Carbon Capture and Storage (CCS) as well as subsurface energy storage in the form of hydrogen are measures to lower carbon emissions to the atmosphere. Large-scale implementation is underway, especially for CCS, and first hydrogen storage projects have been announced recently.

With most CCS projects being planned for offshore locations, public acceptance is less of a determining factor than it used to be 10-20 years ago, where discussions were rather for onshore locations. CO2 leakage has always been a risk highlighted in the public debate, while no or minimal leakage has been reported for current CCS projects worldwide. However, as scientific community, we need to realistically highlight the risk of leakage across sealing units for any fluids stored in the subsurface to inform various stakeholders like regulators, the public and of course also operating companies.

Caprock leakage needs to be studied across various length and time scales, considering the undisturbed matrix as well as fracture networks and faults; we need to consider advective and diffusive flow and transport and incorporate mineral alterations, potentially leading to changes in hydraulic or mechanical properties.

This talk will highlight the current state of research, advancements and future research required for a realistic evaluation of caprock leakage. It will be based on past research related to matrix transport as well as current research focusing on single and multiphase flow along faults and fractures.

In many applications a fluid is injected into rocks for example for CO₂ storage, in enhanced geothermal reservoirs, or during oil and gas recovery. The fluid may be out of equilibrium with the rock resulting in chemical reactions at depth. The correct prediction of reaction front velocities depends on a thorough understanding of the theory of chromatography and the changes of density and porosity in reactive transport models. We study the systematics of reaction fronts in multi-component systems. The methodology is based on a finite difference approach for solving the transport problem in combination with precomputed thermodynamic equilibria. These lookup tables are calculated using Gibbs’ minimization and a linear programming approach. They are validated against full analytical solutions of the Gibbs minimization problem. Porosity and density evolution is predicted based on mass conservation. We focus on ternary ideal fluid or melt solutions in equilibrium with pure phases as exact solutions are feasible and here first consider the isothermal case. For a fixed incoming fluid composition, over twelve reaction sequences may form depending on initial rock composition. Within one type of reaction sequence, bulk rock composition still plays are role in determining the speed of the reaction front as well as the fluid compositions that develop along the path. This theoretical understanding allows better predictions of the formation of reaction sequences and the consequences on rock properties upon injection of fluids with dissolved chemical components.

The Upper Cretaceous Campanian limestones from the Ahlen-Fm. (Beckum-Fm. Submember) of the Münsterland Cretaceous Basin in NW Germany are former high porosity limestones, which consist mostly of detrital components. This study focuses on the petrophysical assessment of these limestones in combination with diagenetic studies to understand the potential interaction with rising mine-water as they unconformably overlie Upper Carboniferous coal-bearing strata. Therefore, outcrop analyses were carried out and samples were taken to study the heterogeneity and controlling factors, as well as diagenetic para-sequence in tight limestones. We show that diagenesis, compaction, authigenic cementation and the detrital composition affect petrophysical properties. Mechanical compaction is seen by elliptically deformed calcispheres and foraminifera at the transition to ductile clay laminae, forming compaction bands. Mechanical compaction and early diagenetic precipitation of inter- and intragranular sparry ferroan calcite reduces porosity and permeability. Porosity ranges between 1.0% to 18.7%, permeability between <0.0001 mD to 0.2 mD, and p-wave velocity ranges between 2089 m/s and 5843 m/s. Furthermore, natural fractures are filled by either ferroan calcite and/or strontianite. Thus, the studied lithologies of the Beckum-Fm. can be considered as seals for potential rising mine-water levels. Furthermore, results indicate, that they may not be potential targets for geothermal utilization. However, open fractures formed during exhumation overprinted the rocks which may enhance the reservoir quality by generating potential fluid pathways close to the present day surface.

Models and simulations allow a prognosis of how processes in the geosphere might occur in the future, considering physical and chemical processes. They are the only way to test future scenarios and hypotheses and to evaluate the long-term evolution of a repository site, e.g. by quantifying potential radionuclide migration in the hydrogeological system of the containment providing rock unit.

An example is used to demonstrate the extent to which simulated migration lengths can vary for a million years, depending on the model concept as well as on the underlying data and parameters. In the case of uranium in the potential host rock Opalinus Clay (Switzerland), the range extends from 5 m applying experimentally determined transport parameters, over 50 m using process-based approaches and taking hydrogeology into account and up to 80 m depending on the thermodynamic data set used.

The degree of reliability of the models is derived from comparison with laboratory tests and data from boreholes and underground laboratories. This is the only way to assess the simulation results. In addition, indications can be provided where new data need to be collected. To reduce the uncertainty related to the migration length of uranium in the Opalinus Clay, the calcite-carbonate ion system as well as the hydrogeological setting at a potential disposal site need to be known, whereas the amount of clay minerals plays a subordinate role as long as it is enough, which is the case in argillaceous formations.