Fluid flow and CO2–fluid–mineral interactions during CO2-storage in sedimentary basins

Modelling the progress of geochemical processes in CO2 storage sites is frustrated by uncertainties in the rates of CO2 flow and dissolution, and in the rates and controlling mechanisms of fluid–mineral reactions that stabilise the CO2 in geological reservoirs. Dissolution of CO2 must be controlled by the complexities of 2-phase flow of CO2 and formation brines and the smaller-scale heterogeneities in the permeability in the reservoirs which increase the fluid contact areas. The subsequent fluid mineral reactions may increase storage security by precipitating CO2 in carbonate minerals but the consequences of fluid–mineral reactions on caprock rocks or potential leakage pathways up fault zones are less certain as the CO2-charged brines may either corrode minerals or decrease permeabilities by precipitating carbonates. Observations from CO2-injection experiments and natural analogues provide important constraints on the rates of CO2 and brine flow and on the progress of CO2 dissolution and mineral–fluid reactions. In these experiments brines in contact with the propagating plume appear to rapidly saturate with CO2. Dissolution of the CO2 drives the dissolution of oxide and carbonate minerals, on times scales of days to weeks. These reactions buffer fluid pH and produce alkalinity such that carbonate dissolution moves to carbonate precipitation over time-scales of weeks to months. The dissolution of Fe-oxide grain coatings and the release of Fe to solution is important in stabilising insoluble Fe–Mg–Ca carbonate minerals but the rate limiting step for carbonate mineral precipitation is the transport of CO2-charged brines and silicate mineral dissolution rates. Observations from CO2-EOR experiments and natural analogues suggest that the silicate mineral dissolution reactions are initially fast in the low pH fluids surrounding the CO2 plume but that reaction progress over months to years drives minerals towards thermodynamic equilibrium and dissolution rates slow over 2–5 orders of magnitude as equilibrium is approached. The sluggish dissolution of silicate minerals is likely to preside over the long-term fate of the CO2 in geological reservoirs. Observations from injection experiments and natural analogues suggest that the potentially harmful trace elements mobilised by the drop in pH are immobilised as adsorbed and precipitated phases as fluid pH is buffered across mineral reaction fronts. There are very few observations of caprock exposed to CO2-rich brines. Preliminary examination of core recently recovered from scientific drilling of a natural CO2 accumulation in Utah suggests that the diffusion of CO2 into reservoir caprocks drives dissolution of Fe-oxides but subsequent precipitation of carbonate minerals likely retards the diffusion distance of the CO2. At this site thin siltstone layers are shown to be effective seals to the CO2-charged fluids, which has significant implications for the long term security of CO2 in geological reservoirs

Kinetics of CO2-fluid-rock reactions in a basalt aquifer, Soda Springs, Idaho

The dissolution of silicate minerals by CO2-rich fluids and the subsequent precipitation of CO2 as carbonate minerals represent a means of permanently storing anthropogenic CO2 waste products in a solid and secure form. Modelling the progression of these reactions is hindered by our poor understanding of the rates of mineral dissolution–precipitation reactions and mineral surface properties in natural systems. This study evaluates the chemical evolution of groundwater flowing through a basalt aquifer, which forms part of the leaking CO2-charged system of the Blackfoot Volcanic Field in south-eastern Idaho, USA. Reaction progress is modelled using changes in groundwater chemistry by inverse mass balance techniques. The CO2-promoted fluid–mineral reactions include the dissolution of primary plagioclase, orthoclase, pyroxene and gypsum which is balanced by the precipitation of secondary albite, calcite, zeolite, kaolinite and silica. Mineral mole transfers and groundwater flow rates estimated from hydraulic head data are used to determine the kinetics of plagioclase and orthoclase feldspar dissolution. Plagioclase surface area measurements were determined using the evolution of the U-series isotope ratios in the groundwater and are compared to published surface area measurements. Calculated rates of dissolution for plagioclase range from 2.4 × 10−12 to 4.6 × 10−16 mol/m2/s and orthoclase from 2.0 × 10−13 to 6.8 × 10−16 mol/m2/s respectively. These feldspar reaction rates, correlate with the degree of mineral–fluid disequilibrium and are similar to the dissolution rates for these mineral measured in other natural CO2-charged groundwater systems

