Atmospheric monitoring of CO2 geological storage has developed from a concept to reality over less than a decade. Measurements of atmospheric composition and surface to air fluxes are now being made at onshore test sites, pilot projects, operational projects and likely future storage regions around the world. The motivation for atmospheric monitoring is usually to detect potential leakage from CO2 storage activities that might affect health and safety or to test the efficacy of carbon capture and storage (CCS) as a climate mitigation option. We have focused on the mitigation requirement, which involves determining whether potential leakage is below a maximum acceptable rate. Climatic considerations suggest that the maximum leakage rate of stored CO2 should be very small, of the order of 0.01% of that stored per year, globally averaged. Monitoring operational CO2 storage sites to confirm that potential leakage to the atmosphere is below this rate and to locate and quantify the any leakage flux can be a challenge, mainly because of the large and variable CO2 concentrations and fluxes in ecosystems and urban environments. We have developed and assessed atmospheric techniques during field experiments, during 4 years of monitoring the CO2CRC Otway Project, and by using model simulations. From this experience we are able to make recommendations about suitable technologies and strategies to optimise the capability of atmospheric monitoring of CCS in different environments. Abstract for paper to be presented at CO2CRC Research Symposium 2010, 1-3 December 2010, Melbourne

The Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) Otway Project in the onshore Otway Basin, Victoria, is Australia's first pilot project for the long term sequestration of CO2. The Otway Project has injected 65,445 tonnes of a mixed CO2-CH4 supercritical fluid (77 mol% CO2, 20 mol% CH4, 3 mol% of minor wet gases and N2) some 2000 m below the surface into the Waarre Formation, which is capped by the Belfast Mudstone regional seal. The site has been comprehensively characterised by a multidisciplinary team and the risk analysis has shown the likelihood of leakage out of the injection horizon, let alone to the land surface, to be exceedingly low. Nevertheless, the objectives of the CO2CRC through the Otway Project are not only to demonstrate safe CO2 injection, but also to develop new methodologies for monitoring and verification (M&V) of carbon storage that might apply to future commercial scale injection. At Otway, this involves M&V at the reservoir level and Assurance Monitoring, in the shallow subsurface (aquifers and soils) and the atmosphere. The groundwater monitoring system represents the most comprehensive system for monitoring freshwater in the vicinity of a CO2 storage demonstration to date. Monitoring the groundwater is of particular significance in demonstrating the ongoing integrity of natural resources to the general community.

The CO2CRC Otway Project in southwestern Victoria, Australia has injected over 17 months 65 445 tonnes of a mixed carbon dioxide-methane fluid into the water leg of a depleted natural gas reservoir at a depth of approximately 2km. Pressurized sub-surface fluids were collected from the Naylor-1 observation well using a tri-level U-tube sampling system located near the crest of the fault-bounded anticline trap, 300 metres up-dip of the CRC-1 gas injection well. Relative to the pre-injection gas-water contact (GWC), only the shallowest U-tube initially accessed the residual methane gas cap. The pre-injection gas cap at Naylor-1 contains CO₂ at 1.5 mol% compared to 75.4 mol% for the injected gas from the Buttress-1 supply well and its CO₂ is depleted in ¹³C by 4.5%₀ VPDB compared to the injected supercritical CO<Sub>2</sub>. Additional assurance of the arrival of injected gas at the observation well is provided by the use of the added tracer compounds, CD₄, Kr and SF₆ in the injected gas stream. Lessons learnt from the CO2CRC Otway Project have enabled us to better anticipate the challenges for rapid deployment of carbon dioxide in a commercial environment at much larger scales.

The critical success factors which control hydrocarbon prospectivity in the Otway Basin have been investigated using petroleum systems approaches. Greater than 99% of the hydrocarbon inventory within the Victorian Otway Basin has been sourced from Austral 2 (Albian-Aptian) source rocks and these accumulations are typically located either within, or within approximately 3,000 m of source rock kitchens which are at peak thermal maturity at present day. Importantly, the zones of greatest prospectivity are located where these source rocks have been actively generating and expelling hydrocarbons throughout the Late Tertiary, primarily as a result of sediment loading associated with progradation of the Heytesbury shelfal carbonates. This peak generation window occurs at an average depth of approximately 2,500-3,500 m 'sub-mud' across much of the basin, which has allowed prospective hydrocarbon fairways to be mapped out, thereby highlighting areas of greatest prospectivity. It is believed that the spatial proximity of the actively generating source rocks to the accumulations is due to several factors, which includes overall poor fault seal in the basin (success cases occur where charge rate exceeds leakage rate) and relatively complex and tortuous migration fairways (which means that large volumes of hydrocarbons are only focussed and migrate for relatively short distances). etc

Geological storage of CO₂ requires fundamental knowledge and predictive capabilities on the transport and reactions of injected CO₂ and associated gases to assess the short and long term consequences. CO₂ can be stored in the subsurface through various mechanisms including structural trapping, solubility trapping and by precipitation of carbonate minerals. While mineral strapping is considered to be the safest storage mechanism as it permanently immobilizes the CO₂, the reaction rates and the likely importance for geosequestration is poorly understood. This project has five objectives, which aim to make CO₂ storage more predictable and safer. A range of approaches will be used including desk top studies, laboratory and field experiments and geochemical modelling.

