Strain-based microstructural evolution of shale caprock during cyclic underground hydrogen storage

Successful large-scale underground hydrogen storage (UHS) in depleted gas reservoirs relies on the integrity of the overlying caprock to prevent hydrogen loss during continuous injection-depletion cycles. Recent experimental work on crushed Marcellus shale, a potential caprock, revealed that cyclic hydrogen injection and depletion induces microstructural changes, such as micro-fracturing along grain boundaries, thus increasing the shale matrix porosity and permeability. This leads to higher hydrogen loss through diffusive flux, although the extent of these changes was not accurately quantified. To address this, a strain-based experiment was designed to measure volumetric strains in the shale matrix as a function of hydrogen injection/depletion cycles under unconstrained stress conditions. A quadrant-shaped Marcellus shale sample without visible fractures was subjected to eight cyclic hydrostatic pore-pressure cycles (injection to 1500 psi and depletion to 500 psi in 250 psi steps) under isothermal conditions. Strains in three orthogonal directions were measured using linear strain gauges. Results showed progressive plastic strain, contributing to a ~13% increase in matrix porosity. The results were used to develop a logarithmic based model to predict porosity changes due to cyclic hydrogen operation in shale caprocks. Results also indicated that intact shales have a higher resistance to micro-fracturing compared to crushed shales.