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For integrating green hydrogen to the future energy system, underground storage facilities are needed. So far, there is no underground hydrogen pore storage facility in Germany. However, there is a lot of experience in town gas, natural gas , and CO2 underground storage which now can be used for hydrogen storage pre-feasibility assessments, explorations and operations.In this approach the Ketzin anticline (Germany) is used for a pre-feasibility assessment for hydrogen underground storage in a saline aquifer by using experimental and measurement data from its time as a CO2 test site during 2004 to 2017.For this first feasibility assessment we focused on the influence of several reservoir parameters on hydrogen solubility and the production rate. Therefore, the influence of the brine´s salinity, capillary pressure, reservoir pressure and temperature of two potential storage formations at Ketzin site is analyzed.The Stuttgart formation was already used as a CO2 storage formation during the projects CO2SINK, CO2MAN, COMPLETE. Due to loss reduction by a optimal structural trapping the potential hydrogen injection and production well is located at the top of the anticline – about 1.5 km north to the former CO2 storage test site. Based on Fleury´s experimental data from 2013, including capillary pressure, six parameter setups are created to simulate six hydrogen production and injection cycles.For the second storage formation, the Exter formation, less experimental and measurement data is available. Therefore, six scenarios were designed on the basis of literature values. The end members represent a low and a high hydrogen recovery scenario which combine the best and worsed possible parameter setup for this formation.The results show that it is important to consider the regional conditions individually and to respond to the specifications of the site to minimize economic and safety risks in hydrogen underground storage.

Seismic and geoelectric methods are used as complementary tools for the identification of fluid/gas effects in underground storage and production scenarios. Both methods present a different resolution. Seismic data tend to be acquired by much more dense geometrical layouts, and geoelectric acquisition as a potential field method shows information integrated over spatial distances. We present results matching seismic and electrical resistivity tomography (ERT) results from two repeat surveys performed during CO 2 storage operations at the Ketzin pilot site in Germany. The approach is based on data acquired in 2009 and 2012 at different stages of total injected CO 2 volume. Masses are obtained from the averaged change in acoustic impedance and are compared to masses inferred from inverted electrical resistivities. Archie's equation is used to obtain saturations from the ERT models. The sensitivity of the background resistivity, R 0 , as a relevant parameter in determining the mass of injected CO 2 is analysed. A specific threshold analysis based on seismic thickness by bin has been applied to investigate the matching of seismically and ERT obtained masses. Both mass estimates match comparatively well, but show inherent sensitive parameters and their corresponding impact as temporal and spatial uncertainties that may cause mass differences for the areas of investigation.

Gypsum crystals are found at the well perforation of observation well Ktzi 202 of the test site for CO2 storage at Ketzin, Germany. XRD analysis confirms pure gypsum. Fluid samples before and after CO2 injection are analyzed. Geochemical modeling is conducted to identify the mechanisms that lead to gypsum formation. The modeling is carried out with PHREEQC and Pitzer database due to the high salinity of up to 5 mol per kg water. Due to their significantly higher reactivity compared to other minerals like silicates, calcite, dolomite, magnesite, gypsum, anhydrite, and halite are considered as primary mineral phases for matching the observed brine compositions in our simulations. Calcite, dolomite, and gypsum are close to saturation before and after CO2 injection. Dolomite shows the highest reactivity and mainly contributes to buffering the brine pH that initially decreased due to CO2 injection. The contribution of calcite to the pH-buffering is only minor. Gypsum and anhydrite are no geochemically active minerals before injection. After CO2 injection, gypsum precipitation may occur by two mechanisms: (i) dissociation of CO2 decreases activity of water and, therefore, increases the saturation of all minerals and (ii) dolomite dissolution due to pH-buffering releases Ca2+ ions into solution and shifts the mass action to gypsum. Gypsum precipitation decreases with increasing temperature but increases with increasing partial CO2 pressure. Our calculations show that calcium sulfate precipitation increases by a factor of 5 to a depth of 2000 m when Ketzin pressure and temperature are extrapolated. In general, gypsum precipitation constitutes a potential clogging hazard during CO2 storage and could negatively impact safe site operation. In the presented Ketzin example, this threat is only minor since the total amount of gypsum precipitation is relatively small.

Large-scale energy storage plants based on power-to-gas-to-power (PtG–GtP) technologies incorporating high temperature electrolysis, catalytic methanation for the provision of synthetic natural gas (SNG) and novel, highly efficient SNG-fired Allam reconversion cycles allow for a confined and circular use of CO2/CH4 and thus an emission-free storage of intermittent renewable energy. This study features a thorough technology assessment for large-scale PtG–GtP storage plants based on highly efficient sCO2 power cycles combined with subsurface CO2 storage. The Allam cycle employs supercritical CO2 as working fluid as well as an oxy-combustion process to reach high efficiencies of up to 66%. The entire PtG–GtP process chain assessed in this study is expected to reach maximum roundtrip efficiencies of 54.2% (with dedicated and sufficient O2 storage) or 49.0% (with a dedicated air separation unit). The implementation of said energy storage systems into existing national energy grids will pose a major challenge, since they will require far-reaching infrastructural changes to the respective systems, such as extensive installations of renewable generation and electrolysis capacities as well as sufficient subsurface storage capacities for both CO2 and CH4. Therefore, this study incorporates an assessment of the present subsurface storage potential for CO2 and CH4 in Germany. Furthermore, a basic forecast study for the German energy system with an assumed mass deployment of the proposed SNG-based PtG–GtP energy storage system for the year 2050 is conducted. In case of a fully circular use of CO2/CH4, when electricity is solely generated by renewable energy sources, 736 GW of renewables, 234 GW of electrolysis and 62 GW of gas-to-power capacities are required in the best case scenario in 2050. The total storage volume on the national scale of Germany for both CO2 and CH4 was determined to be 7.8 billion N m3, respectively, leading to a CH4 storage capacity of 54.5 TW h. The presented investigations illustrate the feasibility of large-scale energy storage systems for renewable electricity based on high temperature electrolysis, catalytic methanation and Allam power cycles paired with large subsurface storages for CO2 and CH4.

