The effect of supercritical CO2, pressure, and temperature on the acoustic and mechanical properties of soft caprock mudstone

• Rapid, cost-effective testing of caprock response to scCO₂ exposure was conducted using small, cutting-sized mudstone samples. • Post-exposure increases in compressional (∼10 %) and shear wave velocities (10–50 %), and shear strength (85–130 %) were observed. • No mineralogical changes were detected by XRD, likely because they were below the technique’s resolution or obscured by the mudstone’s natural heterogeneity. • This suggested that the mechanical strengthening was primarily due to physical changes, such as pore fluid displacement and drying, rather than chemical reactions. • Bridging laboratory results as calibration points with thermo-hydro-mechanical-chemical modelling and full-field imaging is essential to accurately evaluate long-term caprock integrity. Safe storage of carbon dioxide (CO 2 ) in a reservoir depends on the caprock's sealing efficiency and mechanical integrity. For long-term containment of CO 2 , knowledge of rock property alterations due to CO 2 -caprock interaction is crucial. Potentially accessible and permeable reservoirs for CO 2 storage in the southeastern North Sea include Miocene sands, which are topped by mudstones that should act as seals. Here, we expose upper Miocene caprock mudstone recovered from the Lille John-2 well in the Danish North Sea to supercritical CO 2 at field-representative pressure and temperature conditions to simulate the effect of CO 2 sequestration in a laboratory setting. We investigate acoustic, mechanical, and chemical variations in the rock before and after CO 2 exposure, which are essential for understanding the caprock’s strength and integrity. To isolate the geochemical effects, a control set of samples was exposed to inert Ar-gas under equivalent pressure and temperature conditions. The acoustic velocities were measured on cutting-sized (diameter:15 mm, thickness: 3–5 mm) samples using the continuous wave and through-transmission techniques, while shear strength was determined by the punch method. X-ray diffraction studies recorded chemically driven mineralogical alterations. After exposure, both compressional and shear wave velocities, as well as the shear strength of the material, increased by about 10 %, 10–50 %, and 85–130 %, respectively. However, no considerable change in mineralogy was detected. Exposure to scCO 2 displaced pore fluid out of the samples, leading to increased strength and stiffness, possibly due to pore fluid drying. The individual impacts of scCO 2 , Ar-gas, and temperature treatment were ambiguous.