The accuracy of the acknowledgement of these variables determines the assessment of the possible gas production.

2. Active coal mines

Basis for the determination of the exhaustible specific gas volumes in active mines are the gas emissions from the roof and the floor of the workings during the exploitation. This specific gas release (gas make) can be calculated by means of the methods of the prediction of gas emissions. In these calculations, the parameters gas content of the seams, coal thickness and distance from the worked seam mostly are taken into account.

The prediction of gas emissions delivers not only the total gas release, but also the proportion which can be sucked in the roof and - normally secondarily - in the floor of the working by the drainage system.

In Germany the best results are achieved by means of advanced workings. The boreholes are drilled after the passage of the face and in that way they remain stable and productive over a long period of time. If, in contrast, the roadways don't remain open after the passage of the face in the case of a retreating longwall, there is too little time for an effective gas drainage. The boreholes must be arranged before the passage of the face and have only a short lifetime.

The quality of the sucked gas concerning the methane concentration is determined significantly by the sealing of the drainage boreholes. Here the latest developments of the German mining industry with framed rubber as sealing material have proved their worth. The average methane concentrations in the sucked gas stream range between 40 and 45 %.

In some cases in active mines, old fields of exploitation or old roadways are also connected to the gas drainage by sucking the package pipes if the methane concentration is high. Thus, the air stream in the mine is relieved from gas.

3. Abandoned coal mines

3.1 Determination of the gas potential

For the evaluation of the gas potential of abandoned mines, the reduction of the original gas content in the deposit by the exploitation must be calculated.

The interpretations start with the superposition of the exploited areas in the different seams and the digital survey of the numerous intersecting areas (fig. 1). This work is done by means of EDP. Rock sequence, seam thicknesses, dip angle, gas contents and others are attributed to the numerous intersecting areas in terms of data files.

Fig. 1: Superposition of the exploited areas (sector)

The deposit is separated in vertical columns (fig. 2) with the intersecting areas as base and with a constant dip angle of the sequence. In that way a detailed segmentation of the deposit into columns is achieved. In the case of the Westfalen mine, which was examined by DMT, there were more than 3.000 single columns.

Fig. 2: Seam sequence and residual gas contents (example)

The buttom side of each column lies about 100 m in the floor of the lowermost worked seam, because a long-term influence of the exploitation must be taken into account. By subtraction of the exploited seams the residual coal volume is calculated in each column and in the whole deposit.

For the determination of the residual gas volume, the residual gas content in the seams which are still present but influenced by the exploitation must be calculated. This evaluation is done for each of the above described columns by a special method (calculation of gas emissions during exploitation) again by means of EDP. Finally the multiplication of the residual coal and gas volume for each seam in each column and the summing up delivers the residual gas volume in an exploited area in the moment of the last workings. This widely automated procedure was already applied in a number of investigations for German and foreign deposits.

From the residual gas contents (fig. 3), also gas pressures can be calculated by a mathematic or experimental formula (e.g. Langmuir), which are necessary for the modelling of gas streams through the mining-effected rock sequence. The gas pressures are the moving force, which determine the gas emissions of a mine. They are responsible for the gas stream in the abandoned roadways and for the migration of gas through the rock sequence to the surface.

Fig. 3: Medium desorbable residual gas contents in the different constellations of workings (sector)

In order to calculate the gas emissions from abandoned mines for each above described column, a function of the dying out of the gas emissions during the exploitation is determinded. From all columns together a weighted average can be derived. Subsequently the determined function can be extrapolated beyond the end of exploitation. These annual emission rates can be calculated for the time after the closure of the mine. The gas emissions occur into the roadway system of the abandoned mine and from there via the filled shafts and/or the degasification lines within to the surface.

Gas emissions through the overburden rocks to the surface outside the shafts are possible and detected at some places. But the emissions rates are by far less than those via the shafts. Currently, a method of numeric modelling is developed which includes the gas pressure in the coal seams and the permeability of the coal, the intercalated rocks and the overburden strata into the calculation. Thus a more precise evaluation can be expected.

4 Access to the deposit

4.1 Access via filled shafts

Since the end of the 1970ies abandoned shafts in the German coal mines are filled with material of long-term stability. In many cases, especially when the shaft is filled only in part, a degasification pipeline is installed. This pipeline ends either at the bottom of the filled part of the shaft or is connected with the package-pipes at the levels of the abandoned mine. Via such a pipeline the whole roadway system can be put under a depression without additional expenditure by a pump a the surface (fig. 4).

Fig. 4: Mobile power plant at shaft Westfalen 6 (in the foreground connection with the degasification pipeline in the filling of the shaft)

4.2 Access via boreholes

In mines, which were closed more than thirty years ago, in most cases no connection from the surface to the abandoned roadway exists. Here the access to the deposit must be established via boreholes. Normally an old roadway or a crossing of roadways in an upper level of the abandoned mine is the target of drilling. This offers the following advantages:

a) shorter length of the borehole,

b) better gas quality (at least at the beginning of suction),

c) maximum distance from the water level and

d) high probability of an open residual roadway system.

On the other hand it must be looked after a sufficient sealing against the surface in order to minimise leakages to the atmosphere. The localisation of the starting point of the boreholes therefore results from an intensive study of the plans and cross sections of the mine.

In most cases the boreholes are directional drillings in order to hit the target with high probability. The borehole is drilled telescopic, whereas the smallest diameter is dimensioned for the expected gas stream.

5. Evaluation of production times under consideration of the rising water level If just the desorbing gas stream is sucked, which is released from the rock mass without a depression, the pressure in

the abandoned mine changes only slightly. In that case - presumed a constant water level - the residual gas volume in the coal seams would alone be the limiting factor for the gas production. Thus, production times of many decades can be calculated - at least theoretically - for numerous gas fields.

In practice, however, a gas stream is sucked, which is bigger than the "natural" desorption. Thereby, the depression, which is applied to the abandoned mine is increasing gradually (fig. 5). The effect of a decreasing absolute pressure by oversuction of the deposit is still intensified by an increasing water level, because flooded seams - as already mentioned - don't take part in gas release significantly any longer.

Fig. 5: Prognosis of the development of the absolute pressure in the abandoned roadway system.

Thus in practice, not only the existing residual gas volume forms the limiting factor for the production, but also a limit in depression, which is technically realisable. When this limit in depression is reached, the sucked gas volume stream decreases gradually.

The increase of depression over the time takes place approximately linear, if the cavity volume in the mine remains constant. But if the cavity volume and therewith the gas desorption decrease because of an increasing water level, the increase of depression takes place more and more faster. In order to summarise one can establish, that the residual gas volume, the desorption stream into the roadway system, the cavity volume and the development of the water level are the crucial factors for an adapted operation of gas production.