A silica gel was applied in a porous gas reservoir, with the purpose of testing mitigation and remediation of CO 2 leakage from geological storage reservoirs. The gel has a high strength and a very low water-like viscosity, that extends its applicability to small pore diameters and low permeability media. The gel was prepared and applied on-site with oilfield equipment. Mixing was upscaled from laboratory- to field-scale, including one unsuccessful attempt. Environmental concerns and additional health and safety requirements were modest as the formulation was composed only of the non-toxic commercial silica-based product Betol K28 T, acetic acid and fresh water. A well was selected and prepared in the Becej natural CO 2 field in Serbia, the well and reservoir were prepared and the gel was placed into a 600 m deep CO 2 - and CH 4 -bearing sandstone layer. The reservoir was selectively sealed to the gas cap through a fast, CO 2 -selective gelation, while the hydraulic pathways of the liquid-filled part of the reservoir remained open. In the regions where no CO 2 was present the gelation reaction was slower and the kinetic was temperature-accelerated. A numerical model was used to simulate the impact of the pre-injection operational procedures and to quantify the impact on the temperature-dependent gelation time. The workflow aligns the needs of a research project with the interests and practical priorities of an operating company. A© 2019 Elsevier Ltd

Designing low-cost infrastructure networks for transport of hydrogen represents a key step in the adoption and penetration of hydrogen technology in a low-carbon energy future.For hydrogen distribution, network design amounts to creating pipeline systems in which supply is matched to demand through a transportation system that respects multiple constraints (technical, social, environmental) and minimizes cost. This can equate to recycling pre-existing pipelines or building new ones, but also involves the placement of carefully chosen supply nodes.In a multi-level distribution network, supply nodes may assume many roles from large-capacity geological storage facilities, to local relay nodes addressing the end customers.Finding minimum-cost pipeline network designs in which supply node locations are already chosen is itself a well-studied combinatorial optimization problem (Cayley’s formula predicts possible spanning trees for nodes) for which multiple heuristic and exact methods are known [1].Allowing the supply node to take any position within the network renders the problem significantly more complex as the minimum-cost network topology (the specific connections to between nodes) will potentially change for each new supply node position.We propose a heuristic algorithm that finds good solutions in a reasonable amount of time based on a back-and-forth between:- Repositioning optimally the supply node, while maintaining the same connections to the supply node (reduces cost)- Optimizing the network topology, assuming a fixed supply node position (also reduces cost)The algorithm stops once no further cost reductions for the network design are found. The algorithm output is found to be sensitive to the initial guess of the supply node position, the initial guess of the connections to the supply node, and to the specific “path” of the back-and-forth taken to reach the given local minimum. As such, a good initial guess for a “housing polygon”, i.e. the nodes to which supply node is directly connected to, is crucial in finding the minimum-cost solution, and in the shortest time possible. We attempt to make this initial guess with a machine learning algorithm, with features describing the geometrical distribution of node capacity, as well as elementary network concepts.Finally, an example is provided on a model hydrogen network comprised of typical elements and realistic cost-functions. [1]: Brimberg J, Hansen P, Lin K, Mladenovi N, Breton M, Brimberg, J (2003) An oil pipeline design problem. Operations Research, 51(2):228–239. https://doi.org/10.1287/opre.51.2.228.12786

Seismic and geoelectric/electro-magnetic methods are used as complementary tools for the identification of fluid/gas effects in underground storage and production scenarios. Both methods generally have very different resolution. Seismic tends to be acquired by much more dense geometrical layouts and the geoelectric or electro-magnetic acquisition being a potential field method shows information integrated over spatial distances. These inherent scale and design dependent differences require spatial tuning in joint inversion approaches and careful matching in independent interpretations of both methods. We present results matching seismic and electrical resistivity tomography (ERT) results from two repeat surveys acquired during CO2 storage operations at the Ketzin pilot site in Germany. The approach is based on data acquired in 2009 and 2012, at different stages of total injected CO2 volume. Volumes of injected mass are obtained from the averaged acoustic impedance change (seismic) in the vicinity of the injection well and compared to volumes inferred from the ERT cross-well acquisition. The results are compared radially with increasing distances from the injection location. Seismically derived masses of injected CO2 are used as a benchmark for a threshold-driven workflow analyzing the electric resistivity model. The cross-well ERT results have been obtained in a quadrant of the seismic survey acquisitions. Assuming radial symmetry for the ERT makes it possible to compare individual mass balances in the near-vicinity of the injection well. Archie's equation is used to obtain saturations from the tomographic geoelectric models. The sensitivities of parameters relevant in determining the mass of injected CO2 is analyzed. Variations in saturation exponent n, baseline resistivity R0, and porosity Φ enable specifying applicable ranges of the parameters and determining the investigation radii compared to the seismic derived benchmark. This is done for individual threshold levels for saturations derived from the ERT field data. Seismically and ERT obtained masses match comparatively well and subtle variations of the sensitive parameters are capable in explaining differences for individual investigation volumes. Applicable investigation radii lie between 20-100 m. A 10% in- or decrease of the mean parameters is able to match the seismic derived mass in this range. Above a threshold of 10% for the saturations, the derived mass decreases more rapidly showing a larger deviation from the seismic derived mass. Both methods underestimate the total injected mass. This is not surprising as there are both fluid related processes and structural heterogeneities not accounted for in either. Results of surface-downhole measurements support the findings and show applicability of the developed approach. The threshold-based approach may support the monitoring concept of a CO2 storage site and provides a basis for quantitative evaluation of its containment, as investigated in the frame of the EU project SECURe.

The three-year (2017-2020) Pre-ACT project was established as one of the three large projects in the first round of the Accelerating CCS Technologies (ACT) program (https://act-ccs.eu). Pre-ACT was a collaborative effort between partners from six research institutes and four companies with an active role in CCS. The project ambition was to deliver cutting-edge research into the safe storage and monitoring of CO2, particularly to improve strategies for monitoring and management of the pore pressure distribution within the storage reservoir. Pre-ACT has established methodologies for assessing the conformance of a storage site, relative to its expected performance. The primary outputs are quantitative conformance systems (protocols), which rely on cost-effective monitoring, and guidelines on how to act in case of non-conformance. The protocols are applicable to industrial operators and focus on the main storage-related challenges for accelerated deployment of CCS: capacity, confidence and cost. We report here on the lessons learned within the project, both in terms of European collaboration between research institutes and industry partners in the new ACT-program, but most of all in terms of the new knowledge and methods related to monitoring and control over large-scale storage projects in the North Sea. In addition, we describe a unique experimental campaign with brine and CO2 injection at the new ECCSEL Svelvik CO2 Field Lab and touch upon the important work of communicating research findings to external stakeholders such as policy makers, regulators, and potential future CCS project operators.

