The CO₂ Long-term Periodic Injection experiment (CO₂LPIE or hereinafter referred to as CL) investigates the hydraulic, mechanical, and chemical (HMC) effects of CO₂ injection into heterogeneous and anisotropic Opalinus Clay of sandy facies under in-situ conditions at the Mont Terri rock laboratory. The aim is to improve our understanding of caprock behavior and its integrity in the context of CO₂ storage, which is relevant for the following scenario: long-term penetration of CO₂ saturated saline groundwater into the caprock and its diffusive transport and rock-brine-interaction. In order to achieve these goals, new experimental concepts and developments for temporal and spatial high-resolution monitoring were necessary. The innovative experimental concept at CL consists of four monitoring boreholes equipped with a newly developed Modular Multi-sensor Monitoring System (MMMS) which enables active and passive seismic tomography, electrical resistivity tomography (ERT), fiber optical (FO) longitudinal strain monitoring, temperature sensing and pore water pressure monitoring. In addition, two boreholes for injection and extraction were drilled into the experimental volume. Both boreholes are equipped with a five-fold hydraulic multi-packer system (MPS). Injection is achieved via a circulation system, which allows for diffusive exchange between the formation and the pore water in the interval. CO₂ dissolution occurs in the injection cabinet through a membrane. The flux of CO₂ injected into the system is controlled by temporal pressure decay in two connected pressure vessels. These can be operated in series or in parallel. An additional gas flow meter provides the instantaneous flow rate. On the extraction side, three intervals are connected to water circuits with membrane systems connected to gas loops and a custom-made gas spectrometer. An observation interval in the injection borehole builds the fourth monitored interval. For the initial characterization of the experimental volume and the baseline monitoring phase, the MMMS arrangement proved to be extremely valuable. The sensitivity and spatial resolution of the seismic, ERT and FO sensing systems are suitable for the scale of the experiment and the expected effects in the system during the future injection. CO₂ is injected through a circulation system that allows for the assessment of diffusive exchange between CO₂ dissolved in artificial pore water (APW) and pore water from the rock. Injecting with cyclic variations is used for a better signal to noise ratio. We can use wavelet analysis (waveform, amplitude attenuation, frequency attenuation) to check for hydraulic diffusivity and HM-coupled behavior, and we are mimicking tidal effects and reservoir pressure variations. This is the first time that such an injection system has been deployed in the context of rock laboratory-scale carbon capture and storage (CCS) experiments. We expect this new setup to provide a much better understanding of the geochemical exchange than in former in-situ experiments. In this short paper, we describe the test setup and the experimental concept, the installation and deployment of the in-situ experiment, the test performance and the first results as well as preliminary conclusions and an outlook.
Jaeggi et al. (Thu,) studied this question.