Numerical modelling of hydraulic fracture propagation in poro-viscoelastic formation

Modelling of hydraulic fracturing often deals with elastic deformation of rocks and fractures. Some rocks, such as the rock-salt and shale, however, displayed viscoelastic behaviour in both field investigation and lab creep tests. This time-dependent deformation poses a challenge in modelling and characterising of fracture propagation in such formations, thus adding complexity in a multi-physics scenario. In this paper, we intend to numerically model fracture propagation in viscoelastic formation and investigate the effect of creep on crack propagation and crack geometry. To incorporate pore pressure effect, we couple fluid diffusion with shale matrix viscoelasticity in the hydraulic fracturing propagation model. Therefore, a procedure for numerical modelling hydraulicallydriven fracture propagation in poro-viscoelastic formation is developed. The method uses a poroviscoelasticity theory to describe the fluid diffusion and matrix creep in solid skeleton, in conjunction with cohesive law based damage criteria to simulate crack growth and fluid flow in the crack. The modelling took shale as the representative sample. The viscoelastic properties were obtained via creep tests of shale samples from Roseneath formation, Cooper Basin, Australia. Results show that the hydraulically induced fracture tends to be wider and longer in creep formation. What's more, the propagation pressure is also lower than that in poroelastic formation. Fluid diffusion from crack to the matrix is more efficient due to modulus decay. For mixedmode fracturing, the creep of the formation facilitates the process of reorientation with higher fracture propagation speed between kink points.