Carbon dioxide removal and storage is required for limiting global warming to the 2 °C goal of the Paris Agreement. One method is the storage of CO₂ in deeply buried geological formations.

As part of the GEOSTOR project, we created static 3D models for two potential storage sites in the Middle Buntsandstein within the German North Sea Exclusive Economic Zone.

One 3D geological model (~1300 km²) located on the "West Schleswig Block" is based on 2D seismic data from various surveys and geophysical logs from four exploration wells. It encompasses a salt controlled anticline with 40-50 m thick Lower Volpriehausen Sandstones forming the primary reservoir target. The top seal consists of Upper Buntsandstein and unconformable Lower Cretaceous mudstones.

The other 3D geological model (~560 km²) is located within the “Entenschnabel” area and, in contrast, is based on several high-resolution 3D seismic data and geophysical logs from four exploration wells. The reservoir, which also consists of up to 65 m thick Lower Volpriehausen Sandstones, is located within the Mads Graben with erosional discordances at the top. The upper seal consists of Upper Jurassic clays and partly unconformable Lower Cretaceous mudstones.

For both models we conducted petrophysical analyses of all considered well data and calculated reservoir properties to determine the static reservoir capacity for these storage sites.

Finally, we parametrized both models in order to provide two complete reservoir models that are capable of further dynamic capacity simulations, geo-risk and infrastructural analyses aiming at an entire feasibility study within the project framework.

Geological storage of CO₂ contributes to mitigating climate change, but successful storage depends on many subsurface hydrodynamic and geomechanical factors. This study outlines the development of a CO₂ injection strategy for a potential multi-trap storage site in the German North Sea by jointly honouring the geomechanics as well as hydrodynamic and geological constraints. The site comprises three structural closures, each covering an area of tens of km². Based on site-specific geology and petrophysical data, static storage capacities of the individual closures as well as the closure combinations are estimated. Each closure has a distinct configuration and drainage area, resulting in different CO₂ phase dynamics in the subsurface, due to a different interplay of gravity, capillary and viscous forces. A 3D reservoir model is developed and used to simulate CO₂ injection with a rate of 10 Mt/a over 30 years, followed by a 100-year post-injection phase, while accounting for the regional hydraulic boundaries. The allowable pressure limit is derived using dedicated geomechanical simulations. The individual settings of each closure result in varying depths and injectivities, which lead to different numbers of injection wells and their placement due to well interaction. Furthermore, pressure accumulation occurs depending on the relative position of the closure to the hydraulic boundaries, reducing achievable capacity. Consequently, injection well planning and optimisation efforts should prioritise settings that demonstrate high injectivity, high achievable storage capacity and a high allowable pressure window.

CO₂ storage potential in sandstone formations in the German North Sea is investigated by GEOSTOR consortium. This contribution discusses some of the results from the investigation of the geotechnical impact of the storage process. We examine the injection pressure window with respect to the hydro-fractural risk of the reservoir layer or caprock in the vicinity of the injection well by performing coupled hydro-mechanical fracture analyses. Different modelling scales, formulations, failure parameters, and in-situ present day stresses are considered. The results show that 2D- or thin 3D-models may underestimate the material integrity and lead to early fracture initiation. Meanwhile, a medium-scale model of the geoformation may be advantageous in reducing computational burden while maintaining high accuracy. The results also show a good correlation with field-derived fracture gradients and formation strength tests data from the Dutch-German border.

To assess the geotechnical risk of induced seismicity on the offshore infrastructure, we enhanced our boundary element-finite element method dynamic simulator with the formulation to compute arbitrary layered half-space and double-couple dynamic sources. Verification of the numerical method shows an excellent agreement with the analytical solution. The hybrid method is able to take into account the spatially complex geometry of the geological structure. The results show that the impact of the storage process on the formations’ mechanical integrity or offshore infrastructure remains relatively low or manageable for the considered period.

