Geochemical Reactions Research Articles

Analytical solutions for the injection of wettability modifiers in carbonate reservoirs based on a reduced surface complexation model

The potential of tuned-composition waterflooding to enhance oil recovery from carbonate reservoirs has been widely investigated, where wettability alteration is the dominant mechanism. It is hypothesized that the sulfate in seawater adsorbs on the rock surface, removing acidic hydrocarbon species adsorbed thereon. Glycine, the simplest amino acid, has also been investigated as a wettability modifier for carbonates that acts similarly to sulfate.Wettability alteration by the adsorption of sulfate ion and glycine anion has been satisfactorily modeled by extending calcite's surface complexation model (SCM) based on zeta potential measurements. However, determining the relevance of the individual geochemical reactions is obscured by the complexity of the SCM. Moreover, the large number of equilibrium constants and their associated uncertainty result in many degrees of freedom when matching corefloods and spontaneous imbibition experiments.This research determines the chemical reactions in the calcite's SCM that govern wettability alteration and the incremental oil recovery by glycine-enhanced waterflooding in carbonate reservoirs. We found that the wettability alteration by SCM can be approximated by two anion exchange reactions between the wettability modifiers, sulfate and glycine anions, and the carboxylic acids adsorbed on the rock surface. Moreover, the wettability of the system described by the adsorption of carboxylic acids and the incremental oil recovery by waterflooding are linearly correlated with the K value of the reaction of carboxylic acids with the carbonate surface and the concentration of carboxylic acids in the system, which is a function of the total acid number.We present analytical solutions for the two-phase, multicomponent reactive-transport model coupled with anion exchange reactions. To the best of our knowledge, this is the first time the presented analytical solutions are applied to describe enhanced waterflooding in carbonate reservoirs where more than one wettability modifier is injected. The analytical solutions showed that glycine injection enhances oil recovery over waterflooding by the formation of two wettability alteration waves. The number of waves and their types (rarefaction or shock) depend on glycine's and sulfate's concentration at the injection. The improvement in oil recovery by wettability alteration mainly occurs because the more favorable mobility ratio increases the displacement efficiency with no reduction in residual oil saturation. A synergy was found between glycine and sulfate, where the presence of glycine promotes the formation of a wettability-alteration shock by sulfate that improves the displacement efficiency.

New Insights into Hybrid Low-Salinity Polymer Flooding through a Coupled Geochemical-Based Modeling Approach

Summary Low-salinity polymer (LSP) flooding is a synergic emergent enhanced oil recovery (EOR) technique. Previous laboratory experiments showed noticeable improvements in displacement efficiency, polymer rheology, injectivity, and viscoelasticity. Nevertheless, when it comes to modeling LSP flooding, it is still challenging to develop a mechanistic predictive model that captures polymer-brine-rock (PBR) interactions. Therefore, this study uses a coupled MATLAB reservoir simulation toolbox (MRST)-IPhreeqc simulator to investigate the effect of water chemistry on PBR interactions during LSP flooding through varying overall salinity and the concentrations of divalent and monovalent ions. For describing the related geochemistry, the presence of polymer in the aqueous phase was considered by introducing novel solution species (Poly) to the Phreeqc database. The developed model’s parameters were validated and history matched with experimental data reported in the literature. Moreover, different injection schemes were analyzed, including low-salinity (LS) water, LSP injection (1 × LSP), and 5-times spiked LSP injection (5 × LSP) with their related effects on polymer viscosity. Results showed that polymer viscosity during LSP flooding is affected directly by Ca2+ and Mg2+ and indirectly by SO42− owing to PBR interactions on a dolomite rock-forming mineral. Monovalent ions (viz. Na+ and K+) have minor effects on polymer viscosity. Ca2+ and Mg2+ ions discharged from dolomite dissolution create polymer complexes (acrylic acid, C3H4O2) to reduce polymer viscosity significantly. The increased SO42− concentration in the injected LSP solution affects the interactions between the polymer and positively charged aqueous species, leading to minimized polymer viscosity loss. For LSP flood derisking measures, the cation’s effect was related to the charge ratio (CR). Thus, it is key to obtain an optimal CR where viscosity loss is minimal. This paper is among the few to detail the mechanistic geochemical modeling of the LSP flooding technique. The validated MRST-IPhreeqc simulator evaluates the previously overlooked effects of water chemistry on polymer viscosity during the LSP process. Using this coupled simulator, several other geochemical reactions and parameters can be assessed, including rock and injected-water compositions, injection schemes, and other polymer characteristics.

