Natural Analogues

Natural accumulations of CO2 deposits provide natural analogues for the various options of storing CO2 in geologic formations. These, uniquely, allow us to investigate the subsurface processes that control CO2 storage on timescales far beyond those available for engineered systems. Nevertheless, the scientific study of natural CO2 deposits is still at an early phase. Our previous work use noble gas isotopes, major gas composition and stable carbon isotopes to identify physico-chemical processes and trapping mechanisms of CO2 in the short and the long term in geological reservoirs.

Case Study 1: Bravo Dome Natural CO2 Reservoir

Nature cover

This work was featured in Nature

Bravo dome geological setting

The geological setting of Bravo Dome natural carbon dioxide field (click to view larger)

CO2/3He vs 20Ne

CO2/3He variation vs. water derived 20Ne from CO2 rich natural gas fields.

There is a general trend in this data set of decreasing CO2/3He with increasing 20Ne. Because the only subsurface source of the 20Ne is the formation water, the CO2 sink must be linked to the formation water contacted by the gas phase.

figure 4

δ13C(CO2) vs. CO2/3He in Bravo dome reservoir.

The solid line shows the predicted trend for carbonate mineral precipitation and the broken lines show CO2 dissolution trends for the indicated formation-water pH. This data limits the maximum effect of CO2 precipitation in samples to approximately 18%. Solubility trapping is the major mechanism to storing CO2 in Bravo Dome reservoir.

Combined noble gas and stable isotopes provide a new tool to quantify CO2 dissolution and precipitation.

Case Study 2: Jackson Dome Natural CO2 Deposit

Noble gases together with other gases can identify the origin and migration history of a CO2 gas reservoir. Combined noble gas and stable isotopes distinguish between and quantify different mechanisms of CO2 removal from natural CO2 gas deposits.

Geological setting of Jackson Dome (after Studlick et al., 1990)

Geological setting of Jackson Dome

Sample locations (after Studlick et al., 1990)

Sample locations

Role of groundwater

Role of groundwater (Click for larger image)

CO2/3He change correlates with air derived 20Ne† concentrations. It suggests that groundwater plays a key role in controlling CO2/3He. Assuming the highest CO2/3He observed is representative of the original emplaced fluid, groundwater is responsible for >75% of CO2 loss.

Cartoons of Two-stage Groundwater Gas Stripping and Re-dissolution (GGS-R) model (after Gilfillan et al. 2008).

Cartoons of Two-stage Groundwater Gas Stripping and Re-dissolution (GGS-R) model (after Gilfillan et al. 2008).

Fractionation of 20Ne†/36Ar and CO2/3He following the GGS-R model.

Fractionation of 20Ne†/36Ar and CO2/3He following the GGS-R model.

In the GGS-R model 20Ne† and 36Ar in the gas phase are re-dissolved into ASW stripped groundwater in an open system causing Rayleigh fractionation in the residual CO2 phase 20Ne†/36Ar ratio. The CO2/3He is treated as being variably saturated in the groundwater, from 0% saturation (completely stripped) to 100% saturation (in equilibrium with any percolating CO2 phase). Data are consistent with between 100% and 24% CO2 saturation of the groundwater into which re-dissolution occurs.

Identifying mechanisms

Identifying mechanisms

Combined noble gas and stable isotopes distinguish between and quantify different mechanisms of CO2 removal from natural CO2 gas deposits. The solid line shows the predicted trend for carbonate mineral precipitation and the broken line shows the trend as the CO2 re-dissolved into groundwater in its neutral form (GGS-R model). The maximum effect for CO2 precipitation in the samples is approximately 27%.

Reservoir Processes

Naturally occurring noble gas isotopes provide one of the best tools to study gas-fluid-mineral interaction. By injecting and studying artificial noble gas tracers in CO2 EOR or potential sequestration reservoirs, hydrological modelling can be established to quantify the multi-phase flow of fluids and determine the flow path and flow rate of fluids. This will enable further understanding of gas-aqueous fluid-mineral reactions and kinetics.

Case study: CO2 EOR and sequestration in a depleted oil field

Previous noble gas tracer study applied noble gases either as in free gas phase or in dissolution into water, the release mechanism was only by gravity flow into reservoir or formation system. This process of tracer introduction provided tracer pulse in wells. In our recent study, we constructed a continuous tracer injection system which consisted of two HPLC pumps, three high pressure cylinders and a wireless remote system monitoring temperature, pressure and weight in real time. Over 9 days, we injected 2 STP litres of 3He and 129Xe each directly into the EOR CO2 injection stream. Based on the analysis of background 3He/4He and 129Xe/132Xe (0.07Ra and 0.98 respectively), our predicted spiked 3He/4He and 129Xe/132Xe ratios for the mass of injected CO2 are 49.54Ra and 11.64 respectively. The wellhead injector fluid and 4 production wells surrounding the injector were sampled for a period of 5 months.

Tracer injection system on site

Tracer injection system on site

Preliminary results show that the highest 3He/4He and 129Xe/132Xe ratios in the injected CO2 stream reached 49.0Ra and 23.94 respectively and establish the tracer input function. 3He tracer was detected in all producing fluid samples from 4 monitoring wells. Spike breakthrough is consistent with the reservoir geology with the wells updip of the injector seeing tracer arrival earlier than those located downdip of the injector. Spike breakthrough in wells updip of the injector correlated with the well temperature which was an indication of the CO2 breakthrough, but in the wells downdip of the injector spike came through, most probably, dissolved in the water phase. 3He/4He ratio is at a much lower level than the injected ratio and reflect the magnitude of reservoir fluid and CO2 interaction.

Physical Chemistry of Noble Gases

By 2050 at least 40% of all annually produced anthropogenic CO2 (IPCC, 2005) can potentially be captured and pumped underground as a supercritical fluid into naturally occurring fluid traps. The majority of this CO2 is expected to remain within the storage site on geological timescales (105 years) during which it is expected to slowly interact with the ground water, primarily via dissolution and remineralisation. However the rates at which this would occur are unique to each location and are dependent on environmental factors such as temperature, groundwater salinity, groundwater flow rates, and of course the composition of the host rock.

To ensure each storage site is responding as expected to the CO2 it is essential that methods are in place to monitor these sites before, during and after injection. Noble gases are ideal proxies for this task as they are chemically inert and easily quantified at very low concentrations. Due to their physical properties they are soluble in both carbon dioxide and water phases; how they partition between them is dependent is on temperature and salinity. By establishing baseline partitioning values for each noble gas at elevated temperature and pressure regimes it is possible to calculate an expected partitioning coefficient for each noble gas at any storage site in the world. Any deviations from the expected partitioning coefficients of each of the five noble gases would be a result of changing conditions within the sequestration site such as CO2 dissolution into the water phase or migration away from the injection site. By quantifying the degree of digression from the expected partitioning values it is possible to assess the rates at which these occur for each unique system.

Here at the University of Manchester we are dedicated to both experimentally and numerically determining these expected baseline coefficents with close collaborations to the British Geological Survey while also focussing on applying this expected noble gas partitioning to investigate the origins and evolution of natural CO2 storage sites such as Bravo Dome and Jackson Dome. This combined approach has granted us a unique insight into the potential applications of the physical chemistry of noble gases for monitoring the geochemical processes associated with CO2 storage sites.