In this study, we introduce an open-source benchmark gallery for a systematic verification of numerical simulations of coupled multi-field processes in geological storage and sequestration. Here, the focus lies on assessing the integrity of a host rock and/or geological barrier, which requires understanding material failure and behavior under different thermal-hydraulic-mechanical-chemical (THMC) conditions. To support quality assurance of the simulation workflows, the gallery provides automated benchmarking and peer-reviewed code development.

Since closed-form solutions covering all aspects of non-isothermal two-phase flow in deformable media (TH²M) problems are not available, the verification procedure is subdivided into simpler conceptual models, up to single-variable processes (Grunwald et al., 2022). These reduced complexity problems can be described by semi- or fully analytical solutions. Extensive verification of very basic combinations (T/H/M/HM/TH etc.) was already conducted, therefore, focus lies on the investigation of more complex problems.

In the context of TH²M systems, the gallery includes three cases relevant to CCS:

• THM problem: Thermally induced expansion of liquid and solid phases resulting in thermal strain in the surrounding solid matrix and fluid displacement. (Booker and Savvidou, 1985; Chaudhry et al., 2019)

• TH² problem: Phase change and heat transport in a thermal gradient (Udell and Fitch, 1985; Helmig et al. 1997)

• TH²M: Heat loss to under- and overburden due to non-isothermal, supercritical CO₂ injection into reservoir. (LaForce et al. 2014a and 2014b; Green at al. 2021)

The gallery consists of comprehensive browser-integrated problem descriptions, analytical solutions, and numerical simulations obtained with the open-source simulation tool OpenGeoSys (Bilke et al., 2022).

The reliable estimation of the dynamic storage capacity for CO2 storage is a significant challenge due to the uncertainty of process parameters. In our study, we consider storage in the Triassic Bunter sandstone underneath the German sector of the North Sea.

We investigate CO2 injection into a saline aquifer, such as the Volpriehausen Sandstone by means of numerical simulations, to analyze the impact of parameter uncertainty on storage efficiency.

One of the main challenges for capacity estimations is the paucity of measurements regarding storage rock units like the Volpriehausen Sandstone for important process parameters such as relative permeabilities obtained from wells in the German North Sea. This requires the supplementation of measured parameters from the same storage rock units for instance from neighboring countries such as the Netherlands or Denmark.

We conduct flow simulations using TOUGH3 ECO2N and a self-developed framework for sensitivity analysis to investigate the parameter uncertainty of process parameters on storage efficiency.

Our results indicate that the level of uncertainty can significantly affect the estimation of storage capacity, and the accuracy of simulation results is highly dependent on certain input parameters.

This study provides insights into the impact of parameter uncertainty on the efficiency of CO2 storage. These findings are useful for future exploration, characterization, and operation of CO2 storage sites.

For successful and safe underground CO₂ storage, a profound knowledge of the subsurface and its geological structures characterizing the selected reservoir is essential. Besides robust geological models and storage capacity estimations, seal integrity analysis and the identification of potential seal-bypass systems is crucial to ensure save long-term storage of CO₂ in the deep subsurface. Within the framework of the GEOSTOR project, we assess potential Triassic and Jurassic CO₂ reservoirs by analysing the integrity of overlying barrier formations below the German North Sea. For this purpose, we use recently acquired high-resolution 3D seismic data covering an area of 94 km² within the northwestern German North Sea (“Entenschnabel”) and high-resolution 2D seismic data from the central German North Sea (“West Schleswig Block”) covering a total length of about 1500 km. Seismic amplitude anomalies indicate the presence of fluids allowing the investigation of former fluid migration pathways and their connection to faults and salt structures. First results from the “Entenschnabel” show a highly resolved 3D image from the seafloor to the Zechstein covering the salt diapir Belinda. Bright spots indicate fluid migration along the crestal fault system. Direct fluid migration indicators are scarce within the West Schleswig Block. Locally, bright spots indicate fluid migration along crestal faults of salt structures. We compare and discuss the characteristics of identified fluid migration pathways near the salt diapir Belinda with fluid migration and its correlation with faults and salt structures from the central German North Sea in the context of barrier integrity for subsurface CO₂ storage.