Geochemical Reactions Research Articles

Initial and residual trapping of hydrogen and nitrogen in Fontainebleau sandstone using nuclear magnetic resonance core flooding

Hydrogen is among a few promising energy carriers of the future mainly due to its zero-emission combustion nature. It also plays an important role in the transition from fossil fuel to renewable. Hydrogen technology is relatively immature and serious knowledge gaps do exist in its production, transport, storage, and utilization. Although the economical generation of hydrogen to the scale required for such transition is still the biggest technical and environmental challenge, unlocking the large-scale but safe storage is similarly important. It is difficult to store hydrogen in solid and liquid states and storing it in the gaseous phase requires a huge volume which is just available in subsurface porous media. Sandstone is the most abundant and favourable medium for such storage as carbonate rock might not be suitable due to potential geochemical reactions.It is well established in the literature that interaction of the host rock-fluid and injected gas plays a crucial role in fluid flow, residual trapping, withdrawal, and more generally storing capacity. Such data for the hydrogen system is extremely rare and are generally limited to contact angle measurements, while being not representative of the reality of rock-brine-hydrogen interaction(s). Therefore, we have conducted, for the first time, a series of core flooding experiments using Nuclear Magnetic Resonance (NMR) to monitor hydrogen (H2) and Nitrogen (N2) gas saturations during the drainage and imbibition stages under pressure and temperature that represent shallow reservoirs. To avoid any geochemical reaction during the test, we selected a clean sandstone core plug of 99.8% quartz (Fontainebleau with a gas porosity of 9.7% and a permeability of 190 mD).Results show significantly low initial and residual H2 saturations in comparison with N2, regardless of whether the injection flow rate or capillary number were the same or not. For instance, when the same injection flow rate was used, H2 saturation during primary drainage was 4% and it was <2% after imbibition. On other hand, N2 saturation during the primary drainage was 26% and it was 17% after imbibition. However, when the same capillary number of H2 was utilised for the N2 experiment, the N2 saturation values were ∼15% for initial gas saturation and 8% for residual gas saturation. Our results promisingly support the idea of hydrogen underground storage; however, we should emphasise that more sandstone rocks of different clay mineralogy should be investigated before reaching a conclusive outcome.

Experimental Study on the Effect and Mechanism of Chemical Stimulation on Deep High-Temperature Granite

Chemical stimulation has been increasingly applied to improve the performance of geothermal reservoirs since early 1980s. The potential for the successful application of this technique to high-temperature reservoirs hosted in granitic rocks is still uncertain, and one of the keys to assess this potential is to investigate experimentally the geochemical reactivity induced via chemical stimulation on relevant rock specimens. On this premise, we combined high-temperature and high-pressure dynamic simulation and static corrosion experiments to explore the effect of different chemical stimulants on the permeability of granite samples from the Baimiao formation (Hebei Province, China). Experimental results show that NaOH-dominated alkaline stimulants cause only weak dissolution patterns on primary feldspar and quartz, and they do not sensitively affect the original amount of chlorite. The overall effect is a negligible enhancement of the original permeability of all the granite specimens analyzed. Conversely, a large increase in permeability is observed when an acid mixture of 10wt% HCl + 1.5wt% HF is used as a stimulant, with an observed maximum magnifying effect of about 27 times, due to the effective dissolution of feldspar and chlorite. Though quartz appears not to be affected by dissolution, a relatively large amount of secondary neo-formed amorphous silica is also documented.

