Change in the local thermal field due to the removal of the coal layer, Velenje depression, Slovenia

In this paper we attempt to estimate, using the results of simulations, changes in the local thermal field after removal of the coal layer and thus, to establish the influence that mining has had in the area of the Velenje depression, Slovenia. Intensive mining and coal exploitation has been ongoing in the Velenje depression for many years. Subterranean activities affect the thermal field in this area. The exploitation of the useful material substance represents a disturbance in the low temperature system of the area. In this manner the isolation layer, e.g. coal, is removed. The coal layer acts as an isolation barrier in this case, which is clear on the basis of the results of die first model. According to these results, the temperature at the observation point, which lies close to the coal surface is 18.6°C In the second simulation model the coal layer was replaced with material, which was stored above the coal layer. The surface was sunken and filled up with precipitant water. The coefficient of heat conductivity for the rock of change consisting of sand, gravel and fine sandstone is approximate 3.5 times higher than the coefficient of heat conductivity for coal. Consequently, the temperature at the observation point is 24.3°C. The PORFLOW software package (Runchal & Sagar) was used for the simulation. The program is an extensive mathematical model for the simulation of multiphase fluid flow, heat transfer and mass transport problems in variably saturated porous or fractured media. Results obtained through calculations in the first model are compared with the results of well logging and bottom hole temperature (BHT) measurements, which had been carried out in the drill holes near the observed section. The results obtained through calculation show good agreement with the measured results. Some differences appear in the lower part of the profile under 1200 m; I believe this is because the geological structure and the thermal properties of solids in that depth are only estimates.