On discrimination between carbonate and silicate inputs to Himalayan rivers

We review new and published analyses of river waters, bedloads and their constituent minerals from the Dhauli Ganga and Alaknanda, headwaters of the Ganges in Garhwal, and the Marsyandi in Nepal and their tributaries. These data are used to discriminate between the inputs of major cations and Sr from silicate and carbonate sources. Methods of estimating the proportion of the carbonate and silicate inputs to river waters using mixing arrays in Sr-Ca-Mg-Na-K 87Sr/86Sr space are shown to suffer from systematic correlations between the magnitude of the precipitation of secondary calcite and the fraction of the silicate component. This results in factor-of two overestimates of the fractions of silicate-derived Ca, Mg and Sr. To correct for this the magnitude of secondary calcite precipitated and relative fractions of silicate and carbonate-derived cations are instead calculated by modeling the displacement of water compositions from the compositions of the carbonate and silicate components of the bedload in subsets of Sr-Ca-Mg-Na-K 87Sr/86Sr space. The compositions of the carbonate and silicate end-members in the bedload are determined by sequential leaching. The results of this modeling are compared with modeling of the modal mineral inputs to waters where mineral compositions are derived from electronmicroprobe analyses of the minerals in the bedload. In the upper Marsyandi catchment, which drains low-grade Tethyan Sedimentary Series formations, a set of mainstem samples collected over a two-year period define tight correlations in Sr-Ca-Mg-Na-K- 87Sr/86Sr space. Modeling of the magnitude of secondary carbonate precipitation and fractions of silicate-derived Ca, Mg and Sr in Sr-Ca-Mg 87Sr/86Sr space gives selfconsistent results that are compatible with both the calculations of mineral modes and published Mg-isotopic compositions, if the ratio of chlorite to biotite weathering is high or if there is another silicate source of Mg. These calculations imply that between 12 and 31 percent of the Sr and 44 and 72 percent of the Mg is derived from silicate minerals where the range reflects the seasonal change in the ratio of silicate-derived to carbonate-derived cations. Modeling in Sr-Ca-Na and/or K space is inconsistent with the Sr-isotopic and Mg-isotopic constraints and we conclude that in this catchment dissolution of Na and K are incongruent relative to Sr-Ca-Mg. Potassium is preferentially retained in micas whereas the controls on Na are unclear. Modeling of the catchments underlain by High Himalayan Crystalline and Lesser Himalayan Series in Garhwal is complicated by the presence of dolomite as well as calcite in the carbonate and the results imply that dolomite dissolves faster in the acetic acid leaches than in nature. Up to 60 percent of the Sr in the catchment on High Himalayan Crystalline Series and 20 to 30 percent of Sr in the catchments on Lesser Himalayan Series are estimated to be derived from silicates. However it should be noted that the element budgets are not all self-consistent and the use of bedrock-element ratios to model the sources of chemical inputs to river waters remains subject to uncertainties

