Abstract:

In the coal reservoir, the pore of coal is the main accumulation area and migration pathways of coalbed methane. The pore structure not only controls the gas content but also has significant influence on the coalbed methane recovery. On the basis of macromicro observation, vitrinite reflectance and porosity testing, and mercury injection experiment of different metamorphicdeformed coal samples collected from the Huaibei coalfield and Qinshui basin, we studied the pore structure characteristics and its restriction on the recovery of coalbed methane. According to  analysis of experiment data and field observation of representative coal samples in the study area, the coal reservoirs are mainly classified under five different metamorphicdeformed types: the high grade metamorphic and medium deformed to strongly deformed coal reservoir (typeI); the high grade metamorphic and comparatively weakly deformed coal reservoir (typeII); the medium grade metamorphic and comparatively strongly deformed coal reservoir (typeIII); the medium grade metamorphic and comparatively weakly deformed coal reservoir (typeIV); and the low grade metamorphic and strongly deformed coal reservoir (typeV). Furthermore, pore structure characteristics are different among these different types of coal reservoirs. There are more adsorption pores in the coal reservoir of typeI than that in other types; however, the pore connectivity is better in the typeII than that in typeI because of tectonic fractures formed by tectonic action in coal reservoirs of typeII. And there are less adsorption pores in reservoir of typeIII than that in coal reservoir of typeI and typeII, and the pore connectivity is unfavorable. The coal reservoir of typeIV is characterized by better gas content and good pore connectivity caused by a large number of cleats and adsorption pores in coals. As for coal reservoir of typeV, the pore connectivity is worse than other types because of the existence of a large number of thinneck bottle type pores. In conclusion, the coal reservoirs of typeII and typeIV are favorable for the recovery of coalbed methane because of the better absorptive capability and permeability.

Key words: metamorphicdeformed type, coal reservoir, vitrinite reflectance, pore structure, coalbed methane, recovery