Geomechanical Characterization of Natural Shale-Sandstone Interface
Authors: Lloyd-Mills Frank , Cheng Jingyi , Wang Meng , Li Wenfeng 
Published: September 2023

ABSTRACT
Understanding the strength of rock discontinuities has been one of the core research topics in geomechanics over the past decades. This is unsurprising as many structural failures in underground excavations, including mining and tunnelling, occur at rock discontinuities. In oil, gas, and geothermal industry, the strength of rock discontinuities has also attracted a great deal of attentions because, for example, pre-existing discontinuities can significantly change the structures of hydraulic fracture networks. It is also anticipated that the strength of rock discontinuities will become increasingly important for leakage risk assessment of carbon dioxide geological sequestration projects. For all these reasons, we must achieve a good understanding of strength of natural rock discontinuities at the subsurface stress conditions, which, unfortunately, is not well explored. This problem can be primarily attributed to the limited numbers of valid rock samples that contain natural and undisturbed rock discontinuities, especially for the case of cohesive rock discontinuities. In this study, we obtained some rock specimens with naturally bonded shale-sandstone interface. We measured the geomechanical properties of the natural shale-sandstone interface under constant normal loads, using biaxial direct shear apparatus. The measured strength of interface indicates the combined effect of normal loading levels and natural interface roughness. Our characterization provides valuable geomechanical data of natural shale-sandstone interface at the subsurface conditions. These data can benefit all subsurface engineering projects where it is crucial to examine shale-sandstone interface failure for project success, including underground mining, tunnelling, fluid injection/extraction in sandstone reservoirs of shale caprocks.

INTRODUCTION
Geomechanical properties of rock discontinuities are of great importance in many underground engineering projects. In mining engineering, rock discontinuities often govern deformation and failure of rock masses surrounding underground openings, such as the occurrence of coal bumps due to lost of constraint at coal-rock interface (Li et al., 2015a and 2015b). In oil and gas engineering, rock discontinuities are the primary fluid flow pathways in tight formations and can also greatly influence the wellbore stability and hydraulic fracture structures (Karatela et al., 2016; Li et al., 2021a and 2021b; Li et al., 2022; Meng et al., 2021a and 2021b; Welch et al., 2021). In enhanced geothermal system (EGS), rock discontinuities govern the hydrofracking and hydroshearing process and contribute significantly to the stimulated rock volume for effective fluid flow and heat mining (Rinaldi and Rutqvist, 2019; Bijay and Ghazanfari, 2021; Meng et al., 2022). In carbon sequestration projects, rock discontinuities can cause CO2 leakage under rock stress or fluid pressure perturbation (Carey et al., 2009; Pan et al., 2013; Frash et al., 2017). In nuclear waste disposal or underground nuclear explosion detection, rock discontinuities can facilitate transport of the radionuclide gas isotopes from the cavity to the surface (Zhang et al., 2022). It is anticipated that in carbon mineralization projects, rock discontinuities can greatly affect project success because mineral dissolution and solid mineral molar precipitation can either promote or kill the long-term fracture permeability due to the coupled thermos-hydro-mechanical-chemical (THMC) processes (xiong et al., 2017; Menefee et al., 2018). For all those reasons, it is imperative to study the geomechanical properties of rock discontinuities for effective utilization of subsurface resources.