Earth's climate may be stabilized over millennia by solubilization of atmospheric carbon dioxide (CO
) as minerals weather, but the temperature sensitivity of this thermostat is poorly understood. We ...discovered that the temperature dependence of weathering expressed as an activation energy increases from laboratory to watershed as transport, clay precipitation, disaggregation, and fracturing increasingly couple to dissolution. A simple upscaling to the global system indicates that the temperature dependence decreases to ~22 kilojoules per mole because (i) the lack of runoff limits weathering and retains base metal cations on half the land surface and (ii) other landscapes are regolith-shielded and show little weathering response to temperature. By comparing weathering from laboratory to globe, we reconcile some aspects of kinetic and thermodynamic controls on CO
drawdown by natural or enhanced weathering.
Both vertical and lateral flows of rock and water occur within eroding hills. Specifically, when considered over geological timeframes, rock advects vertically upward under hilltops in landscapes ...experiencing uplift and erosion. Once rock particles reach the land surface, they move laterally and down the hillslope because of erosion. At much shorter timescales, meteoric water moves vertically downward until it reaches the regional water table and then moves laterally as groundwater flow. Water can also flow laterally in the shallow subsurface as interflow in zones of permeability contrast. Interflow can be perched or can occur during periods of a high regional water table. The depths of these deep and shallow water tables in hills fluctuate over time. The fluctuations drive biogeochemical reactions between water, CO2, O2, and minerals and these in turn drive fracturing. The depth intervals of water table fluctuation for interflow and groundwater flow are thus reaction fronts characterized by changes in composition, fracture density, porosity, and permeability. The shallow and deep reaction zones can separate over meters in felsic rocks. The zones act like valves that reorient downward unsaturated water flow into lateral saturated flow. The valves also reorient the upward advection of rock into lateral flow through solubilization. In particular, groundwater removes highly soluble, and interflow removes moderately soluble minerals. As rock and water moves through the system, hills may evolve toward a condition where the weathering advance rate, W, approaches the erosion rate, E. If W=E, the slopes of the deep and shallow reaction zones and the hillsides must allow removal of the most soluble, moderately soluble, and least soluble minerals respectively. A permeability architecture thus emerges to partition each evolving hill into dissolved and particulate material fluxes as it approaches steady state.
The empirical equation of R.H. Wood for limiting equivalent NaCl conductance and a set of equations for a proton and other ions in the aqueous solutions HCl, LiCl, NaCl, KCl, Li2SO4, K2SO4 and ...mixtures H2SO4–Na2SO4–H2O are revisited and compared with equations of other authors. The Wood equation is unique in an accurate description of the region close to the critical point of water. In this region the decrease in limiting ion mobilities correlate with the increase in water compressibility. In a remarkable way this effect corresponds to the decisive drop in the limiting proton mobility making it similar to the other univalent ions mobilities. Probably, in this water region the critical decrease in the number of hydrogen bonds per water molecule takes place and finally impeding the jump mechanism of proton mobility. The new high temperature data on electrical conductance of aqueous HCl are represented for molalities 10−5–10−3molkg−1 at temperatures 298–673K and pressures up to 28MPa. The HCl conductivities have been fitted to the Turq, Blum, Bernard and Kunz equation using Wood mixing rule and mean spherical approximation activity coefficients. At 573–663K the adjustable parameters are the proton limiting equivalent conductance and the HCl dissociation constant. At the measured state point of lowest water density (228.8kgm−3; 673K) the additional account of ion triplets gives good fit to the data.
•R.H. Wood equation for limiting equivalent NaCl conductance is represented.•Decrease in ion mobilities is vice versa water compressibility near critical point.•New data on electrical conductance of HCl are represented and analyzed.•Decrease in number of hydrogen bonds per H2O molecule hinders mobility of proton.
► Model baseline 8 components sandstone reservoir was developed. ► Reactive CO2 diffusion from supercritical phase into the sandstone pore brine was modeled. ► 97% of the maximum value (34.5kg CO2 ...per m3 of sandstone) was sequestered by 4000y. ► Varying mineral kinetics changes reaction paths to the equilibrium sequestration. ► To predict sequestration within 1000y requires accurate Olg, Ab, Smct kinetic data.
One idea for mitigating the increase in fossil-fuel generated CO2 in the atmosphere is to inject CO2 into subsurface saline sandstone reservoirs. To decide whether to try such sequestration at a globally significant scale will require the ability to predict the fate of injected CO2. Thus, models are needed to predict the rates and extents of subsurface rock–water–gas interactions. Several reactive transport models for CO2 sequestration created in the last decade predicted sequestration in sandstone reservoirs of ∼17 to ∼90kg CO2 m−3. To build confidence in such models, a baseline problem including rock+water chemistry is proposed as the basis for future modeling so that both the models and the parameterizations can be compared systematically. In addition, a reactive diffusion model is used to investigate the fate of injected supercritical CO2 fluid in the proposed baseline reservoir+brine system. In the baseline problem, injected CO2 is redistributed from the supercritical (SC) free phase by dissolution into pore brine and by formation of carbonates in the sandstone. The numerical transport model incorporates a full kinetic description of mineral–water reactions under the assumption that transport is by diffusion only. Sensitivity tests were also run to understand which mineral kinetics reactions are important for CO2 trapping.
