Geological Storage of CO2 – Long Term Security Aspects

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.

The reliable detection and assessment of potential CO₂ leakages from storage formations require the application of assurance monitoring tools at different spatial scales. Such tools also play an important role in helping to establish a risk assessment strategy at carbon dioxide capture and storage (CCS) facilities. Within the framework of the MONACO project (“Monitoring approach for geological CO₂ storage sites using a hierarchical observation concept”), an integrative hierarchical assurance monitoring concept was developed and validated with the aim of establishing a modular observation strategy including investigations in the shallow subsurface, at ground surface level, and in the atmosphere. Numerous methods and technologies from different disciplines (such as chemistry, hydrogeology, meteorology, and geophysics) were either combined or used complementarily to one another, with results subsequently being jointly interpreted. Patterns of atmospheric CO₂ distributions in terms of leakage detection can be observed on large scales with the help of infrared spectroscopy or micrometeorological methods, which aim to identify zones with unexpected or anomalous atmospheric CO₂ concentrations. On the meso-scale, exchange processes between ground surface level and subsurface structures need to be localized using geophysical methods and soil gas surveys. Subsequently, the resulting images and maps can be used for selecting profiles for detailed in situ soil gas and geophysical monitoring, which helps to constrain the extent of leakages and allows us to understand controlling features of the observable fluid flow patterns. The tools utilized were tested at several natural and industrial analogues with various CO₂ sources. A comprehensive validation of the opportunities and limitations of all applied method combinations is given and it shows that large spatial areas need to be consistently covered in sufficient spatial and temporal resolutions.

The BMBF project CO₂ISO-LABEL (Carbon and Oxygen ISOtopes under extreme conditions LABoratory EvaLuations for CO₂-storage monitoring) investigated stable isotope methods in laboratory studies for transferral to carbon capture and storage (CCS) field sites including enhanced gas and oil recovery (EGR and EOR). The isotope composition of injected CO₂ and water are useful tracers for migration and water-rock-gas interactions during such operations. However, quantification of carbon and oxygen equilibrium isotope effects at elevated pressures and temperatures are so far scarce. They thus need more investigations under p/T conditions that are characteristic for reservoirs and overlying aquifers. With this, the main objective of the project was to improve stable carbon and oxygen isotope methods for monitoring CO₂ storage sites and their impact of injected CO₂ on reservoir geochemistry under controlled laboratory settings. An important finding was that isotope fractionations of carbon between CO₂ and dissolved inorganic carbon (DIC) were not significantly different from each other in experiments with pure CO₂ and pressures between 59 and 190 bar. Furthermore, influences of rock types (limestone, dolomite and sandstone) and fluid salinities were found to be negligible for carbon isotope fractionation between CO₂ and DIC. Another finding was that water oxygen isotope ratios changed systematically in response to different CO₂/H₂O molar ratios in closed system equilibration experiments. This helps to reconstruct the amounts of CO₂ that equilibrated with formation waters. Results of the project will enable better assessment of geochemical conditions in underground carbon storage sites or other subsurface systems where large amounts of CO₂ interact with water and rocks.

The RECOBIO projects (Hoth et al. in Recycling of sequestrated CO₂ by microbial—biogeochemical transformation in the deep subsurface—RECOBIO 2009a; Geotechnol Sci Rep 14:58–65, 2009b; Untersuchung der biogeochemischen transformation von im tiefen Untergrund gespeichertem CO₂—RECOBIO 2 2011) have shown the relevance of biogeochemical processes, related to CO₂ injection. These processes represent an additional pathway for biogeochemical CO₂ storage. The main result was the microbial transformation (binding) of injected CO₂ (formation of organic compounds). This can also influence the pressure behaviour of the system. Furthermore the organic layers can act as nucleation sites and so catalyse the carbonate solid formation. So the main focus of the CO₂BIOPERM project was now to investigate the influence of these processes on the permeability behaviour of the system. Furthermore other aquifer structures, not related to natural gas fields, were characterised by microbiological, molecular genetic investigations. The biocenosis is also often dominated, like in natural gas fields, by sulphate reducers and fermenting bacteria. The study of CO₂ effects to the cultivation of microorganisms showed for deep aquifer microorganisms a strategy to survive the CO₂ stress by spore forming. The proteomic analysis gave a first view how many and which proteins were down and up regulated under CO₂ stress. A part of the flow experiments, which were operated in discontinuously flowed batch mode, are presented in detail. There is no strong influence of the processes on the permeability behaviour for high permeable reservoir sandstones. Nevertheless the sequential extractions on the solid materials, after the tests, underline the ongoing biogeochemical reactions.

In the joint project PROTECT (PRediction Of deformation To Ensure Carbon Traps) we predicted and quantified the distribution and the amount of sub-/seismic strain in the proximity of the CO₂ reservoir in the Otway Basin. Three approaches fill the sub-seismic space: seismic multi-attributes stabilized the interpretation of the 3-D depth model by imaging small lineaments; retro-deformation revealed in the seal ca. 3 % as highest strain magnitudes; numerical forward modelling shows that the minimum horizontal stress at reservoir is locally overprinted by faults. We calibrated our predictions with new near-surface reflection seismic measurements and used advanced visualization tools. Thus, this seismo-mechanical workflow reveals possible migration pathways, and as such provides a tool for prediction and adapted time-dependent monitoring for subsurface storage in general.

