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

• Published in: Acta geologica Sinica (Beijing) Vol. 93; no. 4; pp. 972 - 981
• Main Authors: HOU, Zhenkun, LI, Xianwen, ZHANG, Yanming, LIANG, Huqing, GUO, Ying, CHENG, Hanlie, GAO, Ruchao
• Format: Journal Article
• Language: English
• Published: Richmond Wiley Subscription Services, Inc 01.08.2019; Guangzhou Institute of Building Science Co., Ltd., Guangzhou 510440, China; School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China%Oil and Gas Technology Research Institute, Changqing Oilfield Branch Company of PetroChina, Xi'an 710018, China%Guangzhou Institute of Building Science Co., Ltd., Guangzhou 510440, China%Institute of Geology and Paleontology, Linyi University, Linyi 276000, Shandong, China%LandOcean Energy Services Co.,Ltd.Beijing 100084, China%CCCC Second Harbour Engineering Co., Ltd., Wuhan 430040, China
• Edition: English ed.
• Subjects: Axial strain; Basins; Compression; constitutive model; Constitutive models; Damage; damage variable; Longmaxi Formation shale; Mathematical models; Mechanical properties; Modulus of elasticity; Oil and gas fields; Parameters; Sedimentary rocks; Shale; Shale gas; Silurian; Strain rate; strain rates; Time dependence; Triaxial compression tests; Yield stress; strain rates; triaxial compression tests; damage variable; constitutive model; Longmaxi Formation shale
• ISSN: 1000-9515; 1755-6724
• DOI: 10.1111/1755-6724.13879

Shale, as a kind of brittle rock, often exhibits different nonlinear stress‐strain behavior, failure and time‐dependent 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.