Increasing CO2 emissions resulting from the expansion of coal fired power generation capacity and other industry in Queensland suggests that a long-term high capacity storage solution is needed. Despite some relatively large distances (upwards of 500 km) between sources and sinks, a review of the Galilee Basin suggests that it may have the potential to sequester a significant amount of Queensland's stationary CO2 emissions, however a paucity of data in several significant regions do not allow this potential to be fully assessed at the present time. Sandstones with good porosity and permeability characteristics occur within several formations including the Early Permian Aramac Coal Measures, the Late Permian Colinlea Sandstone and the Triassic Clematis Sandstone. Intraformational and local seals as well as a regional seal, the Triassic Moolayember Formation and the Permian Bandanna Formation, appear sufficient although these have not been tested. Stratigraphic and residual/solution trapping are the most likely CO2 storage mechanisms, as low amplitude structures are a feature of the Galilee Basin. Most of the structures targeted by exploration companies are generally too small to store CO2 in the quantities anticipated to be emitted from potential emission nodes such as the Rockhampton-Gladstone region. Regional reconnaissance indicate small 15-20 km2 structures with a 50-125 m net sandstone section are typical for the Clematis Sandstone Formation in the south eastern area of the Galilee Basin. Covering an area of approximately 247,000 km2 and measuring around 700km north-south and 520 east-west, the Galilee Basin is a significant feature of central Queensland. Three main depocentres the Koburra Trough (east), the Lovelle Depression (west) and the Southern Galilee Basin (south) contain several hundred metres of Late Carboniferous to Middle Triassic sediments (up to 3000m, 730m, and 1400m respectively). Most of the low amplitude structures in the basin, generally trending north-easterly to north-westerly, are the result of reactivation of older basement structures in the underlying Drummond and Adavale Basins. Tectonic events were dominantly compressional resulting in uplift and erosion of parts of the basin during the Late Permian and Triassic. A regional south-westerly tilt was later imposed due to downwarping of the overlying Eromanga Basin, which is up to 1200 m thick over the Galilee strata. Sedimentation in the Galilee Basin was dominated by fluvial to lacustrine (and in part glacial) depositional systems. This resulted in a sequence of sandstones, mudstones, siltstones, coals and minor tuff in what was a relatively shallow intracratonic basin. The entire Galilee sequence is saturated with good to excellent quality fresh water in both the Permian and Triassic strata (Hawkins, unpublished) with probable recharge from the north-east into the outcropping Triassic reservoirs. Sediment composition is mixed as a result of a variety of provenances including older sedimentary rock, metasediments and other metamorphic rocks, granites, volcanics and direct volcanic input (tuffs). Climate varied from glacial to warm and humid to temperate. Forty years or more of exploration in the Galilee Basin has failed to discover any economic accumulations of hydrocarbons, despite the presence of apparently good to very good reservoirs and seals in both the Permian and Triassic sequence. Further geological study and in particular the interpretation of seismic data is required to increase the understanding and assess the quality of the basin for CO2 storage including; fully assessing reservoirs, seals and trapping mechanisms; estimating storage capacity; and addressing issues such as the presence of a potentially large fresh water resource.

This series of cross sections and data show the suitablility of the Sydney Basin for storage of carbon dioxide.Cartography file number 07-1825-1.

