A New Mode of Visible Fracture System in Coal Seams and Its Implications for Coalbed Methane Seepage

Table 1

VFS classification in coal seams.

Channel type	Distribution positions	Space extent range	Connection role	Level
Hydraulic fractures	Around the wellbore	A few meters to tens of meters	Wellbore, exogenetic fractures, and interlayer fractures	I
Exogenetic fractures	Any positions of coal seams	A few meters to hundreds of meters	Hydraulic fractures, interlayer fractures, and gas-expanding fractures	II
Interlayer fractures	Gangue, roof, interlayers	A few meters to hundreds of meters [29]	Connect hydraulic fractures, exogenetic fractures	III
Gas-expanding fractures	Bright coal and semibright coal [15]	Several centimeters to tens of centimeters	Connect hydraulic fractures, exogenetic fractures, and cleats	IV
Cleats	Specular coal and bright coal [30]	Several millimeters to several centimeters	Connect the microfractures in the coal matrix	V