Department Water Resources and Drinking Water

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.

This study focuses on the effects of ozonation on the optical and photochemical properties of dissolved organic matter (DOM). Upon ozonation, a decrease in light absorption properties of DOM was observed concomitantly with a large increase in singlet oxygen (¹O₂) and hydroxyl radical (˙OH) quantum yields (Φ_¹O2 and Φ_˙OH, respectively). The decrease in absorbance was linked to the reaction of DOM chromophores with ozone and ˙OH, formed as a secondary oxidant, while the increase in Φ_¹O2 and Φ_˙OH are linked to the formation of quinone-like moieties from the reaction of ozone with phenolic DOM moieties. Investigations using benzoic acid as a ˙OH probe and methanol as a ˙OH scavenger indicated that not only is ˙OH formed, but that other hydroxylating species (˙OH-like) are also produced upon DOM photo-irradiation.

Carbon capture and storage (CCS) may play a significant role in reducing greenhouse gas emissions. Noble gases are potential tracers to monitor subsurface CO₂ storage sites and verify their containment. Naturally occurring noble gases have been used successfully to refute alleged CO₂ leakage in the past.
We present results from several sampling campaigns at two Norwegian CO₂ capture facilities, the demonstration plant Technology Centre Mongstad (TCM) and the natural gas processing plant with CO₂ capture and storage on Melkøya. The gas streams in the capture plants were monitored with a combination of on-site mass spectrometry and subsequently analysed discrete samples. This allows us to define the factors controlling noble gas concentrations in captured CO₂, to monitor temporal variation of noble gas concentrations and finally evaluate the potential to use noble gases as inherent environmental tracers for labelling CO₂ in storage reservoirs.
At both sites, CO₂ is captured using amine gas treatment. Noble gas concentrations in the gas streams were observed to decrease by several orders of magnitude during the processing. Isotopic ratios are air-like for CO₂ captured after natural gas combustion at TCM and natural gas-like for CO₂ captured from natural gas processing on Melkøya. Further, we detected a solubility trend caused by the amine solvent at TCM with higher solubility for heavier noble gases.
We find that the relative concentrations of noble gases in the captured CO₂ are defined by the gas from which the CO₂ is captured and the design of the amine gas treatment process. Both factors were observed to cause temporal variation in the captured CO₂.
Using mixing and noble gas partitioning calculations we show that the significant depletion in noble gas concentrations, together with degassing of noble gas enriched formation water, means that the injected CO₂ will inherit the noble gas signature of the storage formation, even following the injection of significant CO₂ volumes. Any CO₂ leaked from the storage formation is thus likely to have a crustal noble gas signature, characteristic of the storage site, which can be targeted for monitoring.