Geological Storage Conditions Research Articles

Wetting Preference of Silica Surfaces in the Context of Underground Hydrogen Storage: A Molecular Dynamics Perspective.

The growing interest in large-scale underground hydrogen (H2) storage (UHS) emphasizes the need for a comprehensive understanding of the fundamental characteristics of subsurface environments. The wetting preference of subsurface rock is a crucial parameter influencing the H2 flow behavior during storage and withdrawal processes. In this study, we utilized molecular dynamics simulation to evaluate the wetting preference of the silica surface in subsurface hydrogen systems, with the aim of addressing disparities observed in experimental results. We conducted an initial comprehensive assessment of potential models, comparing the wettability of five common silica surfaces with different surface morphologies and hydroxyl densities in CO2-H2/water/silica systems against experimental data. After introducing the INTERFACE force field as the most accurate potential model for the silica surface, we evaluated the wetting behavior of the α-quartz (101) surface with a hydroxyl density of 5.9 number/nm2 under the impact of actual geological storage conditions (333-413 K and 10-30 MPa), the coexistence of cushion gases (i.e., CO2, CH4, and N2) at various mole fractions, and pH levels ranging from 2 to 11 characterized through considering the negative charges of 0 to -0.12 C/m2 via deprotonation of silanol on the silica surface. Our results indicate that neither pressure nor temperature has a significant impact on the wetness of the silica in the case of pure H2 (single component UHS operations). However, when CO2 coexists with H2, especially at higher mole fractions, an increase in pressure and a decrease in temperature lead to higher contact angles. Moreover, when the mole fraction of cushion gas ranges from 0 to 1, the contact angle increases 20, 9.5, and 4.5° for CO2, CH4, and N2, respectively, on the neutral silica substrate. Interestingly, at higher pH conditions where the silica surface carries a negative charge, the contact angle considerably reduces where surface charges of -0.03 and -0.06 C/m2 result in an average reduction of 20 and 80% in the contact angle, respectively. More importantly, at a pH of ∼11 (-0.12 C/m2), a 0° contact angle is observed for the silica surface under all temperatures, pressures, types of cushion gases, and varying mole fractions.

Caprock Remains Water Wet Under Geologic CO2 Storage Conditions

AbstractCarbon storage technology is primarily targeted in saline formations, which is a porous rock matrix filled with brine, sealed with a low permeability caprock. There are significant variations of CO2 wetting properties, typically reported in the literature as contact angle of CO2 and brine interacting with a rock material, suggesting that CO2 could become wetting under geostorage conditions and negatively impact containment effectiveness. Here, we performed the first controlled laboratory measurements of CO2‐brine contact angles on shale rocks from low permeability sealing formations with distinctive mineralogic properties—calcite‐rich, quartz‐rich, and dolomite‐rich. We targeted temperatures at 40° and 100°C, pressures at 8.3, 34.5, and 62.1 MPa, and salinity at 35,000 and 260,000 ppm. Results show no significant change in contact angle with mineralogy, temperature, pressure, salinity, and CO2 bubble size. We conclude that caprocks will remain water‐wet at geologic CO2 storage conditions and will not lose their capillary sealing capacity.

Investigation of chemical processes in cement exposed to wet ScCO2 and CO2-saturated brine in geological CO2 storage conditions

In geological CO2 storage conditions, wellbore cement can be exposed to both supercritical CO2 (ScCO2) with water vapor, and CO2 dissolved in water. There is a lack of studies that investigate the effects of reaction environments on the extent of CO2-induced cement carbonation and leaching. In this study, four CO2 exposure experiments were designed with wellbore cement samples exposed to both ScCO2 and CO2 dissolved in water to investigate the impacts of evaporation, capillarity, diffusion, and salt deposition on cement carbonation. Severe cement carbonation after 14 and 28 days of CO2 exposure was observed in a wet ScCO2 phase. When water evaporation into ScCO2 phase was minimized by a steel plate between the brine phase and the ScCO2 phase, a strong cement carbonation in ScCO2 phase was still visible. The reason was that imbibition and diffusion drove water to migrate from the lower section to the upper section of the cement sample to participate in the carbonation reaction. The level of cement carbonation in different CO2 exposure environments was ranked as: wet ScCO2 > CO2 dissolved in water > Pure-ScCO2. The corresponding maximum carbonation area ratios were 90 % and 38 % for the wet ScCO2 scenario and the brine scenario, respectively, compared with a maximum carbonation area ratio of 20 % for the Pure-ScCO2 scenario. This study implies that the most altered region in wellbore cement is at the ScCO2—water interface, and the expansion rate of the altered region is the key to evaluate the potential for CO2 leakage through wellbore cement.

