Projects

Projects: Projects for Investigator

Engineered storage of CO₂ within the subsurface will require the injection and storage of CO₂ over thousands of years. Noble gases are conservative tracers within the subsurface which occur with CO₂, or can be miscible with it. Combined with active tracers such as carbon isotopes noble gases have the potential to be extremely useful in monitoring underground CO₂ storage sites. The key objective of this project is to identify and quantify natural geochemical and migration processes sufficiently to enable the potential use of these natural tracers in future engineered CO₂ storage sites.

Recent work has shown that CO₂ gas field's world wide show a clear, and quite amazing, correlation between decreasing CO₂/3He and increasing formation water derived noble gases, 20Ne and 4He. As the noble gases are conservative, the change in CO₂/3He is due to CO₂ loss and this is quantitatively related to the volume of formation water 'seen' by the gas. Using a combination of the noble gases and carbon stable isotopes it is possible to differentiate between the amount of CO₂ stored as carbonate mineral precipitate and the amount dissolved in the reservoir formation water. This technique provides an unusually precise estimate of the pH conditions of the porewaters in the individual reservoirs. This is a critical unknown in geochemical modelling, and very difficult to measure in natural waters, but is essential for accurate prediction of CO₂ dissolution or reaction with reservoir and seal minerals.

The initial phase of the project (months 1-12) will attempt to corroborate a proposed noble gas and stable isotope model using independent geochemical modelling (using a combination of PHREEQC, Geochemists Workbench and RockFlow). Detailed models of three of the key reservoirs (Bravo,McElmo and St. Johns Dome) will be constructed. The pH of the individual reservoirs will be modelled assuming conventional equilibration with calcite and silica. The results will be compared to the values predicted by the noble gas and d13C (CO₂) model.

The second phase (months 13-18) will involve separation of recent carbonate precipitate (in the form of dawsonite mineral) from core samples within the St. Johns Dome reservoir. As the carbon isotopic composition of the CO₂ gas contained within the field is known it is possible to predict the expected isotopic composition of a new mineral precipitating as calcite. However due to a lack of work in this area, the fractionation factor between CO₂ gas precipitating as dawsonite is unknown. This work would produce the first fractionation factor. This tests the common assumption that the fractionation factor is the same for both dawsonite and calcite.

The third phase (months 19-34) will investigate the diffusion of noble gases through mudrocksfrom a variety of different aged CO₂ charge. Cap rock shales from the North Sea Miller (70Ma) field will provide a millennia analogue which will be compared to cap roc ks from both the St.Johns (1Ma) and Bravo Dome fields (10ka). I anticipate that noble gases migrating with CO₂ will leave a 'signature' adsorbed onto organic components, which can be analysed to yield the net long-term seal permeability.

The fourth phase will run in conjunction with the fourth phase and will examine noble gas composition of flue gases formed from the combustion of fossil fuel. Enrichments in heavy noble gases (Ar, Kr and Xe) in organic rich oil and gas rich source rocks have been widely documented. As noble gases are conservative tracers they will be unaffected by the combustion process and therefore the enrichment should be remain in the flue gases. I will investigate if these heavy noble gases are present in high enough concentrations in flue gas to be used to fingerprint individual CO₂ sources, and trace these underground once it is injected.

In months 35-36 the previous months work will be used to produce a comprehensive review of the applications of natural tracers to CO₂ storage.

Since the Industrial Revolution, burning of fossil fuels (coal, oil and gas) has greatly increased the carbon dioxide (CO₂) content of the atmosphere. The higher level of CO₂ is widely accepted to be a major contributor to greenhouse warming of the Earth and acidification of the oceans. The effects of this warming on the world are still controversial. However, many scientists now believe that the Earth's atmosphere will heat up by at least 2 to 3 centigrade over the next 100 years. The effects of this warming include rising sea levels, melting of the polar icecaps and increasing risks of severe weather events such as hurricanes. To try to limit this warming to 2 centigrade, governments from the EU and around the world are looking at ways of reducing CO₂ emissions.

One of the major sources of CO₂ is the generation of electricity. Worldwide fossil fuel burning produces 85% of the world's electricity. In the next 10-20 years, it will be difficult to reduce the use of fossil fuel for electricity generation. Renewable energies need time to be introduced and developed, and the world also needs extra methods of generating electricity to safeguard against shortages of intermittent renewable energy - for example, when the wind does not blow enough to turn windmills.

It is now possible to burn fossil fuels, and capture the CO₂ at power stations, or other concentrated emission sites such as cement works and oil refineries. This CO₂ can then be pressurised to liquefy it, and pumped through pipelines to places where the liquid CO₂ can be injected underground to be stored. This particular proposal examines some of the controls which affect how CO₂ is stored underground and how any leakage out of a reservoir, or to the surface could be detected. To store CO₂ a porousreservoir is needed, overlain by an impermeable seal, such as mudstone which stops the CO₂ from escaping and rising to the surface. The CO₂ must be stored for a long time (thousands of years) to ensure it does not cause further warming. Unfortunately, the first engineered CO₂ storage project has only been operating for 10 years. So to find out more about storing CO₂ over a long time we need to look at natural CO₂ gas fields.

Natural CO₂ fields are similar to oil or methane gas fields except they contain CO₂. Within these gas fields there is also a small amount of unreactive noble gases. These noble gases have different sources and can be used to work out where the natural CO₂ has come from. Recent research has shown that natural CO₂ fields from around the world have trapped CO₂ for millions of years. This research has also shown that a lot of CO₂ istrapped as a result of it dissolving into the porewater within the gas field. This proposalwill firstly develop detailed computer models to independently predict how much chemical dissolution into the porewater could realistically occur within the CO₂ fields.

Several scientists also believe that once CO₂ is pumped underground it will crystallise new mineral. This would 'lock' the CO₂ into the reservoir and is the most secure form of storage. This work would analyze recent minerals formed within a CO₂ field to test if the amount of light carbon (carbon 12) to heavy carbon (carbon 13) was the same as would be expected if the minerals were crystallised from the CO₂ stored in the field.

The project will also investigate if noble gases can be used to record if CO₂ has moved through the mudrock seal from different natural CO₂ fields. As CO₂ moves through the mudrock it is believed that some noble gases contained in it will be left behind, stuck onto organic debris. In a similar fashion noble gases can also be stuck onto natural coals. This project will test if the noble gases derived from coal burnt in a power station and its produced CO₂ exist in large enough quantities to be used to trace the CO₂ once it is injected underground.