Influence mechanism of gas extraction in composite coal seams based on multi-scale mass transfer characteristics of tectonic coal

Abstract

In composite coal seams, the gas transfer relationship of structural coal has a significant impact on gas extraction. Regarding the influence mechanism of multi-scale mass transfer (MSMT) of tectonic coal in composite coal seams on gas extraction, the paper has constructed a corresponding multi-physics field coupling model, which systematically investigates gas migration patterns in composite coal seams and the dominant control mechanisms during coal mining. Results indicate that gas extraction efficiency in tectonic coal exhibits significant sensitivity to key parameters, including Poisson’s ratio, matrix elastic modulus, and extraction negative pressure. This reveals the synergistic extraction mechanism in composite coalbeds: highly permeable primary coal dominates pressure gradient formation during initial extraction, effectively driving gas desorption and cross-scale migration in adjacent tectonic coal. However, the low permeability of tectonic coal causes slow pressure decay, becoming a critical bottleneck constraining overall extraction efficiency. Based on this mechanism, a phased differentiated technical approach was proposed that involves hydraulic fracturing to enhance permeability in primary coal combined with CO₂ displacement and stepwise negative pressure extraction for tectonic coal. These findings provide crucial guidance for optimizing extraction design and achieving efficient, safe mining in composite coal seams.

Keywords

Tectonic coal / Composite coal seam / Multi-scale mass transfer / Main control mechanism / Gas extraction

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