Dissolution Of CO2 In Water Research Articles

New algorithm of three-phase equilibrium calculations for CO2-hydrocarbon-water systems

In this century, global warming caused by the accumulation of atmospheric carbon dioxide has become an essential concern. The industry has two main decarbonization options: CO2 capture for permanent storage in geological formations and CO2 utilization for enhancing oil recovery (EOR). Both methods require accurate reservoir simulation to correctly describe the fluids flowing process and design an optimal injection plan. However, existing reservoir simulation practices often neglect the interaction and dissolution of CO2 and hydrocarbon components in water, leading to inaccurate predictions. Neglecting CO2 dissolution in water undermines storage estimation for capture and results in unreliable EOR descriptions. Therefore, to address this, our study focuses on developing a reliable and accurate phase equilibrium calculation package for hydrocarbon-CO2-water three-phase systems, which meets the demanding efficiency and robustness criteria of reservoir simulation. In this paper, we first introduce the methodology of the three-phase equilibrium calculation algorithm, then illustrate the numerical techniques employed to improve computational efficiency and finally demonstrate superior robustness of this algorithm through comprehensive case studies. Our research contributes three key improvements. First, we have increased the reliability of phase behavior descriptions by considering the dissolution of CO2 and hydrocarbon components in water. Second, we have enhanced the robustness of the three-phase equilibrium calculations algorithm, enabling accurate determination of three-phase statuses and fluid behaviors where standard commercial software may fail. Third, our algorithm exhibits notable efficiency improvements, ensuring its suitability for three-phase compositional simulations. The findings of this research provide valuable insights into the accurate modeling of hydrocarbon-CO2-water three-phase systems and offer practical solutions for designing effective CO2 reduction strategies.

Numerical modelling of CO2-in-water emulsion injection into a water-saturated core

Depressurization method shows promise for methane hydrate (MH) reservoir production, but the reformation of hydrates due to a lack of geothermal heat presents a significant challenge for commercial production. This study proposes a novel approach for MH gas recovery by injecting a CO2-in-water (C/W) emulsion into an MH reservoir aquifer. The exothermic nature of CO2 hydrate formation and liquid CO2 dissolution in water in this method provides heat to the MH layer, effectively preventing hydrate reformation. Subsequently, 1-D numerical models were developed and implemented using the MATLAB Reservoir Simulation Toolbox (MRST). These models were rigorously validated against experimental data under homogeneous conditions and varying injection schemes. The investigation indicated that the average volume fraction of CO2 hydrate within the hydrate particles was found to be 55% of the volume of liquid CO2 droplets. The alternating C/W emulsion injection increased the driving force for CO2 dissolution in water, thereby promoting subsequent hydrate particle decomposition and CO2 hydrate dissolution in water, which effectively mitigated blockage within the porous medium. In conclusion, water alternating C/W emulsion injection, along with the continuous injection of a low volume fraction of liquid CO2 within the C/W emulsion, exhibits promising potential for sustained injection and provides adequate heat to the MH layer, preventing hydrate reformation. Furthermore, the developed numerical model reasonably predicts the behavior of various injection schemes. This developed model can be used for devising long term sub-seabed carbon capture and storage development plan.

Mineralization reaction between CO2 and coal ash produced from underground coal gasification: Implication for in situ carbon sequestration

Underground coal gasification (UCG) represents a promising clean and low-carbon energy technology, which could create a suitable cavity for in situ carbon sequestration by mineralization reaction between CO2 and coal ash produced from UCG. This may assist in achieving a net-zero carbon emission goal for UCG projects. To determine the carbonation efficiency and mineralization potential of CO2 mineral sequestration in abandoned UCG cavity, coal ash produced from the gasification of three different low-rank coal samples was used in this study. A comparative experiment was conducted using a group of high-calcium fly ash from a coal-fired power plant. All mineralization reaction experiments were performed in a high-temperature and high-pressure reactor, with CO2 pressures ranging from 2 to 8 MPa and temperatures ranging from 40 to 80°C. These conditions were selected to simulate the sequestration conditions in the UCG cavity. Real-time monitoring of CO2 pressure in the reactor was used to assess the impact of solid-to-water ratio, temperature, and pressure on the CO2 mineralization. X-ray diffraction and scanning electron microscopy were employed to identify the presence of newly formed carbonate minerals associated with mineralization reaction. The results indicated that the coal ash produced by gasification pretreatment at 900°C contains a substantial quantity of alkaline-earth oxides, such as calcium oxide (CaO), with a relative mass fraction reaching 25.9%. It provided an adequate supply of alkali metal ions for the mineralization reaction, which exhibited comparable efficacy in mineralizing CO2 to that observed in high-calcium fly ash. Without stirring, at lower solid-to-water ratio, the maximum amount of CO2 consumed through mineralization was increased by up to 74.12%. Carbonation efficiency decreased with rising temperatures below 70°C due to the limited dissolution of CO2 in water, it increased with elevated pressure but decreased in the supercritical state, potentially attributable to the dissolution of carbonate minerals in a supercritical CO2 environment. The process of mineralization reaction between CO2 and coal ash aligned well with the Surface Coverage Model (R2 values ranging from 0.92 to 0.99). A preliminary estimation based on Chinese coal reserves revealed that the potential of CO2 sequestration capacity in UCG cavities by mineralization ranging from 29 to 102 Gt, an additional 517 Gt CO2 may be stored when filled with fly ash. This paper provides theoretical bases for predicting the maximum carbonation efficiency under high-pressure and high-temperature conditions in UCG cavities, and research directions for achieving a net-zero carbon emission goal for UCG projects.