Kinetics of CO2-fluid-rock reactions in a basalt aquifer, Soda Springs, Idaho

The dissolution of silicate minerals by CO2-rich fluids and the subsequent precipitation of CO2 as carbonate minerals represent a means of permanently storing anthropogenic CO2 waste products in a solid and secure form. Modelling the progression of these reactions is hindered by our poor understanding of the rates of mineral dissolution–precipitation reactions and mineral surface properties in natural systems. This study evaluates the chemical evolution of groundwater flowing through a basalt aquifer, which forms part of the leaking CO2-charged system of the Blackfoot Volcanic Field in south-eastern Idaho, USA. Reaction progress is modelled using changes in groundwater chemistry by inverse mass balance techniques. The CO2-promoted fluid–mineral reactions include the dissolution of primary plagioclase, orthoclase, pyroxene and gypsum which is balanced by the precipitation of secondary albite, calcite, zeolite, kaolinite and silica. Mineral mole transfers and groundwater flow rates estimated from hydraulic head data are used to determine the kinetics of plagioclase and orthoclase feldspar dissolution. Plagioclase surface area measurements were determined using the evolution of the U-series isotope ratios in the groundwater and are compared to published surface area measurements. Calculated rates of dissolution for plagioclase range from 2.4 × 10−12 to 4.6 × 10−16 mol/m2/s and orthoclase from 2.0 × 10−13 to 6.8 × 10−16 mol/m2/s respectively. These feldspar reaction rates, correlate with the degree of mineral–fluid disequilibrium and are similar to the dissolution rates for these mineral measured in other natural CO2-charged groundwater systems

High-precision determination of lithium and magnesium isotopes utilising single column separation and multi-collector inductively coupled plasma mass spectrometry

Li and Mg isotopes are increasingly used as a combined tool within the geosciences. However, established methods require separate sample purification protocols utilising several column separation procedures. This study presents a single-step cation-exchange method for quantitative separation of trace levels of Li and Mg from multiple sample matrices

Triple oxygen isotope insight into terrestrial pyrite oxidation.

The mass-independent minor oxygen isotope compositions (Δ'17O) of atmospheric O2 and [Formula: see text] are primarily regulated by their relative partial pressures, [Formula: see text]/[Formula: see text] Pyrite oxidation during chemical weathering on land consumes [Formula: see text] and generates sulfate that is carried to the ocean by rivers. The Δ'17O values of marine sulfate deposits have thus been proposed to quantitatively track ancient atmospheric conditions. This proxy assumes direct [Formula: see text] incorporation into terrestrial pyrite oxidation-derived sulfate, but a mechanistic understanding of pyrite oxidation-including oxygen sources-in weathering environments remains elusive. To address this issue, we present sulfate source estimates and Δ'17O measurements from modern rivers transecting the Annapurna Himalaya, Nepal. Sulfate in high-elevation headwaters is quantitatively sourced by pyrite oxidation, but resulting Δ'17O values imply no direct tropospheric [Formula: see text] incorporation. Rather, our results necessitate incorporation of oxygen atoms from alternative, 17O-enriched sources such as reactive oxygen species. Sulfate Δ'17O decreases significantly when moving into warm, low-elevation tributaries draining the same bedrock lithology. We interpret this to reflect overprinting of the pyrite oxidation-derived Δ'17O anomaly by microbial sulfate reduction and reoxidation, consistent with previously described major sulfur and oxygen isotope relationships. The geologic application of sulfate Δ'17O as a proxy for past [Formula: see text]/[Formula: see text] should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling

Sr-isotopic ratios trace mixing and dispersion in CO2 push-pull injection experiments at the CO2CRC Otway Research Facility, Australia

Analysis of 87Sr/86Sr ratios and modelling of formation water, injection water and produced water compositions from the CO2CRC Otway Research Facility in Victoria, Australia are used to test tracer behaviour and response in push-pull experiments. Such experiments are an essential pre-requisite to understanding the controls imposed by reservoir heterogeneities on CO2 dissolution rates which may be an important stabilising mechanism for geological carbon storage. The experiments (Otway stage 2B extension in 2014) comprised two sequential tests in which ~100 t of CO2-saturated water was injected with combinations of Sr and Br or Li and Fluorescein tracers, each injection being followed by two staged extractions of ~10 t and a final extraction of ~50 t all spaced at ~10 day intervals. Analysis of the 87Sr/86Sr ratios of the produced fluids from the first injection, spiked with SrCl2 and NaBr, is consistent with Sr behaving conservatively. This contrasts with previous interpretations in which Br was argued to have behaved conservatively while Sr, which dilutes ~three times as fast as Br, was thought to be lost to a mineral phase. Such Sr-loss cannot explain the evolution of 87Sr/86Sr ratios. The analysis of 87Sr/86Sr ratios in the waters produced after the second injection episode, spiked with LiCl and Fluorescein tracers, allows calculation of the fractions of the formation waters and the injection waters from both tests 1 and 2. The Sr, Li and SO4 tracers (the later formed by oxidation of formation sulphide) all indicate similar rates of dilution that is consistent with conservative behaviour. The results of the two injection episodes with spaced extractions are compared with two subsequent push-pull injections in which the produced waters, spiked with methanol, were extracted continuously. These continuous extraction experiments exhibited significantly less dilution over the same range of produced to injected water volumes (up to only ~0.6) than the earlier experiments with spaced extractions. This implies that some process related to the pauses in extraction enhances mixing of injected and formation waters. Achieving the objective of using push-pull experiments to assess reservoir heterogeneities and CO2 dissolution rates will require better assessment of the various tracers to establish which behave conservatively followed a proper understanding of the causes of the variations in mixing as fluids are extracted from the formations