Determining Reaction Paths by Evaluating Kinetic Isotopic Effects with Density Functional Theory: Example of Methane Thermogenesis

This study demonstrates the determination of reaction pathways by evaluating the carbon kinetic isotopic effect and interpreting isotopic fractionations based on quantum chemical calculations. The reaction under investigation is methane thermogenesis from the decomposition of kerogen, which is a geochemical reaction under temperatures below 150 °C and lasts for tens of millions of years. Investigating its mechanism requires theoretical simulations because lab experiments at practical time-lengths require elevated temperatures, which introduce unwanted side reactions. Density functional theory and kinetic simulations were conducted on isotopic fractionations with the use of two possible pathways (free-radical versus carbonium), and the results were compared to field data sets. Different molecular sizes of kerogen were investigated to account for the hindrance of translation and rotation in modeling a reactant in the solid phase. Both pathways have low reaction barriers, implying that the reaction rates are limited by the concentration of active species (hydrated protons and free radicals). The results support the carbonium pathway and rule out the free-radical pathway as the 13CH4 from the latter would be 30‰ more depleted than the observed data. Additionally, simulations were conducted on hydrocarbon isotope fractionation of the carbonium pathway with consideration of hydrogen exchange between methane and water, successively reproducing the observed abundances of deuterium-containing isotopologues (13CH3D, 13CH3D, and 12CH2D2).

Experimental investigation of shale/hydrogen geochemical interactions

Underground Hydrogen Storage (UHS) is an effective technique that provides safe storage and recovery of the stored hydrogen for long-term energy storage. Shale caprocks are regarded as effective seals of the stored gas, however, one of the key challenges is the in-situ geochemical shale/hydrogen reactions that can impact the seal integrity during injection/production of hydrogen. To address this issue, this work investigates the geochemical reactions between hydrogen and shales, and evaluates the possible alterations in shales’ petro-physical properties. Calcite-rich shale samples are collected from EagleFord (EFS) and Wolfcamp (WCS) shale formations and exposed to pure hydrogen for 108 days under 9.65 MPa at 348 K. Analytical methods including X-ray diffraction, X-ray fluorescence and Scanning Electron Microscope analysis are performed. The results indicate calcite precipitation after hydrogen treatment in both samples due to the presence of organic contents. Sulfate and pyrite reduction reactions with hydrogen are not observed as hydrogen did not all react with sulfate minerals. Consequently, minor changes in the surface pore structure and pore spaces are observed after the treatment, confirming the low reactivity of calcite-rich shale rocks with hydrogen. Gas chromatography analysis shows neither considerable hydrogen loss nor the generation of other gases such as CH4 or CO2 occurred during the experiments. The obtained results suggest that the integrity of shale-caprocks are expected to remain strong during the injection and production of hydrogen, which is favorable for UHS. This work can be a suitable experimental framework to assist in evaluating the hydrogen reactivity to different types of shales with various mineralogy, and assessing the suitability of shale rock for UHS.

Role of geochemical reactions on caprock integrity during underground hydrogen storage

Underground hydrogen storage in depleted gas reservoirs is a promising and economical option for large-scale renewable energy storage to achieve net-zero carbon emission. While caprock plays an important role in sealing capacity, current knowledge is still limited on the effect of H2-brine-rock geochemical interactions on caprock integrity, raising concerns about the viability of long-term UHS. To address this problem, we developed kinetic batch models to characterize the time-dependent redox-reactions which are unique for underground hydrogen storage. This is combined with analytical estimates for the extent of hydrogen penetration into caprock. Our results show that the dissolution degrees of all tested minerals in three types of shales are <1 % in 30 years, indicating a strong caprock integrity and containment ability during underground hydrogen storage from a geochemical perspective. Reactive transport calculations indicate that hydrogen only affects a few metres of the caprock above the reservoir, so that storage integrity of thick caprocks will be unaffected. Similarly, the overall amount of hydrogen penetrating into caprock is likely to be a tiny fraction of the amount stored, typically much <1 %. Overall, our results suggest that H2-brine-shale geochemical interactions may not compromise caprock integrity during underground hydrogen storage.