CO2-brine mass transfer patterns and interface dynamics under geological storage conditions

CO2 geological storage is a promising carbon negative technology with important application prospects. Dissolution trapping is a reliable method throughout the life cycle of a storage project, because it plays a significant role in minimizing risks. A comprehensive understanding of its mechanism is challenging due to the complex mass transfer process controlled by numerous physical and chemical factors, which needs to be clarified at different scales but still significantly limited in current research. Investigating CO2-brine interfacial mass transfer in real rock cores during imbibition provides valuable insights into the behavior of CO2 in geological formations. This study conducted a series of pore-scale CO2-brine interfacial mass transfer experiments during imbibition in three rock cores using an X-ray micro-computed tomography machine. The dissolution patterns and interface dynamics during the dissolving processes were discussed systematically. The results indicate that the overall slice-averaged and per-cluster mass transfer coefficient between CO2 and brine is in the order of 10−8 m/s, but its specific value depends on the dissolution patterns and bubble sizes. Various dissolution patterns were found in different rock cores. A core-length dissolution finger was formed in the Berea core, causing higher per-cluster mass transfer coefficients around the finger, while lower coefficients elsewhere in the dissolution experiments. The interfacial mass transfer process, coupled with gravity instability, induced remobilization of isolated CO2 bubbles at a much lower capillary number than the critical capillary number. The remobilization of CO2 bubbles can even occur at single-pore scale. CO2 remobilization also created a phenomenon similar to intermittent pathway flow. Interface dynamics of CO2 dissolving processes were revealed for the first time, giving rise to non-uniform dissolutions in CO2 singlets, ganglia, and clusters, respectively.

Geological Storage Conditions and Potential Analysis of Carbon Dioxide in the High-steep Fold Belt in Eastern Sichuan Basin

Carbon sequestration refers to the technology of capturing carbon and storing it safely instead of directly discharging CO2 into the atmosphere. Rock salt is a promising CO2 reservoir, and the underground rock salt structure formed by salt cavern reservoirs in geological history has a natural sealing function. Storing carbon dioxide in salt caves can ensure that carbon dioxide is stably stored underground for a long time. However, at present, there are few studies on carbon sequestration in abandoned salt caves. This study analyses the characteristics of carbonate-evaporite strata in the high fold zone of eastern Sichuan and qualitatively evaluates Triassic carbonate-evaporite layers formed in the depth range of 800-3000 m in various sedimentary basins in Chongqing. A large set of carbonates-evaporites are developed in Chongqing, and the carbonate rocks near evaporites have low porosity and permeability, which can be used as CO2 storage caprock in salt cavities. Thick evaporated strata can also provide a good cover for CO2 storage in abandoned salt caves, which has great potential space resources.

Predicting wettability of mineral/CO2/brine systems via data-driven machine learning modeling: Implications for carbon geo-sequestration

Effectively storing carbon dioxide (CO2) in geological formations synergizes with algal-based removal technology, enhancing carbon capture efficiency, leveraging biological processes for sustainable, long-term sequestration while aiding ecosystem restoration. On the other hand, geological carbon storage effectiveness depends on the interactions and wettability of rock, CO2, and brine. Rock wettability during storage determines the CO2/brine distribution, maximum storage capacity, and trapping potential. Due to the high CO2 reactivity and damage risk, an experimental assessment of the CO2 wettability on storage/caprocks is challenging. Data-driven machine learning (ML) models provide an efficient and less strenuous alternative, enabling research at geological storage conditions that are impossible or hazardous to achieve in the laboratory. This study used robust ML models, including fully connected feedforward neural networks (FCFNNs), extreme gradient boosting, k-nearest neighbors, decision trees, adaptive boosting, and random forest, to model the wettability of the CO2/brine and rock minerals (quartz and mica) in a ternary system under varying conditions. Exploratory data analysis methods were used to examine the experimental data. The GridSearchCV and Kfold cross-validation approaches were implemented to augment the performance abilities of the ML models. In addition, sensitivity plots were generated to study the influence of individual parameters on the model performance. The results indicated that the applied ML models accurately predicted the wettability behavior of the mineral/CO2/brine system under various operating conditions, where FCFNN performed better than other ML techniques with an R2 above 0.98 and an error of less than 3%.

