The global energy sector is transitioning to a more sustainable future. As a result, the use of depleted sandstone reservoirs for geo-engineering techniques including the storage of hydrogen gas, compressed air, and carbon dioxide will increase. These storage practises, as well as the production of geothermal energy, all involve the injection and potentially the removal of a pore fluid on a cyclic basis over a range of timescales.
Recent experimental advancements have demonstrated that during the initial depletion of sandstone reservoirs, up to 40% of the total strain measured in the reservoir may be inelastic (i.e. permanent). However, uncertainties remain pertaining to how the portion of inelastic strain increases if the rate of depletion/deformation occurs over decadal time periods, or if repeated cycles of injection and depletion are observed.
To answer these questions, a series of triaxial deformation experiments have been performed on typical reservoir sandstones. These experiments were designed to assess how the rate of depletion and cyclic loading affect the elastic parameters of the rock, as well as the evolution of inelastic strain. Experiments were carried out at the pressure, temperature, and chemical conditions of the Groningen gas field in the Netherlands and ranged from a few minutes to several weeks in duration.
We demonstrate that a non-negligible rate effect is observed, and that the amount of inelastic strain introduced into the samples at an axial strain rate of 10-9s-1 may be 1.5 times that measured at a strain rate of 10-5s-1. Additionally, we show that compared to the initial loading, further loading cycles to the stress levels relevant for typical storage sites, produce only very small amounts of further inelastic strain at realistic numbers of cycles (sub-100). We observed that over 1000 further loading cycles are required to double the total amount of inelastic strain and no failure of the samples occurred. In contrast, at stress levels closer to the conventional yield stress, the effect of large numbers of cycles is much more pronounced, and eventually failure of the sample is observed through fatigue damage.
We describe the rate dependence of inelastic strain generation and the small increase due to cyclic loading through a combination of mechanisms including intergranular clay compaction, stress corrosion cracking, and grain rearrangement and frictional controlled sliding. We then discuss what the implications of our results are in regard to current gas production and future storage techniques.
