Assessing the suitability of geological formations as safe locations for seasonal hydrogen storage requires in-depth geological knowledge and good engineering practice. Establishing the safety and viability conditions of such projects requires solid multidisciplinary experimental evidence. In this context, the role of seal formations is crucial, in particular when considering H₂-tightness and their related transport properties. While there is a wide number of works addressing the performance of low-permeability seal rocks (salt formations, shales, etc.) when exposed to a variety of gasses (CO₂, N₂, CH₄, etc.) there is a significant lack of information on the behavior and interaction of H₂ with natural rocks under realistic geological storage conditions. In this work we will present results of experimental works developed on seal rock materials of potential interest for H₂ storage (Late Miocene marls) under representative field conditions (60 ºC; 17.5 MPa vertical stress; 8 MPa pore pressure; dry density ~1.7 g/cm³). Due to the poor machinability of the rock, a sample reconstitution methodology was required to perform the tests. These included a stepwise sequence of liquid (in situ sampled formation fluid; 4.26 mS/cm) and gas (H₂; 99.9992 purity) steady-state injections aimed at assessing its intrinsic permeability and breakthrough pressure either in single and multiple drainage/re-imbibition stages.

The creep behavior of rocks significantly affects the long-term stability of underground spaces. This phenomenon becomes more remarkable in the case of soft rocks, deep underground construction, and rocks subjected to high stresses. A better understanding of the creep behavior of rocks is crucial for the analysis of the time-dependent behavior of rocks. In this study, a new creep constitutive model is proposed by replacing a Newtonian dashpot in the Burger model with the fractional derivative dashpot and introducing a viscoplastic element to model the accelerated phase of rock creep. The proposed model is validated by the creep test results of soft rocks, which shows that this model can comprehensively describe rock creep characteristics.

The ideal circumstance for accumulation and passage for CBM gas is well developed pores and cracks in gas reservoirs. To explore the multi-scale characteristics of pores, two coal samples have been collected from Raniganj Basin, which has been of interest to scientists and policy-makers due to its abundance of CBM resources and favorable conditions for CBM enrichment in eastern India. Quantitative pore features as evaluated by fluid based method (low-pressure N₂ and Co₂ adsorption) and image based method (Field Emission Scanning Electron Microscope) in this investigation. By utilizing these methods, Characteristics of nano pore including pore size distribution, pore volume, specific surface area and pore shape has been understood. To study the effect of pores on methane adsorption, a full scale pore estimation model has been introduced with the combination of BJH, BET, Langmuir and NLDFT model to establish the micropore (0.4-1.6 nm) and mesopore (1.6-20nm) attributes. According to the case study’s analysis, Raniganj coal samples have a certain specific surface area and mesopore and micropore volume, varying from 8.2913 to 11.8303m²/g and 0.0165 to 0.0188cm³/g for mesopores and 32.042 to 39.416m²/g and 0.014 to 0.017cm³/g for micropore. Mesopore size distributions are multimodal and micropore size distributions are unimodal. The surface fractal dimension of both coal samples is determined using the area-perimeter approach based on microphotographs. The value of the fractal dimension ranges from 1.17 to 1.30. These findings of the study demonstrate that the coal surface has a clear fractal aspect and that fractal theory is a useful tool for understanding the complicacy of pore morphology.

It is not uncommon that seismically active zones are generated away from the region where anthropogenic activities, such as mining excavation and fluid injection, caused a severe change in the in-situ stress. It is still quite challenging to predict the occurrence of such dynamic instability in the remote regions because the stress change in such regions is relatively small and the rock mass is presumed to be stable from a theoretical point of view. It can be deduced from previous field measurements that one of the factors contributing to the occurrence of the rock mass instability is pre-existing stress heterogeneity resulting from the macroscopic stiffness variation of the rock mass. The present study addresses this problem by performing the numerical simulation of the in-situ stress state in a sedimentary basin with a boundary traction method whilst considering the difference in stiffness among geological layers as well as the geometry of the boundary. First, a model parametric study is conducted whilst changing the stiffness of each geological layer. The result indicates that the difference in stiffness between the basement and sedimentary rock significantly disturbs the in-situ stress state, leading to a remarkable increase in deviatoric stress from the hydrostatic stresses applied to the model boundaries. The degree of the stress discrepancy is more pronounced with the decrease in the stiffness of the sedimentary rock situated above the basement. Then, the geometric effect of the geological boundary between the basement and the sedimentary basin is investigated. Interestingly, the result shows that the deviatoric stress of the rock mass increases above a convex geological boundary, indicating a large potential for instability. In contract, when the boundary is concave, the maximum stress decreases above the boundary, but the stress increases in the basement beneath the boundary. This implies that the risk for rock mass instability decreases above the boundary and vice versa. These results clearly shed light on the mechanism of the dynamic instability taking place away from the region where the in-situ stress was severely disturbed due to anthropogenic activities. Importantly, the presented method needs to be combined with the heterogeneity of rock mass strength in the future in order to develop a more reliable prediction method for the dynamic instability of rock mass.