The Calculation Model of Initiation Pressure During Hydraulic Fracturing Process of Perforated Coal Seams

There are lots of cleats, fractures and many other structure weak planes in coal seams, which make the bullet holes and cleats intersecting. During the hydraulic fracturing process in coal seams, fractures will initiate at coal rock body of borehole wall, and cleats or fractures are different from conventional reservoirs. Thus a new model for initiation pressure calculation during coal seams fracturing should be established. Based on the rock mechanics and elasticity mechanics, and also on network distribution characteristics of coal seam cleats and the space position relationships between the intersected bullet holes and cleats, stress distribution around the bullet holes and at the cleats wall were deducted. The model was established in tensile failure condition. The calculated initiation fracture pressure of Well HX-3 was 10.71MPa. The pressure obtained from bottom hole pressure gauge was 11.24MPa. The relative error was 4.72%. The model could be applied for initiation pressure calculation during hydraulic fracturing process in coal seams. The fractures would initiate at the cleats during fracturing.