A long-term safe and reliable abandonment of wells is a crucial prerequisite for a secure abandonment of underground storage sites, while considering the long-lasting environmental impact, e.g. leakage through wells. To study the impact of the cementation on the tightness of wells, both, the cementation of a well during completion and abandonment are investigated in Full-scale laboratory experiments. Different autoclaves from small to Full-scale have been developed to test cementations under various conditions and to perform long-term tests under in situ conditions of a CO₂ storage site. The experiments show that the surface-texture (e.g. roughness) of the drilled well has a significant influence on the formation of mud-channels—for rough surfaces, up to 75 % of the serrations consist of non-displaced mud and only 25 % are well hardened cement, creating possible leakage pathways. Even under idealized cementation conditions using a cement recipe characterized by a very low shrinkage, micro-annuli are formed. These micro-annuli are connected throughout the whole oil-field casing in the Full-scale experiments for which the widths of the micro-annuli in the order of 10–20 µm could be deduced. Along the micro-annuli the cement is carbonated due to the flow of CO₂-bearing fluid. The fluid flow could also be verified by a Spatial Time-Domain-Reflectometry (TDR) setup embedded in the cementation, which was successfully tested as an in situ monitoring system. In the Full-scale experiments, chemical reactions in the system casing—cement—rock—fluid were examined. The geochemical analyses during and after the experiment show variations in pH, conductivity and chemical composition of the brines, which are well described by an interplay of corrosive processes and precipitations of carbonate minerals.

While risk assessment for CO₂-storage often has been conducted by using a lot of simplifications and conservatisms, our approach developed in the CO₂RINA-research project is based on the integration of all models, existing at a moment in time. These models will be coupled by the so called transfer function approach which has been proven to be very powerful in the risk assessment for low level radioactive waste. This concept ensures, that the risk assessment is always consistent with the state of the models existing for a specific site. It can be immediately improved if the existing models will be improved over time. The approach was verified by comparison of the direct coupling of different process models in an overall model with a coupling by transfer functions by conducting a wide range of test calculations showing very good accuracy of the approach. The new coupling approach allows the incorporation of a variety of additional effects which are difficult to handle in an overall model. Examples for such processes are complex chemical and microbiological interactions, geomechanical feedback loops and migration of CO₂ in the atmosphere. In the project there have been developed specified models for a generic site with parameters similar to the Ketzin site. These models include a reservoir model, a model for the alteration of the cementation of a mature well, a fault model, a model describing advection and diffusion through the cap rock, a complex model for the migration in a Quaternary aquifer including complex chemical interactions and a geomechanical model. Additionally there was shown the way of integration of microbiological processes which have been modelled in detail in the CO₂BIOPERM project. The new approach is ready to be adapted to a specific CO₂-storage site.

Saltwater rise is a possible result of CO₂ injection into deep saline aquifers that could threaten aquifer quality. Hence, long-term monitoring of the groundwater system is a required task during and after the injection phase. However, point measurements in boreholes cannot describe large systems with sufficient precision. Therefore cost-efficient monitoring methods are needed, like surface geophysical techniques. Electrical Resistivity Tomography (ERT) is well-suited for saltwater problems and can be deployed using steel-cased boreholes as electrodes, so-called long electrode (LE) ERT. In the project Saltwater Monitoring using Long Electrode Geoelectrics (SaMoLEG) a research institute group focussing on numerical modelling cooperates with a private borehole logging company that has long been active in the region of interest, eastern Brandenburg, where CO₂ injection was planned. A numerical framework for simulation and data analysis is developed and verified using a controlled laboratory experiment. Two differently scaled field sites in the Federal State of Brandenburg, Germany, where natural saltwater rise occurs were repeatedly investigated by LE-ERT. The medium-scale site was permanently wired and allows cost-efficient monitoring. The inversion results agree well with geology and measured in-situ fluid conductivities. The large-scale site, at the area of an existing water works, proves the applicability of the developed method in the catchment scale and gives valuable insight into deeper salinisation. The results show that the LE-ERT method is a suitable and cost-efficient method for saltwater rise and groundwater quality monitoring in the frame of geological CO₂ storage and is ready for application at real storage sites.

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 CO₂BRIM project pursues a combined technical-social sciences approach for investigating participatory modelling applied to different stages of a characterisation of potential CO₂ storage formations. From the technical point of view, the project deals with two topics: The first one is concerned with an early stage site screening, where the Gravitational Number (Gr) is used as an indicator for storage efficiency dependent on the conditions found in German Middle Buntsandstein rock units. The second topic addresses the problem of large-scale salt water displacement due to the injection of CO₂. Both topics are accompanied by a method called Participatory Modelling, where third parties, i.e. stakeholders with different background, are involved into the modelling process. This report gives some details about the methodological aspects of participatory modelling for the two technical topics and discusses the results for the Gr-characterisation in detail; first results and preliminary discussions from work on large-scale brine displacement are added at the end.

This chapter presents two studies on the perception and acceptance of CCS in Germany: the first one is a qualitative case study analysis which examined four German projects which were initiated for CO₂-storage. These include two commercial projects driven by industry (one in North Frisia, the other one in Eastern Brandenburg), a joint research and industry project in the Altmark focusing on Enhanced Gas Recovery (EGR) and a joint research project at Ketzin. Only one of the four projects, the Ketzin project, was successful in proceeding to CO₂ injection and did not elicit local protest. The comparison of the four cases points to differences in project scale and scope, in the perceived risks and benefits and in the communication processes, all of which have possibly influenced project acceptance. The second study investigated and compared the public perception of CO₂ offshore storage, CO₂ onshore storage and CO₂ transport via pipeline based on a national and two regional surveys. It shows that CCS is not unknown amongst the German public; however, the acceptance of CO₂ storage is low independent of the place of storage. Perceived risks and benefits are identified as the main influence factors on attitudes towards CO₂ storage and CO₂ transport via pipeline.