Effects of Temperature Cycling on the Mechanical and Transport Properties of Porous Sandstone: Implications for CO2 Storage in Depleted Hydrocarbon Reservoirs 

Abstract

Carbon capture and storage (CCS) stands as a key technology for mitigating CO2 emissions, with depleted oil and gas fields being excellent candidates for geological storage. However, injection of relatively cold, high-pressure CO2 into higher temperature, low-pressure hydrocarbon reservoirs can induce cooling and potential freezing due to the temperature difference between the injected fluid and the reservoir, as well as Joule-Thomson cooling caused by the rapid expansion of the fluid upon entering the reservoir. This may impact wellbore integrity, and near-wellbore stability and injectivity, posing challenges for safe and cost-effective storage. To be able to accurately predict the impact of cooling on storage operations, it is important to quantify the impact of temperature cycling on the mechanical and transport properties of the rock formations in the near-wellbore area. To address this, we performed thermal cycling experiments under realistic in-situ pressure-temperature conditions on sandstone analogous to typical hydrocarbon reservoir material. We used a novel apparatus comprising a hydrostatic pressure vessel placed inside a climate chamber providing a temperature range of -70°C to +180°C. Bleurswiller sandstone (Vosges, France; 24% porosity) was subjected to temperature changes from 100 °C to +40, +5, or -20°C at constant pore fluid pressure (5 MPa; 0.85 M NaCl brine) and confining pressure (10 or 25 MPa, i.e. similar to reservoirs of up to ~3 km depth). The effect of the rate of temperature change, brine saturation and the number of cycles on the volumetric behaviour of the sandstone were systematically investigated. Thermally treated samples were subsequently subjected to permeability measurements and conventional triaxial compression to evaluate the impact of confined temperature cycling on the transport and mechanical properties. In all our thermal cycling experiments, we observed permanent volume change (compaction) with each cycle, though the amount of compaction decreased with subsequent cycles. Furthermore, our results showed that confined temperature cycling did not significantly alter the mechanical properties (strength, elastic properties) of Bleurswiller sandstone. This is in contrast to the strength reduction observed in other porous sandstones after unconfined thermal cycling. However, our thermally treated samples did exhibit a significant reduction in permeability by several orders of magnitude (κ = 10-15 to 10-16 m2 post-treatment) compared to untreated reference samples (initial κ = ~10-14 m2). Overall, permeability roughly decreased with increasing brine content (i.e. from dry to fully brine saturated), increasing number of thermal cycles, and increased temperature amplitude (i.e. more cooling). Temperature change rate did not affect the permanent volumetric strain or permeability reduction in samples that were only cooled. In experiments achieving sub-zero temperatures, including pore fluid freezing, slower temperature changes resulted in less permeability reduction.