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| Brittleness evaluation index based on energy variation in the whole process of rock failure |
| ZHANG Jun，AI Chi，LI Yuwei，ZENG Jia，QIU Dezhi |
| (Department of Petroleum Engineering，Northeast Petroleum University，Daqing，Heilongjiang 163318，China) |
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Abstract The current indices used to evaluate the shale brittleness consider mostly the mechanical properties before or after the peak separately and cannot reflect the brittleness characteristics in the whole process of rock failure. In this paper，the variation of the various types of strain energy of rock materials in the process of plastic deformation to brittle fracturing was analysed based on the complete stress-strain curves. The levels of the pre-peak dissipation energy and the post-peak fracturing energy are the key factors to determine whether the brittle fracturing occurs. The brittleness estimation indices which represent comprehensively the mechanical properties of rock in pre-peak and post-peak periods were proposed by combining the pre-peak dissipation energy and the post-peak fracturing energy. The brittle characteristics of different rock materials under different confining pressures and the anisotropy of shale brittleness were evaluated with these brittleness indices. The results show that the proposed brittleness indices can simultaneously reflect the difficulty of brittle failure and the scale of brittleness and can evaluate the change of brittle characteristics under different mechanical conditions. The pre-peak dissipation energy and post-peak fracturing energy of different rock materials increase with the increase of the confining pressure. The brittleness decreases constantly with the increase of the confining pressure，but the decrease trends are different. The red sandstone and shale show the transformation of brittleness-plasticity at low and high confining pressures，while the granite has the strong brittleness in the whole process of the confining pressure increasing. The brittleness of shale is anisotropic. The brittleness of shale specimens with different inclination angles of bedding plane has significant difference. The shale brittleness shows a transition process of stable，increase to decrease as the inclination angle of bedding planes ? increases. When ? = 0°，the degree of brittleness of shale is stronger than that of ? = 90°. When ? = 60°，shale has the weakest brittleness and shows the plastic characteristic.
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2017, Vol. 36 