Assessing and modeling hydrogen reactivity in underground hydrogen storage: A review and models simulating the Lobodice town gas storage

Underground Hydrogen storage (UHS) is a promising technology for safe storage of large quantities of hydrogen, in daily to seasonal cycles depending on the consumption requirements. The development of UHS requires anticipating hydrogen behavior to prevent any unexpected economic or environmental impact. An open question is the hydrogen reactivity in underground porous media storages. Indeed, there is no consensus on the effects or lack of geochemical reactions in UHS operations because of the strong coupling with the activity of microbes using hydrogen as electron donor during anaerobic reduction reactions. In this work, we apply different geochemical models to abiotic conditions or including the catalytic effect of bacterial activity in methanogenesis, acetogenesis and sulfate-reduction reactions. The models are applied to Lobodice town gas storage (Czech Republic), where a conversion of hydrogen to methane was measured during seasonal gas storage. Under abiotic conditions, no reaction is simulated. When the classical thermodynamic approach for aqueous redox reactions is applied, the simulated reactivity of hydrogen is too high. The proper way to simulate hydrogen reactivity must include a description of the kinetics of the aqueous redox reactions. Two models are applied to simulate the reactions of hydrogen observed at Lobodice gas storage. One modeling the microbial activity by applying energy threshold limitations and another where microbial activity follows a Monod-type rate law. After successfully calibrating the bio-geochemical models for hydrogen reactivity on existing gas storage data and constraining the conditions where microbial activity will inhibit or enhance hydrogen reactivity, we now have a higher confidence in assessing the hydrogen reactivity in future UHS in aquifers or depleted reservoirs.

Evaluating the hydrogen storage potential of shut down oil and gas fields along the Norwegian continental shelf

The underground hydrogen storage (UHS) capacities of shut down oil and gas (O&G) fields along the Norwegian continental shelf (NCS) are evaluated based on the publicly available geological and hydrocarbon production data. Thermodynamic equilibrium and geochemical models are used to describe contamination of hydrogen, loss of hydrogen and changes in the mineralogy. The contamination spectrum of black oil fields and retrograde gas fields are remarkably similar. Geochemical models suggest limited reactive mineral phases and meter-scale hydrogen diffusion into the caprock. However, geochemical reactions between residual oil, reservoir brine, host rock and hydrogen are not yet studied in detail. For 23 shut down O&G fields, a theoretical maximum UHS capacity of ca. 642 TWh is estimated. We conclude with Frigg, Nordost Frigg, and Odin as the best-suited shut down fields for UHS, having a maximum UHS capacity of ca. 414 TWh. The estimates require verification by site-specific dynamic reservoir models.

In situ/operando XAFS investigation of the sorption/precipitation of Zn(II) on palygorskite surface at the molecular scale: Implications for Zn stable isotope fractionation

In aqueous geological environments, the fate and transport of Zn are controlled by geochemical reactions (e.g., sorption) occurring at the mineral/water interface. However, many of the underlying Zn fixation mechanisms, especially the kinetics of surface-induced precipitation, are still unclear. Additionally, Zn stable isotope fractionation during sorption has recently been shown to be an important process, but the relationship between Zn isotope fractionation and sorption mechanisms (e.g., surface complexation, precipitation) is also not well understood. To address these issues, we employed X-ray absorption fine structure (XAFS) spectroscopy, high-resolution transmission electron microscopy (HRTEM), and in situ/operando quick-scanning XAS (QXAS) spectroscopy to elucidate the sorption/precipitation mechanisms of Zn at palygorskite/solution interfaces. We also measured Zn isotope ratios during some sorption experiments to illustrate how Zn isotope fractionation behavior is affected by the evolution of coordination environments at different pH values, initial concentrations, and reaction times. Our results demonstrate that Zn sorption mechanisms vary as a function of pH, ionic strength, initial concentration, and reaction time. At low pH (pH < 7.0) and low ionic strength (I ≤ 0.01 M), Zn(II) predominantly forms an outer-sphere surface complex in octahedral coordination. At low pH (pH < 7.0) and high ionic strength (I = 0.1 M), Zn(II) is predominantly sorbed as an inner-sphere octahedral surface complex with insignificant isotopic fractionation (Δ66Znsorbed-aqueous = −0.02 ± 0.05‰). At high pH and high initial Zn concentration (pH 7.5, C0 ≥ 0.2 mM), the formation of octahedral Zn phyllosilicate precipitates is observed, yielding a small fractionation with an average Δ66Znsorbed-aqueous of 0.10 ± 0.07‰. In addition, in situ/operando QXAS of Zn sorption at pH 7.5 in a flow cell revealed that the predominant sorbed Zn species shifted from inner-sphere tetrahedral complexes during the initial stage (e.g., within minutes) to inner-sphere octahedral complexes during later stages (e.g., within hours), followed by the formation of Zn-phyllosilicate precipitates (e.g., within days). This molecular evidence coincides with the evolution of Zn isotope fractionation from a large Δ66Znsorbed-aqueous value (0.53 ± 0.05‰) to a small value of 0.05 ± 0.04‰ during the sorption process. The combination of QXAS and isotope fractionation results reveals a change in the Zn local environment from tetrahedral coordination to octahedral coordination. The findings in this study not only provide new insight into surface adsorption/precipitation mechanisms but also demonstrate that stable isotope fractionation is linked with the local molecular/bonding environments at mineral–water interfaces.

