Geomechanical and Ultrasonic Characteristics of Black Shale During Triaxial Deformation Revealed Using Real-Time Ultraso
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ORIGINAL PAPER
Geomechanical and Ultrasonic Characteristics of Black Shale During Triaxial Deformation Revealed Using RealTime Ultrasonic Detection Dependence Upon Bedding Orientation and Confining Pressure Y. Wang . J. Q. Han
Received: 14 January 2020 / Accepted: 11 July 2020 Ó Springer Nature Switzerland AG 2020
Abstract The anisotropic characteristics of the Pand S-wave velocity and their velocity–pressure dependency during triaxial deformation are critical in shale gas exploration and development. However, the geomechanical and ultrasonic properties of shale under continuous triaxial deformation are still not well understood. The aim of this work is to experimentally investigate the velocity characteristics of a black shale plugged normal to bedding, parallel to bedding and at 30° and 60° to bedding, under a confining pressure of 5, 20, and 60 MPa, respectively. The in-situ ultrasonic transmission technique during sample deformation was used to reveal the velocity–pressure characteristics of the anisotropic black shale. The experimental results indicate that strength, P- and S-wave velocities, and final failure morphology present obvious anisotropy, and are influenced by the pronounced bedding plane. Before peak strength, the P-wave velocity almost increases with the sample deformation; however, the S-wave velocity first increases and then
Y. Wang (&) Beijing Key Laboratory of Urban Underground Space Engineering, Department of Civil Engineering, School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China e-mail: [email protected] J. Q. Han Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China e-mail: [email protected]
decreases, which is related to the fracture evolution. The velocity–pressure dependency analysis reveals a different propagation mechanism of ultrasonic wave through shale. Two different kinds of equations were used to fit their relationships and elevated stress. The results suggest that the static geomechanical and ultrasonic properties of black shale are affected by the rock structure and in-field stress level. Keywords Black shale Triaxial deformation Velocity anisotropy Bedding plane Deformation anisotropy
1 Introduction Shale plays an important role in different kinds of rock engineering, including underground engineering, foundation engineering, slope engineering, and especially petroleum engineering. Investigations of its geomechanical properties have attracted sustained attentions from many practitioners and researchers. Shale material, it is characterized by high anisotropy because of the cleavage, foliation, schistosity, bedding planes, and tectonic and non-tectonic natural fractures. Owing to the nonuniformity of those meso-structures, the shales usually present evident anisotropy and heterogeneity characteristics in many aspects, including geomechanical, seismic, thermal and hydraulic properties, and these properties of shale vary with
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