Coal Micro-Fabric as a Trigger for Coal Bursts

Research conducted project C26060 Mechanics of gas related coal bursts indicated that areas of structurally generated micro fractures 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 micro fractures can propagate to form closely spaced macro fractures commonly associated with bursts.

The validity of this mechanism to be a trigger to change “normal coal” into burst prone coal is the topic of investigation in this project which investigated the presence and nature of coal micro fabric characteristics, namely porosity and micro fractures, through microscopic analysis of coal samples in two known burst prone mines in New South Wales.

The underlying concept that was being assessed was:

• Is there an increase in porosity close to burst structures which relate to either pore volume or fracture density?

• Is there an increase in fracture frequency close to structures?

The project has provided an initial understanding of changes in coal porosity with proximity to geological structures to help define burst risk zones.

It became clear that the available optical approach being used was suitable for volumetric data such as open space volume (termed porosity) but was not appropriate for fracture frequency. Therefore, porosity was the primary factor assessed.

A summary of the changes in porosity within proximity to types of geological structure (dykes and faults) is described in details in the report.

Early indications suggest a very high risk burst zone within 1-2m of dykes and faults, and an increased burst risk zone within 13m of a structure. Initial assessment of the boggy zone adjacent to the thrust fault in Mine B suggests a larger burst risk zone about the thrust fault. This is to be the subject of further assessment.

Separately, an increase in porosity was observed with an increase in fracture count in all samples. Fracture frequency and micro fracture set characteristics will be further assessed in the ongoing research.

The variability in coal porosity is an important find in relation to gas energy bursts. It can be assumed that low porosity coal is also likely to exhibit low permeability. The presence of a low permeability coal barrier between a high gas source and an excavation can limit the gas from flowing into the face or roadway, allowing the buildup of free gas pressure. Conversely, high permeability coal can allow for faster drainage of gas into the excavation, limiting the build-up of free gas pressure. Free gas pressure creates stored energy.

The research methodology was highly successful in determining porosity of coal samples. However, it was found that the methodology had limitations in assessing micro fracture frequency and spacing.