Sampling, prior to CO2 injection at the CO2CRC Otway Project, southeastern Victoria at the end of 2007 early 2008, provided a stocktake of the molecular and isotopic (carbon and hydrogen) compositions of the subsurface hydrocarbon and non-hydrocarbon gases (and heavier hydrocarbons) at, and in close proximity to, the injection site. This baseline study is also fundamental to the assessment of present sub-surface petroleum components as natural tracers for injected gases arriving at the monitoring well. The CO2CRC Otway Project will use the CO2-rich natural gas (containing 79% CO2 and 20% methane) from the Buttress-1 well; totalling 100,000 tons of gas injected over 2 years. This gas mixture will be injected supercritically into sandstones of the CRC-1 well below the original gas-water contact at ~2000 m in the Waarre Formation. The depleted natural gas well at Naylor-1 is the monitoring well, situated 300 m updip of the injection well. Gas from the Waarre Formation in Naylor-1 observation well contains <1% CO2, which is isotopically depleted in 13C (13C -15.8) by 9 compared to CO2 (13C -6.8) in Buttress-1. Thus the carbon isotopes of CO2 can act as a primary natural tracer for monitoring purposes. Isotopically, the minimum detection limit would result from an increase of ~20 % in the CO2 concentration at Naylor-1 from the Buttress-derived CO2. On the other hand, the carbon and hydrogen isotopes of methane, wet gases and higher hydrocarbons are very similar between Buttress-1, CRC-1 and Naylor-1, requiring addition of external conservative tracers (Boreham et al., 2007) for the monitoring of hydrocarbon components. Although the content of liquid hydrocarbons in the gases is very low (<1%), there is the potential for supercritical CO2 extraction of these high molecular weight components (e.g. black oil in the Caroline-1 CO2 gas field and solid wax at the Boggy Creek CO2 production plant) that can be either advantageous (lubrication) or detrimental (clogging) to monitoring equipment at Naylor-1. The CRC-1 well provided an opportunity to collect downhole mud gases over many formations. Maximum total hydrocarbon concentration of 0.97 % occurred in the Waarre Formation Unit C. Surprisingly, a free gas zone in the overlying Flaxmans Formation had a lower maximum concentration (0.17 %). Carbon isotopes for the hydrocarbon gases from 1907 to 2249 mRT showed little downhole variation, while the 13C CO2 averaged -16, identical to CO2 at Naylor-1. Interestingly, the condensate recovered from a MDT in the Flaxmans Formation showed depletions in 13C for the C11 to C20 n-alkanes of up to 6 for n-C15 compared to n-alkanes of oils and condensates sourced from the Eumeralla Formation of the eastern Otway Basin (Boreham et al., 2004). Water washing is suspected at CRC-1 but is not expected to be a major factor affecting hydrocarbon compositions in the short term. The results of this subsurface petroleum audit have been pivotal in demonstrating the need for the addition of external tracers, especially for the hydrocarbon components, and provide an integral part of the near-surface, soil gas and atmospheric monitoring activities of the CO2CRC Otway Project. References Boreham, C.J., Hope, J.M., Jackson, P., Davenport, R., Earl, K.L., Edwards, D.S., Logan, G.A., Krassay, A.A., 2004. Gas-oil-source correlations in the Otway Basin, southern Australia. In: Boult, P.J., Johns, D.R., Lang, S.C. (Eds.), Eastern Australasian Basins Symposium II, Petroleum Exploration Society of Australia, Special Publication, pp. 603-627. Boreham, C.J., Underschultz, J., Stalker, L., Freifeld, B., Volk, H., Perkins, E., 2007. Perdeuterated methane as a novel tracer in CO2 geosequestration. In: Farrimond, P. et al. (Eds.), The 23rd International Meeting on Organic Geochemistry, Torquay, England 9th-14th September 2007, Book of Abstracts, 713-714.

Monitoring is an important aspect in verifying the integrity of the geological storage of greenhouse gases. Geoscience Australia is working with CSIRO, the CO2CRC, the Australian National University, the University of Adelaide and the University of Wollongong to develop and evaluate new techniques to detect and quantify greenhouse gas emissions.

Australia's coal-based power-stations produce about 70% of its energy needs and consequently have led, to the adoption of a multi-disciplinary approach to instigating low emission technologies, which include CO2 capture, injection and storage. The onshore Bowen Basin could provide potential multi-scale storage site projects. Storage potential was demonstrated within a 256 square kilometer area on the eastern flank of the 60-km by 20-km Wunger Ridge using a regional model pertaining to a potential commercial-scale 200 megawatt power-station with emission/injection rates of 1.2 million ton/year. Palaeogeography interpretations of the targeted reservoir indicate a dominantly meandering channel system with permeabilities of up to 1 darcy on the ridge's eastern flank, waning to a deltaic system downdip. Seismic interpretation indicates a relatively unfaulted reservoir-to-seal section on the flank with low-relief structures. Depth to reservoir ranges from 2100 to 2700-m. Simulation from a simplified 3-D block model indicates at least two vertical wells are needed to inject at 1.2 million ton/year in permeabilities of 1 darcy, and reservoir thicknesses of about 5-m. The presence of intra-reservoir baffles reduces the injection rate possible, with a subsequent increase in the number of wells required to maintain the project injection rate, also true for a low-permeability trapping scenario. Long-term storage of acceptable volumes would involve intra-reservoir baffle, stratigraphic, residual, and potentially depleted field trapping scenarios along a 10 to 15-km migration route. Trapping success is ultimately a function of optimal reservoir characteristics both estimated from more complex modeling and, ultimately, collection of infill seismic and new wells.