Tracking the interaction between injected CO₂ and reservoir fluids using noble gas isotopes in an analogue of large-scale carbon capture and storage

Industrial scale carbon capture and storage technology relies on the secure long term storage of CO2 in the subsurface. The engineering and safety of a geological storage site is critically dependent on how and where CO2 will be stored over the lifetime of the site. Hence, there is a need to determine how injected CO2 is stored and identify how injected CO2 interacts with sub-surface fluids. Since July 2008 ∼1 Mt of CO2 has been injected into the Cranfield enhanced oil recovery (EOR) field (MS, USA), sourced from a portion of the natural CO2 produced from the nearby Jackson Dome CO2 reservoir. Monitoring and tracking of the amount of recycled CO2 shows that a portion of the injected CO2 has been retained in the reservoir. Here, we show that the noble gases (20Ne, 36Ar, 84Kr, 132Xe) that are intrinsic to the injected CO2 can be combined with CO2/3He and δ13CCO2 measurements to trace both the dissolution of the CO2 into the formation water, and the interaction of CO2 with the residual oil. Samples collected 18 months after CO2 injection commenced show that the CO2 has stripped the noble gases from the formation water. The isotopic composition of He suggests that ∼0.2%, some 7 kt, of the injected CO2 has dissolved into formation water. The CO2/3He and δ13CCO2 values imply that dissolution is occurring at pH = 5.8, consistent with the previous determinations. δ13CCO2 measurements and geochemical modelling rule out significant carbonate precipitation and we determine that the undissolved CO2 after 18 months of injection (1.5 Mt) is stored by stratigraphic or residual trapping. After 45 months of CO2 injection, the noble gas concentrations appear to be affected by CO2-oil interaction, overprinting the signature of the formation water

Noble gases constrain the origin, age and fate of CO2 in the Vaca Muerta Shale in the Neuquén Basin (Argentina)

Unconventional hydrocarbon resources such as shale oil/gas and coal-bed methane have become an increasingly important source of energy over the past decade. The Vaca Muerta Shale (Neuquén Basin, Argentina) contains the second largest technically recoverable quantity of shale gas in the world. Exploitation of the play has been complicated by elevated concentrations of CO2 in several fields, the origin of which is currently poorly understood. Elevated CO2 levels are consistently encountered when deep-rooted faults in the Auquilco Evaporite Formation, present below the Vaca Muerta Shale, overlap with shallower faults that propagate from the top of evaporites into the shale, indicating a sub-evaporate origin of the CO2. Here we report new isotopic analysis of CO2-rich gases from two producing fields. CO2 concentrations increase with C1/(C2 + C3) values (4.8–33.5) and fractionation of δ13CCO2 (−0.9 to −7.7‰), suggest that CH4 have been displaced by CO2 which entered the shale after hydrocarbon maturation. The noble gas composition (3He/4He of 3.43–3.95 RA, where RA is the atmospheric ratio of 1.399 × 10−6, 21Ne/22Ne of 0.0310–0.0455, 20Ne/22Ne of 9.89–10.52, 40Ar/36Ar of 2432–3674 and CO2/3He 6.8–20.2 × 107) of the gases is consistent with mixing of magmatic CO2 with crustal hydrocarbon-rich gases and provides evidence for the loss of significant CO2. Using inverse modelling techniques, we determine that the magmatic gas has a 3He/4He of 3.95–4.08 RA, CO2/3He of 8.8–16 × 108 and 20Ne/22Ne of 12.13−0.10+0.08, 21Ne/22Ne of 0.074−0.003+0.004. Based on the radiogenic He and Ne this is consistent with a depleted asthenosphere mantle source, which has been trapped in the crust since 6.0–22.8 Ma. This is significantly younger than Late Cretaceous maturation of the hydrocarbon source rocks. Mantle melting as a result of asthenosphere upwelling induced by the collision of the South Chile Ridge and the Chile Trench at ~14 Ma is the most likely source of the CO2. Gases from below the shale contain two air saturated water-derived noble gas components, distinguished on the basis of 20Ne†/36Ar, 84Kr/36Ar, 132Xe/36Ar ratios. These are consistent with early and late stage open system Rayleigh fractionation of groundwater-derived noble gases. We find evidence that these mix with the magmatic component prior to entering the Vaca Muerta and mixing with an adsorption derived gas retained in the source kerogen

