Study on The Difference of Pore-Fracture Structure Between High-Low Rank Coal under SEM And Its Influence on Development

The pore-fracture in coal rock is the space of accumulation and migration of coalbed methane, it is the key to affect the development of coalbed methane. Field emission scanning electron microscopy was used to study the pore-fracture characteristics of high-low rank coal reservoirs. Taking the high-rank coal of FJ-1 well in Fanzhuang block, southern Qinshui Basin and the low-rank coal of JM18-2 well in Jiergalangtu sag, Erlian Basin as the research objects, observing and analyzing the pore-fracture types, genesis, structures and spatial distribution.Combined with the development characteristics of its blocks, the differences between the high-low rank coal reservoirs and their impact on development were studied. It is found that the structure of high-rank coal is tight, its pores are mainly micropores, the pore type is mostly gas pore and mould pore, the primary pore is rare affected by metamorphism, occasionally can see the deformed closed cellular pore, the breccia pore are little developed, high-rank coal fractures are mainly developed in vitrinite, mostly are endokinetic fractures and tensile fractures, some of which are filled, poor spatial association and general connectivity, while low-rank coal has loose structure, the pores is dominated by macropores, there are a lot of cellular pores, interclast pores, gas pores and a small number of mould pores, corroded pores and friction pores. The characteristics of pore-fracture development are mainly controlled by the grade of metamorphism, high-rank coal, small pore-fracture scale, poor connectivity, difficult to produce fluid.Low-rank coal, pore-fracture well development, it is characterized by largepore size, good connectivity and favorable for fluid migration, but low-rank coal relatively rich in water and have difficulty to drainage pressure reduction.