Experimental study on the variation of hydro-mechanical properties of reservoir rocks under cyclic loading and cyclic pressurization

Underground storage facilities are receiving increasing interest for a variety of geo-engineering applications in the realm of renewable and sustainable energy: geothermal systems, carbon capture and storage, nuclear waste repository and hydrogen storage. Particularly for the hydrogen storage case, this gaseous fluid in injected and withdrew cyclically, causing variations of the effective stress field of the reservoir and caprock.

Based on the Terzaghi effective stress law, changing the effective stress can be achieved by either changing the principal stress or the pore pressure. The first option, called cyclic loading and unloading, has been used extensively to study the effects of cyclic conditions on the hydro-mechanical properties of different rock types. However, the actual phenomenon occurring in a reservoir rock is the cyclic variation of pore pressure, or cyclic pressurization. The cyclic flow of pressurized fluids may mobilize particles, which can clog fluid pathways, and trigger chemical reactions such as dissolution. This leads to an alteration of the microstructure of the rock matrix differently from that caused by the cyclic loading and unloading case.

Although cyclic pressurization experiments cannot be run on every rock at the laboratory scale due to poor hydraulic properties, we chose a highly porous and permeable rock, Bentheim sandstone, which guarantee us a pore pressure equilibrium throughout a rock sample during this type of experiment. Apart from its hydraulic properties, Bentheim sandstone is regarded as a conventional georeservoir rock even at great depth, due to its mineral composition, homogeneity, micro- and macrostructure. Therefore, it has been extensively tested for a variety of applications to understand its physical and mechanical properties under changing environmental conditions.

As part of the TEN.efzn project, we performed a series of laboratory experiments on both intact and fractured rock samples, carried out in a servo-controlled triaxial apparatus, capable of simulating in-situ pressure and temperature conditions at relevant depths. By combining mechanical and hydraulic data with acoustic emission and ultrasonic velocity data, we observe that cyclic pressurization leads to higher sample compaction compared to cyclic loading and that the presence of a fracture zone leads to higher changes of the hydro-mechanical properties.

Our results suggest that the values of specific properties obtained during cyclic loading experiments underestimate the real values of reservoir rocks under cyclic fluid injection and withdrawal.