Injecting CO(2) into deep geological strata is proposed as a safe and economically favourable means of storing CO(2) captured from industrial point sources. It is difficult, however, to assess the long-term consequences of CO(2) flooding in the subsurface from decadal observations of existing disposal sites. Both the site design and long-term safety modelling critically depend on how and where CO(2) will be stored in the site over its lifetime. Within a geological storage site, the injected CO(2) can dissolve in solution or precipitate as carbonate minerals. Here we identify and quantify the principal mechanism of CO(2) fluid phase removal in nine natural gas fields in North America, China and Europe, using noble gas and carbon isotope tracers. The natural gas fields investigated in our study are dominated by a CO(2) phase and provide a natural analogue for assessing the geological storage of anthropogenic CO(2) over millennial timescales. We find that in seven gas fields with siliciclastic or carbonate-dominated reservoir lithologies, dissolution in formation water at a pH of 5-5.8 is the sole major sink for CO(2). In two fields with siliciclastic reservoir lithologies, some CO(2) loss through precipitation as carbonate minerals cannot be ruled out, but can account for a maximum of 18 per cent of the loss of emplaced CO(2). In view of our findings that geological mineral fixation is a minor CO(2) trapping mechanism in natural gas fields, we suggest that long-term anthropogenic CO(2) storage models in similar geological systems should focus on the potential mobility of CO(2) dissolved in water.

Identification of the source of CO 2 in natural reservoirs and development of physical models to account for the migration and interaction of this CO 2 with the groundwater is essential for developing a quantitative understanding of the long term storage potential of CO 2 in the subsurface. We present the results of 57 noble gas determinations in CO 2 rich fields (>82%) from three natural reservoirs to the east of the Colorado Plateau uplift province, USA (Bravo Dome, NM., Sheep Mountain, CO. and McCallum Dome, CO.), and from two reservoirs from within the uplift area (St. John's Dome, AZ., and McElmo Dome, CO.). We demonstrate that all fields have CO 2 / 3 He ratios consistent with a dominantly magmatic source. The most recent volcanics in the province date from 8 to 10 ka and are associated with the Bravo Dome field. The oldest magmatic activity dates from 42 to 70 Ma and is associated with the McElmo Dome field, located in the tectonically stable centre of the Colorado Plateau: CO 2 can be stored within the subsurface on a millennia timescale.The manner and extent of contact of the CO 2 phase with the groundwater system is a critical parameter in using these systems as natural analogues for geological storage of anthropogenic CO 2 . We show that coherent fractionation of groundwater 20 Ne/ 36 Ar with crustal radiogenic noble gases ( 4 He, 21 Ne, 40 Ar) is explained by a two stage re-dissolution model: Stage 1: Magmatic CO 2 injection into the groundwater system strips dissolved air-derived noble gases (ASW) and accumulated crustal/radiogenic noble gas by CO 2 /water phase partitioning. The CO 2 containing the groundwater stripped gases provides the first reservoir fluid charge. Subsequent charges of CO 2 provide no more ASW or crustal noble gases, and serve only to dilute the original ASW and crustal noble gas rich CO 2 . Reservoir scale preservation of concentration gradients in ASW-derived noble gases thus provide CO 2 filling direction. This is seen in the Bravo Dome and St. John's Dome fields. Stage 2: The noble gases re-dissolve into any available gas stripped groundwater. This is modeled as a Rayleigh distillation process and enables us to quantify for each sample: (1) the volume of groundwater originally 'stripped' on reservoir filling; and (2) the volume of groundwater involved in subsequent interaction. The original water volume that is gas stripped varies from as low as 0.0005 cm 3 groundwater/cm 3 gas (STP) in one Bravo Dome sample, to 2.56 cm 3 groundwater/cm 3 gas (STP) in a St. John's Dome sample. Subsequent gas/groundwater equilibration varies within all fields, each showing a similar range, from zero to $100 cm 3 water/cm 3 gas (at reservoir pressure and temperature).

ABSTRACT:Anthropogenic energy-related CO 2 emissions are higher than ever. With new fossil fuel power plants, growing energy-intensive industries and new sources of fossil fuels in development further emissions increase seems inevitable. The rapid application of carbon capture and storage (CCS) is a much heralded means to tackle emissions from both existing and future sources. However, despite extensive and successful research and development, progress in deploying CCS has stalled. No fossil fuel burning power plants, the greatest source of CO 2 emissions, are currently using CCS, and publicly supported CCS demonstration programmes are struggling to deliver actual projects. Yet, CCS remains a core component of national and global emissions reduction scenarios. Governments have to either increase commitment to CCS through much more active market support and emissions regulation, or accept its failure and recognise that continued expansion of fossil fuel burning energy capacity is a severe threat to attaining climate change mitigation objectives.

