Kevin G. Knauss

Gas-water-rock interactions in Frio Formation following CO2 injection: Implications for the storage of greenhouse gases in sedimentary basins

To investigate the potential for the geologic storage of CO2 in saline sedimentary aquifers, 1600 t of CO2 were injected at 1500 m depth into a 24-m-thick sandstone section of the Frio Formation, a regional brine and oil reservoir in the U.S. Gulf Coast. Fluid samples obtained from the injection and observation wells before CO2 injection showed a Na-CaCl‐type brine with 93,000 mg/L total dissolved solids (TDS) at near saturation with CH4 at reservoir conditions. Following CO2 breakthrough, samples showed sharp drops in pH (6.5‐5.7), pronounced increases in alkalinity (100‐3000 mg/L as HCO3) and Fe (30‐1100 mg/L), and significant shifts in the isotopic compositions of H2O, dissolved inorganic carbon (DIC), and CH4. Geochemical modeling indicates that brine pH would have dropped lower but for the buffering by dissolution of carbonate and iron oxyhydroxides. This rapid dissolution of carbonate and other minerals could ultimately create pathways in the rock seals or well cements for CO2 and brine leakage. Dissolution of minerals, especially iron oxyhydroxides, could mobilize toxic trace metals and, where residual oil or suitable organics are present, the injected CO2 could also mobilize toxic organic compounds. Environmental impacts could be major if large brine volumes with mobilized toxic metals and organics migrated into potable groundwater. The d 18 O values for brine and CO2 samples indicate that supercritical CO2 comprises ;50% of pore-fluid volume ;6 mo after the end of injection. Postinjection sampling, coupled with geochemical modeling, indicates that the brine gradually will return to its preinjection composition.

Dependence of albite dissolution kinetics on ph and time at 25°c and 70°c

Abstract The dissolution rate of albite has been measured as a function of pH and time at 25°C and 70°C in a single-pass flow-through leaching apparatus. Run times extended to 50 days in each experiment. Limited data were obtained at 25°C in the pH range 4–10. More extensive data were obtained at 70°C over the pH range 1.39–11.75. Dissolution rates were defined by release of Si, and in some cases also by Al and Na releases. Speciationsolubility calculations indicate the solutions were well undersaturated for all the likely possible secondary minerals. The fluid was maintained far from equilibrium with albite in all runs. Analysis of the data shows a general consistency with the transition state theory model of Helgeson et al. (1984). Feldspars leached at low and high pH at 70°C showed extensive development of prismatic etch pits demonstrating a surface reaction-controlled dissolution process.

The dissolution kinetics of quartz as a function of pH and time at 70°C

Abstract A single-pass, flow-through apparatus was used to determine the dissolution rate of quartz at 70°C as a function of pH and time. Dissolution rate data were obtained over the pH range 1.4 to 11.8 in nine separate experiments each lasting 50 days. The quartz dissolution rates were defined by the silica release rate to solution. Speciation-solubility calculations using the geochemical modeling code EQ3/6 indicate that the fluid was maintained far from equilibrium with respect to quartz and well-undersaturated with respect to all possible secondary minerals in all runs. The dissolution rates were independent of pH at values (10−15.3 mol/cm2 · s) consistent with the data of Rimstidt and Barnes (1980) up to approximately pH 6, but at higher pH the rates increased with increasing pH, proportional to a H + −0.5 , being almost four orders of magnitude higher at pH 11.8. The rate constants for quartz dissolution at 70°C were 10−15.3 mol/cm2 · s in the pH-independent region extending from acid through neutral solutions, and 10−17.8 mol/cm2 · s in more alkaline solutions. Etch pits were strongly developed in the runs with the more alkaline solutions (pH > 8), in which the rates were the highest. This appears consistent with a surface reaction-controlled dissolution mechanism.

Reactive transport modelling of CO2 storage in saline aquifers to elucidate fundamental processes, trapping mechanisms and sequestration partitioning

