Abstract

Methane emissions into underground coal mines in certain areas of the United States represent a serious hazard and contribute to lowering mining productivity. In order to reduce methane emissions into some of the deep south west Virginia Coal Mines an advance methane drainage program was developed. This paper gives a state of the art account of drilling long in-seam and slant holes into virgin areas of the Pocahontas #3 and #4 coalbeds. Directional drilling techniques are discussed. Methods to evaluate the drainage effectiveness of this program on the emissions in an operating mine are also covered. It is concluded that an effective methane drainage program can reduce methane emissions significantly thereby improving safety conditions in the mine. It also has the potential for increasing mining productivity. The recovered gas could also lead to a better utilization of our available energy resources.

Introduction

Methane liberation from the coal seam and surrounding strata in Island Creek Coal Company's deep Virginia Mines has long been a serious problem. The gas represents a safety hazard because it can be ignited in concentrations of 5% to 15% methane in air through sparks generated by the mining equipment.

Under the current safety regulations the methane has to be diluted to below 1% in all working areas by a sufficient amount of fresh ventilation air. In spite of the large ventilation volumes supplied to the working faces, however, the methane content locally can increase to levels above 1% on occasion. In this event all mining operations have to be stopped until the methane content in the air once again falls to safe levels. Also, through experience the mining machine operators have learned to lower the cutting rate sufficiently as to avoid such high methane concentration levels. The presence of the gas, therefore, also forces a lower than technically possible mining productivity. Gas is emitted into the mine workings productivity. Gas is emitted into the mine workings at the mining face during the cutting process. Large amounts are also liberated from the roof rock, the floor rock, the mined out areas and all unmined portions of the coal seam, like the barrier pillars portions of the coal seam, like the barrier pillars for example. Current control techniques to cope with this gas consist of:

  1. Dilution of gas to below 1% methanewith ventilation air.

  2. Drainage of gas from solid coal byshort (60 m) bleeder holes inlong-wall panels.

  3. Water infusion through short holes to block flow of methane.

  4. Vertical ventilation holes drilled from the surface over longwall panels to liberate gas from mined out areas.

  5. Vertical ventilation holes in advance of mining. These holes, however, represent an experiment on a limited scale only.

In addition to the undesirable effects on safety and mining productivity, the current technique of diluting the methane gas represents a waste of a valuable energy resource. It is our opinion that a suitable methane control technique ahead of mining should be able to recover and utilize much of the gas contained in the coal seam and the rock strata surrounding it. Such a method is currently being developed and was described in some detail elsewhere.

DRAINAGE CONCEPT AND OBJECTIVE

The basic technique consists of drilling long horizontal holes into the coal seam one to several years ahead of mining and allowing the gas to drain from the coal through these holes. A variation to this technique is the drilling of underground slant holes into an overlying coalbed from the working mine level. The gas is collected through an approved piping system and moved to the surface through a piping system and moved to the surface through a vertical borehole.

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