Mechanical behavior and constitutive model of shale under real-time high temperature and high stress conditions

Abstract

The high temperature and high in-situ stress geological environment can significantly affect the mechanical properties, failure modes, and deformation characteristics of deep shale reservoirs. In this study, real-time high temperature triaxial compressive tests simulating the deep shale formation environment (temperature: 25–150 °C, confining pressure: 0–100 MPa) are carried out. The GSI-strength degradation and constitutive models are derived based on the Hoek–Brown criterion. The results show that in low confining pressure conditions, the mechanical behavior of shale is greatly influenced by temperature. Compared with shale at 25 °C, the compressive strength of shale at 150 °C decreases by up to 13.7%, and the elastic modulus decreases by up to 36.9%. The peak strain was increased by a factor of up to 1.4, and the yield stress level was advanced by as much as 7.4%. However, in high confining pressure conditions, the shale plasticity characteristics are significantly enhanced and the failure mode is relatively single. The GSI-strength degradation model can well characterize the variation law of shale strength with confining pressure under high temperature conditions. The statistical damage constitutive model matches the actual stress–strain curve very well, and it can fully reflect the deformation and failure characteristics of deep shale. The findings of this study can help us better understand the variation of mechanical properties of deep shale.

Micro‐Thermomechanical Modeling of Rocks With Temperature‐Dependent Friction and Damage Laws

“…It is noticed that the slope of peak strength fitting lines increases as experimental ambient temperature rises, however, the vertical axis intercept shows opposite negative dependence with regard to the temperature. This can be attributed to the following aspects (Guo et al, 2023;Z. X. Liu et al, 2022;Tang et al, 2022;H.…”

“…As temperature rises, molecular distance and oscillatory motion of molecules within minerals strengthen, leading to the reduction of shear strength at localized adhesion joints consequently (H. T. Yang et al, 2013;Zhong, 2017). Meanwhile, thermal expansion of mesocracks also accounts for decohesion of porous matrix (Guo et al, 2023;Suo et al, 2020). In addition, Y. L. Chen et al (2017); Zuo et al (2017) have revealed that thermal-induced loss of absorbed and crystal water also results in the degradation of fracture toughness.…”

“…Considerable research has confirmed that the friction coefficient exhibits a rate-temperature-stress state dependence (Barbot, 2022;Blanpied et al, 1995;Pranger et al, 2022;Scholz, 2019;Zhang & Ma, 2021). Furthermore, temperature also has a significant influence on fracture toughness and cohesion (Guo et al, 2023;Jefferd et al, 2021;Suo et al, 2020;Xu et al, 2023;C. X. Zhao, Liu, et al, 2023).…”

“…Considerable research has confirmed that the friction coefficient exhibits a rate‐temperature‐stress state dependence (Barbot, 2022; Blanpied et al., 1995; Pranger et al., 2022; Scholz, 2019; Zhang & Ma, 2021). Furthermore, temperature also has a significant influence on fracture toughness and cohesion (Guo et al., 2023; Jefferd et al., 2021; Suo et al., 2020; Xu et al., 2023; C. X. Zhao, Liu, et al., 2023). However, it is regrettable that existing constitutive modeling efforts often resort to constant assumptions or curve fitting to describe the temperature response of mechanical characteristics such as strength, deformation, and damage of geological materials, lacking comprehensive consideration of the temperature‐dependent micromechanical mechanisms of fundamental mechanical properties like the friction coefficient, fracture toughness, and damage evolution.…”

Elasticity and Characteristic Stress Thresholds of Shale under Deep In Situ Geological Conditions

“…Abbas et al [19] found that the strength parameters of shale increased significantly with higher confining pressures, while the elastic modulus varied slightly under different confining pressures. In contrast to the studies that examined only the individual effects of confining pressure or temperature on the mechanical properties of shale, Guo et al [20] and Li et al [21] focused on the coupling effect of both factors. The results of these studies show that increased temperature causes a gradual decrease in peak stress and modulus properties.…”