Speeds of Sound in Binary Mixtures of Water and Carbon Dioxide at Temperatures from 273 K to 313 K and at Pressures up to 50 MPa

Knowledge of thermodynamic properties of aqueous solutions of CO2 is crucial for various applications including climate science, carbon capture, utilisation and storage (CCUS), and seawater desalination. However, there is a lack of reliable experimental data, and the equation of state (EOS) predictions are not reliable, particularly for sound speeds in low CO2 concentrations typical of water resources. For this reason, we have measured speeds of sound in three different aqueous solutions containing CO2. We report speeds of sound in the single-phase liquid region for binary mixtures of water and CO2 for mole fractions of CO2 of 0.0118, 0.0066 and 0.0015 at temperatures from 273.15 K to 313.15 K and at pressures up to 50 MPa, measured using a dual-path pulse-echo apparatus. The relative standard uncertainties of the sound speeds are 0.05 %, 0.03 % and 0.01 % at 0.0118, 0.0066 and 0.0015 CO2 mole fractions, respectively. The change in sound speeds as functions of composition, pressure and temperature are analysed in this study. We find that dissolution of CO2 in water increases its sound speeds at all conditions, with the greatest increase occurring at the highest mole fractions of CO2. Our sound speed data agree well with the limited available experimental data in the literature but deviate from the EOS-CG of Gernert and Span by up to 7 % at the lowest temperatures, highest pressures, and highest CO2 mole fraction. The new low-uncertainty sound speed data presented in this work could provide a basis for development of an improved EOS and in establishing reliable predictions of the change in thermodynamic properties of seawater-like mixtures due to absorption of CO2 gas.

Converting polar silicon surfaces of ordered mesoporous materials to non-polar carbon surfaces for enhanced carbon dioxide capture

The ordered mesoporous carbon-silica composite samples were synthesized by the unremoved template agent (P123) or together with additional sucrose as carbon sources. The effects of sulfuric acid and sucrose added on the properties of the synthesized materials were discussed. Ordered mesoporous structures of the synthesized materials were examined by XRD, TEM, and the 77 ​K N2 adsorption-desorption isotherms characterization. Compared to the silicon SBA-15 sample, the pore walls of carbon-silica composite materials were covered with carbon film, so their pore sizes became smaller and their surface properties were changed from polarity to hydrophobicity. The capturing CO2 performances on two dry/wet carbon-silica samples (SC-1.5, SCC-2) were collected at 275 ​K. Whether or not pre-adsorbed water, the CO2 adsorption capacities of the carbon-silica composite samples were higher than that of SBA-15. For the wet SC-1.5 sample with the mass ratio water/dry sample (Rw) of 3, the capturing CO2 capacity was 32 ​mmol/g at the pressure of 3 ​MPa. The H2O/CO2 mole ratios of the wet SC materials under different Rw were all lower than the theoretical value (5.75). The mechanisms of capturing CO2 on wet carbon-silica composite materials were multiple, which included CO2 dissolution in water, CO2 adsorption in pore space, and CO2 gas hydrate formation. The suddenly increasing CO2 adsorption capacity on the wet SC-1.5 material was mainly due to the formation of CO2 gas hydrates in its pore spaces.

