Behaviour of carbonate reservoir rocks under hydrostatic cyclic loading for hydrogen storage application

Geological hydrogen storage provides a large-scale and long-term solution to balance the seasonal supply-demand mismatch of renewable energy ^[1-2]. Depleted oil reservoirs represent a secure and feasible storage option ^[3], but the heterogeneity and complex microstructure of the carbonate rocks, which comprise approximately 60% of the global oil reserves make the utilization of such reservoirs still challenging ^[4-5]. This work aims to investigate potential performance changes in porous carbonate reservoir rocks under cyclic pressure induced by hydrogen injection and extraction. Triaxial tests under various hydrostatic cyclic loading paths, with continuous permeability measurement and acoustic wave velocity measurement, are conducted on Saint-Maximin limestone (SML) samples ^[6] to observe the mechanical degradation and permeability evolution. In addition, changes in microstructure and percolation characteristics of the compacted samples are characterized by mercury intrusion porosimetry tests. The results show that 50 cycles within the elastic zone result in only minor of irreversible porosity reduction, while permeability and stiffness of SML remain relatively stable. However, even minimal excursions past the plastic onset P^* lead to a noticeable deterioration in the properties of the SML over subsequent cycles, characterized by decreased porosity and permeability, creep and reduced stiffness. The viscoelastic constitutive model calibrated by a creep loading test is used to distinguish time-dependent and cyclic-dependent deformations. Furthermore, mesopore collapses are revealed to be the main source of damage, which leads to an increase in intrusion breakthrough capillary pressure as well as non-wetting phase trapping effects. These findings demonstrate that the historical maximum stress dictates the activation of damage processes, while the cycling intensifies existing damage accumulation without altering the intrinsic damage characteristics. Consequently, controlling the maximum stress level within the reservoir rock emerges as a pivotal parameter in the engineering design of hydrogen storage reservoirs.

References:

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