Experimental study on deformation and fracture characteristics of coal under different true triaxial hydraulic fracture schemes

Hydraulic fracturing techniques are often applied to form permeability enhancement zones near boreholes and/or wellbores in tight low-permeability reservoirs to improve their extraction efficiency. As conventional hydraulic fracturing techniques often generate high breakdown pressure and induced seismic potential, novel alternative production enhancement techniques (such as pulse hydraulic pressurization) have been proposed. However, their applicability requires further verification. Therefore, this study conducted hydraulic fracturing tests of coal under three test schemes: conventional hydraulic fracturing (CHF), stepwise pressurization hydraulic fracturing (SPHF), and stepwise pulse pressurization hydraulic fracturing (SPPHF), considering the true triaxial stress state of an actual coal seam. The results showed that the breakdown pressure of coal decreased with an increase in the horizontal stress difference (σ2-σ3). Moreover, the pulse pressurization produced more intergranular fractures and caused a large number of particles to be conducted as a stress concentrator at the fracture tip spalling, which promoted the unstable propagation of fractures, which decreased the breakdown pressure of coal. The abnormally high breakdown pressure in the SPHF experiments is likely due to the obstruction of high σ2 to the fracture propagation rate and fluid injection, which intensifies the fluid hysteresis effect and the stable fracture development and expansion during the pressure-maintaining stage, thus increasing the breakdown pressure. To describe the relationship between the variation in the breakdown pressure and fracture evolution characteristics, we introduced the equivalent characteristic length l and established a breakdown pressure prediction model based on the tensile stress criterion, which shows effective prediction of the breakdown pressure in different test schemes. Anisotropic principal strain appeared under different injection schemes. During the CHF and SPHF experiments, fluid injection deformed the σ1 direction by compression and the σ3 direction by expansion. In the σ2 direction, the deformation transitioned from expansion at a low stress level to compression at a high stress level. The deformation in the σ2 direction was always compressive in the SPPHF experiments. Continuous and pulse pressurizations can promote the deformation and fracture development. For the fracture morphology, the coal in the CHF experiments was dominated by a single tensile fracture, and macroscopic shear fractures were formed in the SPHF and SPPHF experiments owing to the continuous stimulation of fluid pressure to the bonding of particles and the weak surface structure, such as bedding and cleats during the pressure maintenance and pulse, and the fragmentation effect was significant and accompanied by obvious particle spalling.