Designing low-cost network layouts is an essential step in planning linked infrastructure. For the case of capacitated trees, such as oil or gas pipeline networks, the cost is usually a function of both pipeline diameter (i.e. ability to carry flow or transferred capacity) and pipeline length. Even for the case of incompressible, steady flow, minimizing cost becomes particularly difficult as network topology itself dictates local flow material balances, rendering the optimization space non-linear. The combinatorial nature of potential trees requires the use of graph optimization heuristics to achieve good solutions in reasonable time. In this work we perform a comparison of known literature network optimization heuristics and metaheuristics for finding minimum-cost capacitated trees without Steiner nodes, and propose novel algorithms, including a metaheuristic based on transferring edges of high valency nodes. Our metaheuristic achieves performance above similar algorithms studied, especially for larger graphs, usually producing a significantly higher proportion of optimal solutions, while remaining in line with time-complexity of algorithms found in the literature. Data points for graph node positions and capacities are first randomly generated, and secondly obtained from the German emissions trading CO2 source registry. As political will for applications and storage for hard-to-abate industry CO2 emissions is growing, efficient network design methods become relevant for new large-scale CO2 pipeline networks.

We present the results of a rock physics driven workflow to analyze and steer a planned CO2 migration campaign, it combines a rock-physical seismic model, reservoir model and Bayesian scenario framework to assess monitoring configurations. The injection of CO2 into the subsurface requires a detailed understanding of the petrophysical properties and rock physical frame of the formation under stress. The underlying assumptions made for the unconsolidated high porosity formation have a significant impact on the expected seismic response. For a shallow aquifer pressure and saturation dependent elastic parameters are derived within a rock physical description. Distinguishing pressure and saturation related changes of the acoustic impedance is subject to the sensitivities of the properties used to derive the underlying seismic P- and S-velocities as well as densities. These uncertainties can induce a non-negligible variability in the footprint of a seismic image of the CO2 plume. The rock physics model is based on a solid frame consisting of quartz and clay and saturated with water and gas. The integrated workflow coupled with dynamic simulations provides a possibility to define and evaluate conformance measures during operation. Within this context the workflow is applied to the extended Svelvik CO2 field laboratory, which can be considered as a big sandbox model with capabilities for up-scaling to larger storage formations. An experimental design assessing the sensitivity of the information present for the site is translated into a scenario-based ensemble of dynamic models. Following, a practical quantitative workflow for a-priori assessment of monitoring strategies in probabilistic conformance verification is demonstrated. The results provide insight into the impact of different aspects of geophysical monitoring configurations (e.g., sparsity, noise levels, detection thresholds and timing) on the conformance verification accuracy.

Cheap and efficient groundwater pressure monitoring is a standard task in subsurface hydrology. We present application experience from a tube based pressure monitoring system that is applied to the Svelvik field laboratory for CO2 storage, Norway. In total 13 monitoring points were installed in depths between 51 and 89 m below ground level.The pressure sensor is located above ground. It is temperature compensated to reduce measurement errors due to temperature variations. The pressure sensor is connected to a downhole low diameter tube that has a perforation in the respective measurement depth. The tubes are installed as smart casing installations, i.e. in the borehole annulus. This allows to keep the borehole open during installation of other monitoring devices.Clean pumping of the well was not possible. Some filters were protected with fleece, while others were just perforated tubes. During installation, all tubes had hydraulic contact to the groundwater. After settling of the mud 3 of 4 fleece protected filters show sufficient communication, while all 9 filters that were just perforated were clogged and not usable for pressure monitoring.The system has following advantages: (i) the downhole material is robust and cheap, allowing for multiple measurement points; (ii) has a small diameter (6 mm in the present case); (iii) since the static pressure is removed, a smaller sensor range is required; (iv) the sensors are located at the top of the borehole and can be retrieved after the campaign. Further, it can be installed without downhole metal parts.The system has two disadvantages by design compared to submerged pressure sensors. (i) The absolute pressure can only be approximately determined, limited by the accuracy of the fluid density inside the tube. (ii) Pressure decreases can only be measured up to about 1 bar below piezometric head when the tube is filled with water.The upper metres, that may be exposed to temperatures below 0 °C are filled with antifreeze. The choice of antifreeze allows for a certain static pressure correction. Minimum weight liquid is pure ethanol with a density of about 0.8 kg, allowing to measure pressure up to 2.8 bars below piezometric head for e.g. the 89 m deep measurement.AcknowledgementsThis work has been produced with support from the SINTEF-coordinated Pre-ACT project (Project No. 271497) funded by RCN (Norway), Gassnova (Norway), BEIS (UK), RVO (Netherlands), and BMWi (Ger-many) and co-funded by the European Commission under the Horizon 2020 programme, ACT Grant Agreement No 691712. We also acknowledge the industry partners for their contributions: Total, Equinor, Shell, TAQA. We thank the SINTEF-owned Svelvik CO2 Field Lab (funded by ECCSEL through RCN, with additional support from Pre-ACT and SINTEF) for assistance during installation and for financial support.

One of the major keys to the success of the carbon capture and storage (CCS) is understanding the geochemical effects that CO2 has on the storage reservoir. The injection of CO2 into the reservoir disturbs geochemical equilibrium as it induces acid-generation reactions with subsequent CO2-brine-mineral interactions, including dissolution of certain host minerals and precipitation of secondary minerals. The mineral precipitation, especially precipitation of carbon-bearing minerals in geological formations, is generally a favorable for CO2 trapping mechanism that ensures long-term geologic CO2 sequestration. These precipitates, however, may clog the wellbore and its surroundings, followed by loss of injectivity.The current study is dedicated towards a better understanding of the geochemistry of the geological CO2 storage based on the Ketzin CO2 pilot site. The Ketzin CO2 storage site, the first on-shore geological CO2 storage site in the European mainland, is demonstrated a safe and reliable CO2 storage operation after injection of about 67-kilo tons of CO2 and offers the unique opportunity to work on data sets from all storage life-cycle (Martens et al., 2014). Through both field measurement and modeling studies, this contribution aims to explore the secondary mineral precipitation mechanisms and identify the major influential factors during the CO2 sequestration. This approach supports the H2020 project SECURe establishing best practice in baseline investigations for subsurface geoenergy operations, underpinned by data of pilot and research-scale sites in Europe and internationally. The secondary minerals solubility was investigated as a function of the reservoir temperature, pressure, and CO2 concentration, which occurred in the reservoir. Special focus is set to sulfate minerals, as field evidence exists that gypsum precipitates as a result of reservoir exposition to CO2. Batch modeling was performed using the PHREEQC code version 3 (Parkhurst and Appelo, 2013) with the Pitzer database (pitzer.dat). The coupling interface OGS#IPhreeqc (He et al., 2015) applied reactive transport modeling, and the coupled reactive-transport processes in the reservoir with complex chemistry can be modeled. Our results suggest that the gypsum precipitation was found to increase as CO2 concentration ascends. However, no significant porosity and permeability alterations are observed since the gypsum precipitation acts as a Ca2+ sink and leads to further carbonate dissolution. The results highlight the high reactivity of the near-well zone due to CO2 injection and emphasize the need to be monitored in the injection well to avoid the potential formation of gypsum, which could lead to well clogging.He, W., Beyer, C., Fleckenstein, J.H., Jang, E., Kolditz, O., Naumov, D., Kalbacher, T., 2015. A parallelization scheme to simulate reactive transport in the subsurface environment with OGS#IPhreeqc 5.5.7-3.1.2. Geosci. Model Dev. 8, 3333-3348.Martens, S., Möller, F., Streibel, M., Liebscher, A., 2014. Completion of Five Years of Safe CO2 Injection and Transition to the Post-closure Phase at the Ketzin Pilot Site. Energy Procedia 59, 190-197.Parkhurst, D.L., Appelo, C.A.J., 2013. Description of input and examples for PHREEQC version 3: a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations, Techniques and Methods, Reston, Virginia, USA, p. 519.

