Metanowość złóż węgla kamiennego w Lubelskim Zagłębiu Węglowym

Abstract

METHANE CONTENT IN HARD COAL DEPOSITS OF THE LUBLIN COAL BASIN

Summary 

The methane content and rating of the gas hazards of coal seams in the central part of the Lublin Coal Basin are discussed in the present paper. The deposit examined is connected with coal seams occurring in the Westphalian at the depth of 650 to 960 metres. Coal has been also recognized in the underlying Namurian sediments. The economic series forms an asymetric anticline subdivided into fault blocks. The Carboniferous sediments are overlain by permeable Jurassic carbonate sediments above which marly, practically impermeable Cretaceous sediments occur. The methane content in the deposits is related to gas absorbed by coal seams and it has been defined by direct vaccum degassing techniques on coal samples. Simultaneously the desorption and sorption characteristics of the coals have been obtained. The qualitative evaluation of the gas content in the coal seams has been based on degassing of barren rocks and drilling fluid and on data from comprehensive well logging. The composition of gas in waters has been defined in the course of sample collection from the aquifers. The occurrence of a free water zone to the depth of about 1 000 m has been recognized from hydrochemical data and the isotopic composition of waters (¹⁴C, δ D, δ¹⁸O). Within this zone comprising the overburden and the Westphalian sediments the accumulations of free methane in the reservoir rocks could not have been preserved. Low desorption indices and a small sorption capacity of the coal from the economic series and a low gas pressure in the seams are indicative of a low methane content. The average sorption capacity of coals with respect to methane at a 5 atm pressure is 3.7 cu m of CH₄/t of pure coal substance. The values of the specific desorption index V 2p10 range from 0.15 to 0.51 cu cm of CH₄/1 g of coal, the methane content being 0.32 cu cm of CH₄/1 g of coal The gas pressure in the coal seams reaches 6 atm. The partial pressure in the gas mixture does not exceed 3.6 atm. The methane content in the coal seams of the economic series is up to 5.622 cu m of CH₄/t of pure coal substance, while in the Namurian sediments it reaches 12.528 cu m of CH₄/t of pure coal substance. There is a general trend towards the increase of the methane content in coals with depth. Gas-bearing seams with the methane content exceeding 0.02 cu m/t of pure coal substance occur practically in the whole sequence, methane-free seams predominating at the top to the depth of about 800 m. There is a noticeable increase of the methane content below 800 m, and below 900 m in particular. The methane content drops towards the Kock horst structure which is probably due to natural degassing of the coal seams. The gas occurring in coal is a mixture of nitrogen, methane and carbon dioxide. No gases containing higher hydrocarbons, carbon dioxide or hydrogen sulphide have been found in the economic series. The nitrogen-methane gases zone extends to the depth of 900-1 000 m below which the methane-nitrogen and methane gases zone occurs. Following the binding classification, the seams occurring at the top of the economic series have been classified mostly as non-gaseous, while the underlying seams belong to categories I and II of the gas hazard. Considering the low sorption and desorption abilities the methane seepage from coal seams will be slow and no gas ejections should be expected. Despite the low partial pressure and minor methane content the gas may be expected to ooze into mine workings as a mixture with nitrogen, the process being conroled by the pressure of the gas mixture in the coal seams. However, this will not substantially increase the gas hazard.