A physical simulation experiment on fracture propagation of coal petrography in hydraulic fracturing

It is an important subject worthy of probing into fracture propagation laws of coal petrography in hydraulic fracturing as to how to improve the efficiency of coal-bed methane and reduce the safety risk of mining. According to practical engineering applications,the physical simulation experiment program-“exerting confining pressure on triaxial- drilling hole on the top-injecting water through the down jet pipe”-on fracture propagation of coal petrography in hy- draulic fracturing was established using raw coal samples. Based on the program above combining the existing experi- mental conditions,this paper performed physical simulation experiments and fracture detection experiments. The exper- imental results show that the development degree of fractures is higher on the direction along the bedding plane of coal petrography than on the direction vertical to the bedding plane,and presents that the fracture propagation is a primary waterflooding original transverse form of hole wall with rectilinear-figure and jumping-characteristics extension and a subordinate waterflooding original longitudinal form. And according to the analysis of experimental results,the paper presents that in practical engineering applications jet holes should be placed in waterflooding hole wall with transverse original fractures around,which can help improve the hydraulic-fracturing quality of CBM well. For the borehole wall of reservoir containing many transverse and longitudinal fractures, the fracturing way of “ pressure-control” should be used to improve the fracture-making quality. For thick reservoir,the fracturing way of “ multi stage-multi pressure”, structuring the transverse and longitudinal staggered fracture network,should be used to improve the efficiency of the CBM mining. And jet holes should not be placed at those borehole walls with more vertical original fractures and lager fault for avoiding the unstable failure of coal reservoir roof and bottom to cause safety accidents.