Modelling the near-wellbore rock fracture tortuosity: Role of casing-cement-rock well system, perforation and in-situ stress

Near-wellbore rock fracture is a key subject in subsurface energy extraction. Casing perforation completion is perhaps the most used type of well design mainly in comparison to the open hole completion. However, the effects of the combined casing-cementing-rock structure on pressure transmission in fracturing the rock have not been sufficiently addressed, but is key to understanding some important phenomena in hydraulic fracturing, such as fracture tortuosity. Here we develop a combined analytical-numerical model to investigate the rock fracture mechanism near the wellbore with realistic consideration of the boundary condition imposed by the complete well system. Different well configurations can lead to variation in the pressure applied on the rock formation and hence affect the fracture propagation near the wellbore. Our results show, as the pressure transmission through the well structure is enhanced, the fracture propagates more closely to the perforation orientation with a smaller deflection angle and the breakdown pressure is increased. The near-well tortuosity of the crack can be reduced by decreasing the thickness of casing and cement sheath, reducing Young's modulus of casing and increasing Young's modulus of cement up to around 15 GPa, based on the realistic range of values of the well parameters. The effects of in-situ stress condition and perforation angle and length on the near-well cracking are also investigated. The developed model can be used to aid decision making in terms of improved understanding of hydraulic fracturing technology and well design optimization.