Cobalt hydroxide–cobalt carbonate competitive growth on carbonate surfaces

The presence of mineral surfaces can affect the outcome of geochemical reactions by providing alternative nucleation pathways, but not in ways that can yet be reliably quantified. In this work, the reaction of Co(II) with calcite (CaCO3) and magnesite (MgCO3) powders at room temperature was used to quantify the effects of the nature of the mineral substrate and of solution chemistry on the competitive heterogeneous growth between cobalt carbonate (CoCO3) and cobalt hydroxide (Co(OH)2). Experiments were first performed to determine the appropriate solubility product constants of CoCO3 and Co(OH)2, for which several values have been reported in the literature. X-ray photoelectron spectroscopy measurements were then performed to quantify the relative proportion of each phase in surface precipitates as a function of the nature of the substrate, initial saturation level, and pH. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were also used to characterize the morphology and composition of surface precipitates. On calcite powders, Co(OH)2 formed predominantly despite the initial solutions being more supersaturated with respect to CoCO3 than to Co(OH)2, indicating that the kinetics of heterogenous growth were faster for Co(OH)2 than for CoCO3. In contrast, magnesite powders were much more favorable to the growth of CoCO3 because of the low lattice mismatch between the two phases, which allowed for heteroepitaxial growth. However, the proportion of CoCO3 in surface precipitates decreased with increasing initial supersaturation, likely due to the resulting decrease of the free energy barrier for Co(OH)2 nucleation and the rapid kinetics of Co(OH)2 growth. Lowering pH increased the proportion of CoCO3 on both substrates. These findings highlight how the interplay between lattice mismatch, heterogeneous nucleation barriers, and rates of heterogenous growth can influence competitive heterogeneous growth and dramatically affect the outcome of geochemical reactions.

Low salinity water flooding: Evaluating the effect of salinity on oil and water relative permeability curves using coupling of DLVO and geochemical reactions

Enhancing the oil recovery of carbonate reservoirs with acidic oil depends on injected brine composition and salinity throughout the low salinity water flooding. The typical description of this phenomenon on oil and water relative permeability curves is being put forward by the process-dependent interpolation function, which barely addresses the underlying fundamentals governing the direct impact of wettability alteration and geochemical reactions on flow saturation functions. The main objective of this work is to perform an integrated investigation of the effect of salinity on oil and water relative permeability curves by dynamic coupling of the Derjaguin, Landau, Verwey, and Overbeek (DVLO) theory and geochemical reactions. We studied both the single and two-phase systems using the basis of the geochemical reactions to track the variations of the wetting and non-wetting phase relative permeability curves. In the single-phase scenario, the results show the superiority of the reactive flow modeling technique with kinetic-controlled reactions, in the range between [2–4] of the Damkohler number, to yield a better result for the experimental effluent ions' concentrations than the equilibrium approach. In addition, the single-phase results under dynamic conditions propose Civan correlation as the best predictor of residual resistance factor (RRF) by numerically adjusting the correlation of coefficients based on the physical variables - salinity, dimensionless consumed ion concentration, and Peclet number. Accordingly, the results indicate a novel linkage between the single-phase and two-phase results in the sense that RRF under single-phase mode can be used to estimate the change in the wetting phase relative permeability under the two-phase mode. The single-phase methodology provides the capability for an improvement – up to 15% - in the prediction of the pressure drop curve. For two-phase cases, our procedure involved coupling the characterized geochemical reactions and the DLVO theory to consider the effect of injected water composition on contact angle as the static two-phase parameter. The phase-field method is coupled with DLVO theory to calculate the relative permeability curves for the non-wetting phase, which corresponds to the contact angle at each simulation time step. The two-phase results indicate an improvement – up to 30% - in the prediction of the recovery factor curve, on a history-matching basis, using the DLVO theory with contact angle-dependent correlation of non-wetting phase relative permeability curve. The main novelties of this study are using the single-phase dynamic permeability impairment results in calculating the variation of the wetting-phase relative permeability curve. Moreover, dynamic coupling of DLVO and non-linear contact angle-dependent correlation is used to calculate the non-wetting phase relative permeability curve. According to these novelties, more accurate prediction of the laboratory production profiles can be obtained.

An extension of gases-brine mutual solubility models to include geochemical reactions: Modeling and applications

Accurate and quantitative numerical modeling of phase behavior and speciation of gas-water-rock systems is crucial for fluid and rock evolution analysis, and important in scientific research and engineering applications. In our previous work, mutual solubility models of series gases and brine have been developed for wide temperature, pressure and composition ranges. The gas components included CO2, CH4, H2S, N2 and O2. The existing experimental data can be reproduced accurately. In this paper, the models are extended to include water-rock interactions by means of a full coupling with the open-source software package PHREEQC. With the new coupled model, the solubility of gases and various minerals can be correctly and reliably reproduced. The proposed numerical framework is properly implemented and it can be applied in different scenarios of gas-water-rock interactions. Three practical applications have been investigated: calculation of uranium in situ leaching, CO2 geological storage and natural gas souring are provided in the paper. The application cases have additionally proven the stability and reliability of the proposed model.

How do chlorite coatings form on quartz surface?

