The study of filtration permeability of the barrier pillar between the isolated fire section and the worked longwall ventilation drift was completed using the mathematical model developed by the authors for coupled processes of rock deformation and gas filtration. The problem is solved involving a finite element method implemented in the author's programme. When solving, the parameters of width of the barrier pillar and thickness of the mined coal seam were varied. The article presents the results of calculating the values of geomechanical parameters, permeability coefficients and gas pressure in the filtration area. It is shown that with an increase in thickness of the coal seam, intensity of the crack formation process increases in the barrier pillar. With small pillar width, the coal seam along its entire length is in a disturbed state, as well as the rocks of its roof and soil. The filtration areas around the worked longwall ventilation drift and the isolated conveyor drift are connected both along the coal seam and along the host rocks. This means that at a certain value of the pressure drop, filtration of fire gases into the mine workings of the worked excavation section is possible. With a greater width of the pillar, between disturbed rocks around the mine workings of the worked section and isolated fire one, there is a barrier made of undisturbed rocks. That impermeable barrier prevents gas filtration from one mine workings to another. Its width decreases with an increase in the thickness of the coal seam. Based on the results of calculating gas pressure, it is shown that methane from the coal seam moves into atmosphere of the mine workings, where gas pressure has lower values. At the same time, for the accepted conditions, with a pillar width of 20 m and a seam thickness of 0.4 m, gas is filtered from the mine workings of isolated fire section into the adjacent excavation section along the disturbed rocks of the seam roof. In the rest of the cases, the areas of low pressure around the mine workings are separated by zones of higher pressure, exceeding pressure of fire gases in the isolated section. Filtration of fire gases into atmosphere of the worked excavation section is impossible under such conditions.