Solubility and interfacial tension models for CO2–brine systems under CO2 geological storage conditions

Thermodynamic properties of the CO2–brine pseudo-binary system are essential for the design of geological carbon storage (GCS) projects, especially those utilizing saline aquifers. The gas–liquid–solid interactions manifest in the interfacial tensions (IFTs) and contact angle determine the injectability, sealing capacity, and storage security of the GCS process. Dissolution of CO2 in the reservoir brine occurs throughout the entire GCS process, leading to enhanced storage capacity but also to acidification of the brine, possibly leading to reservoir or seal damage. Two of the most important thermodynamic properties of the fluids are the mutual solubility and the IFT of the CO2–brine pseudo-binary system. In this work, we report a new correlative model for the IFT between CO2- and water-rich phases over wide ranges of temperature (273 to 473 K) and pressure (up to 100 MPa). The model is parameterized for brines comprising any combinations of sodium, potassium, calcium and magnesium cations with chloride, sulphate and bicarbonate anions up to a total molality of at least 5 mol·kg−1. The independent variables in this new model are reduced temperature, ion molalities and the mole fraction of CO2 dissolved in the aqueous phase. The latter is related to temperature, pressure and ion molalities by an improved model for the mutual solubility. More than 2000 experimental data points were used in the development of the two models. For the IFT of the CO2-H2O binary system, the overall root-mean-square deviation (RMSD) is 0.65mN·m−1 while the absolute average relative deviation (AARD) is 1.8%. In the case of mutual solubility, the RMSD of CO2 mole fraction in the aqueous phase is 0.0003 and the AARD is 5.5% while, in the non-aqueous phase, the RMSD of H2O mole fraction is 0.0035 and the corresponding AARD is 8.7%. Similar results are found for the CO2-brine systems.

3D micro-structural changes of an artificial flow channel in wellbore cement under geologic CO2 storage conditions: Combined effect of effective stress and flow

Understanding CO2-induced micro-structural changes at the imperfections in wellbore cement is vital for assessing the risk of CO2 leakage through wellbore cement under geologic CO2 storage (GCS) conditions. To investigate the evolution of a flow channel width in cement under GCS conditions and the influence of effective stress and flow on the micro-structural changes of the flow channel in cement, we carried out a set of experiments in which the flow condition (flow-through v.s. static) and the effective stress (3 MPa effective stress v.s. no effective stress) were varied. Micro-structural changes of an artificial flow channel were investigated by X-ray micro-computed tomography (CT). CT images revealed a clear micro-structural change of the flow channel and distributions of Ca(OH)2/C-S-H dissolution and calcite precipitation zones near the channel after reacting with CO2-saturated brine. CT results showed that a flow rate of 0.01 mL/min through the channel turned channel self-sealing (as observed in the static scenario) into channel opening. Effective stress accelerated the dominant chemical reaction, i.e., enhancement of Ca(OH)2/C-S-H dissolution around the channel in a flow-through scenario and enhancement of calcite precipitation around the channel in a static scenario. It seems that effective stress and flow have a combined contribution to micro-structural change of the flow channel in hydrated Portland cement, which may increase the risk of CO2 leakage through wellbore cement when exposed to high concentration CO2.