In Situ Recovery of Copper: Identifying Mineralogical Controls via Model-Based Analysis of Multistage Column Leach Experiments

In situ recovery (ISR) has for several decades been considered an alternative mining method. It can substantially reduce the scarring of landscapes and eliminate tailings, dams, and waste rock while potentially minimizing energy consumption and carbon emissions. However, ISR applications beyond those for uranium have been limited. One key factor that limits the widespread application of the ISR approach is that many metals are largely hosted by orebodies with low hydraulic conductivity and/or a more pronounced hydrogeological and mineralogical heterogeneity. In those cases, the viability of ISR relies on a thorough understanding of the coupled transport and geochemical reaction processes and, more specifically, which subset of processes and parameters controls metal recovery. Laboratory experiments represent an essential first step toward developing this knowledge. Here, we describe a series of column experiments in which copper ore was leached during multistage experiments. Constrained by the collected data, we developed a reactive transport model and applied it to identify the main controls for copper recovery in the studied setup. The experiments showed copper recoveries above 60%, with a strong mineralogical control exerted by the composition of the copper mineral assemblage and a dependency of the copper recovery rates on the supplied lixiviant, i.e., the specific combination of acid and oxidant concentrations. The developed modeling framework was capable of accurately reproducing those observations and will provide a sound basis for optimizing copper recovery and establishing the reagent consumption.

Evolution of Mineral-Accessible Surface Area Induced by Geochemical Reactions

Mineral dissolution and precipitation reactions occur in a wide range of porous media systems, driven by deviations from an equilibrium between solid and fluid phases. Reactions occur at mineral surfaces in contact with reactive fluids or accessible mineral surface areas. These mineral surface areas can be determined using a multiscale imaging approach and have been shown to improve the simulation of mineral reaction rates in porous media compared to other estimates of reactive surface area. As reactions progress, mineral surface area evolves, and reactive transport simulations often use a simplified model assuming spherical grains to estimate mineral surface area evolution. This, however, does not depict the evolution of reactive surfaces in porous media systems with varying mineral accessibility. This work aims to quantitatively assess the evolution of accessible mineral surface area in porous media for a multimineralic system undergoing mineral dissolution reactions induced by acid exposure. Before and after the reaction, accessible mineral surface areas are determined from 2D scanning electron microscopy, and 3D X-ray computed tomography imaging of a sandstone sample. The quantified evolution of accessible surface area is compared to the calculated mineral surface area evolution using current approaches. Results show an overall increase in total surface area due to the reaction; however, individual mineral surface areas may increase or decrease. Variation is observed in mineral surface area values measured from imaging and equations used in models. The evolution of mineral surface area is largely impacted by the total surface area as well as the pore connectivity rather than porosity and volume fraction evolution.

