Influence of stress-induced microcracks on viscoplastic creep deformation in Marcellus shale
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RESEARCH PAPER
Influence of stress-induced microcracks on viscoplastic creep deformation in Marcellus shale Neel Gupta1
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Brijes Mishra1
Received: 21 December 2019 / Accepted: 6 November 2020 Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Time is one of the often-neglected factors in the assessment of the erratic failure of shale rock. Laboratory-creep experiments showed that constant stress induces time-dependent failure in brittle shale. However, the microscopic reason for time-dependent deformation in shale is still unknown. In the current study, triaxial creep and recovery experiments showed that the brittle shale specimens exhibit viscoelastic and viscoplastic creep deformation at constant stress state. In addition, the X-ray computed tomography scan of Marcellus shale showed that the shale specimens contained significant volume of preexisting microcracks. The statistical correlation among permanent strain during the triaxial experiment and stress-induced change in the three-dimensional geometry of microcracks showed that the microcracking is the microscopic reason for viscoplastic creep deformation in shale. In addition to the time and level of constant differential stress, factors such as orientation of bedding planes and specimen heterogeneity also influence the nature of creep deformation. Keywords Creep Marcellus shale Microcracking X-ray computed tomography
1 Introduction In the past, only a few researchers have studied the timedependent deformation in shale at constant stress and temperature. The experimental creep studies showed that the constant stress induces instantaneous and time-dependent strain [14, 38]. The rate of creep strain and time to reach failure depends on the level of differential stress and confining stress [35, 48]. In addition, the laboratory experiments on shale from deep oil and gas wells, underground tunnels, and prospective sites of nuclear waste repository proved that temperature, mineralogy, porepressure, moisture content, and stress history can also affect the rate of creep deformation [3, 23, 49, 56]. Researchers also used several viscoelastic and viscoplastic rheological constitutive models to simulate and predict the creep behavior of shale. For example, Rouabhi et al. [43] and Li et al. [27] simulated creep behavior of Tournemire shale and understood the excavation distributed zone & Brijes Mishra [email protected] 1
Department of Mining Engineering, West Virginia University, Morgantown, WV 26505, USA
around underground excavations. Li and Ghassemi [26] proposed a linear viscoelastic Burger model to predict the creep and closure rate of hydraulic fractures in shale. Sone and Zoback [50] used viscoplastic constitutive model to simulate creep deformation in organic-rich shale. Although the experimental studies identified the vital parameters influencing creep deformation in shale and numerical studies reproduced the creep behavior, the studies did not accurately describe the physical
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