Chlorite-coats on quartz surfaces are ubiquitous in various sedimentary environments. Chlorite-coats shield the surface of quartz from quartz cement overgrowths, thus preserving anomalously high porosity in deeply buried sandstone reservoirs. The inhibition of the quartz cement implies that the chlorite-coats on the surface of quartz grains can significantly influence the physicochemical behavior of the quartz grains. Therefore, failure to notice the initial thin microscale coatings forming during deposition can have serious consequences for modeling several geochemical reactions occurring at liquid-solid interfaces. Despite this huge implication, the fundamental mechanisms involved in chlorite-coat formation is not well understood. Here we present an experimental study to determine the parameters that control chlorite-coat formation on the surface of quartz grains. The batch experiments were conducted in a concoction of quartz and chlorite under different conditions of ionic strength, pH, and presence of humic acid (HA), iron- (Fe) and aluminum (Al) oxides). HA, Fe- and Al oxides are suggested to aid the emplacement of chlorite-coat precursors. At pH 7, the quartz–chlorite and quartz–chlorite–Fe/Al-oxides mixing experiments performed in saline and non-saline solution result in equal chlorite-coat coverages, suggesting neither salinity nor Fe and Al-oxides explain the mechanisms of chlorite-coat formation. At pH 5 and 9, however the chlorite-coat coverage was superior only in saline solution, indicating differences in coat coverage may be caused by variable electrokinetic charge distribution due to the distribution and transport of dissolved salt. The chlorite-coat barely formed in experiments that contain HA in quartz–chlorite mixtures regardless of ionic strength and pH. Against a long-standing notion, the presence of organic matter cannot necessarily be prerequisites for binding chlorite on the surface of quartz grains. The dynamic interactions between solution chemistry and surface chemistry of solid phases (quartz, chlorite, HA, Fe and Al oxides) can result in changing the electrokinetic properties in a region near the solid phases and at mineral-solution interfaces. We therefore propose that the electrokinetic response that arises in heterogeneous systems may explain the mechanisms of chlorite-coat formation.

Simulating the effects of model parameters on stagnation points position during seawater intrusion.

Several works have been performed in order to understand seawater intrusion by simulating the Henry problem. Investigations were implemented by simulating the effect of many parameters (fractured aquifer, dispersion and diffusion, geochemical reactions, heterogeneity, anisotropy, boundary conditions) on the flow and transport. This paper focus on the concept of the "stagnation point," and this concept plays an important role in modeling, management and characterization of coastal aquifer. A total of forty-eight simulations, including the base case, were made to explore the effect of molecular diffusion coefficient, dispersivity, and seawater density on the position of the stagnation point. It was found that the increase of the molecular diffusion coefficient or the dispersivity leads to a downward displacement of the stagnation to the aquifer bottom and the lowest point position is reached for a Pe value of 0.35. For the seawater density effect, numerical results predict a nonlinear behavior of the stagnation point position, where the downward displacement is detected only for a ρs ranging from 1025 to 1045kg/m3.

CO2-Water-Rock Interactions in Carbonate Formations at the Tazhong Uplift, Tarim Basin, China

The effects of CO2-water-rock interactions on the injectivity and safety of CO2 geological storage have drawn wide attention. The geochemical reaction mechanisms in carbonate formations after CO2 injection are still controversial. To better understand the transformation of injected CO2 in carbonates and the involved geochemical reactions, we first conducted autoclave experiments reproducing the in-situ conditions of the Lianglitage Formation, Yingshan Formation, and Qiulitage Formation at the Tazhong Uplift in the Tarim Basin. We then established a batch model using TOUGHREACT-ECO2H, validated with the experimental results, to simulate the long-term CO2-water-rock interactions. It was found that the initial mineral compositions and water chemistry have important effects on the CO2-water-rock interactions in carbonate formations. The experiment results show that the dissolution of calcite and dolomite dominates in the early reaction period. However, we still observed some secondary minerals, such as ankerite, montmorillonite, calcite, and dolomite. The CO2-water-rock reactions can be more dramatic when the contents of calcite and dolomite in carbonates are closer. Moreover, the long-term simulation results show that calcite, magnesite, and hematite are the main formed secondary minerals, whereas dolomite is the major dissolved mineral. This study is helpful for a better understanding of the CO2 mineral trapping mechanism in carbonate formations.

