In recent years, the deep geological subsurface gained more and more attention as it offers various reservoirs potentially applicable for different geotechnical use options, e.g. deep geothermal energy, geological storage of CO2 and H2/energy. In this context, knowledge about the occurrence and the controlling processes of fluid-rock reactions in space and time is important to guarantee sustainable long-term reservoir utilization. While fluid-rock reactions occur at pore scale, they can significantly influence both reservoir-scale (transport) processes and regional flow regimes. Based on field observations, exemplary fluid-rock reactions in connection with their wide-ranging effects on geotechnical utilizations will be discussed.

During drilling of the Gotthard Base Tunnel through the Central Alps the exposed fractured rocks and the frequent water inflows provided a deep insight into regional hydrogeological processes in orogenic crystalline basements. Here we report data from the 10 km long central Sedrun section. 211 water samples were collected from frequent inflow points at 900 to 2350 m below the surface. The singular samples and data provide a comprehension of the deep hydrochemical cross-section through the Central Alps. The investigated tunnel section cuts across gneisses and schists of the pre-Alpine basement and across two narrow zones of meta-sediments. Rock temperature varies from 30 °C to 45 °C depending on the thickness of the overburden. The fracture water is of meteoric origin and acquires its composition exclusively by chemical interaction with the surrounding rocks along the flow path.

Water from inflow points in the basement of the Gotthard massif has typically a high pH of about 10 and total dissolved solids in the range of 100 to 300 mg L^-1. Sodium is the prime cation of most waters. Although plentiful in the rocks, calcium, potassium, and magnesium are low to very low in the water. The anions associated with Na are carbonate/bicarbonate, sulfate, fluoride and chloride in widely varying proportions. High fluoride concentrations of up to 15.4 mg L^-1 are characteristic for most waters. As a result of the high pH dissolved silica (SiO₂) reached concentrations of up to 58 mg L^-1 and represents 25 - 30 wt.% of the solutes.

During IODP Expedition 382, two sites were drilled at 53.2°S at the northern edge of the Scotia Sea and three sites at 57.4°–59.4°S in the southern Scotia Sea within the Atlantic sector of the Southern Ocean. Sediments at both locations alternate between dominant terrigenous components during glacials and dominant biogenic components, carbonate at the northerly sites and opal in the southern Scotia Sea, during interglacials. Here we constrain the geochemical environment in interstitial waters using the boron (δ¹¹B), silicon (δ³⁰Si) and ⁸⁷Sr/⁸⁶Sr isotopic composition.

Interstitial water δ¹¹B and δ³⁰Si decrease in the uppermost tens of meters downcore, most likely due to in situ weathering processes preferentially releasing light isotopes to interstitial waters. This process is partly also reflected by strongly increasing alkalinities in this depth interval. While δ³⁰Si at all sites increase already at shallow sediment depth where organic matter degradation is intense, δ¹¹B remain relatively low beyond the lower boundary of elevated dissolved phosphate concentrations at every core site. Below this depth δ¹¹B follow isotopic trends seen in δ³⁰Si towards heavy compositions, presumably because of dominating secondary clay formation.

Interstitial waters obtained as deep as 550 and 670 mbsf from the southern Scotia Sea sites reveal an increasing importance of off-axis hydrothermal fluids within the basement underlying the sediments. This feature is detectable by lowest ⁸⁷Sr/⁸⁶Sr alongside lowest Mg/Sr and strongly decreasing δ¹¹B at the lower end of the cores. Our key aim is to illustrate the dominant diagenetic process at each depth downcore, and how to identify these.

Following a thermal and photogrammetric outcrop mapping campaign undertaken at the Waiwera geothermal reservoir in 2019, a pre-existing 3D hydrogeological model was revised in the present study to assess the impact of the updated structural and lithological interpretation on the existing numerical model calibration. For the latter, well data comprising measured temperature and salinity profiles were employed to reconstruct the reservoir’s natural thermal state and spatial distribution of salinity, supported by numerical simulations of density-driven fluid flow coupled with the transport of heat and sodium chloride. In this context, the previously applied fluid equations of state were extended to consider all relevant parameters as functions of temperature and salinity. Our simulation results demonstrate that the undertaken revisions of the static model and fluid properties substantially improve the agreement between the simulated and observed temperature profiles in the monitoring wells, while the achieved match of the simulation results with early recordings on seawater intrusion emphasizes the general model validity. Ongoing work focusses on applying the newly calibrated numerical model to support the sustainable management of the reservoir and to investigate the reappearance of natural seeps at the Waiwera beach, triggered by a decrease in the past excessive groundwater abstraction.

Hydrate formation from dissolved methane in saline solutions is a hydrochemical process, resulting in the accumulation of gas hydrates in sedimentary strata under the seafloor or overlain by permafrost regions. In the scope of the SUGAR framework, LARS has been established to study gas hydrate formation processes and dissociation strategies under in-situ conditions. In the latest hydrate formation experiments, key parameters have been applied to mimic the local marine environment of the Mallik site, Canada. LARS was equipped with temperature sensors and an electrical resistivity tomography (ERT) array for these tests to monitor the dynamic temperature changes and spatial hydrate distribution. Numerical simulation on the hydrate formation process in LARS has not yet been successfully conducted, so that the equations of state relevant to describe equilibrium hydrate formation from dissolved methane have been implemented into a numerical framework and integrated with the TRANsport Simulation Environment to study and quantify the temporal of CH₄-hydrate formation in our present study. We present our model implementation, its verification against HydrateResSim and the findings of the model calibration and validation against the temperature and ERT data from the corresponding hydrate formation experiment. The simulation results demonstrate that our numerical implementation can reproduce the spatial temperature distribution and hydrate formation processes in LARS. Furthermore, spatial hydrate distribution is in good agreement with that produced by ERT measurements undertaken during experiment. Consequently, our numerical simulation framework can be applied for the design of new experiments and to investigate hydrate formation in representative geological settings.