The formation of NeH+ in static vacuum mass spectrometers and re-determination of 21Ne/20Ne of air

Air-derived neon is used for routine calibration of magnetic sector mass spectrometers, principally for determining sensitivity and mass discrimination for Ne isotope determinations. The commonly accepted 21Ne/20Ne ratio of air (0.002959 ± 0.000022; Eberhardt et al. (1965) does not take account of the contribution of 20NeH+ at m/z = 21. Honda et al. (2015) and Wielandt and Storey (2019) have recently re-determined the 21Ne/20Neair by resolving 20NeH+ from 21Ne+. The 21Ne/20Neair values of the two studies differ by 1.8%, beyond the uncertainty of the measurements (± <0.1%). We have developed a protocol for precise determination of NeH+ in air using a low-resolution Thermo Fisher ARGUS VI mass spectrometer and use it to re-determine the 21Ne/20Ne of air. 22NeH+/22Ne+ measured at different H2+ and Ne+ intensities reveal that (i) the partial pressure of H2+ in the instrument is the primary control on NeH+ production, and (ii) increasing Ne+ pressure suppresses the formation of NeH+. Calibration curves of 22NeH+/22Ne+ vs. 22Ne+ at constant H2+ are used to calculate the 20NeH+ production in aliquots of air-derived Ne and allow for hydride correction at m/z = 21. The fully isobaric interference-corrected Ne isotope compositions measured at different electron energy (eV) settings define a single mass fractionation line in 22Ne/20Ne vs. 21Ne/20Ne space. The 20NeH+/21Ne+ ratio varies between 0.4% (90 eV) and 2.3% (60 and 70 eV). Correcting for 20NeH+ assuming 22NeH+/20NeH+ = 22Ne/20Ne yields an over-correction of up to 0.7% and the data do not plot on a single mass fractionation line. Our study defines 21Ne/20Neair to be 0.002959 ± 0.14% (1σ) assuming 22Ne/20Ne = 0.102 (Eberhardt et al., 1965). This overlaps the value determined by Wielandt and Storey (2019), albeit with a slightly higher uncertainty. However, our value is statistically more robust and accounts for the dependency on hydride formation by Ne partial pressure. From this we conclude that high precision Ne isotope ratio determinations in future require the quantification of 20NeH+. The improved precision of air 21Ne/20Ne will result in more precise cosmogenic 21Ne surface exposure and (U + Th)/Ne ages

New system for measuring cosmogenic Ne in terrestrial and extra-terrestrial rocks

Cosmogenic Ne isotopes are used for constraining the timing and rate of cosmological and Earth surface processes. We combined an automated gas extraction (laser) and purification system with a Thermo Fisher ARGUS VI mass spectrometer for high through-put, high precision Ne isotope analysis. For extra-terrestrial material with high cosmogenic Ne concentrations, we used multi-collection on Faraday detectors. Multiple measurements (n = 26) of 1.67 × 10−8 cm3 air-derived 20Ne yielded an uncertainty of 0.32%, and 21Ne/20Ne = 0.17% and 22Ne/20Ne = 0.09%. We reproduced the isotope composition of cosmogenic Ne in the Bruderheim chondrite and Imilac pallasite in a sub-ten mg sample. For lower Ne amounts that are typical of terrestrial samples, an electron multiplier detector was used in peak jumping mode. Repeated analysis of 3.2 × 10−11 cm3 STP 20Ne from air reproduced 21Ne/20Ne and 22Ne/20Ne with 1.1% and 0.58%, respectively, and 20Ne intensity with 1.7% (n = 103) over a 4-month period. Multiple (n = 8) analysis of cosmogenic Ne in CREU-1 quartz yielded 3.25 ± 0.24 × 108 atoms/g (2 s), which overlaps with the global mean value. The repeatability is comparable to the best data reported in the international experiments performed so far on samples that are 2–5× smaller. The ability to make precise Ne isotope determinations in terrestrial and extra-terrestrial samples that are significantly smaller than previously analysed suggests that the new system holds great promise for studies with limited material

Multi-Isotope Geochemical Baseline Study of the Carbon Management Canada Research Institutes CCS Field Research Station (Alberta, Canada), Prior to CO2 Injection