Combination of Gibbs and Helmholtz Energy Equations of State in a Multiparameter Mixture Model Using the IAPWS Seawater Model as an Example

For carbon capture and storage (CCS) applications different sets of equations of state are used to describe the whole CCS-chain. While for the transport and pipeline sections highly accurate equations of state (EOS) explicit in the Helmholtz energy are used, properties under typical geological storage conditions are described by more simple, mostly cubic EOS, and brines are described by Gibbs energy models. Combining the transport and storage sections leads to inconsistent calculations. Since the used models are formulated in different independent variables (temperature and density versus temperature and pressure), mass and energy balances are challenging and equilibria in the injection region are difficult to model. To overcome these limitations, a predictive combination of the Gibbs energy-based IAPWS seawater model (IAPWS R13-08, 2008) with Helmholtz energy-based multi-parameter EOS is presented within this work. The Helmholtz energy model used in this work is based on the EOS-CG-2016 of Gernert and Span (J Chem Thermodyn 93:274–293, https://doi.org/10.1016/j.jct.2015.05.015, 2016). The results prove that a consistent combination of the two different models is possible. Furthermore, it is shown, that a more complex brine model needs to be combined with Helmholtz energy EOS for calculations at storage conditions.

Experimental investigation on effect of water film thickness in unsaturated sandstone cores on CO2 transport during geologic storage

Characteristics of the water distribution and film thickness within pores control CO2 transport and determine the effectiveness of geologic storage. Although studies of adsorbed water films on solid surfaces are abundant, reports about the dependence of CO2 transport behavior on film thickness under geologic CO2 storage conditions are still lacking. In this study, the water film thickness was estimated through water adsorption equilibrium experiments on four tight sandstone samples at varying relative humidities. The calculated results reveal that the adsorbed water exerts a greater influence on smaller pores, as micro- and mesopores are first filled with condensed water, while macropores only contain absorbed water films, and that the measured critical brine film thicknesses are strongly controlled by the clay mineral content. The influence of water film thickness on CO2 transport was evaluated based on the DLVO theory for cylindrical pores. Considering the mobility difference of water films in different pores, the effective disjoining pressure measured by the most probable pore radius can be regarded as the preliminary basis for the selection of reservoir or caprock and evaluation of the sealing performance of rock. Increasing the water film thickness results in a smaller effective pore radius and flowable pore space, and the movement of water in pore channel causes an electroviscous effect. Both conditions have a negative effect on CO2 movement and eventually decrease the CO2 effective permeability.

Carbonation rate and microstructural alterations of class G cement under geological storage conditions

Cementitious grouts are a vital component for the economically-viable implementation of the geological storage of CO2 in providing an engineered long-term seal. In this study a class G cement was carbonated at 80 bar, at either 60 °C or 120 °C, whilst immersed in a synthetic brine for durations of up to 5 months. X-ray computed tomography was used to evaluate the advancement of carbonation depth, whilst SEM/EDXA and XRD were used to characterise microstructural alteration of the cement phases. The microstructure of the ‘main carbonation front’ was found to be representative of the governing reactive transport mechanism. An ill-defined ‘main carbonation front’ during carbonation at 80 bar/60 °C showed a carbonation mechanism controlled by the rate or precipitation/dissolution reactions; diffusion in that case was not the controlling factor. The faster local supersaturation conditions in the pores at 60 °C (with respect to Ca2+ and HCO3−) created a dynamic system of aragonite precipitation from the carbonated to the inner regions of the cement. At 80 bar/120 °C a clearly defined ‘main carbonation front’ with higher compositional density than at 60 °C, was correlated with the fast reactions and diffusion limited evolution of the ‘main carbonation front’. Calcite, as the main result of those fast reactions at 120 °C, filled ubiquitously previously unmineralized voids, creating a system less prone to compositional alterations by chemical changes due to the CO2 plume. This study showed, that the formation of calcium carbonate polymorphs depends on the kinetics of carbonation reactions for a class G cement that is determined by temperature and time. The findings of the current paper can be further used for the understanding of reaction processes within the cements of the CO2 injection wells and assess their long-term chemical stability.

Evidence of self-sealing in wellbore cement under geologic CO2 storage conditions by micro-computed tomography (CT), scanning electron microscopy (SEM) and Raman observations

In this study, reaction experiments between CO2-saturated brine and wellbore cement samples cured under different pressures were conducted to study microstructural and mineral composition changes using micro-computed tomography (micro-CT), scanning electron microscopy (SEM) and Raman spectroscopy. The CT images of post-CO2 exposure cement samples showed a dissolution-precipitation-dissolution pattern at the exterior of the samples. The dissolution in the inner hole of the samples, however, was not significant. Instead, only CaCO3 precipitation was observed in the inner hole. CaCO3 precipitation in the inner hole contributes to self-sealing of the cement, which reduces the risk of CO2 leakage through wellbore cement. According to CT and SEM observations, a higher curing pressure caused more precipitation of CaCO3, which favored cement self-sealing when exposed to CO2. The appearance of a CO peak and the disappearance of a –OH peak after reaction with CO2 were observed by Raman spectroscopy, which was attributed to cement carbonation that converted Ca(OH)2 into CaCO3. This study provides solid evidence of cement self-sealing due to cement carbonation under geologic CO2 storage circumstances.