Coupled H, H2, OH, and H2O lunar exosphere simulation framework and impacts of conversion reactions

Numerical models of the lunar exosphere have been available for several decades to allow investigations of the Moon’s tenuous atmosphere beyond the few measurements made by satellite or ground-based experiments. We present a novel approach to modeling a system of elements, namely the hydrogen-bearing species H, H2, OH, and H2O, in a multi-element simulation, implementing exospheric and geochemical conversion reactions to connect the different species through their loss and source processes. The model resembles the lunar water cycle, including several source and sink mechanisms like the solar wind proton influx and the eventual loss through Jean’s escape, photodissociation, or cold trapping. Unknown and uncertain parameters have been modeled using probability distributions that propagate their uncertainty through the system to allow subsequent statistical analysis of the predictions. In eight studies we varied the reactivity of the lunar environment and investigated the response of the exosphere’s densities and dynamics. The results show that the new framework is able to reproduce many published values for the surface number densities and reported dynamics like the H2 dawn-dusk asymmetry, even with a very simple approach to many of the complex input parameters. The most diverging results can be found for the hydroxyl and water predictions. Nonetheless, we show that the multi-element approach using conversion reactions offers an alternative explanation to the observed H and H2 exosphere without having to assume these volatiles as condensable species.

Impacts of hydrogeochemical processes and land use practices on groundwater quality of Shwan sub-Basin, Kirkuk, northern Iraq

Shwan sub-Basin is one of the substantial groundwater sources in northern Iraq. Along with an increase in population, agricultural and industrial activities synced with the change in climate conditions, all could have a negative impact on the hydrochemistry of groundwater. Therefore, it becomes crucial to investigate the different processes that could affect hydrochemistry and water quality. Thirty-two groundwater samples were collected from wells distributed in the study area, and one surface water sample from Lesser Zab River, all water samples were gathered during two seasons. Hydrogeochemical model was performed on physiochemical analysis results by using PHREEQC software to understand the geochemical reactions occurring in groundwater. The results of the Saturated Index showed supersaturated values for calcite, aragonite and dolomite in groundwater samples during the first season in a percent of 84%. While the second season samples were supersaturated in percent of 40.6%, 37.5% and 46.8% for aragonite, calcite and dolomite minerals respectively. The Saturated Index shows supersaturated values of quartz mineral in most groundwater samples, which are sourced from the abundance of silicate minerals that are primarily included within the ambient rock materials of the tertiary and quaternary clastic aquifer system in the study region. The saturated index showed undersaturated values with most minerals of feldspar, halide and sulfate. However, these minerals were in a dissolution state, releasing significant amounts of Ca2+, Na+, Mg2+, HCO3−, Cl⁻ and SO42− ions into the solution. Most of the groundwater samples were classified as earth-alkaline water with an increased portion of alkali with prevailing bicarbonate for two seasons, except the groundwater sample W2 was classified as earth-alkaline water with an increased portion of alkali with prevailing SO42⁻ and Cl⁻. The water quality for human drinking was evaluated using the water quality index (WQI). The values of WQI were from 51.9 to 99.2 and from 53.9 to 88.5 for the first and the second seasons respectively. WQI revealed that most of the samples were classified as poor to very poor water quality, except the Lesser Zab River sample for the second season was good water quality and the sample W2 for the first season was unsuitable for drinking purposes.

Moving towards sustainable mine waste rock management through automated mineral and geochemical analysis

