Suitability of Existing Numerical Model Codes and Thermodynamic Databases for the Prognosis of Calcite Dissolution Processes in Near-Surface Sediments Due to a CO2 Leakage Investigated by Column Experiments

Among the risks of CO₂ storage is the potential of CO₂ leakage into overlaying formations and near-surface potable aquifers. Through a leakage, the CO₂ can intrude into protected groundwater resources, which can lead to groundwater acidification followed by potential mobilisation of heavy metals and other trace metals through mineral dissolution or ion exchange processes. The prediction of pH buffer reactions in the formations overlaying a CO₂ storage site is essential for assessing the impact of CO₂ leakages in terms of trace metal mobilisation. For buffering the pH-value, calcite dissolution is one of the most important mechanisms. Although calcite dissolution has been studied for decades, experiments conducted under elevated CO₂ partial pressures are rare. Here, the first study for column experiments is presented applying CO₂ partial pressures from 6 to 43 bars and realising a near-natural flow regime. Geochemical calculations of calcite dissolution kinetics were conducted using PHREEQC together with different thermodynamic databases. Applying calcite surface areas, which were previously acquired by N₂-BET or calculated based on grain diameters, respectively, to the rate laws according to Plummer et al. (Am J Sci 278:179–216, doi:10.2475/ajs.278.2.179, 1978) or Palandri and Kharaka (US Geol Surv Open file Rep 2004–1068:71, 2004) in the numerical simulations led to an overestimation of the calcite dissolution rate by up to three orders of magnitude compared to the results of the column experiments. Only reduction of the calcite surface area in the simulations as a fitting procedure allowed reproducing the experimental results. A reason may be that the diffusion boundary layer (DBL), which depends on the groundwater flow velocity and develops at the calcite grain surface separating it from the bulk of the solution, has to be regarded: The DBL leads to a decrease in the calcite dissolution rate under natural laminar flow conditions compared to turbulent mixing in traditional batch experiments. However, varying the rate constants by three orders of magnitudes in a field scale PHREEQC model simulating a CO₂ leakage produced minor variations in the pH buffering through calcite dissolution. This justifies the use of equilibrium models when calculating the calcite dissolution in CO₂ leakage scenarios for porous aquifers and slow or moderate groundwater flow velocities. However, the selection of the thermodynamic database has an impact on the dissolved calcium concentration, leading to an uncertainty in the simulation results. The resulting uncertainty, which applies also to the calculated propagation of an aquifer zone depleted in calcite through dissolution, seems negligible for shallow aquifers of approximately 60 m depth, but amounts to 35 % of the calcium concentration for aquifers at a depth of approximately 400 m.     

A geological database for parameterization in numerical modeling of subsurface storage in northern Germany

Underground land use can play a significant role in future concepts of energy and gas storage and requires an improved understanding of the parameters of potential storage formations (saline aquifers), for instance of porosity and permeability, and also of mineralogical and gas compositions. This study aims at providing data examples and calculating vertical spatial variations through variogram analyses of important North German geological reservoirs from Dogger, Rhaetian, Middle Buntsandstein, and Rotliegend (Sub)Groups and Formations, focusing on the western part of the North German Basin. Vertical correlation lengths of porosity and permeability data range between 0 and 30 m, while most results are calculated at approximately 2–4 m and do not show relevant differences among the evaluated formations. In the majority of the regarded formations, the Kozeny–Carman relationship between porosity and permeability is supported as long as low porosity and permeability values are excluded from the evaluation. Mineral percentages varied significantly among the evaluated sediments. Besides quartz, ankerite is the main compound in the Dogger Group, while feldspars and clay minerals were more frequent in the Rhaetian, Middle Buntsandstein, and Rotliegend sediments. Methane was the main gas compound in the reservoirs, followed by nitrogen, ethane, and carbon dioxide. This study serves as preparatory work to allow for the parameterization of geological models and a subsequent simulation of fluid transport to evaluate (long-term) safety and impacts of geothermal and gas storage projects.     

500-yr. precipitation variability in Southern Taihang Mountains,China, and its linkages to ENSO and PDO

An annually resolved and absolutely dated ring-width chronology spanning 657 yrs. is constructed with Whitebark pine (Pinus bungeana Zucc.) samples from the southern Taihang Mountains, Eastern China. On the basis of a significant correlation between the tree-ring width index and observed instrumental data, precipitation in current May is reconstructed for the region since AD 1510, with predictor variables accounting for 37.9 % of the variance in precipitation data. In agreement with other drought reconstructions, notable dry spells occur in the 1630s–1650s, 1680s–1700s, and 1770s–1800s, whereas wet periods prevail in the 1530s–1570s, 1840s–1870s, and 1950s-present. Wavelet analysis reveals clear 2–8, 20–40, and 80–130 yrs cycles at the 95 % confidence level for the reconstructed series over the past 500 yrs, suggesting possible linkages with the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Correlation analysis between the tree-ring series, ENSO, and PDO index further demonstrates that precipitation is negatively correlated with PDO and ENSO in the long term.     

Degradation of chlorinated ethylenes by Fe0: inhibition processes and mineral precipitation

. Granular zero-valent iron was used for the treatment of groundwater pollution caused by chlorinated ethylenes, mainly TCE, cis-DCE and VC at an industrial site. The rapidly decreasing rates of de-chlorination in the initial phase were attributed to the precipitation of carbonates and the development of hydrogen by anaerobic corrosion. After 70 pore volumes, sulphate was reduced by bacteria. From this point in time, the degradation of TCE was slightly accelerated whereas the de-chlorination rates of the other chlorinated ethylenes decreased only slowly. This relative improvement was assumed to be caused by the uptake of electron-transfer-blocking hydrogen by bacteria. Because the overall trend of the degradation rates is negative we conclude that the inhibitive effect of carbonate precipitation and hydrogen formation cannot be compensated for by the positive influence of the activity of sulphate-reducing bacteria.