Potential for uranium release under geologic CO2 storage conditions: The impact of Fe(III)

Saline aquifers are considered ideal subsurface sinks for large amounts of CO2 storage. It is common to have trace levels of uranium-bearing minerals (naturally occurring radioactive material, NORM) in sandstone saline aquifers and CO2 injection may cause uranium mobilization due to the coupled chemical and physical interactions at mineral-water interfaces. In this study, we developed a TOUGHREACT model to assess the uranium mobilization potential from uraninite (UO2) dissolution induced by CO2 injection into a hypothetical CO2 storage reservoir with Fe(III)-bearing hematite in a time scale of 30 years CO2 injection and 100 years after CO2 injection. Numerical simulation results show that injection of CO2 reduced pH and induced small amounts of hematite dissolution, which released Fe3+ into aqueous phase. Fe3+ together with dissolved bicarbonate species caused oxidative dissolution of UO2 (Fe3+ serving as an oxidant), resulting in an increase of dissolved uranium concentrations in the CO2 storage reservoir. However, the released uranium was limited to a small region close to Fe3+ supply source and the maximum concentration of released uranium was only 9.00 × 10−10 mol/kg in the CO2 storage reservoir at t = 30 years. The availability of Fe3+ is the main factor that limits mineralized uranium release because the pH drop induced by CO2 injection is not low enough to cause significant Fe3+ release. For sorbed uranium, the main factor that influences sorbed uranium release is pH because uranium sorption is amphoteric. Based on our simulation results, the pH range in the CO2 storage reservoir does not cause a substantial release of sorbed uranium. Also, the potential for released uranium to migrate through a permeable zone in the caprock to the layer above the caprock and cause groundwater contamination is negligible. In summary, results from this study imply a very low risk of environmental contamination by bicarbonate and Fe(III)-induced uranium mobilization.

High-Temperature, Resistant, Argillite-Based, Alkali-Activated Materials with Improved Post-Thermal Treatment Mechanical Strength

AbstractFire resistance performance is one of the most important requirements in geological storage conditions in order to improve the resistance of storage packages to high thermal constraints (in the case of a fire for example). With the need to develop new fire-resistant materials, the aim of the present study was to develop fire-resistant geopolymer binders based on Callovo-Oxfordian (COx) argillite. Two types of kaolin with different degrees of purity were mixed with argillite in various proportions. These mixtures were calcined at 600 or 750°C. In order to assess the fire resistance of activated materials, thermal treatment at 1000°C was performed. The compressive strength and mineralogical composition of the samples were investigated before and after heat treatment. The results showed that the addition of argillite improved significantly the thermomechanical properties of kaolin-based geopolymers containing impurities, especially the mixture containing 67% argillite and calcined at 750°C. This phenomenon was not observed for the pure-kaolin geopolymer. Improvement of fire resistance was due to the formation in situ of leucite and zeolite-type phases (KAlSi2O6 and KAlSiO4) and of wollastonite (CaSiO3) at high temperature, which is linked to the Ca available in the raw materials.

