Guest editorial for the topical collection: geomechanics for deep resource and energy exploitation

1 Introduction

With the rapid depletion of resources from shallow georeservoirs, increasing attention has been paid to deep mineral, ore and energy resources, e.g., coal, metal ore, coal seams, conventional and unconventional oil and gas reservoirs, and geothermal reservoirs. Up to now, deep resource and energy exploitation over the upper 1000 m of the Earth’s crust has been widely reported worldwide for different types of resources. The depths of coal mines, geothermal exploitation, non-ferrous metal mines and oil or gas exploitation have reached 1500, 5000, 4500 and 7500 m, respectively, in past decades (Ranjith et al. 2017; Xie et al. 2019). Therefore, resource and energy exploitation from deeper reservoirs will become more prevalent in the future as the resources from shallow geosystems have gradually become exhausted. In addition to tapping the potential of deep energy resources, deep energy storage and geological repositories are becoming part of the man-made geoenergy cycle (Kolditz et al. 2021).

The exploitation of these deep resources always involves multi-physical processes and coupled behaviors as the related geosystems are exposed to elevated conditions, including high temperature, pore pressure and in-situ stress that will trigger significant time-dependent effect and dynamic disturbance. For example, stress at depths of 1000 to 5000 m may be as high as 250 MPa due to the weight of the overburden and stress concentration caused by the geological structures and man-made activities. While ambient temperature and hydraulic pressure at a depth of around 1000 m may raise nominally be around 40 °C and 10 MPa, respectively, and temperatures as high as 400 °C have been measured in geothermal energy fields. Thus, the distinct features of deep geosystems result in the complicity of deep resource and energy exploitation and surely unforeseen challenges and problems. This may lead to increasingly unprecedented disasters or abnormal failures during deep energy and resource exploitation, e.g., rockburst, induced seismicity, coal-gas outburst and large caving.

Therefore, there is indeed a growing demand for new theories and techniques for resource and energy exploitation from deep georeservoirs considering the extreme in-situ conditions, e.g., the high temperature, pore pressure and in-situ stress along with dynamic disturbance. Additionally, a thorough understanding of geomechanics of deeper systems is a prerequisite for the successful and safe exploitation of resource and energy from deep geosystems. This Special Issue is a collection to highlight recent progress on the fundamental mechanisms and in-situ applications in deep resources, energy exploitation and deep geological repositories. Thus, this special issue will bring forward advanced research and promote cutting-edge, innovative, and creative techniques in geomechanics for resource and energy exploitation from deep geosystems.

2 Scope of the special issue/topical collection

This special issue “Geomechanics for Deep Resource and Energy Exploitation” aims at addressing these challenges and to present recent progress in geomechanics for deep resource and energy exploitation. The special issue has collected 25 novel research papers covering experimental, analytical, and numerical progresses of deep geomechanics. A series of experimental devices, e.g., a mining dynamic stress test system, a creep and dynamic disturbance impact loading test system, a modified triaxial split Hopkinson pressure bar system and a hydraulic impact test system are developed to quantify the dynamic responses of geomaterials subjected to complex loading conditions such as nonuniform load, disturbance stress and coupled hydraulic-mechanical loading and to simulate the rockburst. New findings are achieved through conducting a number of cross-scale experiments including friction-permeability testing on rock fracture, compressive testing on flawed rocks, direct tensile testing on shale, biaxial shear testing on a single saw cut granite fracture, triaxial hydraulic fracturing testing on coal sample, triaxial compression testing on coal mass and large-scale experimental roadways stability experiments conducted on the physical model test system. Numerical studies of non-isothermal two-phase flow in deformable porous, hydraulic fracturing, the CO₂ long-term periodic injection and fault-slip induced seismicity are also included in this Special Issue. Theoretical studies of rockburst criterion, heat extraction capability and pressure solution creep are included to insight into the fundamental issue of deep geomechanics. Field studies regarding blasting, rockburst control, outburst characteristics of CO₂ gas-coal mixtures, stability of large-section multi-chamber group and strong earthquakes induced by deep coal mining are demonstrated in this issue. The Guest Editors are appreciating and acknowledging the contributions of these authors, reviewers and the editorial office to make this Special Issue published successfully. The contributions of this special issue are summarized as follows:

• 1. “Non-isothermal two-phase flow in deformable porous media: systematic open-source implementation and verification procedure” by Grunwald et al. (2022).

• 2. “Experimental characterization of time-dependent mechanical behaviours of frac sand at high compressive stresses and implication on long-term proppant conductivity” by Sanchez et al. (2022).

• 3. “Development and application of a series of experimental devices for coal mining dynamic behavior research” by Wen et al. (2022).

• 4. “Dynamic response of rock mass subjected to blasting disturbance during tunnel shaft excavation: a field study” by Xie et al. (2022).

• 5. “Experimental study on the mechanism of pressure releasing control in deep coal mine roadways located in faulted zone” by Wang et al. (2022).

• 6. “Stability of roadway along hard roof goaf by stress relief technique in deep mines: a theoretical, numerical and field study” by Sun et al. (2022).

• 7. “Optimization of hydraulic fracturing with rod-shaped proppants for improved recovery in tight gas reservoirs” by Mehmood et al. (2022).

• 8. “A criterion of rockburst in coal mines considering the influence of working face mining velocity” by Li et al. (2022).

• 9. “Outburst characteristics of CO2 gas-coal mixture with tunnel outburst simulator in deep mining” by Ding and Yue (2022).

• 10. “A modified triaxial split Hopkinson pressure bar (SHPB) system for quantifying the dynamic compressive response of porous rocks subjected to coupled hydraulic-mechanical loading” by Zhao et al. (2022).

• 11. “Impact of injection rate ramp-up on nucleation and arrest of dynamic fault slip” by Ciardo and Rinaldi (2022).

• 12. “Laboratory friction-permeability response of rock fractures: a review and new insights” by Fang and Wu (2022).

• 13. “Stability analysis and determination of large-section multi-chamber group in deep coal mine” by Tan et al. (2022).

• 14. “Development and application of a hydraulic impact test machine for simulating rockburst conditions” by Pan et al. (2022).

• 15. “Innovative measures for thermal performance enhancement of single well-based deep geothermal systems : Existing solutions and some viable options” by Xiao et al. (2022).

• 16. “Hydro-Mechanical response of Opalinus Clay in the CO2 long-term periodic injection experiment (CO2LPIE) at the Mont Terri rock laboratory” by Sciandra et al. (2022).

• 17. “Experimental investigation of mechanical properties, impact tendency, and brittleness characteristics of coal mass under different gas adsorption pressures” by Xue et al. (2022)

• 18. “Full-field quantification of time-dependent and -independent deformation and fracturing of double-notch flawed rock using digital image correlation” by Xue et al. (2021).

• 19. “On the strong earthquakes induced by deep coal mining under thick strata-a case study” by Jiao et al. (2021).

• 20. “Experimental investigation on failure behaviors and mechanism of an anisotropic shale in direct tension” by Li et al. (2021).

• 21. “Thermally induced slip of a single sawcut granite fracture under biaxial loading” by Sun and Zhuang (2021).

• 22. “Characterization of hydraulic crack initiation of coal seams under the coupling effects of geostress difference and complexity of pre-existing natural fractures” by Liu et al. (2021).

• 23. “A displacement-dependent moment tensor method for simulating fault-slip induced seismicity” by Bai et al. (2021).

• 24. “What process causes the slowdown of pressure solution creep” by Lu et al. (2021).

• 25. “Discrete element numerical simulation of two-hole synchronous hydraulic fracturing” by Yang et al. (2021).