Experiment on the liquid CO2 fracturing process for increasing permeability and the characteristics of crack propagation in coal seam

In terms of low permeability in China coal seams,based on the fact that liquid CO2 has the characteristics of low viscosity,low resistance,acidification-dispel and phase supercharge,the authors performed a field experiment of hydraulic CO2 fracturing for increasing coal seam permeability,and studied the crack developing process. The results show that the flow curve presents the characteristics of fluctuation with the pressure increase. Under the action of CO2 pressure,some cracks appear and extend forward gradually around the borehole. The speed of cracks extension is fast at the initial phase of the fracturing,and then decreases gradually. Cracks extend both in axial and radial directions a- long the fracture hole. There are two turning points of pressure curve at 1. 8 MPa and 2. 2 MPa. The fracturing process is divided into three stages,and the coal damage forms at each stage,including initial rupture in broken zone around borehole,weak plane extending and micro pore damage during fracturing process respectively. The results of testing pressure and temperature show that the fracture radius can reach 10 meters,so it is possible to increase coal seam per- meability by applying liquid CO2 fracturing.