Carbon Capture and Storage technology is considered to be able to contribute to a carbon neutral society and is again receiving increased attention in the efforts to reduce CO2 emissions. To ensure safe operation of such CO2 storage projects, reliable monitoring technologies are required. Due to the generally high electrical resistivity contrast between CO2 and formation water, Electrical Resistivity Tomography (ERT) can be considered one of the most effective geophysical techniques in the monitoring of CO2 migration in the subsurface.Within the ERA-NET co-funded ACT project Pre-ACT (Pressure control and conformance management for safe and efficient CO2 storage - Accelerating CCS Technologies) a CO2 injection and monitoring experiment was planned at the Svelvik CO2 Field Lab, located on the Svelvik ridge at the outlet of the Drammensfjord in Norway. The Svelvik field lab consists of four 100 m deep monitoring wells, drilled in July 2019, surrounding an existing well used for brine and CO2 injection. Each monitoring well is equipped with modern sensing systems including five types of fiber-optic cables, conventional and capillary pressure monitoring systems, as well as 16 ERT electrodes with a spacing of five meters.With 64 installed electrodes, a large number of measurement configurations is possible. We combine the free and open-source geophysical modeling library pyGIMLI with ECLIPSE reservoir modeling to simulate the expected behavior of all cross-well electrode configurations during a CO2 injection experiment. Simulated CO2 saturations are converted to changes in apparent resistivity using Archie's law. Different considerations have to be made to select a suitable set of electrode configurations, i.e. not too large geometric factors, maximum response to the predicted change, as well as sensitivity in the target area. We select sets of configurations based on different criteria, i.e. the ratio between the measured change in resistivity in relation to the geometric factor, the maximum change in apparent resistivity, and maximum sensitivity in the target area. The individually selected measurement schedules are tested by inverting them with different assumed data errors. The numerical results show adequate resolution of the CO2 plume.The CO2 injection took place between 27th October 2019 and 5th November 2019. Approximately two metric tonnes of CO2 were injected in 65 m depth. Preliminary field results indicate a considerably lower response than predicted by our model. These discrepancies can potentially be explained by oversimplified simulations as well as operational uncertainties. Results from baseline and repeat surveys can therefore support an integrated approach towards a revised static and dynamic model for the test site.Acknowledgements:This work was produced within the SINTEF-coordinated Pre-ACT project (Project No. 271497) funded by RCN (Norway), Gassnova (Norway), BEIS (UK), RVO (Netherlands), and BMWi (Germany) and co-funded by the European Commission under the Horizon 2020 programme, ACT Grant Agreement No 691712. We also acknowledge industry partners Total, Equinor, Shell, TAQA.Finally, we thank the SINTEF-owned Svelvik CO2 Field Lab (funded by ECCSEL through RCN, with additional support from Pre-ACT and SINTEF) for assistance during installations and for financial support.

13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland / Edited by Tim Dixon, Lyesse Laloui, Sian Twinning 13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, Lausanne, Switzerland, 14 Nov 2016 - 18 Nov 2016; Amsterdam [u.a.] : Elsevier, Energy Procedia 114, 3948-3955 (2017). doi:10.1016/j.egypro.2017.03.1526, Published by Elsevier, Amsterdam [u.a.]

In accordance with the European Union target of 80-95% reduction in net greenhouse gas emissions for the year 2050 relative to 1990 levels, as well as the 2050 German directive for Net-Zero greenhouse gas emissions, the Helmholtz Inititive for Climate Adaptation and Mitigation aims to provide the innovation and decision-making guidance required to both minimize future emissions and improve societal resilience to the negative consequences of anthropogenic climate change.While the rapid development of clean energy generation infrastructure, associated with cross-sector energy efficiency improvements and progress of low-carbon technologies such as electric vehicles represent tangible contributions to this goal, a large degree of emissions remain tied to industrial processes at the core of German economic output, notably within refineries, the iron and steel industry and the cement and lime industry.In parallel to searching for low-carbon process alternatives, or utilisation scenarios for tough-to-decarbonise emissions, the case for underground CO2 storage remains attractive both economically and from a safety point of view [1].The German onshore territory presents a large potential for carbon storage owing to a number of stratigraphic layers presenting favourable storage characteristics (depth, thickness, porosity, as well as surrounding rock properties) with considerable geographical extent.A significant aspect in establishing cost-effective carbon networks that are engaging for both the public and industrial partners is the creation of advantageous organisational structures that take into account viable placement of storage sites, minimal-cost pipeline networks, coherent regional grouping, sensitivity to public concerns, as well as awareness of future emission landscapes.As a consequence, we propose a national-scale study in which we address the aforementioned constraints to create hypothetical CO2 networks based on varying regional clustering methods, number of storage sites, pipeline scalability costs and underground storage and transport constraints relating to public acceptance.We make use of recently published graph-optimisation algorithms to ensure we achieve close-to-optimal network structures for each input CO2 sources and storage sites, and usable land space for transport [2].The geological data used is based on literature work establishing potential CO2 storage sites, as well as a catalogue of faults of which the fault-zone conductivity is not necessarily known [3]. CO2 emission data is taken from the EU carbon trading emitters register and future emission scenarios exclude fossil energy generation.Our results show that a large diversity of CO2 networks can be envisioned for a 2050 German Net-Zero landscape while still maintaining acceptable regional exclusivity, owing primarily to the large degree of underground storage potential available. One aspect that is not considered is the prospect of trans-national CO2 networks that could benefit both locally certain large isolated point sources close to country borders or more globally through infrastructure economy of scale. [1] Global Status of CCS: 2019, Global CCS Institute[2] Heijnen, P., Chappin, E., Herder, P. (2019): A method for designing minimum‐cost multisource multisink network layouts. - Systems Engineering, Volume 23, Issue 1, Pages 14-35[3] Schulz, R., Suchi, E., Öhlschläger, D., Dittmann, J., Knopf, S. & Müller, C. (2013): Geothermie-Atlas zur Darstellung möglicher Nutzungskonkurrenzen zwischen CCS und Tiefer Geothermie. – Endbericht, LIAG-Bericht

