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A Multifarious Approach to Evaluate the CO2 Migration and Associated Hydrochemical Reactions in Natural and Artificial CO2 Leakage Conditions

  • Hyun-Kwon Do (도현권, 대학원 지구환경과학과 지질환경학전공)

초록/요약

Part 1: Unusual episodic fluctuations of electrical conductivity (EC) were observed twice a year in a national groundwater monitoring network well in South Korea where EC was automatically monitored at a depth of 20 m below ground level (bgl). This study examined the depth profile of groundwater in the 70-meter-deep monitoring well to address the reasons for the EC fluctuations. The results of well logging, borehole video recording, and hydrochemical analysis of groundwater indicated that the CO2-rich groundwater entering through the screen zones below 50 m bgl was physicochemically stratified into three layers with distinct EC that were separated by two transition zones in the well: a bottom layer (70−43 m bgl)) with an EC of ~3900 μS/cm, intermediate layer (35−24 m bgl) of ~1800 μS/cm, and top layer (16−3 m bgl) of ~300 μS/cm. The first transition zone at depths of 43−35 m bgl was attributed to CO2 exsolution in the open system and the subsequent physicochemical changes of groundwater, while the second transition zone at depths of 24− 16 m bgl was formed by the precipitation of hydrous ferric oxides with consequent sorption of remaining ions due to a sudden change toward the oxidizing environment. The monitoring probe installed at a depth of 20 m bgl was found to be located within the upper transition zone, which caused EC peaks when the well was purged at a depth of 25 m bgl for well maintenance twice a year. This study shows that automated groundwater monitoring systems may misguide one about the groundwater quality if an unexpected physicochemical variation (such as stratification) occurs in a monitoring well. Therefore, the presence of interface zones caused by abrupt changes in EC must be carefully considered when an automated monitoring well is designed. In particular, it is important to appropriately set the depth of geochemical logging and sampling to accurately monitor the groundwater chemistry in an aquifer that is under the influence of inputs of low-pH and high-TDS fluids (e.g., CO2-rich groundwater and acid mine drainage).

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초록/요약

Part 2: Eighteen water samples collected from eight CO2-rich springs in the northern part of the Gyeongsang sedimentary basin (GSB), South Korea showed distinct hydrochemistry, in particular, pH, total dissolved solids (TDS), and Na contents, and were classified into four groups: 1) Group I with low pH (average of 5.14) and TDS (269.8 mg/L), 2) Group II with high TDS (2681.0 mg/L) and Na-enriched (202.9 mg/L), 3) Group III with intermediate Na content (97.5 mg/L), and 4) Group IV with Na-depleted (42.3 mg/L). However, they showed the similar partial pressure of CO2 (0.47 to 2.19 atm) and stable carbon isotope ratios of dissolved inorganic carbon (–6.3 to –0.6‰), indicating the inflow of deep-seated CO2 into aquifers along faults. In order to elucidate the evolutionary process for each group of CO2-rich springs, a multifarious approach was used combining stable hydrogen (D), oxygen (18O) and carbon (13C) and radioactive carbon (14C) isotopic, geophysical, and hydrochemical data. The highest D and 18O ratios of water and the relatively young 14C ages in Group I and the lowest D and 18O in Group II indicated the short and long residence time in Group I and II, respectively. The electrical resistivity tomography (ERT) survey results also supported the fast rising through open fractures in Group I, while a relatively deep CO2-rich aquifer for Group III. Group II had high contents of Mg, K, F, Cl, SO4, HCO3, Li, and As, while Group I showed low contents for all elements analyzed in this study except for Al, which exceeded the World Health Organization (WHO) guideline for drinking-water quality probably due to the low pH. Meanwhile Group IV showed the highest Ca/Na as well as Ca, Fe, Mn, Sr, Zn, U, and Ba, probably due to the low-temperature dissolution of plagioclase based on the geology and the ERT result. The levels of Fe, Mn, and U exceeded the WHO guidelines in Group IV, while As in Group II. The different hydrochemistry suggests a distinct evolutionary process for each group. Group I seems to represent a fast discharge from the CO2-rich aquifer to a discharge point, experiencing a low degree of water-rock interaction, while Group II seems to represent a slow discharge with a high degree of water-rock interaction. GSB is a potential site for geological carbon storage (GCS), and injected CO2 may leak through various evolutionary processes given heterogenous geology as CO2-rich springs. The study result suggests that the combined use of pH, Na, K, Li, and Ca/Na are effective hydrochemical monitoring parameters to assess the leakage stage in silicate rocks in GCS projects. Besides, aluminum (Al) can be risky at the early stage of CO2 leakage, while Fe, Mn, U, and As at the later stage of CO2 leakage.

