The movement of methane gas in the coal seam has been observed by use of tracer gases in several coal mines in HOKKAIDO.
Radioactive methane and freon gas were used as tracer gases. Results gained are as follows.
(1) Almost all quantity of the methane gas in the coal seam moves through cracks, And seams which have no crack are scarcely permeable.
(2) There are three stages of crack generation in coal seams. 1-st stage;(as shown Fig.18-I) The coal seam has not been fractured by the effect of openings and flow rate of gas is expressed by formula (1). But in this stage flow rate of gas is very small. 2-nd stage;(as shown Fig.18-II) Cracks in the coal seam are being increased by the effect of openings and most of gas flow comes through cracks. The flow rate of gas varies widely by places. 3-rd stage;(as shown Fig.18-III) Cracks in the coal seam have been increased in number and grown up in length. Flow rate of gas is calculated basing on fomula (1).
(3) To express the movement of gas in the coal seam, it is not always correct to use diffusion formula (1) only for it.Because in the coal seams where gas drainage has been extensively conducted, it is to think that coal seams are in 2-nd stage usually.
(4) In the coal seam which has few or no cracks, gases hardly move in it. In the case of multi-coal seams extraction, it is very important to drain gas before operation is commenced. Because the seam which must be extracted first has so few cracks that gas drainage from it is usually very difficult. To estimate permeability of gas of the coal seams in-situ which have no cracks and to develop gas drainage technique from them are two problems remained till now.