Suitability of depleted gas fields for underground hydrogen storage in Australia

If there is a significant adoption of hydrogen in Australia as an energy carrier, it will be necessary to have storage options to buffer the fluctuations in supply and demand, both for domestic use and for export. For large-scale storage in a single location, underground hydrogen storage (UHS) is the preferred option for reasons of both cost and safety. The search for suitable sites for UHS will depend on the proximity to potential hydrogen generation, ports, and processing infrastructure, as well as CO2 storage options for blue hydrogen. Although UHS in salt caverns is an established technology, most of the suitable salt deposits in Australia (in the Canning Basin in WA, the Adavale Basin in Qld, and the Amadeus Basin in the NT) are not always well-located for production and transport. Depleted gas fields have been used previously for storage of hydrogen-rich gas mixtures as well as natural gas storage and appear to be the most promising and widely available UHS option in Australia. There appears to be sufficient storage capacity in depleted gas fields in most of the geographic areas with hydrogen production potential. However, there are still technical challenges to be addressed, such as the extent of possible contamination of the stored hydrogen with residual hydrocarbons, and the possible effects of geochemical reactions and microbial processes.

Diavik Waste Rock Project: Simulation of the geochemical evolution of a large test pile using a scaled temperature and sulfide-content dependent reactive transport model

The Diavik Waste Rock Project (DWRP) project included four principal components focused on the development of techniques for assessing the environmental impacts of waste rock at mine sites. These components were small-volume laboratory experiments, intermediate- and large-volume field experiments, and assessment of the operational-scale waste-rock stockpiles, which facilitated characterization of waste-rock weathering at different scales. The heavily instrumented large-scale field experiments (test piles) were constructed to replicate, as closely as practicable, the temperature, water flow, and gas transport regimes of a waste-rock pile that is exposed to annual freezing and thawing cycles and to facilitate characterization of the long-term weathering of a low-sulfide waste rock. An integrated conceptual model of sulfide-bearing waste-rock weathering, developed at the small scale, was applied to assess the capacity of the conceptual model to capture the geochemical evolution of the waste rock at the large field-scale test-pile experiment. The integrated conceptual model was implemented using reactive transport code MIN3P, taking into account scale-dependent mechanisms. The test-pile mineralogy was similar to the small-scale laboratory experiments and included low-sulfide waste rock with an S content of 0.053 wt% (primarily pyrrhotite). The flow regime of the test pile was simulated using parameters measured as part of other DWRP investigations, including temporally variable infiltration estimates that represented the measured precipitation events at the site. The temporally and spatially variable temperature of the test pile was interpolated from values measured using instrumentation installed at the beginning of the experiment and was included in the simulation to refine the temperature dependence of the geochemical reactions. To allow continuous, multi-year simulation, freezing was also simulated to represent the conditions experienced at the test-pile experiment. Normalized root mean square error analysis of the large-scale field experiment simulation results indicated most parameters compare well to measured daily mass flux (i.e., the fraction of the range of annual values encompassed in the residual was less than 0.5 for SO4, Fe, Ni, Si, Ca, K, Mg, Na, and pH and 1.0 or less for all parameters except Cu). The method of using an integrated conceptual model developed from the results of humidity cell experiments to implement a mechanistic approach for assessing the primary geochemical processes of waste-rock weathering on a large scale was shown to provide reasonable results; however, the results are specific to the study site and the approach requires application to various sites under different geological and climatological conditions to facilitate further refinement.

Groundwater geochemistry evolution and geogenic contaminants in the Sulaimani-Warmawa Sub-basin, Sulaimani, Kurdistan Region, Iraq.

Evolution of groundwater geochemistry in the Sulaimani-Warmawa Sub-basin in the Kurdistan Region of Iraq has been investigated using hydrogeochemical and isotopic methods. This is a semiarid region with seasonal precipitation in winter. Water chemistry generally evolves from Ca-HCO3 groundwater type close to the basin boundaries towards Ca-Mg-HCO3 groundwater type close to the Tanjero River along the axis of the basin. Some samples have increased concentrations of Na, Cl, and SO4 as a consequence of dissolution of halite and gypsum embedded in carbonates. Values of pH are slightly alkaline or alkaline, and redox parameters indicate a moderately reducing environment. Isotopes δ2H and δ18O indicate recharge from winter precipitation with no evaporation. Values of dissolved 13C(DIC) correspond to equilibrium with carbonates and C4 plants as the source of CO2. Values of 87Sr/86Sr in groundwater are in a good agreement with carbonate dissolution as a principal process. The principal geogenic contaminant is Ba with concentrations up to 0.383mg/L. Dissolved concentrations of other geogenic contaminants such as As, F, Mn, and Cr are low or below the detection limit as expected based on their low contents in carbonate rocks. Inverse geochemical modeling on selected profiles calibrated using δ13C values provided mass transfer coefficients for possible geochemical reactions. Future work should focus on interactions in the hyporheic zone of the Tanjero River.