EGU2020: Sharing Geoscience Online, 4-8 may 2020, Underground hydrogen storage (UHS) in porous media has been proposed as an effective and sustainable energy storage method to balance renewable energy supply and seasonal demand. To determine the potential for and conduct realistic risk assessments of the UHS technology, learnings from more mature underground fluid storage technologies, such as underground storage of natural gas (UGS) or CO2 (UCS), can be used. Here we discuss the caveats related to the use of these technologies as analogues to UHS and highlight current knowledge gaps that need to be addressed in future research to make UHS a secure and efficient technology. Abiotic and biotic reactions between the rock and the fluids, often not considered in UCS and UGS operations, play an important role in UHS and can change the chemical environment in the reservoir dramatically. The mineralogy of the reservoir and cap rocks, as well as the in-situ pore fluid chemistry, is of vital importance and the characterisation efforts should not be limited to the reservoir quality. The risk assessment of UHS operation may follow similar production cycles as in UGS, but there are important lessons to be learnt from UCS. UCS aims to store injected gas permanently and different CO2 trapping mechanisms are contributing to storage security. Residual trapping, which locks parts of the CO2 within the pore space, may reduce the commercial profitability in UHS, but can assist to mitigate potential leakage of hydrogen. The dissolution of hydrogen in the pore water will likely play a minor role in UHS compared to UCS, while the precipitation of minerals containing hydrogen during UHS has not yet been appropriately investigated. The main storage process in gas storage is the accumulation of buoyant fluid underneath a low-permeability cap rock in a three-dimensional trap. Storage sites are determined by different parameters: UGS is mainly used in depleted gas fields (hence sites with proven gas storage security), while UCS sites are usually located deeper than 800m for efficiency reasons, under conditions at which CO2 is present as a high-density supercritical phase. None of these restrictions are a pivotal for UHS and a new set of constrains should be formulated specifically designed to the properties of hydrogen. These must involve: The unique properties of hydrogen (high diffusivity and low density and, thus, high buoyancy) require potential storage sites to have well-understood cap rocks with minimal diffusion and capillary leakage risk. A reservoir architecture and heterogeneity that guarantees economically sensible injection and withdrawal rates by choosing sites, which minimise the isolation of hydrogen from the main plume during UHS operations. Site monitoring protocols will also need to be re-evaluated for different scales, as well as for the dynamic properties of hydrogen, such as low density and fluid mobility. It is certain that leakage along abandoned wells, the main risk for leakage in UCS and UGS, will also pose a risk to the containment of injected hydrogen. Therefore, hydrogen storage site locations require a comprehensive investigation into abandoned and operational (deep) petroleum and (shallow) water exploration and production wells.

This chapter provides a review of the life-cycle operations at the Ketzin pilot site for CO2 storage. The injection phase at the Ketzin site lasted from 2008 to 2013 and a comprehensive geophysical monitoring program was applied, including seismic and geoelectric surface and surface-downhole measurements, as well as crosshole tomographic investigations. Furthermore, borehole logging campaigns using the pulsed-neutron-gamma method and permanent pressure and temperature sensing were applied. The geophysical data were used to set up and iteratively optimize the reservoir model for simulating the storage behavior over a wide range of scales, including the long-term stabilization of the reservoir in the postinjection phase.

At the Ketzin pilot site, a permanent downhole electrode array has been utilized as part of the multi-disciplinary monitoring concept during all phases of the CO2 storage project. After more than seven years under the present CO2/brine prevailing subsurface conditions, including the five years of regular CO2 injection, a first long-term evaluation of the general technical performance of the electrode array can be given. This article reports on the application of the crosshole electrical resistivity tomography (ERT) technique considering the site specific requirements and challenges. It describes the installation procedure of the electrode array, field data acquisition schemes and automated pre-processing routines applied to the continuously growing data archive. It has been found that the evaluation of contact resistance measurements provides useful information about the current condition of the downhole installation. The 3D time-lapse inversion of one of the major observation planes yields resistivity distributions from various operational stages during the injection and post-injection phases of the CO2 storage reservoir. A simple saturation approach converts averaged resistivity signatures from the target reservoir zone into CO2 saturation estimates, which are in good agreement with saturation results measured by borehole logging campaigns. The permanent ERT array has shown a promising lifespan under downhole conditions, and is able to provide complementary information in conjunction with other monitoring systems. For further practical applications, a more standardized workflow for data acquisition and processing might be beneficial.

Between the years 2008 and 2013, approximately 67 kilotons of CO2 have been injected at the Ketzin site, Germany. As part of the geophysical monitoring programme, time-lapse electrical resistivity tomography has been applied using crosshole and surface-downhole measurements of electrical resistivity tomography. The data collection of electrical resistivity tomography is partly based on electrodes that are permanently installed in three wells at the site (one injection well and two observation wells). Both types of ERT measurements consistently show the build-up of a CO2-related resistivity signature near the injection point. Based on the imaged resistivity changes and a petrophysical model, CO2 saturation levels are estimated. These CO2 saturations are interpreted in conjunction with CO2 saturations inferred from neutron-gamma loggings. Apart from the CO2–brine substitution response in the observed resistivity changes, significant imprints from the dynamic behaviour of the CO2 in the reservoir are observed.

To meet the ambitious target set out in the Paris Agreement to keep the temperature rise well below 2°C, all the tools available for reducing CO2 emissions, including CO2 Capture, Utilisation and Storage (CCUS), are needed to meet the challenge. Global collaboration is key in advancing CCUS. CO2GeoNet, a pan-European scientific body on CO2 geological storage, has gained visibility and recognition in the European and global arenas, participating in research and providing scientific advice, training and capacity building, and information and communication. A summary of CO2GeoNet’s cooperation activities with countries outside Europe and with international bodies is given here. CO2GeoNet is open to further opportunities as the Association views global cooperation as critical to accelerating the development, recognition and deployment of CCUS as an important and flexible climate change mitigation technology.

