Abstract:

Minerals in coal can influence coal reservoir physical properties and the development of coalbed methane. Mineral species, content and occurrence characteristics in coal of southern Qinshui Basin were studied by using optical microscope and scanning electron microscope (SEM). Based on the test of isothermal adsorption experiment and mercury-injection method, the effects of minerals on adsorption capacity, porosity and permeability of coal reservoir were analyzed. The results show that the mineral content of No.3 and No.15 coal seams of Shizhuang block are 10.68% and 12.81%, respectively, and the content of sulfide minerals in No.15 coal seam is higher than that in No.3 coal seam. Calcite, pyrite, kaolinite and quartz filling in cell pores, intergranular pores and microfractures can be identified under SEM. The values of langmuir volume, proportion of macropores and mesopores, porosity and permeability decrease with the increasing of ash content, indicating that minerals in coal can decrease the adsorption capacity, porosity and permeability of coal reservoirs. Minerals in coal mainly fill in macropores, mesopores and microfractures of coal and they can influence the diffusion of coalbed methane in cleats and fractures, causing the decrease of porosity and permeability. However, a small part of clay minerals which fill in micropores of coal reservoir can lead to the decrease of adsorption capacity.

Key words: mineral in coal, occurrence characteristic, coal reservoir, physical property, mineral filling, Qinshui Basin