Key Takeaways & Executive Findings
- •• In-situ high-temperature triaxial tests reveal that sandstone peak stress decreases with temperature but increases with confining pressure, with the confining pressure effect diminishing at higher temperatures. • Thermal conductivity and diffusivity of sandstone show a linear relationship with temperature, while mass loss increases with temperature. • Poisson's ratio and internal friction angle decrease with rising temperature, with 400°C identified as a threshold for friction angle reduction. • Under confining pressures of 10 and 20 MPa, damage stress decreases with temperature, whereas uniaxial damage stress is less temperature-sensitive, providing critical data for caprock integrity in oil shale conversion.
Abstract
Low- to medium-maturity oil shale resources display substantial reserves, offering promising prospects for in-situ conversion in China. Investigating the evolution of the mechanical properties of the reservoir and caprock under in-situ high-temperature and confinement conditions is of considerable importance. Compared to conventional mechanical experiments on rock samples after high-temperature treatment, in-situ high-temperature experiments can more accurately characterize the behavior of rocks in practical engineering, thereby providing a more realistic reflection of their mechanical properties. In this study, an in-situ high-temperature triaxial compression testing machine is developed to conduct in-situ compression tests on sandstone at different temperatures (25, 200, 400, 500, and 650°C) and confining pressures (0, 10, and 20 MPa). Based on the experimental results, the temperature-dependent changes in compressive strength, peak strain, elastic modulus, Poisson’s ratio, cohesion, and internal friction angle are thoroughly analyzed and discussed. Results indicate that the mass of sandstone gradually decreases as the temperature increases. The thermal conductivity and thermal diffusivity of sandstone exhibit a linear relationship with temperature. Peak stress decreases as the temperature rises, while it increases with higher confining pressures. Notably, the influence of confining pressure on peak stress diminishes at higher temperatures. Additionally, as the temperature rises, the Poisson’s ratio of sandstone decreases. The internal friction angle also decreases with increasing temperature, with 400°C acting as the threshold temperature. Interestingly, under uniaxial conditions, the damage stress of sandstone is less affected by temperature. However, when the confining pressure is 10 or 20 MPa, the damage stress decreases as the temperature increases. This study enhances our understanding of the influence of in-situ high-temperature and confinement conditions on the mechanical properties of sandstone strata. The study also provides valuable references and experimental data that support the development of low- to medium-maturity oil shale resources.
1. Introduction
China has abundant recoverable resources of low- to medium-maturity oil shale resources, which are comparable to the total recoverable resources from conventional oil technologies [1–3]. For low- to medium-maturity oil shale, in-situ conversion technology is generally used to convert the organic matter in the oil shale layers into oil and natural gas through heating [4–5]. During in-situ heat injection and oil extraction, high temperatures can alter the composition and organization of the caprock, making it an unstable factor for oil and gas production [6–7]. Therefore, investigating the evolution of sandstone physico-mechanical properties under in-situ high-temperature and confinement conditions is essential to advance the extraction of low- to medium-maturity oil shale resources.
With the development of low- to medium-maturity oil shale reservoirs, extensive studies have been conducted on the mechanical properties of caprock sandstone that has naturally cooled at high temperatures and different confining pressures [8–15]. The influence of these conditions on the stability and integrity of the caprock can be evaluated by examining the changes in the physical and mechanical properties of sandstone under high temperatures and different confining pressures [16]. Notably, at high temperatures, the failure mode of sandstone may transition from brittle to ductile, a shift that carries considerable implications for engineering design and the prevention of geological disasters [17]. Furthermore, high temperatures can impact the permeability of sandstone, which, in turn, can alter fluid flow pathways and efficiency, affecting the effectiveness of oil and gas extraction operations [18]. Recent studies have also concentrated on the dynamic mechanical properties of sandstone under high-temperature conditions to effectively understand rock behavior under extreme conditions.
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Liyuan Liu, Juan Jin, Jiandong Liu, Wei Cheng, Minghui Zhao, Shengwen Luo, Yifan Luo, Tao Wang (2025). Mechanical properties of sandstone under in-situ high-temperature and confinement conditions. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-3047-9
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Frequently Asked Questions
What is the significance of in-situ high-temperature testing for sandstone?
In-situ high-temperature testing more accurately replicates real engineering conditions compared to post-treatment tests, providing realistic mechanical property data for caprock stability in oil shale conversion.
How does temperature affect the peak stress of sandstone?
Peak stress decreases with increasing temperature, while higher confining pressures increase peak stress. However, the influence of confining pressure diminishes at elevated temperatures.
What is the threshold temperature for internal friction angle changes?
The internal friction angle decreases with temperature, with 400°C identified as the threshold temperature beyond which significant reduction occurs.
How does confining pressure influence damage stress at high temperatures?
Under confining pressures of 10 or 20 MPa, damage stress decreases with temperature, whereas under uniaxial conditions, damage stress is less affected by temperature.
What are the practical implications of this study?
The findings provide critical experimental data and references for evaluating caprock integrity and supporting the development of low- to medium-maturity oil shale resources through in-situ conversion.
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