State of the art reservoir monitoring delivers numerous property data with high resolution. Especially the consistent interpretation of pressure data with different geophysical methods requires multi-physical modelling and inversion workflows. Such a workflow is developed based on the reservoir monitoring concept of the Ketzin pilot site for CO2 storage, Germany. The workflow consists of three physical models, (i) a single phase hydraulic model, (ii) a multiphase CO2 migration model and (iii) a geoelectrical model. Calibration is carried out to match observation data groups hydraulic pressure, CO2 pressure, CO2 arrival time and geoelectrical cross-hole observations. Calibration parameters are spatially distributed hydraulic permeability and porosity, compressibility, the relative permeability function and the geoelectrical saturation exponent. Geoelectrical measurements with low coverage that cannot be inverted with traditional methods could be included, since the multiphysical reservoir model acts as physical regularisation. The indirect nature of geophysical data is overcome by implementation of petrophysical relations between permeability and porosity and between CO2 saturation and electrical resistivity. Stability against field data is increased by reducing the impact of structural noise through preprocessing the observation data. Stability against the overparameterisation is added by Tikhonov regularisation and singular value decomposition, the latter combined with super parameter definition reducing the problem dimensions and simulation time by three quarters. A synthetic case study demonstrates that the model resolves the spatial permeability and identifies the petrophysical relation between CO2 saturation and electrical resistivity. The weighting scheme balances different observation data groups and measurement intervals. The model to measurement misfit is reduced proprotionally for all observation data groups, while the geoelectrical data are most difficult to match.

Pre-ACT is a three-year (2017-2020) research project on CO2 storage which brings together a partnership of 10 research institutes and industrial companies. Pre-ACT is delivering cutting-edge research into the safe storage and monitoring of CO2. The focus is on improving strategies for monitoring and management of pore pressure within the storage reservoir. Pre-ACT is establishing methodologies to assess the conformance of a storage site, relative to its expected performance, through research in five work packages. The primary output will be a quantitative conformance system that utilises a cost-effective monitoring strategy to define a series of pressure-driven support protocols. The protocols are directly applicable to industrial operators and focus on the main storage-related challenges for accelerated deployment of CCS: capacity, confidence and cost. Most of the work during the first year in Pre-ACT has addressed pre-injection (pressure) modelling, pressure and saturation monitoring, and conformance assessment. Examples of initial work include: (1) studies at the Horda Platform (North Sea) showing the importance of uncertainty in interpretation of faults and their sealing properties for Smeaheia pre-injection pressure modelling; (2) planning of an experiment for controlled pressure- and saturation discrimination at the Svelvik Field Lab and development of an approach to separate the effects of pressure and saturation changes; and (3) the development of a conformance assessment tool and value-of-information workflow for comparison of different monitoring configurations (expected gain and deployment cost).

New concepts are required to accelerate the global development of high-temperature geothermal resources in a cost effective and environmentally safe way. Keeping the geothermal system in operation for several decades is challenging - yet key to economic success. Real-time monitoring of temperature and strain along the well bore is a powerful tool to evaluate the performance and integrity of installed subsurface components. A fiber optic cable was installed behind the casing of a high temperature geothermal well in Iceland (IDDP2). A second cable was installed in a low temperature well in Germany. During cementation, distributed strain data (Rayleigh backscatter) as well as distributed temperature sensing data (Raman backscatter) was acquired. In a subsequent measurement campaign, data shall be acquired in these wells during load changes (production/injection). In combination with concurrent laboratory investigations and conventional borehole logging data, this study will identify processes influencing the long-term integrity of casing and cement in high-temperature geothermal environments.

Monitoring the fate of the injected CO2 and possible associated effects, such as hydro-mechanical and chemical effects in the target reservoir and its surroundings, is essential for safe operation of a storage facility. In this chapter, we shall first provide an overview of the technologies available and used for monitoring of CO2. We shall then proceed to describe specific methods and finally present some important case studies that will demonstrate the use of the discussed monitoring technologies under specific field settings.

13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland / edited by Tim Dixon, Lyesse Laloui, Sian Twinning 13th International Conference on Greenhouse Gas Control Technologies, GHGT 13, Lausanne, Switzerland, 14 Nov 2016 - 18 Nov 2016; Elsevier, Energy Procedia 114, 3588-3596 (2017). doi:10.1016/j.egypro.2017.03.1490, Published by Elsevier

Between the years 2008 and 2013, approximately 67 kilotons of CO2 have been injected at the Ketzin site, Germany. As part of the geophysical monitoring programme, time-lapse electrical resistivity tomography has been applied using crosshole and surface-downhole measurements of electrical resistivity tomography. The data collection of electrical resistivity tomography is partly based on electrodes that are permanently installed in three wells at the site (one injection well and two observation wells). Both types of ERT measurements consistently show the build-up of a CO2-related resistivity signature near the injection point. Based on the imaged resistivity changes and a petrophysical model, CO2 saturation levels are estimated. These CO2 saturations are interpreted in conjunction with CO2 saturations inferred from neutron-gamma loggings. Apart from the CO2–brine substitution response in the observed resistivity changes, significant imprints from the dynamic behaviour of the CO2 in the reservoir are observed.

Surface-downhole electrical resistivity tomography (SD-ERT) surveys were repeatedly carried out to image [Formula: see text] injected at the pilot storage Ketzin, Germany. The experimental setup combines surface with downhole measurements by using a permanent electrode array that has been deployed in three wells. Two baseline experiments were performed during the site startup and three repeat experiments were performed during the first year of CO2 injection. By the time of the third repeat, approximately 13,500 tons of [Formula: see text] had been injected into the reservoir sandstones at about 650 m depth. Field data and inverted resistivity models showed a resistivity increase over time at the [Formula: see text] injector. The lateral extent of the related resistivity signature indicated a preferential [Formula: see text] migration toward the northwest. Using an experimental resistivity-saturation relationship, we mapped [Formula: see text] saturations by means of the resistivity index method. For the latest repeat, [Formula: see text] saturations show values of up to 70% near the injection well, which matches well with [Formula: see text] saturations determined from pulsed neutron-gamma logging. The presence of environmental noise, reservoir heterogeneities, and irregularities in the well completions are the main sources of uncertainty for the interpretations. The degradation of the permanently installed downhole components is monitored by means of frequently performed resistance checks. In consistency with the SD-ERT data, these resistance checks indicate a long-term resistivity increase near the [Formula: see text] injector. In conclusion, the investigations demonstrate the capability of surface-downhole electrical resistivity tomography to image geologically stored [Formula: see text] at the Ketzin site.

