AUTHOR=Liu Yi , Li Zihan , Zhao Shisen , Jiang Jian , Yuan Chao 

TITLE=Effects of inclination angle and confining pressure on triaxial unloading-induced slip behaviors of shale fractures

JOURNAL=Frontiers in Earth Science

VOLUME=12

YEAR=2024

URL=https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2024.1372839

DOI=10.3389/feart.2024.1372839

ISSN=2296-6463

ABSTRACT=<p>To explore the effects of fracture inclination angle <italic>θ</italic> and confining pressure <italic>σ</italic><sub>3t</sub> on the slip behaviors and friction properties of fractures, the triaxial unloading-induced slip experiments were performed on the shale fractures. The results show that the <italic>σ</italic><sub>3t</sub> controls the slip modes of fractures, while the <italic>θ</italic> affects the occurrence of the stick-slip events during the quasi-static slip stage. With the increase in <italic>σ</italic><sub>3t</sub>, the main slip modes of fracture transform from the stable-slip to stick-slip, and eventually to the creep-slip. The increase in <italic>θ</italic> facilitated the occurrence of stick-slip events. As the <italic>θ</italic> increased from 30° to 50°, the number of stick-slip events increased from 0 to 3 and from 2 to 4 for <italic>σ</italic><sub>3t</sub> = 10 MPa and 20 MPa, respectively. For <italic>σ</italic><sub>3t</sub> = 40 MPa, no stick-slip event occurred in the slipping process. The <italic>θ</italic> and <italic>σ</italic><sub>3t</sub> have great effects on interaction modes between asperities, which directly affected the friction properties of fractures. With increasing <italic>σ</italic><sub>3t</sub>, the void spaces between the asperities were further compacted, resulting in the transition of asperity interaction from overriding mode to shear-off mode. The transition of asperity interaction model therefore brought about the weakening of friction coefficient at the activation point and the onset of dynamic slip stage. There is a competitive relationship between the <italic>θ</italic> and <italic>σ</italic><sub>3t</sub> for the evolution of the friction properties of fractures. As the <italic>θ</italic> increase from 30° to 50°, for <italic>σ</italic><sub>3t</sub> = 10 MPa, the mean sheared-off thickness decreased from 0.502 mm to 0.433 mm, while for <italic>σ</italic><sub>3t</sub> = 40 MPa, the mean sheared-off thickness decreased from 0.505 mm to 0.319 mm. With the increment of <italic>θ</italic>, the anisotropy of joint roughness coefficient was weakened. We suggested that by adjusting the fracturing angle of hydro-fracturing, the earthquakes with large seismic moments may be effectively mitigated.</p>