Hydraulic Fracturing Mechanical Mechanism Analyses for Soft Seams Considering Strain Softening Character

Abstract

Mathematical model based on elasticity is not suitable for soft seam hydraulic fracturing mechanism study because its intensity is small, Poisson's ratio is relatively large, and its prone to plastic deformation. Based on plastic mechanics, the theory of large deformation and fracture mechanics theory, hydraulic fracturing of soft coal seam is divided into three phases, namely, coal bed compaction, fracture initiation and crack propagation from the view of the deformation mechanism, the occurring and developing mechanism. The initiation pressure of soft seams considered strain softening character after plastic deformation is obtained on the basis of above. The result shows that the initiation pressure is related to elastic modulus, Poisson's ratio, the angle of internal friction and residual strength. Elastic modulus is inversely proportional to the initiation pressure, the greater its value, the smaller the initiation pressure; but Poisson's ratio, the angle of internal friction and the residual strength and fracture initiation pressure is directly proportional relationship, the greater its value, since the smaller the crack pressure.