The major task of a safety surveillance concept is to support the closed loop of reservoir management, i.e. to guarantee an iterative interaction between reservoir monitoring and numerical simulation. The presented research is based on the experience gained on the permanent geoelectrical monitoring at the Ketzin CO2 pilot storage site continuously operating since 2007. We have structured this experience in a modular monitoring workflow which offers well prepared links to actual process data, petrophysical data, and reservoir simulation. First results have been reached in major topics such as site-specific customization of technical tools, data acquisition and processing, and data evaluation. Structuring this approach in a modular manner allows the geoelectric monitoring at the Ketzin site to be efficiently scaled up and adapted to the fit-for-purpose requirements of future CCS demonstrations.

The Ketzin pilot site, led by the GFZ German Research Centre for Geosciences, is Europe’s longest-operating on-shore CO2 storage site with the aim of increasing the understanding of geological storage of CO2 in saline aquifers. Located near Berlin, the Ketzin pilot site is an in situ laboratory for CO2 storage in an anticlinal structure in the Northeast German Basin. Starting research within the framework of the EU project CO2SINK in 2004, Ketzin is Germany’s first CO2 storage site and fully in use since the injection began in June 2008. After 39 months of operation, about 53,000 tonnes of CO2 have been stored in 630–650 m deep sandstone units of the Upper Triassic Stuttgart Formation. An extensive monitoring program integrates geological, geophysical and geochemical investigations at Ketzin for a comprehensive characterization of the reservoir and the CO2 migration at various scales. Integrating a unique field and laboratory data set, both static geological modeling and dynamic simulations are regularly updated. The Ketzin project successfully demonstrates CO2 storage in a saline aquifer on a research scale. The results of monitoring and modeling can be summarized as follows: (1) Since the start of the CO2 injection in June 2008, the operation has been running reliably and safely. (2) Downhole pressure data prove correlation between the injection rate and the reservoir pressure and indicates the presence of an overall dynamic equilibrium within the reservoir. (3) The extensive geochemical and geophysical monitoring program is capable of detecting CO2 on different scales and gives no indication for any leakage. (4) Numerical simulations (history matching) are in good agreement with the monitoring results.

This paper deals with the electrical resistivity monitoring of the Ketzin CO 2 injection pilot (CO2ReMoVe EC project) through time-lapse CSEM measurements. There, 3 boreholes about 800 m deep have been especially designed for current injection at reservoir (sandstone) depth. CO 2 is directly injected in a saline (∼240 g/l) aquifer. Prior modelling results indicated that the increase of electrical resistivity generated by the CO 2 plume (gaseous and liquid CO 2 phases) supposed to be highly resisitive, would generate measurable changes in the EM fields on the surface, when injecting current directly inside the reservoir. In order to highlight and follow these expected resistivity changes, 3 CSEM surveys were performed in August 2008 (baseline prior to injection), June 2009 and August 2010. Each time, 13 EM stations have been recorded during current injection of a square wave at 3 frequencies (0.125 Hz, 0.5 Hz and 4 Hz) in two configurations (“double mise a la masse” (2×MAM) and “mise a la masse–surface” (MAM-Surface)). This paper only presents results of the 2×MAM configuration at 0.5 Hz. In spite of a very noisy area (gas pipes, high voltage power lines), we measured signal amplitude 10 times higher than noise amplitude. We show that EM fields vectors (both inphase and quadrature components) measured on the surface are very similar to the forward modelling EM responses computed with COMSOL Multiphysics ® . Models also show that electric field spatial distribution is strongly affected by a thin and resistive layer (35 m–200 Ωm) of anhydrite above the reservoir, making E field diverging from the boreholes whereas a dipolar pattern was expected for the dipole current injection used here. Moreover, while June 2009 survey highlighted the expected strong increase of electric field (increase of resistivity), August 2010 survey showed electric field amplitudes similar to the 2008 baseline survey, revealing therefore major changes of the reservoir properties. Finally, the directional sensitivity of the 2×MAM array is tested through modelling residuals computed for five CO 2 plume spatial distributions. Results show that a north-eastward migration of the CO 2 plume is expected to fit field data.

Geophysical monitoring activities were an important component of the CO2 injection program at the Ketzin site, Germany. Here we report on the seismic and electrical resistivity tomography (ERT) measurements performed during the period of the site development and CO2 injection. Details on the site geology, the history of the CO2 injection operation, and petrophysical models relevant for the interpretation of the geophysical data are presented. The seismic measurements comprise 2D and 3D surface seismic surveys, vertical seismic profilings, and crosshole measurements. Apart from the measurements, results from advanced processing methods, such as impedance inversion and full-waveform inversion are also presented. In addition, results from crosshole ERT and surface-downhole ERT are presented. If operational efforts are taken into consideration we conclude that a combination of several geophysical methods is preferable given the demands of a spatiotemporally comprehensive monitoring program. We base this conclusion on that the different imaging characteristics and petrophysical sensitivities of different methods can complement each other. An important finding is, based on signal quality and reduced operational costs, that the use of permanent installations is promising. Generally, specific monitoring layouts will depend on site-specific characteristics, such as reservoir depth, availability of wells, petrophysical characteristics, and accessibility of surface locations. Given the comprehensive range of methods applied, the reported results are not only relevant to the operation of CO2 storage sites, but are also of interest to other monitoring projects dealing with fluid injection or production. © 2015 Elsevier B.V.

Permanent geophysical crosshole methods offer opportunities to provide spatiotemporal information on CO2 migration, since the physical properties change in regions affected by fluid substitution. The direct and quantitative integration of geophysical methods in a reservoir modeling context is not straightforward and requires multi-physical process models coupled through petrophysical parameter transformations. Based on an example from the Ketzin CO2 storage site, Germany, we present a hydrogeophysical modeling workflow, in which simulated CO2 distributions are transfered to changes in electrical properties used for geoelectrical forward modeling. Simultaneous inversion of the electrical time-lapse response, reservoir pressure and CO2 arrival times has not been reported before and enables estimation of the permeability distribution. The calibrated model captures the known migration direction of the CO2 and is able to explain the multi-physical calibration dataset. The presented framework allows to integrate various types of observations into a single hydrogeophysical model leading to increased confidence in permeability parameterization and, in perspective, to improved predictive assessments of the storage reservoir.

The Ketzin pilot site for geological storage of CO2 located about 25 km west of Berlin was the first European pilot site for onshore storage of CO2 in saline aquifers. A total amount of 67 kilotons of CO2 was injected without any safety issues between June 2008 and August 2013 when the injection ceased and the site entered its post-closure phase. Within this phase a multidisciplinary monitoring program continues and the five research wells are stepwise abandoned in order to address and finally close the entire life cycle of the storage site. In October 2014, a two-week field experiment was carried out with the aim to back-produce parts of the injected CO2 and to study the reservoir and wellbore behaviour. In this contribution, we present a comprehensive overview on the status of the Ketzin site in its second year of the post-closure phase and the main results of the field experiment on CO2 back-production.

