Hydro-mechanical response of opalinus clay in the CO<inf>2</inf> long-term periodic injection experiment (CO<inf>2</inf>LPIE) at the Mont Terri rock laboratory

Abstract: Guaranteeing the sealing capacity of caprocks becomes paramount as CO2 storage scales up to the gigaton scale. A significant number of laboratory experiments have been performed with samples of intact rock, showing that low-permeability and high-entry pressure caprocks have excellent sealing capacities to contain CO2 deep underground. However, discontinuities, such as bedding planes, fractures and faults, affect the rock properties at the field scale, being at the same time challenging to monitor in industrial-scale applications. To bridge these two spatial scales, Underground Research Laboratories (URLs) provide a perfect setting to investigate the field-scale sealing capacity of caprocks under a well-monitored environment. In particular, the CO2 Long-term Periodic Injection Experiment (CO2LPIE) at the Mont Terri rock laboratory, Switzerland, aims at quantifying the advance of CO2 in Opalinus Clay, an anisotropic clay-rich rock with bedding planes dipping 45° at the experiment location. To assist in the design of CO2LPIE and have an initial estimate of the system response, we perform plane-strain coupled Hydro-Mechanical simulations using a linear transversely isotropic poroelastic model of periodic CO2 injection for 20 years. Simulation results show that pore pressure changes and the resulting stress variations are controlled by the anisotropic behavior of the material, producing a preferential advance along the bedding planes. CO2 cannot penetrate into Opalinus Clay due to the strong capillary effects in the nanoscale pores, but advances dissolved into the resident brine. We find that the pore pressure oscillations imposed at the injection well are attenuated within tens of cm, requiring a close location of the monitoring boreholes with respect to the injection interval to observe the periodic signal. Article highlights: Underground rock laboratory experiments permit examining the caprock sealing capacity at a representative scale for CO2 storage;We perform coupled transverse isotropic hydro-mechanical simulations to gain insight on the response of shaly rock to CO2 periodic injection;Simulation results assist in the design of the injection amplitude and period and monitoring of the long-term periodic CO2 injection experiment.D.S., I.R.K. and V.V. acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Program through the Starting Grant GEoREST (www.georest.eu) under Grant agreement No. 801809. I.R.K. also acknowledges support by the PCI2021-122077-2B project (www.easygeocarbon.com) funded by MCIN/AEI/10.13039/501100011033 and the European Union NextGenerationEU/PRTR. IDAEA-CSIC is a Centre of Excellence Severo Ochoa (Spanish Ministry of Science and Innovation, Grant CEX2018-000794-S, funded by MCIN/AEI/10.13039/501100011033 ). The authors would like to thank swisstopo and the Mont Terri Consortium for their comprehensive information and very valuable discussions. R.Y.M. acknowledges the support from US DOE through Carbon SAFE Illinois Corridor Project DE-FE0031892.Peer reviewe