The diffusion transport model shows that for the first ∼20years (20a) after CO2 diffusion initiates, CO2 is mostly consumed by dissolution into the brine to form CO2,aq (solubility trapping). From 20 to 200a, both solubility and mineral trapping are important as calcite precipitation is driven by dissolution of oligoclase. From 200 to 1000a, mineral trapping is the most important sequestration mechanism, as smectite dissolves and calcite precipitates. Beyond 2000a most trapping is due to formation of aqueous HCO3-. Ninety-seven percent of the maximum CO2 sequestration, 34.5kg CO2 per m3 of sandstone, is attained by 4000a even though the system does not achieve chemical equilibrium until ∼25,000a. This maximum represents about 20% CO2 dissolved as CO2,aq, 50% dissolved as HCO3,aq-, and 30% precipitated as calcite. The extent of sequestration as HCO3- at equilibrium can be calculated from equilibrium thermodynamics and is roughly equivalent to the amount of Na+ in the initial sandstone in a soluble mineral (here, oligoclase). Similarly, the extent of trapping in calcite is determined by the amount of Ca2+ in the initial oligoclase and smectite. Sensitivity analyses show that the rate of CO2 sequestration is sensitive to the mineral–water reaction kinetic constants between approximately 10 and 4000a. The sensitivity of CO2 sequestration to the rate constants decreases in magnitude respectively from oligoclase to albite to smectite.
The electrical conductivities of aqueous solutions of Na2SO4, H2SO4, and their mixtures have been measured at 373−673 K at 12−28 MPa in dilute solutions for molalities up to 10-2 mol kg-1. These ...conductivities have been fit to the conductance equation of Turq et al. with a consensus mixing rule and mean spherical approximation activity coefficients. Provided the concentration is not too high, all of the data can be fitted by a solution model that includes ion association to form NaSO4 -, Na2SO4 0, HSO4 -, H2SO4 0, and NaHSO4 0. The adjustable parameters of this model are the dissociation constants of the SO4 - species and the H+, SO4 -2, and HSO4 - conductances (ion mobilities) at infinite dilution. For the 673 K and 230 kg m-3 state point with the lowest dielectric constant, ε = 3.5, where the Coulomb interactions are the strongest, this model does not fit the experimental data above a solution molality of 0.016. Including the species H9(SO4)5 - gave satisfactory fits to the conductance data at the higher concentrations.
•CO2 reacts and diffuses in a sandstone reservoir capped by shale.•Chlorite reacts and ankerite precipitates, sealing the shale porosity by 7500 a.•CO2 trapped as bicarbonate in solution depends on ...the initial moles of Na+K.•CO2 trapped as carbonate mineral depends on initial moles of Ca+Mg+Fe.•Timing of occlusion is extremely sensitive to the kinetic constants of clay minerals.
We use a reactive diffusion model to investigate what happens to CO2 injected into a subsurface sandstone reservoir capped by a chlorite- and illite-containing shale seal. The calculations simulate reaction and transport of supercritical (SC) CO2 at 348.15K and 30MPa up to 20,000 a. Given the low shale porosity (5%), chemical reactions mostly occurred in the sandstone for the first 2000 a with some precipitation at the ss/sh interface. From 2000 to 4000 a, ankerite, dolomite and illite began replacing Mg–Fe chlorite at the sandstone/shale interface. Transformation of chlorite to ankerite is the dominant reaction occluding the shale porosity in most simulations: from 4000 to 7500 a, this carbonation seals the reservoir and terminates reaction. Overall, the carbonates (calcite, ankerite, dolomite), chlorite and goethite all remain close to local chemical equilibrium with brine. Quartz is almost inert from the point of its dissolution/precipitation. However, the rate of quartz reaction controls the long-term decline in aqueous silica activity and its evolution toward equilibrium. The reactions of feldspars and clays depend strongly on their reaction rate constants (microcline is closer to local equilibrium than albite). The timing of porosity occlusion mostly therefore depends on the kinetic constants of kaolinite and illite. For example, an increase in the kaolinite kinetic constant by 0.25 logarithmic units hastened porosity closure by 4300 a. The earliest simulated closure of porosity occurred at approximately 108 a for simulations designed as sensitivity tests for the rate constants.
These simulations also emphasize that the rate of CO2 immobilization as aqueous bicarbonate (solubility trapping) or as carbonate minerals (mineral trapping) in sandstone reservoirs depends upon reaction kinetics – but the relative fraction of each trapped CO2 species only depends upon the initial chemical composition of the host sandstone. For example, at the point of porosity occlusion the fraction of bicarbonate remaining in solution depends upon the initial Na and K content in the host rock but the fraction of carbonate mineralization depends only on the Ca, Mg, Fe content. Since ankerite is the dominant mineral that occludes porosity, the dissolved concentration of ferrous iron is also an important parameter. Future efforts should focus on cross-comparisons and ground-truthing of simulations made for standard case studies as well as laboratory measurements of the reactivities of clay minerals.
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