Abstract The ultimate fate of CO2 injected into saline aquifers for environmental isolation is governed by three interdependent yet conceptually distinct processes: CO2 migration as a buoyant immiscible fluid phase, direct chemical interaction of this rising plume with ambient saline waters, and its indirect chemical interaction with aquifer and caprock minerals through the aqueous wetting phase. Each process is directly linked to a corresponding trapping mechanism: immiscible plume migration to hydrodynamic trapping, plume-water interaction to solubility trapping, and plume-mineral interaction to mineral trapping. In this study, reactive transport modelling of CO2 storage in a shele-capped sandstone aquifer at Sleipner has elucidated and established key parametric dependencies of these fundamental processes, the associated trapping mechanisms, and sequestration partitioning among them during consecutive ten-year prograde (active-injection) and retrograde (post-injection) regimes. Intra-aquifer permeability structure controls the path of immiscible CO2 migration, thereby establishing the spatial framework of plume-aquifer interaction and the potential effectiveness of solubility and mineral trapping. Inter-bedded thin shales, which occur at Sleipner, retard vertical and promote lateral plume migration, thereby significantly expanding this framework and enhancing this potential. Actual efficacy of these trapping mechanisms is determined by compositional characteristics of the aquifer and caprock: the degree of solubility trapping decreases with increasing formation-water salinity, whereas that of mineral trapping is proportional to the bulk concentration of carbonate-forming elements, principally Fe, Mg, Ca, Na and Al. In the near-field environment of Sleipner-like settings, 80–85% by mass of injected CO2 remains and migrates as an immiscible fluid phase, 15–20% dissolves into formation waters, and less than 1% precipitates as carbonate minerals. This partitioning defines the relative effectiveness of hydrodynamic, solubility, and mineral trapping on a mass basis. Seemingly inconsequential, mineral trapping has enormous strategic significance: it maintains injectivity, delineates the storage volume, and improves caprock integrity. Four distinct mechanisms have been identified: dawsonite [NaAlCO3(OH)2] cementation occurs throughout the intra-aquifer plume, while calcite-group carbonates [principally (Fe,Mg,Ca)CO3] precipitate via disparate processes along lateral and upper plume margins, and by yet another process within inter-bedded and caprock shales. The coupled mineral dissolution/precipitation reaction associated with each mechanism reduces local porosity and permeability. For Sleipner-like settings, the magnitude of such reduction for dawsonite cementation is near negligible; hence, this process effectively maintains initial CO2 injectivity. Of similarly small magnitude is the reduction associated with formation of carbonate rind along upper and lateral plume boundaries; these processes effectively delineate the CO2 storage volume, and for saline aquifers anomalously rich in Fe-Mg-Ca may partially self-seal the plume. Porosity and permeability reduction is most extreme within shales, because their clay-rich mineralogy defines bulk Fe-Mg concentrations much greater than those of saline aquifers. In the basal caprock shale of our models, these reductions amount to 4.5 and 13%, respectively, after the prograde regime. During the retrograde phase, residual saturation of immiscible CO2 maintains the prograde extent of solubility trapping while continuously enhancing that of mineral trapping. At the close of our 20-year simulations, initial porosity and permeability of the basal caprock shale have been reduced by 8 and 22%, respectively. Extrapolating to hypothetical complete consumption of Fe-Mg-bearing shale minerals (here 10 vol.% Mg-chlorite) yields an ultimate reduction of about 52 and 90%, respectively, after 130 years. Hence, the most crucial strategic impact of mineral trapping in Sleipner-like settings: it continuously improves hydrodynamic seal integrity of the caprock and, therefore, containment of the immiscible plume and solubility-trapped CO2.

Muscovite dissolution kinetics as a function of pH and time at 70°C

Abstract The dissolution rate of muscovite at 70°C was determined using a single-pass, flow-through apparatus. Dissolution rate data were obtained at approximately unit pH interval over the pH range 1.4 to 11.8 in 11 separate experiments, each lasting 50 days. The muscovite dissolution rates were defined by the silica, aluminum and/or potassium release rates to solution. Under most pH conditions the dissolution at steady-state was congruent or nearly so. Speciation-solubility calculations made using the geochemical modeling code EQ3NR indicate that the fluids were maintained far from equilibrium with respect to muscovite and undersaturated with respect to possible secondary minerals. Under all pH conditions we initially observe a transiently elevated dissolution rate. The dissolution rate gradually approaches a limiting (steady-state) value, which we interpret as the dissolution rate of the bulk mineral. From pH 1 to approximately pH 5, the limiting rate for muscovite dissolution at 70°C varies negatively with pH, and the rate constant, k, is 10−14.7 mol/cm2 · s. Under acidic conditions the dissolution rate is proportional to−0.37 pH. As pH is increased at 70°C, the rate of dissolution becomes essentially pH independent, and k is 10−16.6 mol/cm2 · s. At about pH 7 and higher the limiting rate for muscovite dissolution varies positively with hydroxyl ion activity, and the dissolution rate is proportional to +0.22 pH. Under alkaline conditions k is 10−18.1 mol/cm 2 · s at 70°C.

Th230-U234 dating of pedogenic carbonates in gravelly desert soils of Vidal Valley, southeastern California

Radioactive disequilibrium relationships among Th 230 , U 234 , and U 238 can be used to date pedogenic carbonates formed in regions of arid to semiarid climate. Samples suitable for dating consist of dense carbonate rinds around pebbles from the Cca soil horizon. Analytically, the method involves leaching the samples with dilute hydrochloric acid and measuring U 238 , U 234 , Th 230 , and Th 232 in both the leachate and residue fractions. As the soil carbonate commonly incorporates silicate mineral-bearing detritus, corrections are made to account for possible introduction of detrital Th 230 and U 234 into the acid leachate. The corrections are based on the assumptions that (1) the carbonate initially contains negligible amounts of Th 232 and Th 230 , or has a Th 230 /Th 232 ratio similar to that in detrital minerals, (2) U 238 , U 234 , and Th 230 in the detrital silicate phase are in secular equilibrium with each other, and (3) the thorium isotopes in the detrital phase are not fractionated by the acid leaching. Application of the method to calcareous soils developed on upper Quaternary alluvial deposits of the eastern Mojave Desert in southern California gives ages that are internally consistent and that agree with the geomorphic and stratigraphic relative age relationships. Fourteen samples from an upper Pleistocene geomorphic surface, Q2b, yielded an average age of 83,000 ± 10,000 yr. This age is confirmed by a different assessment of the data independent of the aforementioned assumptions. On one specimen two different layers of a pebble coating were dated, and a carbonate accumulation rate of about 1 mm/8,000 yr was obtained. It is hoped that this study will serve as a basis for further research into the absolute dating of various types of impure carbonates of late Quaternary origin.

