A Strain Rate Dependent Constitutive Model for the Lower Silurian Longmaxi Formation Shale in the Fuling Gas Field of the Sichuan Basin,China

【Author】

HOU Zhenkun;LI Xianwen;ZHANG Yanming;LIANG Huqing;GUO Ying;CHENG Hanlie;GAO Ruchao;Guangzhou Institute of Building Science Co., Ltd.;School of Materials Science and Engineering, South China University of Technology;Oil and Gas Technology Research Institute, Changqing Oilfield Branch Company of PetroChina;Institute of Geology and Paleontology, Linyi University;LandOcean Energy Services Co.,Ltd.;CCCC Second Harbour Engineering Co., Ltd.;

【Institution】

Guangzhou Institute of Building Science Co., Ltd.;School of Materials Science and Engineering, South China University of Technology;Oil and Gas Technology Research Institute, Changqing Oilfield Branch Company of PetroChina;Institute of Geology and Paleontology, Linyi University;LandOcean Energy Services Co.,Ltd.;CCCC Second Harbour Engineering Co., Ltd.;

【Abstract】

Shale, as a kind of brittle rock, often exhibits different nonlinear stress-strain behavior, failure and timedependent behavior under different strain rates. To capture these features, this work conducted triaxial compression tests under axial strain rates ranging from 5×10-6 s-1 to 1×10-3 s-1. The results show that both elastic modulus and peak strength have a positive correlation relationship with strain rates. These strain rate-dependent mechanical behaviors of shale are originated from damage growth, which is described by a damage parameter. When axial strain is the same, the damage parameter is positively correlated with strain rate. When strain rate is the same, with an increase of axial strain, the damage parameter decreases firstly from an initial value(about 0.1 to 0.2), soon reaches its minimum(about 0.1), and then increases to an asymptotic value of 0.8. Based on the experimental results, taking yield stress as the cut-off point and considering damage variable evolution, a new measure of micro-mechanical strength is proposed. Based on the Lemaitre's equivalent strain assumption and the new measure of micro-mechanical strength, a statistical strain-rate dependent damage constitutive model for shale that couples physically meaningful model parameters was established. Numerical back-calculations of these triaxial compression tests results demonstrate the ability of the model to reproduce the primary features of the strain rate dependent mechanical behavior of shale.

【Keywords】

strain rates;;triaxial compression tests;;damage variable;;constitutive model;;Longmaxi Formation shale

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