Carbon capture and storage (CCS) can help nations meet their Paris CO2 reduction commitments cost-effectively. However, lack of confidence in geologic CO2 storage security remains a barrier to CCS implementation. Here we present a numerical program that calculates CO2 storage security and leakage to the atmosphere over 10,000 years. This combines quantitative estimates of geological subsurface CO2 retention, and of surface CO2 leakage. We calculate that realistically well-regulated storage in regions with moderate well densities has a 50% probability that leakage remains below 0.0008% per year, with over 98% of the injected CO2 retained in the subsurface over 10,000 years. An unrealistic scenario, where CO2 storage is inadequately regulated, estimates that more than 78% will be retained over 10,000 years. Our modelling results suggest that geological storage of CO2 can be a secure climate change mitigation option, but we note that long-term behaviour of CO2 in the subsurface remains a key uncertainty.

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25For carbon capture and storage to successfully contribute to climate mitigation efforts, the 26 captured and stored CO2 must be securely isolated from the atmosphere and oceans for a 27 minimum of 10,000 years. As it is not possible to undertake experiments over such timescales, 28here we investigate natural occurrences of CO2, trapped for 10 4 -10 6 yr to understand the 29 geologic controls on long term storage performance. We present the most comprehensive 30 natural CO2 reservoir dataset compiled to date, containing 76 naturally occurring natural CO2 31 stores, located in a range of geological environments around the world. We use this dataset 32to perform a critical analysis of the controls on long-term CO2 retention in the subsurface. We 33 find no evidence of measureable CO2 migration at 66 sites and hence use these sites as 34 examples of secure CO2 retention over geological timescales. We find unequivocal evidence 35 of CO2 migration to the Earth's surface at only 6 sites, with inconclusive evidence of migration 36 at 4 reservoirs. Our analysis shows that successful CO2 retention is controlled by: thick and 37 multiple caprocks, reservoir depths of >1200m, and high density CO2. Where CO2 has 38 migrated to surface, the pathways by which it has done so are focused along faults, illustrating 39 that CO2 migration via faults is the biggest risk to secure storage. However, we also find that 40 many naturally occurring CO2 reservoirs are fault bound illustrating that faults can also 41 securely retain CO2 over geological timescales. Hence, we conclude that the sealing ability of 42 fault or damage zones to CO2 must be fully characterised during the appraisal process to fully 43 assess the risk of CO2 migration they pose. We propose new engineered storage site selection 44 criteria informed directly from on our observations from naturally occurring CO2 reservoirs. 45These criteria are similar to, but more prescriptive than, existing best-practise guidance for 46 selecting sites for engineered CO2 storage and we believe that if adopted will increase CO2 47 storage security in engineered CO2 stores.

Following the landmark 2015 United Nations Paris Agreement, a growing number of countries are committing to the transition to net-zero emissions. Carbon capture and storage (CCS) has been consistently heralded to directly address emissions from the energy and industrial sectors and forms a significant component of plans to reach net-zero. However, despite the critical importance of the technology and substantial research and development to date, CCS deployment has been slow. This review examines deployment efforts over the last decade. We reveal that facility deployment must increase dramatically from current levels, and much work remains to maximize storage of CO 2 in vast subsurface reserves. Using current rates of deployment, CO 2 storage capacity by 2050 is projected to be around 700 million tons per year, just 10% of what is required. Meeting the net-zero targets via CCS ambitions seems unlikely unless worldwide coordinated efforts and rapid changes in policy take place. ll

The Pah Tempe hot springs discharge ~260 L/s of water at ~40 °C into the Virgin River in the footwall damage zone of the Hurricane fault at Timpoweap Canyon, near Hurricane, Utah, USA. Although these are Na-Cl waters, they actively discharge CO 2 gas and contain signifi cant quantities of CO 2 (~34.6 mmol/kg), predominantly as H 2 CO 3 and HCO 3 -. Because of excellent exposures, Pah Tempe provides an exceptional opportunity to observe the effects of enhanced fracture permeability in an active extensional fault.Pah Tempe waters have been deeply circulated (>5 km; >150 °C) into basement rock as illustrated by the clear water-rock exchange of oxygen isotopes. Waters were probably recharged under colder climate conditions than present and therefore have a prolonged subsurface residence. Discharge of both water and gas in the springs correlates to the density of fractures in carbonate rocks above stream level. This observation suggests that clusters of high fracture density in the faultdamage zone act as pathways from the likely regional aquifer, the eolian Queantoweap Sandstone, through the overlying confi ning unit, the gypsiferous silty Seligman Member of the Kaibab Formation.Mass-balance modeling suggests that the majority of CO 2 discharge is the product of the quantitative dissolution of CO 2 gas at depth within the fault zone. Upon discharge, most of the carbon is released to the surface as dissolved species. It appears that the subsurface production rate of CO 2 is relatively low because Pah Tempe waters are grossly undersaturated in CO 2 at inferred minimum circulation depths and temperatures. Geological and geochemical data also suggest that the CO 2 is dominated by a crustal component complemented by minor mantle contributions.