Mining activities generate vast amounts of waste rock (WR); if this waste is sulphide- and sulphosalt-bearing, it can pose a risk to ecosystems due to contaminated mine drainage. Strategic long-term planning is necessary to prevent and mitigate adverse environmental impacts. This study includes a detailed characterization of WR from a mine in northern Canada and defines the diameter of physical locking of sulphides and sulphosalts (DPLS) that can be used to separate the WR into two fractions according to the geochemical reactivity. An automated mineralogy system (QEMSCAN®) and computed tomography (CT) were used to quantify the liberation degrees of acid-generating (sulphides) and neutralizing minerals (carbonates). Chemical and mineralogical analysis indicated that sulphides, mostly pyrite (FeS2) and gersdorffite (NiAsS) (dominant sulphides), were enriched in the fine- to mid-sized fractions compared to the coarse fractions. These minerals were more liberated within the fine fractions and their liberation degree was considered negligible at sizes >2.5 mm. Consequently, 2.5 mm was defined as the critical diameter of sulphide reactivity (DPLS) for the studied WR. Furthermore, kinetic tests using humidity cells were conducted to confirm this result and to assess the geochemical behaviour of the total fraction (< 10 mm), the coarse fraction (> 2.5 mm) and the fine fraction (< 2.5 mm). Sulphides associated with the coarse fraction were not significantly exposed to weathering conditions; sulphides were associated with non-sulphide gangue minerals, which was confirmed by QEMSCAN® and computed tomography. Moreover, mineral weathering occurred primarily in the fine fraction (< 2.5 mm) due to the higher available exposed surface area of the minerals. The geochemical analysis of humidity cells leachate revealed that the pH values remained within the range of 7.07 and 8.61, and that the instantaneous metal concentrations (Fe, Cu, and Ni) were consistently below environmental limits during the154- day test period. However, As concentrations from fine and total fractions exceeded environmental standards. Sample reactivity showed that the fine fraction controlled the geochemical behaviour of the studied WR, where the fine fraction was 1.5 times more reactive than the total fraction and 10 times more reactive than the coarse fraction which represents about 60 wt% of the total sample. Consequently, screening waste rock in accordance with the DPLS could prove to be an effective method for managing WR to prevent contaminated drainage. This approach may allow a reduction in the amount of WR to be stored in surface piles (which need to be managed) and will reduce the costs associated with the reclamation of waste rock piles.

Response of redox zoning and microbial community structure in riverbed sediments to the riverbed scouring during bank filtration

Riverbed sediments are an important zone and interface for the interaction of surface water and groundwater; hence, biochemical processes in the riverbed sediments are critical to maintain the ecology of the system and water quality. Riverbed sediment scouring can substantially alter the hydrodynamic conditions of river water infiltration and redox conditions at the surface–groundwater interface, which in turn affects the biochemical processes. Currently, in-depth understanding of the distribution characteristics of the microbial community structure in riverbed sediments, especially their response to riverbed scouring, is lacking. This study revealed the temporal and spatial changes in redox indicators in the Liaohe riverbed sediment and discussed the response of the microbial community to the influence of riverbed scouring. The results showed that during the investigation period (September to November 2020), the riverbed sediments in the river reach were generally in a state of scouring in the low water stage, and the scour depth was 4–9 cm. The redox zone in the nearshore area moved up by 5.5–23 cm, and moved down by 0.1–10 cm in the intermediate transition area and the center of riverbed, and the maximum overlapping depth of the redox zone could reach 50 cm. The relative abundance of dominant bacteria species had a good correlation with the intensity of geochemical reactions, decreasing in low-water levels close to the center of the riverbed, which further indicated the weakening of the reduction rate. Influenced by riverbed scouring, the species of denitrifying bacteria in the surface layer increased, multiple dominant bacteria of iron, manganese and sulfate often coexist in deep riverbed sediments, and were slightly different in the low water level. The results provide a better understanding of the chemical composition of the pore water in the riverbed sediments, which are of great significance for maintaining the ecological safety of river water.

A new surfactant wettability alteration model for reservoir simulators

AbstractSurfactants enhance oil recovery in naturally‐fractured oil‐wet rocks by wettability alteration and interfacial tension reduction. The oil‐wet state is ascribed to the adsorption of soap on the rock surface. Soaps are the dissociated forms of carboxylic acids in the crude oil, that is, carboxylate surfactants. This paper describes a new mechanistic surfactant wettability alteration model that was developed for and implemented in a reservoir simulator. The model captures the geochemical reactions of acid/soap, the formation of mixed micelles, Henry's law adsorption, and the formation of cationic surfactant‐anionic soap ion‐pairs. A new wettability scaling factor is used to interpolate between the oil‐wet and water‐wet relative permeability and capillary pressure curves. The new model also accounts for the effect of salinity and pH, so it should also be useful for modeling low‐salinity flooding without surfactant. Previous surfactant wettability alteration models ignored the underlying mechanisms and were not predictive. Simulations of both static and dynamic imbibition were performed to better understand the key surfactant parameters and the dynamics of wettability alteration, microemulsion phase behavior, and interfacial tension reduction on oil recovery. Optimizing surfactant formulations for wettability alteration is discussed.

