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

Abstract

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 removing gas from the Upper Pottsville Formation. The radii of influence of the GGVs were in the order of 330-380 m, exceeding the widths of the panels, due to bedding plane separations and stress relieved by fracturing. Material balance analyses indicated that the initial pressure of the Pratt coals, which was around 648 KPa when longwall mining started, decreased to approximately 150 KPa as the result of strata fracturing and production of released gas. Approximately 70% of the initial gas-in-place within the area of influence of the GGVs was captured during a period of one year.

Keywords: Coal gas; Gob gas ventholes; Longwall mining; Material balance; Methane control; Pratt coal; Upper Pottsville Formation.

Fig. 1

A schematic representation of deformed overburden as a response to longwall mining, and location of GGV to control strata gas.

Fig. 2

Location of study area and a plan view with vertical degasification wellbore locations, mine outline, and major geologic structures with vertical displacements. The figure on the right is an expanded section of the panels that shows the horizontal, in-seam degasification boreholes.

Fig. 3

Stratigraphic section of the Upper Pottsville Formation in the Black Warrior basin.

Fig. 4

Monthly averaged daily advance rate of the longwall face while mining the E-1 panel.

Fig. 5

Detailed map of the mine, the E-1 panel layout, and locations of the GGVs.

Fig. 6

Well design specifications of the GGVs used at the mine site and the completion intervals. The numbers in parentheses are diameters.

Fig. 7

Methane production profiles of GGVs during and after mining of the E-1 panel.

Fig. 8

Rate and cumulative methane production from all GGVs (a) and the correlation of mining advance rate to methane production rate (B).

Fig. 9

Change in gas-in-place with time within the drainage area of each GGV (A), and cumulative gas productions from these wells during the same duration (B).

Fig. 10

Pressure change in the deformed coal strata due to gas production from GGVs.

Fig. 11

Incremental gas production and formation pressure drop relation (A) and the history of change in pressure drop (B) during mining of the E-1 panel. The error bars in 10-A show 10% error margin of the value of the data points, whereas the 100 KPa level in B show the limit up to which the relation is linear for all GGVs.