Geochemical and physical alteration of clay-rich shales under supercritical CO2 conditions

The injection of Carbon Dioxide (CO2) into shale gas formations is a promising approach that not only reduces the impact of greenhouse gas on climate change but also enhances the gas recovery due to geochemical interactions between CO2 and clay minerals. However, these geochemical interactions have not been fully explored and changes in the petrophysical properties of shales have been reported over time. This paper evaluates the geochemical reactions and physical changes caused by the injection of supercritical carbon dioxide (ScCO2) on the properties of clay-rich shales. Samples from the Eagle Ford formation were collected and exposed to ScCO2 at different pressures ranging from 10 to 24 MPa at 70 °C. Analytical methods such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thin-section microscopy were used to characterize the ScCO2-treated samples. The results showed dissolution of clays and precipitation of quartz with increasing ScCO2 treatment pressure. The content of carbonate minerals was also reduced at high pressure, which can be attributed to the reactions of the dolomite with H+ to form magnesium carbonate. The percentage of absorption of aromatic hydrocarbons and oxygenated groups gradually increased with increasing pressure, which can be attributed to the increase in CO2 adsorption. On the other hand, the absorption of aliphatic and hydroxyl groups decreased after treatment. ScCO2 treatment pressure is an important factor to evaluate the CO2 adsorption capacity of clay-rich shales. The presented results enrich the understanding of the interactions between CO2 and shale under different pressures, which may be helpful to determine the feasibility of long-term injection and storage in shale.

Scaling CO2 convection in confined aquifers: Effects of dispersion, permeability anisotropy and geochemistry

Solubility trapping of carbon dioxide (CO2) in deep saline aquifers is considered an effective mechanism of carbon storage. The dissolution of CO2 in aquifer brine may result in gravitational instability, which triggers convective mixing (similar to the well-known Rayleigh–Bénard convection problem). The process starts with the diffusion of CO2 into the brine, followed by the onset of convection (tc), which may be followed by the constant-flux regime, F=Fc, where F is the dissolution flux. Scaling tc and Fc with characteristic measures (such as Rayleigh number as a measure of buoyancy to diffusive driving forces) is important for the characterization of CO2 injection at a large scale. Previously, natural complexities such as hydrodynamic dispersion, permeability anisotropy and geochemical interactions have been neglected in the scaling relations of convective mixing. This work explores the effects of such simplifications on the scaling relations of convective mixing using high-resolution 2D numerical simulations. Our results provide new insights from revisiting the convective mixing scaling. We show that the Sherwood number (Sh) depends on the domain height in the presence of the hydrodynamic dispersion. In line with such findings, we show that Sh-Rayleigh scaling prefactor is directly related to longitudinal dispersivity (αL). Moreover, we show that increasing the permeability in each direction is beneficial for the process by decreasing convection onset time (tc) and increasing the dissolution flux (F). Smaller permeability anisotropy ratio (defined as the ratio of vertical to horizontal permeability, ℘=Kz/Kx) results in earlier onset time and higher dissolution flux, which scales with ℘0.54 and ℘−0.21 compared to convection–diffusion, respectively. Finally, our simulation results show that earlier tc and higher F are expected due to carbonate geochemical reactions. The scaling relations of convective mixing in deep saline aquifers guide prioritization and selection of appropriate aquifers for geological CO2storage and designing efficient injection schemes.

