosity zone with a low permeability barrier from calcite infill or low levels of mining induced fractures from the stress shadow next to the dyke. For the thrust fault, this risk zone consists of variable microfracture networks with zones of typical microfracture networks and reduced fracture frequency zones indicative of low permeability.

The presence of a consistent and connected microfracture network in a typical coal scenario (not structurally altered) suggests that a pocket of gas encountered on development drivage, is likely to flow into the roadway, without causing a buildup of pressure in the rib or face.

The potential for a gas related coal burst occurs when:

  • The gas pressure can build quickly from either propagating microfractures creating a high fracture frequency and corresponding high desorption rate or very high porosity creating high volume of free gas and large surface area for rapid desorption, or,
  • There is a low permeability barrier restricting pressure release or gas flow into the roadway e.g. reduced microfracture frequency.

The presence of low permeability zones observed adjacent to structure highlights that gas drainage may not be as efficient as experienced in typical coal.

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