Jonathan Pearce

Natural occurrences as analogues for the geological disposal of carbon dioxide

It is now generally accepted that anthropogenic CO2 emissions are contributing to the global rise in atmospheric CO2 concentrations. One possibility for reducing carbon dioxide emissions is to remove it from the flue gases of coal-fired power stations and dispose of it in underground geological reservoirs, possibly offshore in the North Sea. The feasibility of this option has been studied in detail by a consortium of European partners. As part of this study, natural occurrences of carbon dioxide were identified and preliminary information from these was obtained. The best characterised are found in the United States where the carbon dioxide reserves are exploited for use in tertiary enhanced oil recovery (EOR) programs in the Texas oilfields. The carbon dioxide reserves occur in geological structures and lithologies which are similar to those present in the North Sea. As such, these fields offer an ideal natural analogue for the disposal of carbon dioxide, since the interactions with groundwaters and reservoir lithologies have occurred on both spatial and temporal scales relevant to geological processes. Those carbon dioxide fields currently being exploited have already been studied to a limited extent by the oil companies involved. However, further study is required to provide information on the potential effects that disposing of large quantities of carbon dioxide might have on groundwaters and reservoir quality. In addition, more detailed information will be obtained on the interactions which occur during EOR using carbon dioxide. This paper presents data on some of the natural carbon dioxide fields, and compares the effects of these natural fluid-rock interactions with those observed in laboratory experiments performed to establish what reactions occur during the geological disposal of carbon dioxide.

A review of natural CO2 accumulations in Europe as analogues for geological sequestration

Abstract Natural geological accumulations of carbon dioxide occur widely throughout Europe, often close to population centres. Some of these CO2 deposits leak, whereas others are sealed. Understanding these deposits is critical for selecting and designing underground storage sites for anthropogenic CO2. To provide confidence that the potential risks of geological CO2 storage are understood, geologists are required to predict how CO2 may behave once stored underground. Natural CO2 accumulations provide a unique opportunity to study long-term geochemical and geomechanical processes that may occur following geological storage of anthropogenic CO2. In addition, natural CO2 springs and gas vents can provide information on the mechanisms of gas migration and the potential effects of CO2 leakage to the surface. This paper provides a description of some natural, European CO2 occurrences. CO2 accumulations occur in many basins across Europe. In addition, volcanic areas and seismically active areas allow CO2-rich fluids to migrate to the near surface. Many of these occur in areas that have been populated for hundreds and thousands of years. Stratigraphic traps have allowed CO2 to accumulate below evaporite, limestone and mudstone caprocks. Comparisons between reservoir sandstone and equivalent nearby sandstones that contain no CO2 indicate that reservoir sandstones may experience increased secondary porosity development through feldspar dissolution. Where fracture reactivation allows CO2-rich fluids to migrate, limited self-sealing may take place through calcite precipitation. Gas migration experiments indicate that, due to geochemical interactions, fine-grained seals would be able to trap smaller volumes of CO2 compared to, for example CH4. In natural systems most leakage from depth occurs along fractures and is typically extremely localized on a metre-scale.

Review of monitoring issues and technologies associated with the long-term underground storage of carbon dioxide

Abstract Large-scale underground storage of CO2 has the potential to play a key role in reducing global greenhouse gas emissions. Typical underground storage reservoirs would lie at depths of 1000 m or more and contain tens or even hundreds of millions of tonnes of CO2. A likely regulatory requirement is that storage sites would have to be monitored both to prove their efficacy in emissions reduction and to ensure site safety. A diverse portfolio of potential monitoring tools is available, some tried and tested in the oil industry, others as yet unproven. Shallow-focused techniques are likely to be deployed to demonstrate short-term site performance and, in the longer term, to ensure early warning of potential surface leakage. Deeper focused methods, notably time-lapse seismic, will be used to track CO2 migration in the subsurface, to assess reservoir performance and to calibrate/validate site performance simulation models. The duration of a monitoring programme is likely to be highly site specific, but conformance between predicted and observed site performance may form an acceptable basis for site closure.

