Study of Pore-Pressure Induced Hydromechanics Effects on Permeability of Fractured Rock with Curved and Dead-End Fractures

Abstract:

The pore-pressure induced hydromechanical effects on a typical two-dimensional fractured domain at 500 m depth below the land surface were studied. The domain contains fracture sets with curved fractures and dead-end fractures. The study investigates the effects of tension and shear failures in the rock matrix and fractures, respectively, as a result of increases in pore pressure. Two in situ stress boundary conditions are considered: an isotropic case SR1 with the two horizontal boundary stresses having the same magnitude and an anisotropic case SR2 with the ratio between these stress components set to be 2. Stress concentration points are found in the rock matrix near fracture endings or between adjacent fractures in close vicinity with each other. The maximum ratios of the maximum to minimum principal stresses found at points over the fractured domain are found to be as much as doubling those at the boundaries of the fractured domain, for both isotropic SR1 and anisotropic SR2 loading cases. Changes in the stress and stress ratio distributions due to increasing pore pressures were then evaluated. It was found that although the maximum principal stress magnitude generally decreases over the fractured domain, there is a significant increase in the magnitude of this stress component at the connections of the existing fractures. Changes in flow paths and permeability of the fractured rock domain due to the induced connections between the existing fractures were also investigated. It was found that because rock matrix cracking and consequent fractures propagation is more prevalent in anisotropic case SR2, these changes are more significant than in isotropic case SR1. It was found that changes in permeability are not significant under changes in pore pressure until the pore pressure reaches 85% of the magnitude of the in situ stresses at the boundaries of the fractured domain.