Analysis of gob gas venthole production performances for strata gascontrol in longwall mining

摘要

Longwall mining of coal seams affects a large area of overburden by deforming it and creating stress-relief fractures, as well as bedding plane separations, as the mining face progresses. Stress-relief fractures and bedding plane separations are recognized as major pathways for gas migration from gas-bearing strata into sealed and active areas of the mines. In order for strata gas not to enter and inundate the ventilation system of a mine, gob gas ventholes (GGVs) can be used as a methane control measure. The aim of this paper is to analyze production performances of GGVs drilled over a longwall panel. These boreholes were drilled to control methane emissions from the Pratt group of coals due to stress-relief fracturing and bedding plane separations into a longwall mine operating in the Mary Lee/Blue Creek coal seam of the Upper Pottsville Formation in the Black Warrior Basin, Alabama. During the course of the study, Pratt coal's reservoir properties were integrated with production data of the GGVs. These data were analyzed by using material balance techniques to estimate radius of influence of GGVs, gas-in-place and coal pressures, as well as their variations during mining.The results show that the GGVs drilled to extract gas from the stress-relief zone of the Pratt coal interval is highly effective in removinggas from the Upper Pottsville Formation. The radii of influence of the GGVs werein the order of 330–380 m, exceeding the widths of the panels, due tobedding plane separations and stress relieved by fracturing. Material balanceanalyses indicated that the initial pressure of the Pratt coals, which wasaround 648 KPa when longwall mining started, decreased to approximately 150 KPaas the result of strata fracturing and production of released gas. Approximately70% of the initial gas-in-place within the area of influence of the GGVswas captured during a period of one year.