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<p>Methane in-situ combustion fracturing (MISEF) technology has emerged as a promising water-free stimulation method for enhancing shale gas production by inducing transient high-temperature and high-pressure conditions that effectively modify the fracture network within shale reservoirs. However, the intense thermal effects associated with MISEF can severely degrade the mechanical integrity of the wellbore-surrounding rock, leading to enhanced time-dependent deformation (creep) under sustained geostatic stress and consequently jeopardizing long-term wellbore stability. In this study, standard cylindrical shale specimens subjected to controlled thermal treatments at 25&#xa0;°C, 80&#xa0;°C, 150&#xa0;°C, and 225&#xa0;°C were examined using scanning electron microscopy to elucidate temperature-induced microstructural alterations. Triaxial graded-loading creep tests were subsequently conducted to characterize the creep behavior and rate of thermally damaged shale. To accurately describe the observed nonlinear creep response, a novel fractional-order creep model incorporating a soft-body element was proposed based on fractional calculus theory. A sensitivity analysis of the fractional-order parameter <i>γ</i> was also performed. For comparative validation, classical Burgers and Merchant models were applied to the same dataset. Experimental results reveal that elevated temperatures significantly accelerate the evolution of internal microcracks and pore structures, leading to pronounced microstructural deterioration. Creep rates at both decelerated and steady stages increase substantially with temperature, accompanied by a reduction in creep damage strength and amplified axial deformation, highlighting the loss of long-term mechanical stability. The proposed model achieved a coefficient of determination (R<sup>2</sup>) exceeding 0.95 across all conditions, demonstrating superior fitting accuracy and robustness compared to traditional models. These findings not only provide a comprehensive understanding of the thermally induced creep mechanisms in shale but also offer critical theoretical insights for evaluating and predicting the long-term integrity of shale formations under high-temperature, high-pressure conditions typical of MISEF operations.</p>

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Fractional Creep Modeling of Thermally Damaged Shales Under Post-High-Temperature Conditions

  • 期刊论文

  • 发表日期：

    2025年8月30日

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  • Qiaojun Chen,
  • Chun Liu,
  • Hu Qu,
  • Zhaoqian Wang,
  • Jihui Tian

摘要

Methane in-situ combustion fracturing (MISEF) technology has emerged as a promising water-free stimulation method for enhancing shale gas production by inducing transient high-temperature and high-pressure conditions that effectively modify the fracture network within shale reservoirs. However, the intense thermal effects associated with MISEF can severely degrade the mechanical integrity of the wellbore-surrounding rock, leading to enhanced time-dependent deformation (creep) under sustained geostatic stress and consequently jeopardizing long-term wellbore stability. In this study, standard cylindrical shale specimens subjected to controlled thermal treatments at 25 °C, 80 °C, 150 °C, and 225 °C were examined using scanning electron microscopy to elucidate temperature-induced microstructural alterations. Triaxial graded-loading creep tests were subsequently conducted to characterize the creep behavior and rate of thermally damaged shale. To accurately describe the observed nonlinear creep response, a novel fractional-order creep model incorporating a soft-body element was proposed based on fractional calculus theory. A sensitivity analysis of the fractional-order parameter γ was also performed. For comparative validation, classical Burgers and Merchant models were applied to the same dataset. Experimental results reveal that elevated temperatures significantly accelerate the evolution of internal microcracks and pore structures, leading to pronounced microstructural deterioration. Creep rates at both decelerated and steady stages increase substantially with temperature, accompanied by a reduction in creep damage strength and amplified axial deformation, highlighting the loss of long-term mechanical stability. The proposed model achieved a coefficient of determination (R2) exceeding 0.95 across all conditions, demonstrating superior fitting accuracy and robustness compared to traditional models. These findings not only provide a comprehensive understanding of the thermally induced creep mechanisms in shale but also offer critical theoretical insights for evaluating and predicting the long-term integrity of shale formations under high-temperature, high-pressure conditions typical of MISEF operations.