Uranium release surrounding a single fracture in a uranium-rich reservoir under geologic carbon storage conditions

Though geologic carbon storage (GCS) is widely recognized as a promising strategy to reduce emissions of greenhouse gas (GHG), the potential for mobilization of radioactive uranium (U) from U-bearing minerals in deep subsurface due to CO2 injection remains a concern. In this study, supercritical CO2 and brine flowing through a fracture surrounded by reservoir rock containing uranium is simulated so as to study the potential of uranium release as a result of CO2 injection and the impact of various factors on total uranium release rate. Mineral compositions of the reservoir rock are from previously published literature, which mimics typical sandstone mineral compositions. The reservoir rock is assumed to have 2 × 10−4 vol% UO2 in solid phase. Simulation results show that CO2 injection induces UO2 dissolution, and both CO2 and mobilized uranium are able to migrate in both the fracture and the rock matrix surrounding the fracture. However, the released uranium concentration is quite low in a 60-day simulation period. Mineral dissolution causes a very small porosity increase surrounding the fracture in the simulation period. Sensitivity analysis shows that an increase of UO2 specific surface area and UO2 content in reservoir rock causes a significant increase in released uranium concentration. In other words, total uranium release rate is positively correlated with the specific surface area of UO2 and UO2 content in reservoir rock because the increase of specific surface area and UO2 content increases the total area of UO2 in contact with HCO3− and O2, which raises total uranium release rate. In summary, uranium release in and surrounding the fracture is mainly controlled by uranium supply of the reservoir rock, and the risk of environmental contamination by CO2-induced uranium release is quite low in the scenario reported.

Application of arbitrary polynomial chaos (aPC) expansion for global sensitivity analysis of mineral dissolution and precipitation modeling under geologic carbon storage conditions

Numerical modeling of geochemistry associated with geologic CO2 storage involves many conceptual and quantitative uncertainties. In this study, a time efficient arbitrary polynomial chaos (aPC) expansion approach was proposed to do global sensitivity analysis of mineral dissolution and precipitation modeling in geologic carbon storage scenarios. To demonstrate the workflow of the aPC approach, a numerical model to predict permeability evolution of a Lower Tuscaloosa sandstone core exposed to CO2 saturated brine was used. The modeled sandstone core permeability by the aPC approach was 2095.5 mD ± 504.5 mD after 180 days of CO2 exposure. The measured permeability of the core after 180 days of CO2 exposure was 1925.0 mD, which was within the uncertainty range. Keq (SiO2 (am)) was the most important modeling parameter that influenced permeability results, implying that SiO2 (am) is a key mineral that governs permeability evolution of sandstone in geologic carbon storage scenarios. The aPC approach can reduce 99% of simulation time needed to do global sensitivity analysis of a complicated geochemical model, compared with traditional Monte Carlo approach.

Simulation of uranium mobilization potential in a deep aquifer under geological carbon storage conditions

Geological carbon storage is considered as a promising strategy to reduce greenhouse gas emissions. For those subsurface systems with uranium-bearing minerals in rock formations, there is an uncharacterized possibility for injected CO2 to cause uranium mobilization due to the coupled chemical and physical interactions at mineral--water interfaces. We developed a TOUGHREACT model to assess the uranium mobilization potential from uraninite (UO2) dissolution induced by CO2 injection into a hypothetical CO2 storage reservoir in a time scale of 30 years of CO2 injection and 100 years after. Numerical simulation results show that the concentration of HCO3− increased after CO2 injection, and CO2 was able to migrate toward the shallow aquifer through existing leakage pathways. HCO3−-assisted UO2 oxidative dissolution was the dominant mechanism that led to the increase of dissolved uranium concentration in a form of uranyl carbonate complex and the mechanism of H+-assisted UO2 dissolution could be ignored. Sensitivity tests suggested that the increase of UO2 content and O2 level in the CO2 storage reservoir resulted in the most significant increase of dissolved uranium concentration, but no sign of uranium intrusion was predicted up to the shallow aquifer. In summary, uranium release induced by CO2 injection is possible if the target CO2 storage reservoir contains U-bearing minerals and is exposed to oxidizing environment, but our simulation assured that significant upward migration of uranium from deep CO2 storage reservoir driven by CO2 injection is unlikely even at the worst scenario.