Performance assessments for the geological storage of carbon dioxide: Learning from the radioactive waste disposal experience

The geological storage of carbon dioxide is currently being considered as a possible technology for reducing emissions to atmosphere. Although there are several operational sites where carbon dioxide is stored in this way, methods for assessing the long-term performance and safety of geological storage are at an early stage of development. In this paper the similarities and differences between this field and the geological disposal of radioactive wastes are considered. Priorities are suggested for the development of performance assessment methods for carbon dioxide storage based on areas where experience from radioactive waste disposal can be usefully applied. These include, inter alia, dealing with the various types of uncertainty, using systematic methodologies to ensure an auditable and transparent assessment process, developing whole system models and gaining confidence to model the long-term system evolution by considering information from natural systems. An important area of data shortage remains the potential impacts on humans and ecosystems.

Recarbonation of metamorphosed marls, Jordan

Abstract The Maqarin area, northern Jordan, hosts some unusual, hyperalkaline (pH= 12.5) groundwaters discharging from thermally metamorphosed bituminous marls which formed through spontaneous, in situ combustion of the bitumen. The groundwaters have evolved geochemically through hydration, recarbonation and sulphatization of high temperature minerals. Mineralogical relations of the carbonate phases were examined by XRD and cathodoluminescence in conjunction with a detailed investigation of stable isotope ratios by Nd-YAG laser microsampling. Carbon-13 contents trace the sequence of alteration reactions, involving high temperature decarbonation of host biomic marl, followed by in situ recarbonation of secondary calcium hydroxide and calcium-silicate-hydrates (CSH). Carbonation took place shortly after thermal metamorphism, when non-saturated conditions allowed an atmosphere rich in CO2 from adjacent combustion zones to access reaction sites. Low δ18OCaCO3 values suggest that the earliest phase of recarbonation took place by reaction with hydroxide at elevated temperatures while later phases formed at cooler temperatures. Variable14 activities show that soil CO2 was a component of the later recarbonating atmosphere. Once saturated conditions prevailed in the alteration zone, recarbonation ended and alteration evolved to hydroxide and sulphate dissolution reactions. The recarbonation reactions are a field-scale analogue of recarbonation and14C attenuation in cementitious barriers for radioactive waste repositories.

Comparison of long-term geochemical interactions at two natural CO2-analogues: Montmiral (Southeast Basin, France) and Messokampos (Florina Basin, Greece) case studies

Publisher Summary This chapter assesses the impact of long-term CO2 accumulations in two sandy reservoirs of different origin: the high-temperature and high-pressure reservoir at great depth at Montmira and the shallow, low-temperature and low-pressure reservoir at Messokampos. Petrographic characterization of the reservoirs enabled the identification of both the effects of CO2-induced geochemical interactions as well as their impact on reservoir lithologies. Subsequently, geochemical modeling was applied to reproduce the observed effects, identify their driving parameters and to assess their impact in terms of potential mineral trapping and porosity changes. It indicates that the porosity increase attributed to this reactivity requires that the sediment is flushed intensively with CO2-rich pore waters and that a flow regime in the reservoir must have been in place at a certain point in the reservoirs geological history. The impact of these reactions is minor and does not seem to influence the porosity of the sediment. Comparison of the geochemical interactions at the two sites shows that a reservoirs temperature and pressure conditions determine the impact of CO2 interactions, with elevated temperatures significantly increasing the reaction rates of mineral-trapping reactions.

Potential hazards of CO2 leakage in storage systems : learning from natural systems

Publisher Summary The aim of this chapter is to focus on the effects of increasing global atmospheric CO2 concentrations on plants and marine ecosystems, less is known about the effects of high, but very localized, CO2 concentrations originating from depth. It summarizes some of the findings from the EC-funded natural analogues for the storage of CO2 in the geological environment project (NASCENT) project, which studied a number of natural CO2 seeps throughout Europe. Of these, four sites were chosen for this chapter which, allowed different leakage processes and impacts to be assessed. One site was located in northern Greece, around the commercially producing florina CO2 gas field. The other three sites were in central Italy, including: a selected area of the latera geothermal complex, where natural deep CO2 migrates upwards along faults and is emitted to the atmosphere, the San Vittorino intermontane basin where CO2-charged ground waters cause extensive dissolution of limestone to form large sinkholes, and the Ciampino area to the southeast of Rome, where CO2 derived from deep-seated volcanism within the Alban hills complex migrates along faults in a residential area. Results indicate modifications in groundwater chemistry, sinkhole formation, and elevated toxic gas exposure risks in these areas caused by the occurrence of numerous active CO2 vents, however data also show that effects can be spatially restricted, and that health risks can be minimized with simple and inexpensive approaches and regulations.