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초록/요약

Part 3: A critical environmental issue in carbon capture and storage (CCS) is to verify the amounts of leaked CO2 and also to trace the migration of injected CO2 which accompanies geochemical reactions with surrounding aquifer materials. To investigate potential impacts of CO2 leakage on shallow groundwater and to evaluate which hydrochemical and isotopic parameters can be used as a tracer of CO2 migration, a controlled CO2 release experiment was performed at the Environmental Impact Evaluation Test (EIT) site in Eumseong. For the experiment, CO2-infused water was injected into a shallow siliciclastic aquifer for 26 days under an artificially induced pressure gradient condition. Prior to the injection of dissolved CO2, baseline levels of hydrochemical and carbon isotopic (δ13CDIC) parameters were obtained. Following the injection, the arrival of dissolved CO2 plume at the monitoring wells were easily detected by both the pH decrease, the EC increase, and the significant increase of total dissolved inorganic carbon (TDIC). The spatial and temporal trends of hydrochemical evolution indicated that the dissolved CO2 plume migrated along a preferential flow and that plume sank to some degree as it traveled in aquifer. Concentrations of chemical elements displayed three types of behavior: (1) pulse-like (HCO3, Ca, Mg, Na, K, Sr, and Ba), which consisted of a rapid increase immediately after the arrival of the dissolved CO2 plume and a decrease despite the continuous inflow of the plume, (2) pH dependent (SiO2 and Mn), which showed relatively slow increase and decrease in concentrations compared to other hydrochemical elements, and (3) rapid increase and slow decrease (Li). This observation indicates the occurrence of hydrochemical processes which consist of the dissolution of a certain amount of soluble minerals such as calcite, together with cation exchange reactions, even in a predominantly siliciclastic aquifer. Despite of such hydrochemical changes, the δ13CDIC values of groundwater showed a good correlation with the measured PCO2 and TDIC values, because the δ13CDIC values of groundwater were mainly affected by a prevailing carbon source (i.e., injected CO2).

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목차

Preface - 1 -
References - 6 -
Part 1. Lessons from the physicochemical stratification of CO2-rich groundwater in a well: an implication for automated groundwater monitoring systems - 9 -
Abstract - 9 -
1. Introduction - 11 -
2. Study area - 14 -
2.1. Geology - 14 -
2.2. Andong-Giran monitoring station - 16 -
3. Methodology - 18 -
3.1. Physicochemical analyses - 18 -
3.2. Imaging methods and geophysical survey - 20 -
3.3. Computation of thermodynamics - 21 -
4. Results - 23 -
4.1. Fluctuations of automated monitoring data - 23 -
4.2. Vertical well logging - 25 -
4.3. Borehole imaging using nano camera and SEM - 27 -
4.4. Hydrochemical analysis of groundwater and electrical resistivity tomography - 29 -
5. Discussion - 31 -
5.1. Origin of gas bubbles - 31 -
5.2. Hydrochemical processes at each transition zone and the stability - 33 -
5.2.1. First transition zone at depths of 43−35 m bgl attributed to CO2 exsolution - 34 -
5.2.2. Second transition zone at depths of 24–16 m caused by the precipitation of hydrous ferric oxides - 36 -
5.2.3 The effect of purging - 38 -
5.2.4. Stability of physicochemically stratified water bodies in the monitoring well - 39 -
5.3. Representativeness of wellbore water for automated monitoring - 42 -
5.4. Guide for geochemical monitoring using a real-time data logger - 43 -
6. Conclusion and suggestion - 47 -
References - 50 -
Figures - 59 -
Supplementary Figure - 74 -
Tables - 76 -
Part 2. Hydrochemical parameters to assess the evolutionary processes of CO2-rich spring water: A suggestion for evaluating CO2 leakage stages in silicate rocks. - 80 -
Abstract - 80 -
1. Introduction - 83 -
2. Material and Method - 87 -
2.1. Study area - 87 -
2.2. Sampling and analytical methods - 89 -
2.3. Geophysical exploration - 91 -
3. Results - 93 -
3.1. Hydrochemical and isotopic compositions - 93 -
3.2. Geophysical survey - 96 -
4. Discussion - 98 -
4.1. Source and discharge pathways of CO2 - 98 -
4.2. Hydrochemical evolutionary processes - 100 -
4.2.1. Group I and II - 100 -
4.2.2. Group III and IV - 104 -
4.3. Useful hydrochemical parameters - 107 -
5. Conclusion - 109 -
Reference - 111 -
Figures - 121 -
Tables - 128 -
Part 3. Tracing CO2 leakage using changes of hydrochemical and carbon isotope compositions of groundwater during the controlled CO2 release field test - 134 -
Abstract - 134 -
1. Introduction - 136 -
2. Materials and methods - 142 -
2.1. Geology and hydrogeology - 142 -
2.2. Design for controlled CO2 injection and migration - 144 -
2.2.1. Forced-gradient field - 144 -
2.2.2. Injection period - 144 -
2.2.3. Post-injection period - 145 -
2.3. Sample collection and analysis - 146 -
3. Results - 149 -
3.1. Baseline survey - 149 -
3.2. Physicochemical changes - 151 -
3.3. Changes in carbon isotope ratio - 154 -
3.4. Evolution of major ions and trace elements - 155 -
4. Discussion - 157 -
4.1. Migration of dissolved CO2 plume - 157 -
4.1.1. Horizontal migration - 157 -
4.1.2. Vertical migration - 159 -
4.2. Hydrochemical responses to dissolved CO2 plume - 161 -
4.2.1. Carbonates dissolution and Ca-driven ion exchange - 162 -
4.2.2. Enhanced mineral weathering attributed to low pH - 165 -
4.2.3. Ion exchange and dissolution of silicate minerals - 167 -
4.3. Carbon isotope - 168 -
5. Conclusion - 171 -
References - 173 -
Figure - 180 -
Tables - 191 -
Conclusion and perspectives - 192 -

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