STOWENG

Subsurface energy technologies will play a central role in the smooth transition towards a lower-carbon future. Storage of energy into underground reservoirs is one of the current solutions to solve energy production and consumption irregularities. Two types of underground storage are particularly important: the short-term storage of subsurface energies such as compressed air, hydrogen, and water, and the long-term storage/sequestration of hydrocarbons (liquid or liquefied) and CO2. In this project the attention focuses on this last topic. Petrophysical and hydro-thermo-mechanical properties of underground reservoir materials are key properties to evaluate yield and storage potential; however they are locally different than the rest of the basin. The interface between the reservoir rock and the near field, in particular the caprock, is therefore a potential area for critical hydro-mechanical behavior, generating possible instabilities and high environmental risks. The most relevant issue, for all underground storages, concerns the environmental impact and the health and safety of people in the occurrence of the loss of sealing capacity of the reservoir,. Fracture triggering and propagation caused by temperature or pressure variations, particularly sudden changes, may compromise the storage confinement, cause gas/fluid migration through groundwater and induce micro-seismicity. The main objective of this project is to investigate the trigger conditions of these critical states, focusing in particular on solid fracturing and fluid fingering, at the interface between the underground reservoir and the host formation, and attempting to establish proper causality relations between them. The particular case of low permeability clayey geomaterials is addressed, which typically constitute the near field (in particular the caprock) of the reservoir. A multi disciplinary approach will be adopted, which aims at merging competences in micro-scale and laboratory scale experimental geomechanics, constitutive laws and numerical modeling, to identify the main characteristics of low permeability geomaterials in terms of hydraulic tightness and resistance to applied stresses. The main interest, and the originality, of the project reside in covering the gap of knowledge and the lack of modeling tools of the micro-scale physical processes occurring within low permeability clayey geomaterials as well as in describing the effects of these phenomena on the macro-scale properties of the clayey soil. In order to understand to what extent tightness of underground reservoirs is affected by the occurrence of the above mentioned critical states, prototype low permeability geomaterials, sand-clay mixtures, prepared in the laboratory, and natural clayey geomaterials will be tested imaged and modeled, under different hydro-mechanical loading conditions. First of all the experimental activity will concentrate on the analysis, at the laboratory scale, of suitable analog materials: on the one hand, granular materials saturated by highly viscous fluids, which simulate low hydraulic conductivity materials; on the other hand macro-crystals of clay, which will be used to investigate chemo-mechanical couplings, but at a much larger scale than usual. Secondly, micro-scale experiments will be implemented to analyze the behavior of sand-clay mixtures and natural geomaterials, under biaxial and in-situ loading conditions. The modeling activity will be devoted to the formulation and the numerical implementation of a phase-field model for describing the condition of partial saturation. The presence of more than one fluid phase within the porous network, and the possible occurrence of strain localization and fracture of the solid component will be considered.