Experimental simulation of the alkaline disturbed zone around a cementitious radioactive waste repository: numerical modelling and column experiments

Abstract One approach to describe the migration of an alkaline plume from the cementitious engineered barriers of a geological disposal facility for radioactive wastes is to employ coupled chemistry and flow computer models. Although evidence from natural systems is useful to constrain reaction mechanisms and minerals to be incorporated into such models, time-dependent information is generally lacking. A series of laboratory column experiments has been conducted in order to test the capabilities of two of the currently available, coupled models to predict product solids and output fluid compositions with time. The coupled models PRECIP and CHEQMATE were used to provide predictive calculations based upon known experimental parameters and data available from the literature. The predictions did not replicate all the variations in mineralogy observed in the experiments, primarily due to restrictions in the availability of kinetic and thermdynamic data for the range of secondary phases of interest. However, the model predictions did reproduce the general variation of secondary phases with time and distance along the columns.

Natural CO2 Accumulations in Europe: Understanding Long-Term Geological Processes in CO2 Sequestration

Approximately one-third of anthropogenic CO 2 emissions arises from transport, one-third from industrial and domestic sources, and one-third from power generation. While achieving substantial reductions in emissions from either of the first two will be a long-term process, the technology to capture CO 2 from power plants is available and could lead quickly to significant reductions in emissions—provided mechanisms are available to dispose of the CO 2 thus captured. The capture and underground storage of industrial quantities of carbon dioxide is currently being demonstrated at the Sleipner West gas field in the Norwegian sector of the North Sea. Natural CO 2 accumulations offer the potential to understand the long-term geological processes involved in CO 2 sequestration. By identifying the effects of CO 2 on rock properties, such as changes in permeability and porosity or rock strength, models can be corroborated against empirical data. This can build confidence in their ability to predict likely responses of reservoirs and cap-rocks to geological sequestration. In addition, where CO 2 is actively leaking to the surface, the effects of CO 2 on groundwaters and ecosystems can be identified, and migration mechanisms can be described. The interactions between CO 2 -charged porewaters and both reservoirs and their caprocks through petrographic characterization, porewater and gas geochemistry, geomechanical testing, and gas migration studies in low permeability caprocks have been described. Leakage pathways are identified through soil gas surveys for CO 2 and associated tracer gases. Geochemical analyses of carbonated waters are assessing the effects of CO 2 on groundwaters. An understanding of these processes will be subsequently gained through geochemical and geomechanical modeling.

The underground sequestration of carbon dioxide: containment by chemical reactions in the deep geosphere

Abstract Anthropogenic emissions of carbon dioxide (CO2) have been linked to increasing levels in the atmosphere and to potential global climate change. The capture of CO2 from large point sources, followed by its sequestration as a supercritical fluid into the deep geosphere, is one potential method for reducing such emissions without a drastic change in our energy-producing technologies. Once emplaced underground, geochemical and hydrogeological processes will act to ‘trap’ the CO2 as dissolved species and in carbonate minerals. Although dry supercritical CO2 appears to cause little reaction with the host rocks, once dissolved in water mineral dissolution and precipitation reactions can result. From a geochemical standpoint, sandstones appear to be preferable to carbonates for sequestration operations because fluid-mineral reactions within them have a better capacity for pH buffering. However, individual host lithologies will vary in structure, mineralogy and hydrogeology, and individual sequestration operations will have to take account of local geological, fluid chemical and hydrogeological conditions. This paper summarizes some of the recent laboratory experimental, natural analogue and computer modelling approaches directed at understanding reactions involved in the chemical containment of CO2.