Temperature and pressure dependence of n-hexadecane cracking

The rates and product compositions of hexadecane cracking are reported for temperatures ranging from 300 to 370°C and pressures of 150 to 600 bar. The overall apparent activation energy at an intermediate pressure of 300–350 bar is about 74 kcal/mol. This is higher than the overall 60 kcal/mol energy consistent with higher temperature measurements, even though the rates from our highest temperatures overlap with those from earlier reports. The product composition is consistent with a free radical mechanism in which alkene intermediates react with primary and secondary radicals to form branched and normal liquid products both smaller and larger than the starting material. Pressure has a retarding effect on the rate of reaction, but the dependence is not measured precisely enough to say more than that the survival of oil is likely to vary by a factor of two or so under typical geologic conditions. A simple kinetic model having five first-order reactions is presented that predicts the lumped kinetic species of C1, C2–C4, C5–C9, C10–C15, C16, and C16+. The gas is depleted in methane compared to most natural gas.

Radium and thorium isotopes in the surface waters of the East Pacific and coastal Southern California

Abstract A fiber extraction technique is used to concentrate Ra and Th isotopes from 1000 liters or more of seawater. Natural 226 Ra and 234 Th are used as yield tracers. In the equatorial Pacific the 228 Ra activity of surface water varies from 20 to 1 dpm/1000 kg and generally decreases away from continental shelf areas. Across the Peru Current System, this decrease is modeled as one-dimensional diffusion and indicates the possibility of two flow regimes with distinct characteristic mixing lengths and apparent eddy diffusivities of 10 5 and 10 7 cm 2 /s. The perturbing effects of advection and equatorial upwelling west of the Galapagos Islands are noted. Off the coast of Southern California a vertical 228 Ra distribution gives an apparent diffusivity of 1.6 cm 2 /s for the upper thermocline. 226 Ra concentrations near the coast appear to be higher than the open ocean values at comparable depths, which may reflect supply of this isotope from continental shelf sediments and/or upwelling. The insoluble daughter/soluble parent activity ratios 228 Th/ 228 Ra and 234 Th/ 238 U in the equatorial Pacific surface water display latitudinal trends which may be correlated with productivity variations. Near the coast of California these ratios reflect the differing oceanographic conditions north and south of Pt. Conception indicating a mean chemical removal time constant on the order of 4 months for Th and other highly reactive elements within the Southern California Bight. The 232 Th content of seawater sampled is less than 0.1 μg/1000 1; most of the published values for seawater 232 Th could well be too high. A comparison of the two methods of determining 228 Ra (via 228 Ac and via 228 Th) made on 64 seawater samples shows that the time delay required by the 228 Th method is more than compensated by its better analytical simplicity and precision.

A Kinetic Model for Borosilicate Glass Dissolution Based on the Dissolution Affinity of a Surface Alteration Layer

A kinetic model for the dissolution of borosilicate glass, incorporated into the EQ3/6 geochemical modeling code, is used to predict the dissolution rate of a nuclear waste glass. In the model, the glass dissolution rate is controlled by the rate of dissolution of an alkalidepleted amorphous surface (gel) layer. Assuming that the gel layer dissolution affinity controls glass dissolution rates is similar to the silica saturation concept of Grambow [1] except that our model predicts that all components concentrated in the surface layer, not just silica, affect glass dissolution rates. The good agreement between predicted and observed elemental dissolution rates suggests that the dissolution rate of the gel layer limits the overall rate of glass dissolution. The model predicts that the long-term rate of glass dissolution will depend mainly on ion concentrations in solution, and therefore on the secondary phases which precipitate and control ion concentrations.

Dissolution kinetics of magnesite in acidic aqueous solution, a hydrothermal atomic force microscopy (HAFM) study: Step orientation and kink dynamics

The dissolution kinetics of features on the magnesite (104) surface were studied in aqueous solutions from pH 4.2 to 2 and at temperatures between 60 and 90°C by hydrothermal atomic force microscopy (HAFM). At pH = 4.2, HAFM images showed magnesite step orientations that are comparable to the step orientations on calcite. Similar to calcite (104), there is anisotropy in the step velocity, but the magnitude of the anisotropy is much greater for magnesite. Furthermore, below pH = 4.2, changes in the dominant step orientation were observed. These results are discussed in terms of a nearest neighbor kink dynamic model, and the associated kink dynamics were tested with kinetic Monte Carlo (KMC) simulations. The KMC results suggest that the kink dynamic model does not account for the experimental observations and that further details such as second-nearest neighbor interactions or surface/edge diffusion cannot be excluded from the model. The dominant step orientations at low pH also point toward mechanisms stabilizing steps along periodic bond chain directions.