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Rising atmospheric emissions as a result of fossil fuel consumption is a major concern for the developed and developing countries, considering the role it plays in the greenhouse effect and hence global climate change. Various schemes for underground CO storage (viz. geologic disposal into coal seams, depleted oil/gas reservoirs, salt caverns, and deep oceans) have already been reported in the literature. Subsurface CO storage through clathrate hydrate formation is a novel option for the reduction of atmospheric carbon content and permanent underground CO disposal over geological periods. Depths of CO2 injection, respective pressure-temperature conditions, water salinity etc. are all important factors for successful CO sequestration. Furthermore if CO2 is injected/stored in methane hydrate reservoirs it could be possible to produce low-carbon methane energy, thereby offsetting the cost of CO transportation and disposal. In this communication, we present the results of experiments carried out to understand the mechanisms of CH displacement in hydrate structure by injected CO and the formation of simple CO or mixed CH-CO hydrates, thereby simulating the conditions of CO injection into CH hydrate reservoirs. We used two sets of experimental rigs specifically designed for studying gas hydrates in porous media. They are the Medium Pressure Glass Micromodel (80 bar) for visual observation of gas hydrate formation / dissociation and distribution in porous media, and the Ultrasonic Rig (400 bar) for studying CO sequestration in CH hydrates in synthetic porous media.

The timescales needed for the geological storage of carbon dioxide (CO) are potentially thousands of years. Therefore, before large-scale underground CO storage can take place, it will be necessary to demonstrate that the processes are well understood, risks to the environment and human populations are low, and environmental disturbances can be minimised. One way of demonstrating that CO can remain trapped underground for geologically significant times is to provide evidence from existing naturally occurring accumulations. These accumulations occur in a variety of geological environments and many can be demonstrated to have retained CO for periods longer than those being considered for CO storage. This fact will build confidence in the concept with non-specialist policy-makers, environmental NGOs and the public. Studies of natural analogues can be used to further validate predictive geochemical and geomechanical models, increasing confidence in these models to predict how CO will behave during and after storage, helping to determine how much of the CO will be permanently trapped through mineral reactions. The results have identified that kinetic reaction data need to be improved. It is unlikely that in reservoirs similar to those investigated here, significant mineral trapping can be expected, except over long geological timescales. Natural accumulations can be used to test methodologies for monitoring CO leakage that may be appropriate for use above repositories, both onshore and offshore, to establish baseline conditions and to monitor sites at the surface during and after storage. Soil gas surveys and analyses of gas leakage rates can define how CO migrates through the near surface environment. Techniques for determining the sealing capacity of caprocks have been tested on natural seals known to retain CO and caprocks from future potential storage sites can be compared with these datasets.

As is well known the long-term effects and stability of a man-made CO geological storage facility is very difficult to predict with laboratory or modeling experiments due to the size and long time scales involved. Instead attractive additional sources of information are natural sites where CO produced at great depths is either trapped in porous reservoirs or leaks to the surface. These sites can be considered as “natural analogues„ of what may occur over geological time spans within an engineered CO geological storage site. The study of these sites can be subdivided into three broad fields: i) understanding why some reservoirs leak while others don't; ii) understanding the possible effects of CO should it leak into the near-surface environment; and iii) using the leaking sites to develop, test and optimise various monitoring technologies. The present article summaries many of the near-surface gas geochemistry results obtained in central Italy during the EC-funded NASCENT project (Natural Analogues for the Storage of CO in the Geological Environment). These include a comparison of leaking (Latera) and a non-leaking (Sesta) CO reservoirs, detailed soil gas surveys to outline migration pathways, the development of a geochemical continuous-monitoring station to study temporal changes in CO concentrations, and field experiments involving the injection of a gas mixture in the shallow subsurface to outline migration pathways and to understand the behaviour of various gas species based on their different chemical-physical-biological characteristics. Put together this data provides useful information for site selection, risk assessment and monitoring techniques, which is needed if CO geological storage is to become an accepted and widely-applied technology.

