Repeated pressure build-up tests to characterise mining-induced permeability changes around in-seam gas drainage boreholes

Mining-induced permeability enhancement around in-seam boreholes offers a critical opportunity to improve gas drainage efficiency in low-permeability coal seams, yet its quantitative characterisation under longwall top coal caving (LTCC) remains largely unexplored. This study presents a unique field-validated, quantitative characterisation of this permeability evolution in an active LTCC setting, through a programme of sixteen repeat pressure build-up tests conducted in a thick lignite deposit. Eight tests with sufficient pressure recovery were interpreted using Horner analysis and log-log pressure-derivative diagnostics. The initial coal seam permeability confirms dual-porosity behaviour characteristic of natural coal. As the longwall face advanced, systematic contraction of the wellbore storage regime provided progressive evidence of permeability enhancement, increasing by approximately one order of magnitude at intermediate face distances and reaching two orders of magnitude as the face approached the borehole. Independent boundary inversion corroborated this trend, yielding enhancements approaching 200-fold at close face proximity. Late-time diagnostics revealed an evolving drainage geometry, transitioning from a channelled no-flow boundary configuration to a fully enclosed rectangular drainage domain as the expanding drainage radius intercepted additional boundaries. These findings demonstrate that LTCC-induced stress redistribution can dramatically and progressively enhance coal seam permeability, with direct implications for optimising borehole placement and drainage scheduling in gassy LTCC panels.