Comparison of water-caprock and CO2-water-caprock geochemical reactivity in the Triassic Heshanggou Formation of Ordos Basin

This study investigates the influence of geochemical reactions among CO2, water and key minerals on caprock sealing using laboratory experiments and numerical simulations, based on mudstone caprock of the Triassic Heshanggou Formation at the Shenhua CCS demonstration project in Ordos Basin. The results show that there is an obvious dissolution occurring in caprock after long-term water-rock interactions without CO2 injection, with a maximum increase of 11% in caprock permeability. The dissolution of kaolinite, chlorite, K-feldspar and quartz results in the increase of caprock permeability, and the precipitation of illite and hematite is the key factor effectively slowing down the change of permeability. However, the self-sealing capability of the caprock is continuously enhanced under the condition of CO2 injection, with permeability reduced by 40% of the original permeability at maximum. The precipitation of carbonate minerals directly induces the change of caprock permeability, and the precipitation of Ca-smectite and quartz further promotes the caprock sealing. Dolomite, siderite, magnesite and dawsonite are the key CO2 trapping minerals for the enhancement of caprock sealing, and the maximum amount of CO2 trapped in mineral phase is 35.44 kg/m3 medium. The dissolution of albite, chlorite and calcite provides the necessary Na+, Mg2+, Ca2+ and Fe2+ for the precipitation of carbonate minerals, which is conducive to the enhancement of caprock sealing, while chlorite and calcite are the key minerals affecting the caprock sealing. The dissolution of K-feldspar is not conducive to the enhancement of caprock sealing. This study could provide a theoretical basis for selection of reservoir-caprock and safety evaluation of CO2 geological storage.

Estimation of Mineral Accessible Surface Area from Mineral Abundance and Clay Content

The mineral reactive surface area is often quantified through a wide range of approaches (e.g., Brunauer–Emmett–Teller adsorption, geometry approximation, and imaging techniques). As such, values vary 1–5 orders of magnitude which can result in large discrepancies when used in reactive transport models to simulate geochemical reaction rates. Simulations carried out using mineral accessible surface areas (ASAs) determined from a coupled 2D and 3D imaging approach have shown better match with reaction rates measured in core-flood experiments. However, such image processing requires large amounts of time and resources. In this work, the possibility of estimating mineral ASAs from easily measured properties like mineral abundance and porosity is explored. Six sandstone samples of varying compositions were studied along with data from three additional samples from the previous literature. Mineral ASAs were quantified using a combined 2D scanning electron microscopy and 3D X-ray nano-computed tomography imaging approach. Sample properties like mineral accessibility, mineral ASAs, connected porosity, and clay content were compared to explore potential correlations between properties. Overall, it was observed that mineral accessibility can be predicted where feldspar mineral accessibility generally increases with increasing abundance and quartz accessibility decreases with increasing clay content. Mineral ASAs vary between samples, depending on the relative abundance of minerals and overall pore connectivity. While the ASA of quartz decreases with abundance, albite and carbonate mineral ASAs increase with abundance. Quantitative observations, including predictive relationships for ASAs from porosity and mineral volume fraction, are developed. Estimations of ASAs and mineral accessibility from more easily quantifiable properties can largely reduce the required extent of image analysis.

Geochemical reactions altering the mineralogical and multiscale pore characteristics of uranium-bearing reservoirs during CO2 + O2in situ leaching

CO2 + O2in situ leaching has been extensively applied in uranium recovery in sandstone-type uranium deposits of China. The geochemical processes impact and constrain the leaching reaction and leaching solution migration; thus, it is necessary to study the CO2 + O2–water–rock geochemical reaction process and its influence on the physical properties of uranium-bearing reservoirs. In this work, a CO2 + O2–water–rock geochemical reaction simulation experiment was carried out, and the mineralogical and multiscale pore characteristics of typical samples before and after this simulation experiment were compared by X-ray diffraction and high-pressure mercury intrusion porosimetry (HPMIP). The results show that the CO2 + O2–water–rock geochemical reaction has complicated effects on the mineral compositions due to the various reaction modes and types. After the CO2 + O2–water–rock geochemical reaction, the femic minerals decrease and the clay minerals in the coarse sandstone, medium sandstone, fine sandstone, and siltstone increase, while the femic minerals and clay minerals in sandy mudstone show a contrary changing trend. The CO2 + O2–water–rock geochemical reaction decreases the total pore volume of uranium-bearing reservoirs and then promotes pore transformation from small scale to large scale. The fractal dimensions of macropores are decreased, and the fractal dimensions of mesopores, transition pores, and micropores are increased. The effects of felsic mineral and carbonate dissolution, secondary mineral precipitate, clay mineral swelling, and mineral particle migration are simultaneously present in the CO2 + O2in situ leaching process, which exhibit the positive transformation and the negative transformation for the uranium-bearing reservoirs. The mineral dissolution may improve reservoir permeability to a certain degree, while the siltation effect will gradually reveal with the extension of CO2 + O2in situ leaching. This research will provide a deep understanding of the physical property response of uranium-bearing reservoirs during CO2 + O2in situ leaching and indicate the direction for the efficient recovery of uranium resources.

