to mitigate climate change, carbon capture and storage (CCS) is considered to be one of the most promising technologies to meet CO2 emission reduction targets. During CCS, CO2 is captured at the source (e.g. power plant or CO2-heavy industrial plant), transported and injected at depth into the subsurface. In the Netherlands, re-use of depleted oil and gas fields is currently being considered for long-term CO2 storage through the Porthos and Aramis projects. However, injection of high-pressure CO2 into a low-pressure, depleted reservoir can affect mechanical, transport and thermal properties of the host reservoir and the overlying caprock, as it can lead to substantial, local, cooling. This could impact injectivity, near-well stability and well integrity, thereby impacting storage safety. Several studies have investigated the impact of thermal cycling on reservoir behavior, though most of these studies were performed under unconfined conditions. However, little is known of the effect of freeze-thaw cycles under elevated pressures representative for realistic in-situ conditions.
Therefore, we conducted a series of hydrostatic experiments (up to 30 MPa confinement) under cyclic temperature conditions (100°C down to 5 or -20°C) to assess the potential damage and volumetric response of an analogue reservoir sandstone (Bleurswiller sandstone, 24% porosity). We investigated the impact of saturation, number of thermal cycles, temperature amplitude, and confining pressure. Following the thermal treatment, triaxial compressive tests, coupled with permeametry, were performed to determine the impact of thermal cycling on mechanical properties and permeability.
Our results demonstrated that freeze-thaw cycling under confinement, leads to permanent bulk volume reduction, which decreases with increasing number of the cycles. However, no significant impact on peak strength and elastic moduli was observed, though a significant decrease in permeability was measured after the thermal treatments. Furthermore, we found that this impact varies depending on the specific conditions investigated. Notably, a greater reduction in permeability was observed when either the number of freeze/thaw cycles or the saturation level of the sample was increased. To assess the grain-scale mechanisms causing the damage induced by freeze-thaw cycling, detailed microstructural analysis will be performed. Eventually, based on the predominant damage mechanisms, a constitutive law will be formulated to establish the relationship between thermal cycling and the observed damage.
