Three-zone characterisation of coupled strata and gas behaviour in multi-seam mining

• We characterise coupled strata and gas behaviours in multi-seam mining.
• Overlying coal-measure strata are characterised into three zones.
• Method of discriminating the zones is developed with key-stratum hypothesis.
• The gas three-zone characterisation helps optimise coal mine gas drainage.

We propose here a three-zone conceptual model in overlying strata of a longwall panel that accounts for the coupled behaviour of strata deformation and gas flow. The model comprises a fractured gas-interflow zone, a de-stressed gas-desorption zone, and a confined gas-adsorption zone. The fractured gas-interflow zone represents the area where mining-induced cross-strata fractures and bedding separations are well developed with high permeability in both the vertical and horizontal directions. Coal seam gas can easily be released from this lower zone to flow down into the mine workings. The de-stressed gas-desorption zone, which lies above the fractured gas-interflow zone, is another significant gas-producing zone in which strata are highly de-stressed. However, mining-induced fractures in this zone are mainly created in the form of bedding separations, which only increase horizontal permeability, and thus the gas cannot easily flow vertically down to the mine workings. In the upper confined gas-adsorption zone, strata depressurisation is limited; the major proportion of coal seam gas in this zone remains adsorbed and cannot be effectively captured. While both lower zones are the targets of gas drainage, the fractured gas-interflow zone is the main source of ventilation gas emission and the prime area of gas control. We have developed an approach to determine the height of these three zones based on the hypothesis of key stratum in strata movement, and verified the approach using gas drainage experience at a Chinese coal mine. The applications of the three-zone concept in selecting appropriate gas drainage methods for varied mining conditions, assessment of methane recovery efficiency, and gas drainage optimisation and maximisation in a mining district of China are also discussed.