Wisdom and Geology, the North German Basin, and the Significance of Thrown Dices

For thousands of years, mankind is observing the surrounding nature. Often, they found no obvious clues for inexplicable and complex facts, leading to the belief that their wisdom was limited. This is in the majority of cases still true today, but based on hundreds of years of (geo-) scientific work some older thoughts can now be readjusted by combining newer geological, environmental, historical and philosophical clues. Facts about the development of the North German Basin are used to demonstrate the variability of geological systems and how these can be described by taking dice as a metaphor for ruling geological parameters. This includes all kinds of plate tectonically controlled basin forming processes, especially metamorphism of the lower crust due to a fixed mantle plume, basin filling processes with their galactic and lunar overprints, basin modifying tectonics due to internal (halokinesis, inversion) or external forces (one-sided loads at the surface due to mighty Delta sediments or glacial ice sheets) and geochemical reactions as a result of pressure and temperature changes in course of subsidence. Especially, the Rotliegend (Lower Permian) Gas Play is one of the possible illustrations of the entity of the North German Basin with its more than 70 - 90 independent parameters belonging to a global set of very complex hydrocarbon systems. Processes on Earth like the formation of systems of hydrocarbon fields as well as environmental systems (e.g. river systems, lakes, islands, sedimentary basins) are subordinated to the dices of nature and are steered invisibly by a selection of rules of the game that one understands as natural laws. The facts and remaining uncertainties as well as problems with subsurface-related processes (e.g. manmade tectonics, subsidence and uplift) guide the thoughts of engaged individuals on how to proceed wisely with limited predictability of challenges and dangers of a subsurface system. This work will be a trial to associate once more the natural sciences (geology) and the humanities (philosophy) for the benefit of both.

A Mechanistic Study of Wettability Alterations in Sandstone by Low Salinity Water Injection (LSWI) and CO2 Low Salinity Water-Alternating-Gas (WAG) Injection

Low salinity water injection (LSWI), an emerging Enhanced Oil Recovery (EOR) method, has proven to be effective in increasing oil recovery by wettability alteration. As low salinity water is injected into the reservoir, the pre-established equilibrium is disturbed. The chemical reactions among the oil/brine/rock system alters the existing wettability, resulting in enhanced oil recovery. Water-alternating-gas (WAG) injection is also a leading EOR flooding process in light to medium oil sandstone and carbonate reservoirs. A recently proposed hybrid EOR method, CO2 low salinity (LS) WAG injection, shows promise based on experimental and simulation studies, compared to LSWI or CO2 injection alone. Wettability alteration is considered as the dominant mechanism for CO2 LSWAG injection. In this study, a new displacement contact angle measurement which better mimics the actual displacement process taking place in a reservoir is used, aiming to investigate the effect of monovalent and divalent cations, CO2, and injection schemes. It is found that the injection of NaCl low salinity water alters the wettability towards slightly water-wet, and the injection of CaCl2 low salinity water alters the wettability towards slightly oil-wet. The injection of CO2 promotes water-wetness and geochemical reactions between oil and brine. Injection scheme of CO2 and NaCl low salinity water is more efficient than WAG cycle of CO2/NaCl in wettability alteration towards more water-wet. However, the opposite trend is observed with CaCl2 low salinity water, of which WAG cycle of CO2/CaCl2 is more efficient in altering wettability towards water-wet. The oil drop deformation process during LSWI resembles the process of oil removal using surfactant. As CO2 is introduced, due to the acidic effect of CO2 and ion exchange, it acts to wet the rock surface, leading to a more water-wet state. With introduction of CO2, the oil drop deformation resembles the “roll-up” oil removal process.