The degree of reactivity between CO, pore-waters and minerals may have significant consequences on CO storage capacity, the injection process, and long-term safety and stability. Geochemical reactions are highly site-specific and time-dependent. They need to be assessed on a site-to-site basis according to best practises by combining numerical modelling and observations from laboratory experiments, field monitoring, and natural analogues. A selection of lessons learned from three European projects about the reactivity of CO with reservoir rocks and cap rocks is presented here for three sites: Sleipner (Norway) and Weyburn (Canada) where more than 1 Mt of CO per year has been injected underground since 1996 and 2000 respectively, and Montmiral, a natural CO field in France.

Natural gas emissions represent extremely attractive surrogates for the study of CO-effects both on the environment and human life. Three Italian case histories demonstrate the possible co-existence of CO natural emissions and people since Roman times. The Solfatara crater (Phlegraean fields caldera, Southern Italy) is an ancient Roman spa. The area is characterized by intense and diffuse fumarole and hydrothermal activity. Soil gas flux measurements show that the entire area discharges between 1200 and 1500 tons of CO a day. In proximity of Panarea island (Aeolian islands, Southern Italy), a huge submarine volcanic-hydrothermal gas burst occurred in November, 2002. The submarine gas emissions locally modified seawater pH (from 8.0 to 5.0) and Eh (from +80 mV to -200 mV), causing a strong modification of the marine ecosystem. Collected data suggest an intriguing correlation between the gas/water vent location/evolution and the main local and regional faults. CO degassing also characterizes the Telese area (Southern Italy), one of the most seismically active segments of the southern Apennine belt with the occurrence of five large destructive earthquakes in the last 500 years. Geochemical surveys in this area reveal the presence of high CO content in ground-water. Carbon isotopic analysis of CO reveal its deep origin, probably caused by the presence of a cooling magmatic intrusion inside the carbonate basement. All the above mentioned areas are constantly monitored since they are densely populated. Although natural phenomena are not always predictable, local people have nevertheless learnt to manage and, in some case, exploit these phenomena, suggesting significant human adaptability even in extreme situations.

Soil gas surveys were performed along the Adriatic foredeep (Vasto, Ferrandina, Pisticci basins), in order to evaluate the relationship between neotectonics and gas leakage from hydrocarbon reservoirs. More than 4000 soil gas samples were collected in the area and analyzed for helium, which is a good fracture tracer due to its chemical inertness and high mobility. Furthermore, helium is enriched in more than 90% of known reservoirs, and displays many characteristics of the ideal geochemical tracer for buried faults and gas and oil reservoirs (abiogenic, non-reactive, and mobile). Statistical analysis shows an average helium concentration of 5.7 ppm in the foredeep basin areas (atmospheric value is 5.2 ppm). This value is higher than that calculated using over 30,000 soil gas samples collected throughout Italy in different geological scenarios (about 5.4 ppm). Results obtained using a geostatistical approach are consistent with the presence of high helium concentrations as linear or spot anomalies due to irregular, channelled flow along faults above hydrocarbon accumulations. Considering the Plio-Miocene age of the Adriatic foredeep reservoirs, the magnitude of these diffuse gas microseepages highlights that losses from hydrocarbon reservoirs should be low. Results indicate that despite the fact that the area is heavily faulted and that gas seepage has occurred from the reservoir over geological periods of time, no environmental effects are observed at surface. This fact lends support to the idea that geological sequestration of CO within a less structurallyactive area would result in the safe, long-term isolation of this green-house gas.

