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High-temperature rock mechanics and thermoplastic theory with emphasis on hot dry rock fracturing: advances and perspectives

Year: 2026

Type: article

Abstract: High temperatures in geothermal reservoirs increase the complexity of rock mechanical properties, constraining the effectiveness of hydraulic fracturing stimulation for enhanced permeability in hot dry rock (HDR). Research on the high-temperature rock mechanics relevant to HDR fracturing has primarily focused on two thermal mechanisms: intrinsic high-temperature environments and fluid-rock thermal shocks. Studies have found that, in general, heat treatment at ≥200 ℃ and real-time high temperatures of ≥100 ℃ significantly reduce the strength parameters of granite under compression, and alter the initial and critical states of the yield surface as well as its evolutionary process between them. This indicates that temperature alters the plastic mechanical properties of rock. Other experiments observed that increasing real-time temperature initially increases and then decreases the rock compressive strength. Thermal shock from lower temperatures leads to an approximately linear decrease in both compressive and tensile strength. Temperature increase causes the rock fracture toughness to first increase and then decrease, with the critical transition temperature generally lying between 100 ℃ and 200 ℃. The fracture process zone (FPZ) at the rock crack tip, characterized by plastic softening, notably expands with increasing temperature above 150 ℃, indicating enhanced rock thermoplasticity at high temperatures. Changes in rock microstructure with temperature primarily manifest as three competing mechanisms: the release of different types of bound water, the development of microcracks, and thermal expansion-induced mutual compaction of minerals. These competing mechanisms cause the observed trend of initial increase followed by decrease in rock strength and fracture toughness with rising temperature. Physical and numerical modeling of HDR fracturing indicates that thermal shock from cooler fracturing fluid reduces fracture initiation pressure, induces earlier initiation and delayed propagation of the hydraulic fracture, activates weak planes, alters the stress field around the fracture, and promotes the formation of branch fractures. A temperature-dependent microcrack zone exists around the hydraulic fracture, suggesting that the propagation of hydraulic fracture at high temperatures exhibits thermoplastic mechanical characteristics. Based on the above high-temperature mechanical properties, a theory of rock thermoplasticity has been developed. Focusing on the three thermoplastic zones around the hydraulic fracture—comprising the fracture process zone, the microcrack zone, and the compressive-shear plastic zone—a framework for thermoplastic constitutive modeling has been established. This includes fully coupled stress-strain-temperature constitutive relations and temperature-dependent yield/hardening/unloading criteria that distinguish between hydrostatic pressure and deviatoric stress. By linking the thermoplastic softening in the FPZ, a thermoplastic fracture model integrating plastic and fracture mechanics was developed, characterized by seven parameters that comprehensively describe high-temperature rock fracture. Three thermoplastic response factors were proposed to characterize the microcrack zone around a high-temperature hydraulic fracture, to evaluate the permeability enhancement effectiveness through fracturing. Future research should focus on the dependence of rock mechanical properties on thermal stress under boundary displacement constraints and should distinguish, both experimentally and in modeling, between the effects of heat treatment and real-time high temperature on rock mechanical behavior. It is also necessary to enhance the integration of thermoplastic theory with fracturing models within a coupled Thermo-Hydro-Mechanical-Chemical (THMC) framework and to further elucidate the mechanisms governing the formation and propagation of multiple fractures during HDR fracturing.

Source: Chinese journal of rock mechanics and engineering

DOI: 10.3724/1000-6915.jrme.2025.0335

Authors: Yuekun XING, Guangqing ZHANG

Institutions: China University of Petroleum, Beijing, China University of Mining and Technology

Language: English

Topic: Rock Mechanics and Modeling

Subfield: Mechanics of Materials

Field: Engineering

Domain: Physical Sciences

Open Access status: bronze

Funders: National Natural Science Foundation of China, China University of Mining and Technology, Fundamental Research Funds for the Central Universities

Award: G4405926543