The global energy transition away from fossil fuels requires robust energy storage solutions to address the inherent intermittency of renewable sources. Underground hydrogen storage in depleted hydrocarbon reservoirs and saline aquifers is a promising candidate for such a solution. Although cyclic injection and extraction of hydrogen gas are not typically conducted under stress levels that can cause rock failure, it may lead to mechanical pore space degradation. Over time, cyclic pore pressure loading may reduce the initial pore volume and permeability, thus reducing reservoir storage capacity and impairing the efficiency of hydrogen injection and extraction operations at scale.
This study investigates the hydromechanical response of reservoir rock formations (Berea sandstone and Austin Chalk from the U.S.) to simulated cyclic hydrogen storage conditions. Laboratory-scale induced loading experiments (both of pore pressure and confining pressure) permit to reproduce conditions in reservoirs at a few hundreds of meters depth. During loading, we measured the evolution of pore volume as a function of amplitude and frequency of pore pressure or confining pressure cycles. Preliminary findings indicate a measurable permanent degradation in porosity of over 1% in chalks, demonstrating the long-term impact of these operations. We found that the most influential factor regarding the effect of the pore degradation is the pore pressure injected into the rock with higher pore pressures yielding the most degradation. These findings provide insights for a comprehensive understanding of reservoir rock-fluid-stress interactions under dynamic conditions, and highlight the need for further investigation and modelling of the long-term geomechanical performance of reservoir rocks used for hydrogen storage.