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

Safe storage of carbon dioxide (CO₂) in a reservoir depends on the caprock's sealing efficiency and mechanical integrity. For long-term containment of CO₂, knowledge of rock property alterations due to CO₂-caprock interaction is crucial. Potentially accessible and permeable reservoirs for CO₂ 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 CO₂ at field-representative pressure and temperature conditions to simulate the effect of CO₂ sequestration in a laboratory setting. We investigate acoustic, mechanical, and chemical variations in the rock before and after CO₂ 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 scCO₂ displaced pore fluid out of the samples, leading to increased strength and stiffness, possibly due to pore fluid drying. The individual impacts of scCO₂, Ar-gas, and temperature treatment were ambiguous.