Characterisation of wellbore cement microstructure alteration under geologic carbon storage using X-ray computed micro-tomography: A framework for fast CT image registration and carbonate shell morphology quantification

An aqueous CO2–cement interaction experiment along with X-ray computed micro-tomography characterisation of pre- and post-exposure cement samples was carried out to investigate the cement structure evolution under geologic carbon storage conditions. An image processing framework was proposed for mapping mineral dissolution and precipitation and characterisation of carbonate shell morphology. The main workflow covered in this framework is to, 1) register cement CT images before and after reaction; 2) generate the difference image showing chemical alteration and map the difference image to demonstrate local content changes of pore space, calcite and portlandite; 3) segment carbonate shell from the difference image; 4) generate auxiliary images including skeleton, 3D local thickness and surface boundaries of the carbonate region, and 5) spatial quantification of the area, thickness, penetration depth and pore/calcite/portlandite content changes of the carbonate shell. The effectiveness of the framework was validated through step-by-step demonstration of results when deploying the framework to process the CT images of six cement samples acquired before and after reaction with CO2. The 3D mineral precipitation and dissolution (or local mineral content change) map and the internal and external carbonate shells were visualised. The spatial distribution of the shell area, thickness, penetration depth and pore/calcite/portlandite content changes along the height of the sample was revealed. Overall, the dissolution and precipitation map gives more intuitive and interpretable results of CO2-induced chemical alteration than direct visual comparison from the original CT images, and the morphological quantification of the carbonate shell gives reasonable interpretation of the spatial distribution of the carbonate reaction.

Quantifying pore scale and matrix interactions of SCCO2 with the Marcellus shale

Marcellus Shale was investigated as a storage reservoir for CO2, applying CO2 as an enhanced hydrocarbon recovery agent, and examining the CO2 alteration of the shale as a sealing unit. The investigation was comprised of observing the geochemical interaction of CO2 and water with two Marcellus Shale samples (MS-1 [silicate-rich] and MS-4 [carbonate-rich]) using various analytical techniques. In-situ Fourier Transform infrared (FT-IR) spectroscopy at geological storage conditions (high temperature and pressure) was used to examine the geochemical interaction of CO2 (with and without water) and Marcellus Shale. FT-IR results showed that CO2 interacted physically with the clay and kerogen components of dehydrated Marcellus Shale samples while dissolving in the water layer in the hydrated Marcellus Shale samples to promote carbonate dissolution. Feature relocation scanning electron microscopy was utilized to document the physical changes and the pore alterations based on exposure to CO2 with and without water. SEM indicated significant carbonate dissolution to the MS-4 sample after exposure to CO2 and water. Density function theory (DFT) and sorption isotherms were used for pore size analysis and to quantify CO2 sorption. The results were very different for the two Marcellus Shale samples studied. The MS-4 showed dissolution of carbonate to be a major reaction under hydrated conditions with a loss of mesopore volume from 68% to 40%, while MS-1 often showed an opposite trend. Both the MS-1 and MS-4 samples showed that significant volume of macropores larger than 200 nm were present. Isotherm results suggested uptake of CO2 of the MS-4 sample is four times greater than the MS-1 sample.

Micro-CT Characterization of Wellbore Cement Degradation in SO42-–Bearing Brine under Geological CO2 Storage Environment

In order to explore the process of acid- and CO2-induced degradation of wellbore cement and the development of pre-existing leakage channels in wellbore cement under sulfate-rich geological CO2 storage conditions, wellbore cement samples were immersed in SO42--bearing brine solution for 7 days, and the samples after reacting with the low and circumneutral pH solutions were scanned by a micro-CT scanner. HCl+Na2SO4 solution was used to simulate the low-pH condition in deep formation waters and the possible existence of high sulfate ion content in deep formation waters. The acidification and carbonation results were compared, and the results given different pH values and different curing conditions were compared as well. The results show that the degradation of cement was related to the pH value of the reaction solution. There was a significant dissolution in the exterior of the cement sample after exposure to the low-pH solution, but the dissolution surrounding a penetrating borehole at the center of the sample (mimicking a leakage pathway within the wellbore cement in geological CO2 storage environment) was limited. Comparison between acidification and carbonation results in this study shows formation of a thick carbonate layer due to cement carbonation, and this layer was not observed in the acidification result. As for different curing conditions of cement samples, no significant difference in cement alteration depth was observed for the acidification case. For the carbonation case, precipitations in the borehole occurred in the cement sample cured at ambient pressure, while the cement sample cured at high pressure did not produce any precipitation in the borehole. This study provides valuable information on how low pH-induced corrosion and HCO3--induced cement carbonation contribute to structure evolution of wellbore cement in SO42--bearing brine under geological CO2 storage environment.