Natural fractures initiation and fracture type prediction in coal reservoir under different in-situ stresses during hydraulic fracturing

Hydraulic fracturing is one of the most effective ways of formation stimulation for enhancing coalbed methane (CBM) recovery. The fracture initiation and propagation during hydraulic fracturing is primary constrained by local stress field, hydraulic pressure magnitude, coal properties, and natural fractures. In this work, the fracture initiation region and propagation direction during hydraulic fracturing under various in-situ stresses were investigated both experimentally and numerically. For experimental study, fracturing under true tri-axial stresses was designed to determine the fracture propagation under two sets of in-situ stresses. And for numerical study, a single-fracture model was constructed by using finite element method (FEM3D) ANSYS workbench, which indicates that high lateral stress coefficient is conducive to create new compressive fractures instead of shear fractures. σ_H = σ_v or σ_h = σ_v has a great influence on the initiation location that determined by θ (from −90° to 90°) of the compressive fracture, while there is no significant influence on shear fracture. With the same σ_v applied, the horizontal stress difference has slightly impact on the type of newly-created fracture. The value of lateral stress coefficient that ranges from 0.35 to 1.20 decides the fracture type, which plays a crucial role in determining the stress magnitude and orientation around the fracture tip. The horizontal and vertical fracture are the main types when σ_v>σ_H>σ_h and σ_H>σ_v>σ_h respectively. The inclined fractures are possibly created in the coals with strong heterogeneity. Therefore, they can be used to predict the direction of initiation and propagation for the type of newly-created fracture during hydraulic fracturing for coalbed methane (CBM) development.