Effect of mechanical weakening and crack formation on caprock integrity during underground hydrogen storage in depleted gas reservoirs – A comprehensive review

This paper comprehensively reviews mechanical weakening and crack development in the caprock during underground hydrogen storage in depleted gas reservoirs. Hydrogen loss due to the geochemical interactions of hydrogen and caprock minerals critically impacts caprock integrity. As shown in the review, it is conspicuous that the mechanical properties of the caprock also change with the hydrogen injection, affecting its brittle-ductile behaviour. Furthermore, the stress–strain behaviour of the caprock is changed, and it undergoes irreversible deformations under the influence of confining pressure, cyclic loading, and changes in the mineral composition.The fracturing of the caprock is another critical impact on the storage integrity, which may create new routes for the hydrogen permeation through the caprock. Cracks may form in the caprock in multiple ways, mainly 03 ways; 1) Highly pressurized hydrogen injection creates critical cracks in the caprock when the pore pressure exceeds the fracture toughness, called critical cracks, 2) The injected hydrogen accumulates under the caprock due to gravity segregation and, eventually, diffuses into the caprock, displacing its pore fluid (brine). Consequently, capillary stress on the caprock minerals and the pores may increase with developing cracks, called shrinkage cracks, and 3) Geomechanical interaction between hydron-pore fluid-rock minerals under the biotic environment (micro-organisms) available at underground storage sites can cause mechanical properties degradation in caprock, forming new cracks under low injection pressure conditions, called sub-critical cracks.Although the critical or tensile crack formation process has been widely studied in the existing studies, minor attention has been given to other possible crack formation processes in the caprock, including the hydrogen-induced shrinkage cracking and the geomechanical reactions causing sub-critical cracking. In addition, the impact of this mechanical weakening of the caprock on its overall structure and flow characteristics hasn't been properly understood, adding extra uncertainty to the caprock's integrity during the underground hydrogen storage process.