Visualization of the dynamic propagation of two simultaneously-stimulated hydraulic fractures: Competition and interaction

Deepening the understanding of the dynamic propagation and interaction of multiple hydraulic fractures is the key to the optimization of hydraulic fracturing design. By conducting two-hole hydraulic fracturing experiments on transparent polymethyl methacrylate (PMMA) samples, we visualize the dynamic propagation of fractures stimulated from two holes in three dimensions with the aid of high-speed cameras and image reconstruction methods. The characteristics of hydraulic fracture growth were discussed in conjunction with extended finite element method (XFEM) simulation and theoretical analysis. The competition between the boundary stress and the internal stress from the holes controls the growth mode of the fractures. The reduction of boundary stress difference intensifies the stress concentration between holes, resulting in the transformation of planar fractures formed from a single hole into spiral fractures connecting two holes. The propagation of double-hole spiral fractures can be divided into hole connection, deflection and rapid propagation stages. The fractures first connect the two holes dominated by the stress concentration, and then reoriente to the σ_H direction under the control of the boundary stress. The propagation of single-hole planar fractures can be divided into upward propagation, bilateral synchronization and downward propagation stages. The fracture propagating in the σ_H direction first appears on the single-hole side under the control of boundary stresses, and then deflects towards the adjacent hole influenced by the attraction stresses from adjacent holes. The propagation of two-hole hydraulic fractures has obvious sequence, and the stress repulsion of the primary fracture makes the secondary fracture propagate in opposite direction.