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. σH = σv or σh = σv 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 σv 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 σv>σH>σh and σH>σv>σh 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.
This research was funded by the National Natural Science Foundation of China (grant no. 41602170), the Fundamental Research Funds for Central Universities (grant no. 35832015136) and Key Project of Coal-based Science and Technology in Shanxi Province-CBM accumulation model and reservoir evaluation in Shanxi province (grant no. MQ2014-01).
- Finite element method
- Hydraulic fracturing
- Lateral stress coefficient
- Natural fracture initiation