Methane, which is at least partly stored in the bottom sediments of Lake Baikal as gas hydrates, is released on the lake floor in the deeper parts of the basin along major faults, forming venting structures similar to small mud volcanoes. The CH venting structures are considered to be the surface expression of escape pathways for excess CH generated by the dissociation of pre-existing hydrates. The existence of a local heat flow anomaly associated with the seep area is most likely due to a heat pulse causing the dissociation of the underlying gas hydrates. The heat pulse may be caused by upward flow of geothermal fluids along segments of active faults, possibly accelerated by seismic pumping. It is assumed that this fluid flow is tectonically triggered, considering that left-lateral strike-slip movements along the border faults act as a major factor in fluid accumulation: even a reduced lateral displacement is able to generate fluid flow in the compressional direction, resulting in fluid escape along faults directed along the main direction of extension. The tectonic effect may be coupled to the sediment compaction due to a high sedimentation rate in the area of mud volcanism. Both processes may generate a large-scale convective fluid loop within the basin-fill sediments which advects deeper gases and fluids to the shallow sub-surface. Even in the extensional tectonic environment of Lake Baikal, local compressional forces related to a strike-slip component, may play a role in fluid flow, accumulation and gas escape along active faults. The mechanisms that result in the expulsion of the CH in the Lake Baikal sediments are considered as an analogue of what could happen during CO sequestration in a similar tectonic environment.

The IEA Weyburn CO Monitoring and Storage Project has analysed the effects of a miscible CO flood into a carbonate reservoir rock at an onshore Canadian oilfield. Anthropogenic CO is being injected as part of an enhanced oil recovery operation. The European research was aimed at analysing longterm migration pathways of CO and the effects of CO on the hydrochemical and mineralogical properties of the reservoir rock. The long term safety and performance of CO storage was assessed by the construction of a Features, Events and Processes (FEP) database which provides a comprehensive knowledge base for the geological storage of CO. The pre-CO injection hydrogeological, hydrochemical and petrographical conditions in the reservoir were investigated in order to recognise changes caused by the CO flood and to assess the fate of the CO. The Mississippian aquifer has a salinity gradient in the Weyburn area, where flows are oriented SW-NE.

CO is being injected into a 1450-m deep oil reservoir located in Weyburn, Saskatchewan, Canada, for enhanced oil recovery. To complement this commercial activity, a major research project to study geological sequestration and storage of CO, known as the International Energy Agency (IEA) Weyburn CO Monitoring and Storage Project, was launched in July 2000. Phase 1 of this project was completed in 2004. This paper discusses the longterm assessment of the fate of CO in the IEA Weyburn Project, describing the underlying methodology as well as the modeling approach and the results obtained. The conclusion from the modeling predictions is that if the Weyburn CO storage system evolves as expected, long-term geological storage of greenhouse gas CO will be achieved.

EnCana's CO injection EOR project at Weyburn Saskatchewan (Canada) is the focal point of a multi-faceted research program, sponsored by the IEA GHG R&D and numerous international industrial and government partners. More than yearly strontium isotope, trace element and dissolved gas surveys were conducted by INGV in conjunction with the thrice yearly borehole fluid sampling trips performed by the Canadian partners. This paper focuses on the Sr isotope monitoring. Approximately 25 samples were collected over three years for Sr/Sr analyses. At Weyburn, a water-alternating-gas (WAG) EOR technique is used to inject water and CO into the Mississippian Midale reservoir. Sr/Sr ratios for produced fluids fall between 0.7077 and 0.7082, consistent with published values for Mississippian fluids and carbonate minerals. A small Sr/Sr component of this produced fluid is derived from waters of the Cretaceous Mannville aquifer, which has been used for waterflooding EOR since 1959. The progressively more positive Sr isotope trend from 2001 to 2003 may be due to: 1) a smaller Mannville aquifer component in the water flooding process; and/or 2) the dissolution of Mississippian host rocks during the ongoing CO injection. Evidence that Sr/Sr values are approaching those of Mississippian host-rock values may point towards zones of carbonate dissolution as a result of continuing CO injection. This hypothesis is strengthened by i) δC data; ii) preliminary “gross composition„ of dissolved gases (HS, CO, CH, He, H) and iii) by trace elements data.