Carbon capture and storage (CCS) is an industrial scale mitigation strategy for reducing anthropogenic CO2 from entering the atmosphere. However, for CCS to be routinely deployed, it is critical that the security of the stored CO2 can be verified and that unplanned migration from a storage site can be identified. A number of geochemical monitoring tools have been developed for this purpose, however, their effectiveness critically depends on robust geochemical baselines being established prior to CO2 injection. Here we present the first multi-well gas and groundwater characterisation of the geochemical baseline at the Carbon Management Canada Research Institutes Field Research Station. We find that all gases exhibit CO2 concentrations that are below 1%, implying that bulk gas monitoring may be an effective first step to identify CO2 migration. However, we also find that predominantly biogenic CH4 (∼90%–99%) is pervasive in both groundwater and gases within the shallow succession, which contain numerous coal seams. Hence, it is probable that any upwardly migrating CO2 could be absorbed onto the coal seams, displacing CH4. Importantly, 4He concentrations in all gas samples lie on a mixing line between the atmosphere and the elevated 4He concentration present in a hydrocarbon well sampled from a reservoir located below the Field Research Station (FRS) implying a diffusive or advective crustal flux of 4He at the site. In contrast, the measured 4He concentrations in shallow groundwaters at the site are much lower and may be explained by gas loss from the system or in situ production generated by radioactive decay of U and Th within the host rocks. Additionally, the injected CO2 is low in He, Ne and Ar concentrations, yet enriched in 84Kr and 132Xe relative to 36Ar, highlighting that inherent noble gas isotopic fingerprints could be effective as a distinct geochemical tracer of injected CO2 at the FRS

Origin of dawsonite-forming fluids in the Mihályi-Répcelak field (Pannonian Basin) using stable H, C and O isotope compositions: implication for mineral storage of carbon-dioxide

Natural CO2 reservoirs provide an opportunity to study long-term fluid-rock interactions, which are essential to reassure the safety of mineral storage of carbon-dioxide. The Mihályi-Répcelak field (Pannonian Basin, Central Europe) is one of the largest natural CO2-bearing reservoirs in Europe (25 Mt). The CO2 was trapped in Neogene sandstones, which contain various carbonate minerals (dolomite, ankerite, siderite, dawsonite). To reveal the origin of the parent fluid, from which these minerals precipitated, dawsonite and siderite were separated by a new physical method to minimise the uncertainties in the analysis of their stable isotope composition. The δ13CDaw values range from +1.3‰ to +1.6‰ and the calculated δ13CCO2 values in equilibrium with dawsonite (−4.8‰ - –2.0‰) overlap with the carbon isotope compositions of the local CO2 and the European Subcontinental Lithospheric Mantle (−3.9‰ - –2.1‰). This indicates that the dawsonite-forming CO2 had a magmatic origin. The siderite data indicates that some formed from the magmatic CO2, possibly simultaneously with dawsonite (−6.0‰ - –3.9‰), whereas the rest (−8.4‰ - –6.1‰) formed either from a fractionated CO2 with magmatic origin or before the CO2 invasion. The hydrogen isotope composition of structural OH− of dawsonite (−57‰ to −74‰) was determined and was used to estimate the origin of the interacting porewater. The calculated porewater data (δD: −69‰ - –103‰ and δ18O: −1.4‰ - +4.7‰) indicate that the parent fluid was meteoric water modified by water-rock interaction. Our data allows estimation of the total amount of CO2 stored in the dawsonite-bearing sandstone reservoir to be 25 kg/m3, well in line with previous modelling works, which gives a total of 2.01 × 106 t of CO2, higher than previous estimates. We suggest that individual mineral analysis complemented by hydrogen isotope analysis is to be employed to effectively trace in-reservoir fluid-rock interactions in CO2 reservoirs and provide valuable input data for geochemical modelling for better predicting conditions for mineral storage of CO2