High-precision determination of lithium and magnesium isotopes utilising single column separation and multi-collector inductively coupled plasma mass spectrometry.

RATIONALE: Li and Mg isotopes are increasingly used as a combined tool within the geosciences. However, established methods require separate sample purification protocols utilising several column separation procedures. This study presents a single-step cation-exchange method for quantitative separation of trace levels of Li and Mg from multiple sample matrices. METHODS: The column method utilises the macro-porous AGMP-50 resin and a high-aspect ratio column, allowing quantitative separation of Li and Mg from natural waters, sediments, rocks and carbonate matrices following the same elution protocol. High-precision isotope determination was conducted by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) on the Thermo Scientific™ NEPTUNE Plus™ fitted with 1013  Ω amplifiers which allow accurate and precise measurements at ion beams ≤0.51 V. RESULTS: Sub-nanogram Li samples (0.3-0.5 ng) were regularly separated (yielding Mg masses of 1-70 μg) using the presented column method. The total sample consumption during isotopic analysis is <0.5 ng Li and <115 ng Mg with long-term external 2σ precisions of ±0.39‰ for δ7 Li and ±0.07‰ for δ26 Mg. The results for geological reference standards and seawater analysed by our method are in excellent agreement with published values despite the order of magnitude lower sample consumption. CONCLUSIONS: The possibility of eluting small sample masses and the low analytical sample consumption make this method ideal for samples of limited mass or low Li concentration, such as foraminifera, mineral separates or dilute river waters

Validating reactive transport models of CO2-brine-rock reactions in caprocks using observations from a natural CO2 reservoir

Storage of anthropogenic CO2 in geological formations relies on impermeable caprocks as the primary seal preventing buoyant super-critical CO2 escaping. Although natural CO2 reservoirs demonstrate that CO2 may be stored safely for millions of years, uncertainty remains in predicting how caprocks will react with acid CO2-bearing brines. This uncertainty poses a challenge to the assessment of carbon capture and storage schemes. Prediction of caprock behaviour is based primarily on theoretical modelling and laboratory experiments. However, the reactive transport phenomena cannot be reproduced in laboratory experiments over sufficient timescales, theoretical models need calibration against observational data and existing studies on natural caprocks have not resolved mineral reactions. Here we report a detailed description of a stacked sequence of CO2 reservoir-caprock systems exposed to CO2-rich fluids over ∼ 105 years, a time-scale comparable with that needed for effective geological carbon storage. Fluid-mineral reactions in the base of multiple caprocks is driven by diffusion of CO2 and minor H2S from the underlying reservoirs. The reactions include dissolution of hematite, dolomite and K-feldspar and precipitation of Fe-bearing dolomite, gypsum, pyrite and illite over centimetre length-scales. The mineral dissolution reactions generate transient increases in porosity, as determined by neutron scattering measurements, but the propagation of mineral reaction fronts is retarded by the reaction stoichiometry and mineral precipitation. Modelling of the mineral reaction fronts shows that the alteration is sluggish, developing over a >104 year period. The results attest to the significance of transport-limited reactions to the long-term integrity of sealing behaviour in caprocks exposed to CO2