An analysis of reservoir conditions and responses in longwall panel overburden during mining and its effect on gob gas well performance

NIOSH conducted a cooperative research study to provide direct measurements of changing reservoir conditions in longwall panel overburden. The field measurements documented changes in permeabilities, methane concentrations, fluid pressures, and the effects of adjacent gob gas ventholes (GGVs) on NIOSH boreholes drilled in the study panel. Three different stratigraphic horizons were monitored by the NIOSH boreholes. Results indicated that the gob gas venthole fracture network formed 24 to 46 m (80 to 150 ft) ahead of the mining face. Overburden permeabilities within the same overburden test zones were ~ 1 md prior to undermining, increasing to hundreds or thousands of md during undermining. Permeabilities measured seven months after undermining showed additional increases. The relationship between changing reservoir conditions, longwall face position, and surface movement is discussed. Recommendations are made to optimize GGV performance by evaluating changes in subsidence produced by mining, resulting in rock stresses that substantially influence induced fracture permeability. Mechanisms to account for the observed changes in reservoir conditions are reported.

► The removal of coal bed gas is critical for the safe mining of gassy coal beds.
► Changing reservoir conditions in longwall panel overburden are summarized.
► Induced fractures form the dominant gas transport network in the overburden following mining.
► Surface subsidence profiles can predict the distribution of fracture permeability regimes.
► Recommendations are made to enhance coal bed gas removal through borehole design improvements.