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| Study on fracture propagation and formation mechanism of core discing at depth under high in-situ stresses |
| ZHANG Fengshou1，2，LI Mengli1，2，ZHANG Chongyuan3，4，HE Manchao1，2，4，ZHANG Shengsheng5，HENG De6 |
(1. Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education，Tongji University，Shanghai 200092，China；2. College of Civil Engineering，Tongji University，Shanghai 200092，China；3. Key Laboratory of Neotectonic Movement and Geohazard，Institute of Geomechanics，Chinese Academy of Geological Sciences，Beijing 100081，China；4. School of Mechanics and Civil Engineering，China University of Mining and Technology(Beijing)，Beijing 100083，China；5. The Survey Institute of Hydrogeology，Engineering Geology and Environment Geology of Qinghai Province，Xining，Qinghai 810008，China；
6. Sichuan Changning Natural Gas Development Co.，Ltd.，Chengdu，Sichuan 610051，China) |
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Abstract In this paper，the macro characteristics and micro scanning results of the discing cores from hot dry rock wells in Gonghe Basin，Qinghai Province were analyzed，and then，the fracture evolution in the process of core drilling under different types of stress states was analyzed to explore the underlying mechanism of core discing from the micro perspective using the hybrid DEM-continuum method. The following results can be drawn. Firstly，the observed shapes of discing cores can be divided into four types according to the thickness，i.e.，crushed discing，thin discing，discing and thick discing，and the fracture surfaces，most of which are rough and uneven，present saddle，flat，cup-shaped concave and ellipse shapes. Secondly，the rock core discing is attributed to the tensile failure. Thirdly，the high horizonal principal stress is the main factor inducing core discing and leads to the zonal distribution of fractures and the cup-shaped concave surface at the root of the cores. The vertical stress restrains the zonal distribution of fractures，and the high vertical stress could result in a flat-shaped section in the case of low horizonal stress. Fourthly，under the influence of the stress redistribution，a large amount of fractures generate at the root of the core and the existing fractures at the upper part of the core continue to develop，augmenting core discing. Fifthly，the horizontal stress difference affects both the distribution and the strike of the fractures. The strike of the fractures is mainly along the direction of the maximum horizontal stress，and the fractures gather near the core edge. Sixthly，the strike of the fractures is also affected by the three-dimensional stress level. The proportion of the fractures striking along the maximum horizontal stress increases with decreasing the minimum horizontal stress or increasing the vertical stress. However，the indication effect of the fracture strike on the stress direction is minimized under high maximum horizontal principal stress.
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2022, Vol. 41 