Fingerprinting coal-derived gases from the UK

The large-scale extraction of unconventional hydrocarbons in the United States has led to fears of methane contamination of shallow groundwaters. Differentiating between the deep gas released during extraction (shale gas, coal bed methane and underground coal gasification) and natural shallow-sourced methane is imperative for the monitoring and managing of environmental risks related to the extraction process. Here, for the first time, we present measurements of the major gas, and stable and noble gas isotope composition of coal bed methane (CBM) from central Scotland and coal mine methane (CMM) from central England, UK. The molecular (C1/(C2+C3) = 21 to 120) and stable isotope compositions (δ13CCH4 = -39.5 to -51.1‰; δDCH4 = -163 to -238‰) indicate a thermogenic origin for the methane. They are distinct from the majority of shallow-sourced gases in UK. Both sample suites exhibit high He concentrations (338 to 2980 ppmv) that are considerably above atmospheric and groundwater levels. Simple modelling shows that these high 4He concentrations cannot be solely derived from in situ production since coal deposition, and hence the majority is derived from the surrounding crust. The Scottish CBM contains a resolvable mantle He, Ne and Ar contribution that may originate from melts in the deep crust, demonstrating the UK coals have acted as a store for deep volatiles for 10s of millions of years. The high 4He in the coal-derived gases has the potential to be used as a novel diagnostic fingerprint to track fugitive release of deep methane from future unconventional gas extraction operations in the UK

Tracing injected CO2 in the Cranfield enhanced oil recovery field (MS, USA) using He, Ne and Ar isotopes

The He, Ne and Ar isotopic composition of gases collected in 2009 and 2012 from 13 production wells, injection wells and the CO2 supply pipeline at the Cranfield CO2-enhanced oil recovery field (MS, USA) have been measured in order to determine the extent to which they trace the fate of injected CO2 in the reservoir. In the absence of samples of CO2 pre-injection reservoir gas we use the Ne isotope composition of the production and injection well gases to determine the isotopic composition of the natural gas. The noble gas isotopes display binary mixing trends between the injected CO2 and a CH4-rich natural gas that is characterised by radiogenic He, Ne and Ar isotope ratios. 3He/4He and 40Ar*/4He ratios (where 40Ar* represents the non-atmospheric 40Ar) display coherent relationships with CO2 concentrations that can be used to trace and quantify the injected CO2 in an engineered site over a sustained period of injection. The presence of a small amount of air-derived Ar, from a non-atmospheric source, in many gas samples rules out using 40Ar/36Ar to track the injected CO2. The noble gases identify the loss of a significant proportion of the CO2 from the gas phase sampled by five production wells in 2009. Using 3He/4He and 40Ar*/4He ratios to reconstruct the major gas composition, it appears that between 22% and 96% of the CO2 has been lost in individual wells. This study demonstrates that the naturally occurring noble gases have the potential to trace the fate and quantify the sequestration of CO2 at injection sites

Surface and groundwater hydrochemistry of the Menengai caldera geothermal field and surrounding Nakuru County, Kenya

In order to assess the sustainability and impact of production from geothermal reservoirs on hydrological systems, a thorough understanding of local and regional hydrogeological systematics is a prerequisite. The Menengai Caldera in the Kenya Great Rift Valley is one of the largest explored geothermal fields in the country. This paper presents a hydrochemical investigation of the Menengai Caldera geothermal field and the ground and surface waters of the surrounding Nakuru County. Our results demonstrated a similar, sodium-alkaline dominated, ionic composition across all water types. Geothermal wells return the highest cation/anion concentrations and largely demonstrate a meteoric source from their δ18O and δ2H signature. Wells MW-09 (central part of the caldera), MW-18 (eastern part) and MW-20 (central part) showed a more evaporitic signature, closely matching with our own calculated Lake Evaporation Line, suggesting an increased mixing influence of Lake Nakuru waters. MW-09 also showed evidence of high-temperature oxygen isotopic exchange and significant water-rock interaction. Lake samples largely demonstrated seasonal shifts in ionic and isotopic values. Lake Nakuru ionic composition and isotopic values increased throughout the 12-month wet-dry-wet season sampling period. This correlated with a decrease in area which suggests a lessening of water inflow and facilitates increased evaporation. Groundwaters demonstrated clear evidence of mixing between meteoric, irrigation and lake waters. These observations enhanced the understanding of the hydrological system surrounding the Menengai Caldera and, when combined with future studies, will provide a powerful tool to assess the sustainability and impact of soon-to-be completed geothermal power production operations