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| New brittleness index based on energy balance and analysis of failure mechanism of shale |
| LIU Junxin1，2，LI Junrun1，2，YIN Binrui1，2，TANG Wei1，2，3，YUAN Yusong4，ZHU Baolong1，2 |
(1. Shock and Vibration of Engineering Materials and Structures Key Laboratory of Sichuan Province，Southwest University of Science and Technology，Mianyang，Sichuan 621010，China；2. School of Civil Engineering and Architecture，Southwest
University of Science and Technology，Mianyang，Sichuan 621010，China；3. College of Civil Engineering，Guizhou
University，Guiyang，Guizhou 550025，China；4. Petroleum Exploration and Production Research
Institute，SINOPEC，Beijing 100083，China) |
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Abstract Aiming to establish the brittleness index for characterizing deep shale gas reservoirs，a new brittleness evaluation method，characterizing the brittleness of rock from three aspects such as rock failure rate()，post-peak instability() and pre-peak plastic yield()，was proposed based on energy balance theory in consideration of the energy conversion rate of the rock system to the outside and the effect of radial expansion on brittleness characteristics，and a series of triaxial compression tests designed according to shale-formation information were conducted to verify the applicability of the index. The test results show that the brittleness characteristics of shale decrease with increasing “formation level”(i.e.，the confining pressure and the temperature)，and compared with the existing brittleness indices，the new brittleness index is superior in describing the brittleness characteristics of shale under different test conditions，especially in the condition of high confining pressure(at 100 MPa) and high temperature(at 190 ℃). Besides，the brittle failure mechanism of shale was analyzed in terms of strength factors，and it is revealed that the strength envelope curve of shale presents bilinear characteristic. Meanwhile，a critical condition for macroscopic fracture patterns of shale was quantified by establishing the correlation of the macroscopic fracture pattern，the brittleness characteristics and the critical principal stress ratio. It is also found that the damage degree of shale can reduce the brittleness characteristics of shale by increasing the pre-peak plastic yield and the contribution rate of the elastic energy.
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2022, Vol. 41 