Three-dimensional reactive transport simulation of Uranium in situ recovery: Large-scale well field applications in Shu Saryssu Bassin, Tortkuduk deposit (Kazakhstan)

Uranium in situ recovery (ISR) is the most widely used uranium mining technique worldwide. It consists of the dissolution of the ore by a mining solution, directly within the deposit. By predicting fluid flow and geochemical reactions in reservoirs, reactive transport (RT) modelling is a powerful tool to better understand and pilot ISR production. However, very few industrial uses have been reported thus far. This paper fills the gaps by illustrating a large-scale well-field application of RT modelling at one of the largest ISR mines worldwide, which is operated by Katco. This study highlights the robustness of a complex workflow based on the coupled reactive transport software HYTEC and its added value for the operator in the context of uranium ISR. The robustness demonstration is performed on 2394 wells covering 39 different production areas (blocks). The model reproduces the observed uranium concentrations and pH of pumped solutions over time scales up to 12 years. Only three parameters are manually adjusted to calibrate the model: global initial grades in clays (beidellite), calcite, and iron hydroxide (goethite). The discrepancy between simulated and observed uranium production and acid consumption decreases as the observation scale widens, showing that local errors compensate for each other. These deviations are mainly explained by the uncertainties of the 3D geological models and not by the RT simulations. Furthermore, the robustness of the model is a key asset for decision-making as it enables accurate predictions. This accuracy is illustrated through a case study of four of the simulated blocks. In 2019, after 10 years of production, the well field was redesigned to target the remaining uranium using the RT-based workflow. Several scenarios were simulated and sequentially optimised using a geometallurgical approach. The final adopted design predicted a 28% increase in uranium production and 35% in economic gains over the first two years of simulation alone (2019–2021). These theoretical gains were validated in practice as the comparison between the 2019 predictions and observations over 16 months showed a deviation less than 10% in the total uranium production, which decreased to 1.9% using the observed operational conditions, which reinforces the predictability of the workflow and validates the forecasted gains.

Subsurface multiphase reactive flow in geologic CO2 storage: Key impact factors and characterization approaches

Subsurface multiphase reactive flow in geologic CO2 storage: Key impact factors and characterization approaches

Fluid-rock interaction during low-salinity water flooding of North Sea chalks

The injection of low-salinity water into carbonate oil-reservoirs has been the subject of much interest as a potential method to enhance oil recovery (EOR). Low salinity water injection has been shown to alter the geochemical equilibrium in the reservoir, and consequentially potentially change the surface wettability of the reservoir rocks. Nevertheless, the geochemical reaction pathways during low salinity water flooding in carbonates remain poorly understood.In order to provide a more comprehensive insight into the geochemical reaction pathways, a series of batch and core-flooding experiments have been performed at 70 °C. The experiments made use of analogous and authentic reservoir material from the Danish North Sea oil reservoirs, and brines with chemical composition similar to the formation water and injection water available in offshore flooding operations.Results show that both mineral dissolution and the precipitation of new secondary phases can accurately describe the geochemical changes observed in the experimental effluents during the course of a low-salinity water core-flooding scenario.The dissolution of amorphous silica (SiO2) present in the chalk, and the precipitation of a Mg–Si clay mineral, identified here as sepiolite, are key reactions that take place during low-salinity water core-flooding in chalks. The suggested geochemical pathway was successfully fitted by a simplified geochemical simulation which can accurately describe the experimental observations.Surface reactions which take place during modified-salinity water flooding were put into perspective by quantifying their contribution to the overall effluent composition.

Subsurface Mixing Dynamics Across the Salt‐Freshwater Interface

AbstractMixing along the salt‐freshwater interface is critical for geochemical reactions, transport, and transformation of nutrients and contaminants in coastal ecosystems. However, the mechanisms and controls of mixing are not well understood. We develop an analytical model, based on the coupling between flow deformation and dispersion, which predicts the mixing dynamics along the interface for steady‐state flow in coastal aquifers. The analytical predictions are compared with the results of detailed numerical simulations, which show that nonuniform flow fields, inherent to seawater intrusion in coastal aquifer, result in a non‐monotonic evolution of mixing width and mixing rates along the interface. The analytical model accurately captures these dynamics over a range of freshwater flow rates and dispersivities. It predicts the evolution of the mixing width and mixing rates along the interface, offering a new framework for understanding and modeling mixing and reaction processes in coastal aquifers.

