Hydraulic properties and mechanical properties of rock fractures are intimately linked and the accurate understanding of the poromechanical link/behavior is essential for the crustal energy utilizations, such as geothermal and hydrocarbon. In particular, such a relationship under high pressure environment (more than 50 MPa) is important but has remained unclear due to the difficulties of its insitu monitoring and laboratory experiment. The present study explores this relationship via the pressurized water injection experiments on granite fractures with rough surfaces. We first imitate and hold the critical-stressed state of granite fracture, then gradually increase the pore fluid pressure to trigger the shear slip on fracture. During the experiments, we record the fracture permeability, normal/shear stresses on fracture, and shear displacement. In addition, to classify the failure mode (tensile or shear), we acquire the full waveforms of acoustic emissions and evaluate RA values and average frequency, which are proposed by Ohno and Ohtsu (2010), for the respective waveforms. As a result, we find that pressurized water injection first triggers the aseismic motion, which can be simply interpreted as jack-up opening of fracture, and contributes on the reversible permeability enhancement. The onset of the aseismic motion is possibly constrained by the Mohr-Coulomb failure criteria. This aseismic motion then induces the shear slip, which leads to the irreversible self-propped deformation and the associated permeability enhancement. New fractures concurrently propagate from the contacting asperities with stress concentrations and these newly created fractures significantly enhance the reservoir permeability. Our results reveal the importance of the mixed-mode failure behavior during reservoir pressurizations.

1. Introduction

Hydraulic stimulation on the geothermal reservoir is the well-known operation for improving or maintaining the transmissivity and fracture connectivity within the reservoir (Norbeck et al., 2018). In this operation, by injecting pressurized water into the reservoirs, pre-existing fractures are reactivated in shearing mode with the opportunity for self-propping on asperities. Although it is important to explore the mechanical and hydraulic coupling of rock fracture/fracture network during hydraulic shearing, the laboratory experiments for exploring such coupling are very limited (Ye and Ghassemi, 2018; Ishibashi et al., 2018).

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