Abstract
Abstract. The success of geological carbon storage depends on the assurance of permanent containment for injected carbon dioxide (CO2) in the storage formation at depth. One of the critical elements of the safekeeping of CO2 is the sealing capacity of the caprock overlying the storage formation despite faults and/or fractures, which may occur in it. In this work, we present an ongoing injection experiment performed in a fault hosted in clay at the Mont Terri underground rock laboratory (NW Switzerland). The experiment aims to improve our understanding of the main physical and chemical mechanisms controlling (i) the migration of CO2 through a fault damage zone, (ii) the interaction of the CO2 with the neighboring intact rock, and (iii) the impact of the injection on the transmissivity in the fault. To this end, we inject CO2-saturated saline water in the top of a 3 m thick fault in the Opalinus Clay, a clay formation that is a good analog of common caprock for CO2 storage at depth. The mobility of the CO2 within the fault is studied at the decameter scale by using a comprehensive monitoring system. Our experiment aims to close the knowledge gap between laboratory and reservoir scales. Therefore, an important aspect of the experiment is the decameter scale and the prolonged duration of observations over many months. We collect observations and data from a wide range of monitoring systems, such as a seismic network, pressure temperature and electrical conductivity sensors, fiber optics, extensometers, and an in situ mass spectrometer for dissolved gas monitoring. The observations are complemented by laboratory data on collected fluids and rock samples. Here we show the details of the experimental concept and installed instrumentation, as well as the first results of the preliminary characterization. An analysis of borehole logging allows for identifying potential hydraulic transmissive structures within the fault zone. A preliminary analysis of the injection tests helped estimate the transmissivity of such structures within the fault zone and the pressure required to mechanically open such features. The preliminary tests did not record any induced microseismic events. Active seismic tomography enabled sharp imaging the fault zone.
Highlights
Carbon capture and storage (CCS) has a fundamental role in reducing the amount of anthropogenic CO2 in the atmosphere and achieving the Paris Agreement’s challenging objective of keeping global temperature rise below 2 ◦C above pre-industrial levels (IPCC, 2018, 2019; Cozier, 2015).Published by Copernicus Publications on behalf of the European Geosciences Union.A
Experiments in the Mont Terri rock laboratory (MTRL) investigate the properties of a pristine claystone, the Opalinus Clay, that has been indicated as a possible host rock for radioactive waste in Switzerland (Bossart et al, 2017, and references therein)
The decameter-scale experimental setup allows for close monitoring of fluid injected into a fault zone in the Opalinus Clay, simulating leakage through a faulted caprock at shallow depth
Summary
Carbon capture and storage (CCS) has a fundamental role in reducing the amount of anthropogenic CO2 in the atmosphere and achieving the Paris Agreement’s challenging objective of keeping global temperature rise below 2 ◦C above pre-industrial levels (IPCC, 2018, 2019; Cozier, 2015). We are currently running an experiment that aims to cover this knowledge gap by providing observations of CO2 migration in a fault system at a decameter scale, which is under well-controlled conditions, but targeting a rock volume that can capture heterogeneities representative of a large-scale in situ injection. The MTRL located in northwestern Switzerland (Fig. 2) allows in situ access to a fault (the Mont Terri main fault) hosted in a clay formation, the Opalinus Clay, and offers a unique opportunity for a prolonged (multiple months) decameter-scale CO2 injection experiment (CS-D: Carbon Sequestration – Series D) into a fault to study relevant geomechanical and geochemical processes of leakage and fault properties. We discuss the implications of the current observations and speculate on the potential impact of the longterm experiment
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