New approach in analysing rock property changes in vuggy carbonates

X Field is a HPHT carbonate gas field containing about 70% of CO2, and is located in offshore East Malaysia. In order for this field to be economical and environmental friendly, PETRONAS plans to reinject produced CO2 back into the reservoir. With this objective, PETRONAS Research Sdn Bhd is conducting a feasibility study on changes of reservoir rock properties due to this CO2 sequestration activity. As the available vuggy-carbonate core-plugs have a higher risk of damage, the team employed digital core analysis (DCA) technology for this study. The study consists of three phases. In the first phase, the selected core plugs were analysed using micro CT scan and image processing software. To mimic the gas reservoir, the core plugs were then saturated with synthethic brine, before undergoing centrifuge until Sw-irr from Pc data was reached. The second phase consisted of aging the core-plugs by immersion in supercritical CO2 at pressure and temperature of the reservoir for 45 days. In the third phase the core plugs were analysed via DCA again to determine changes caused by CO2 reaction in rocks. Due to the fragility of the selected samples, they the post aging DCA analysis revealed both mechanical and geochemical changes. However, the central part of the core-plugs was not mechanically damaged and could be used to analyse the changes in porosity mainly due to geochemical reaction. Our results indicate that there will be small impact of porosity changes due to CO2 storage activity in X Field. DCA has proven to be a very useful method in determining porosity changes in reservoir rocks. Not only can DCA determine porosity of broken core plugs, but it also can extract the unbroken parts of the rock for more accurate analysis.

Nitrogen biogeochemical reactions during bank filtration constrained by hydrogeochemical and isotopic evidence: A case study in a riverbank filtration site along the Second Songhua River, NE China

River eutrophication and nitrogen pollution poses a potential threat to the groundwater quality in riverbank filtration systems. The river filtration process is often accompanied by complex redox reactions. Therefore, identification of nitrogen biogeochemical processes is essential to scientifically characterize the cause of NO3− attenuation and NH4+ accumulation in groundwater, optimize the design of groundwater pumping wells, and design protection strategies in sudden environmental pollution accidents. Hydrogeochemical and stable isotope tracing techniques were employed in this study at a typical riverbank filtration site located in Songyuan, NE China, to explore the main geochemical reactions controlling the migration and transformation of nitrogen along the groundwater flow path during riverbank filtration. The results indicate that mixing, adsorption, organic nitrogen mineralization, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) represent the major geochemical reactions. Denitrification primarily occurs within 10–20 m from the riverbed surface along the filtration path and is the primary reaction accounting for NO3− attenuation, whereas DNRA typically occurs between 1.5 and 6 m from the riverbed surface along the filtration path and is more active in the wet season, characterized by high temperatures (>15–17 °C), low dissolved oxygen (DO, < 2–4 mg/L), and high carbon load i.e., organic carbon (OC):NO3− > 10–15. This work confirms that DNRA is an important nitrogen geochemical reaction during riverbank filtration, and emphasizes its role in NH4+ enrichment.

Generalizable Framework of Unpaired Domain Transfer and Deep Learning for the Processing of Real-Time Synchrotron-Based X-Ray Microcomputed Tomography Images of Complex Structures

Mitigating greenhouse gas emissions by underground carbon dioxide storage or by coupling intermittent renewable energy with underground hydrogen storage are solutions essential to the future of energy. Of particular importance to the success of underground storage is the fundamental understanding of geochemical reactions with mineralogical phases and flow behavior at the length scale at which interfaces are well resolved. Fast synchrotron-based three-dimensional x-ray microcomputed tomography (\textmu{}CT) of rocks is a widely used technique that provides real-time visualization of fluid flow and transport mechanisms. However, fast imaging results in significant noise and artifacts that complicate the extraction of quantitative data beyond the basic identification of solid and void regions. To address this issue, an image-processing workflow is introduced that begins with unpaired domain transfer by cycle-consistent adversarial network, which is used to transfer synchrotron-based \textmu{}CT images containing fast-imaging-associated noise to long-scan high-quality \textmu{}CT images that have paired ground truth labels for all phases. The second part of the workflow is multimineral segmentation of images using convolutional neural networks (CNNs). Four CNNs are trained using the transferred dynamic-style \textmu{}CT images. A quantitative assessment of physically meaningful parameters and material properties is carried out. In terms of physical accuracy, the results show a high variance for each network output, which indicates that the segmentation performance cannot be fully revealed by pixel-wise accuracy alone. Overall, the integration of unpaired domain transfer with CNN-based multimineral segmentation provides a generalizable digital material framework to study the physics of porous materials for energy-related applications, such as underground ${\mathrm{CO}}_{2}$ and ${\mathrm{H}}_{2}$ storage.