At the Ketzin pilot site (Germany), a multi-disciplinary monitoring concept was established in order to image CO2 injected into a saline aquifer. Within this surveillance program a behind-casing borehole electrode array was deployed as permanent reservoir monitoring (PRM) tool. During the period from 2008 until 2014, weekly geo-electrical cross-hole data were recorded. This comprehensive archive of time-lapse resistivity data forms a respectable base for CO2 saturation estimation. Moreover, the long-term evaluation of the borehole electrode array provided the evidence for its endurance under subsurface conditions, reliability and usefulness for integrative geophysical data analysis.

Crosshole resistivity tomography has received consideration as a tool for quantitative imaging of carbon dioxide stored in deep saline aquifers. With regard to the monitoring responsibility of site operators and the substantial expenses associated with permanent downhole installations, optimized experimental design gains particular importance. Based on an iterative appraisal of the formal model resolution matrix, we developed a method to estimate optimum electrode locations along the borehole trajectories with the objective to maximize the imaging capability within a prescribed target horizon. For the presented crosshole case, these layouts were found to be symmetric, exhibiting refined electrode spacings within the target horizon. Our results revealed that a sparse but well conceived set of electrodes can provide a large part of the information content offered by comparably dense electrode distributions. In addition, the optimized layout outperformed equidistant setups with the same number of electrodes because its resolution was focused on the monitoring target. The optimized electrode layouts presented provided a powerful and cost-efficient opportunity to complement permanent installations, particularly at, but not limited to, future [Formula: see text] storage sites. Although preliminarily developed to support the design of crosshole resistivity layouts, our approach is directly applicable to other survey geometries including surface and surface-to-hole acquisitions.

Brine was a scientific joint-project implemented to accompany a prospective CO2 storage site in Eastern Brandenburg, Germany. In this context, we investigated if pore pressure elevation in a CO2 storage reservoir can result in shallow freshwater salinization involving the conceptual design of a geophysical early warning system. Furthermore, assessments of a potential synergetic geothermal heat recovery from the CO2 storage reservoir and hydro-mechanical integrity were carried out. The project results demonstrate that potential freshwater salinization is strongly depending on the presence and characteristics of geological weakness zones. The integrated geophysical early warning system allows for reliable monitoring of these potential leakage pathways at different spatial and time scales.

The joint project CO2MAN (CO2 Reservoir Management) was a scientific programme accompanying geological CO2 storage at the Ketzin pilot site in the German Federal State of Brandenburg. The project which was funded by the German Federal Ministry of Education and Research (BMBF) from 1 September 2010 to 31 December 2013 enclosed six scientific institutions and seven industry partners. The Ketzin pilot site is the longest-operating on-shore CO2 storage site in Europe. In advance of the CO2MAN project, CO2 injection had already started in June 2008 and storage operation had been accompanied by one of the world’s most extensive scientific research and development programmes. The CO2MAN project took advantage of this unique potential of the site in order to answer further technical and scientific questions on CO2 storage and to inform about this highly debated technology. The CO2MAN project demonstrates safe geological CO2 storage at the Ketzin site on a pilot scale.

A combination of seismic and geoelectric processing was studied by means of a structurally constrained inversion approach. Structural constraints were interpreted from the seismic data and integrated into the geoelectric inversion through a local regularization, which allowed inverted resistivities to behave discontinuously across defined boundaries. This arranged seismic processing and constrained resistivity inversion in a sequential workflow, making the generic assumption that the petrophysical parameters of both methods change across common lithostructural boundaries. We evaluated the approach using a numerical example and a real data example from the Ketzin [Formula: see text] pilot storage site, Germany. The latter demonstrated the efficiency of this approach for combining 4D seismic and surface-downhole geoelectric data. In consistence with the synthetic example, the constrained resistivity inversions produced clearer delineated images along the boundary between caprock and reservoir formation. Near the [Formula: see text]-flooded reservoir, the seismic and geoelectric time-lapse anomalies correlated well. At some distance to the downhole electrodes, however, the geoelectric images conveyed a notably lower resolution in comparison to the corresponding seismic images. Both methods confirm a northwesterly trend for the [Formula: see text] migration at the Ketzin site, although a rather northerly direction was initially expected. The results demonstrate the relevance of the presented approach for the combination of both methods for integrated geophysical [Formula: see text] storage monitoring.

Reliable and cost-efficient reservoir surveillance techniques play an important role in the context of geological carbon dioxide storage. Electrical methods are particularly suited to image the displacement of highly conductive formation fluids by a more resistive CO2 plume. At the Ketzin site close to Berlin, Germany, a permanent installation of 45 ring-shaped stainless steel electrodes distributed in an injection well and two observation wells is in operation since the start of injection in June 2008. The depth of the installation intervals in the three wells ranges from 590 m to 735 m. Based on our practical experience after more than five years of injection (until August 2013), we draw conclusions with regard to the usefulness and versatility of such an electrode array as well as associated challenges under the aspect of technical longevity. With regard to future installations, we present an experimental design approach to estimate optimum electrode locations along the borehole trajectories with the objective to maximize the resolution within a prescribed target horizon. Our results show that a sparse but well-conceived set of electrodes with a refinement of the electrode spacing in the target region can provide a large portion of the information content offered by very dense electrode layouts. Using Ketzin as a case study for realistic CO2 plume migration, we compare the optimized layout to the existing equidistant installation at Ketzin with a focus on the offered resolution and the inversion performance. The presented results can assist practitioners in the design of reliable and cost-efficient monitoring systems for future underground storage sites.

A combination of seismic and geoelectric processing is presented using a structurally constrained inversion approach. Structural constraints are interpreted from seismic data and inserted into the geoelectric inversion through a local regularisation, which allows inverted resistivities to behave also discontinuously across these a priori constraints. This sequential arrangement of seismic processing and constrained resistivity inversion makes the generic assumption that the petrophysical parameters of both methods change across common lithostructural boundaries. We evaluate the approach using a numerical example and a real 4D data example from the CO2 pilot storage site, Ketzin, Germany. The real data case shows that the time-lapse anomalies, which have independently been imaged by surface seismic and surface-downhole geoelectric methods, correlate well at the CO2-flooded reservoir. However, at some distance to the downhole electrodes the geoelectric images provide a notably lower resolution in comparison to the corresponding seismic images. The results demonstrate the relevance of the presented approach for the combination of both methods in geophysical CO2 storage monitoring operations.