Hydromechanical Behaviour of Intact and Recompacted Crushed Caprock for Underground CO2 Storage

Abstract This work investigates the hydromechanical behaviour of Opalinus Clay (OPA) shale as a caprock material for geological storage of CO $$_2$$ 2 . In addition to intact OPA, the hydromechanical response of crushed material is evaluated to simulate damaged zones around injection wells or faults. An initial experimental campaign was conducted to establish a recompaction protocol and to determine the main hydromechanical properties of crushed OPA, from both sandy and shaly-rich facies. The acquired hydromechanical and sealing properties of recompacted OPA were then compared to intact sandy-rich OPA under the same effective stress conditions. Although comparable bulk density and porosity were achieved, the recompacted material remained significantly weaker, exhibiting an elastic modulus approximately one order of magnitude lower than that of the intact material. The water permeability of the recompacted samples approximately one order of magnitude higher, with an average value of $$4 \times 10^{-19}$$ 4 × 10 - 19 m $$^2$$ 2 , compared with $$2 \times 10^{-19}$$ 2 × 10 - 19 m $$^2$$ 2 for intact sandy-rich OPA. Constant rate gaseous CO $$_2$$ 2 injection tests resulted in comparable levels of breakthrough pressure, with average pressure equal to 1.60 MPa for the recompacted and 2.17 MPa for the intact OPA material. To further analyse and interpret the hydromechanical and sealing response of the caprock material, X-ray tomographies of intact, pre-fissured and crushed OPA samples were acquired at different levels of confining and pore water pressure. The observed microstructural evolution of crushed OPA during mechanical compaction and water resaturation, highlighted the role of mineralogical and structural heterogeneity in the material’s self-sealing response.