Sequential Formation of CO2 Hydrates in a Confined Environment: Description of Phase Equilibrium Boundary, Gas Consumption, Formation Rate and Memory Effect

Since 1980, one of the most promising solutions for the exploitation of natural gas hydrate reservoirs was found to be the replacement of methane with carbon dioxide in order to improve the efficiency of methane recovery and, at the same time, permanently store carbon dioxide. However, the process efficiency is still too low and far from reaching technical maturity and becoming economically competitive. In this sense, studying the intrinsic properties of CO2 hydrates formation and dissociation processes may help in better defining the reasons for this low efficiency and finding feasible solutions. This work deals with carbon dioxide hydrates formation in a natural silica-based porous medium and in fresh water. A lab-scale apparatus was used for experiments, which were carried out consecutively and with the same gas–water mixture in order to detect the possible occurrence of the “memory effect”. Six tests were carried out: the quantity of gas available for the formation of hydrates led to an initial pressure equal to 39.4 bar within the reactor (the initial pressure was 46 bar; however, the dissolution of CO2 in water during the first test caused a reduction in the quantity of gas available for the process). Each experiment started and ended at temperatures equal or higher than 20 °C. Considering the local pressures, these temperatures ensured the complete dissociation of hydrates. Besides thermodynamic parameters, the gas consumption and the rate constant were evaluated throughout the whole of the experiments. Conversely to what is asserted in the literature, the results demonstrated the weak persistence of the memory effect at a temperature slightly above 25 °C. As expected, ice formation competed with hydrates; however, during tests, it caused the partial release of carbon dioxide previously trapped into hydrates or dissolved in water. Finally, the rate constant completely agreed with the labile Cluster Theory and proved that primordial clusters and hydrate crystals formed and dissociated during the whole test. The first phenomenon was predominant during the formation phase, while the opposite occurred during the following step. The rate constant was found to be an effective parameter to quantify differences between measured and real equilibrium conditions for the system.

Periodic CO2 Injection for Improved Storage Capacity and Pressure Management under Intermittent CO2 Supply

Storing CO2 in geological formations is an important component of reducing greenhouse gases emissions. The Carbon Capture and Storage (CCS) industry is now in its establishing phase, and if successful, massive storage volumes would be needed. It will hence be important to utilize each storage site to its maximum, without challenging the formation integrity. For different reasons, supply of CO2 to the injection sites may be periodical or unstable, often considered as a risk element reducing the overall efficiency and economics of CCS projects. In this paper we present outcomes of investigations focusing on a variety of positive aspects of periodic CO2 injection, including pressure management and storage capacity, also highlighting reservoir monitoring opportunities. A feasibility study of periodic injection into an infinite saline aquifer using a mechanistic reservoir model has indicated significant improvement in storage capacity compared to continuous injection. The reservoir pressure and CO2 plume behavior were further studied revealing a ‘CO2 expansion squeeze’ effect that governs the improved storage capacity observed in the feasibility study. Finally, the improved pressure measurement and storage capacity by periodic injection was confirmed by field-scale simulations based on a real geological set-up. The field-scale simulations have confirmed that ‘CO2 expansion squeeze’ governs the positive effect, which is also influenced by well location in the geological structure and aquifer size, while CO2 dissolution in water showed minor influence. Additional reservoir effects and risks not covered in this paper are then highlighted as a scope for further studies. The value of the periodic injection with intermittent CO2 supply is finally discussed in the context of deployment and integration of this technology in the establishing CCS industry.

Competitive adsorption between CO2 and CH4 in tight sandstone and its influence on CO2-injection enhanced gas recovery (EGR)

CO2 flooding is a new promising way of achieving enhanced natural gas recovery (EGR) and CO2 sequestration in tight gas reservoirs. CO2 adsorption is a reality that cannot be ignored during CO2 flooding process. However, the adsorption behavior of CO2 in tight cores remains unclear. Here, the CO2CH4 competitive adsorption characteristics in tight sandstone were experimentally investigated. The results indicate that the CO2 show significantly higher adsorption capacity in the tight cores compared to CH4 and that competitive adsorption occurs after CO2 is injected into tight core preadsorbed with CH4, with over 30% of the CH4 being replaced by the injected CO2 under an ambient pressure of 24 MPa. Further, the results of sensitivity experiments demonstrated that CO2CH4 competitive adsorption is more significant in tight cores with lower permeability, higher rock fragment content, and lower water saturation, while experiments on CO2-EGR and CO2 storage showed that for tight cores with lower permeability or higher rock fragment content, the higher degree of competitive adsorption resulted in greater EGR efficiency and CO2 storage capacity. For water saturated cores, however, the dissolution of CO2 in water has a greater impact on EGR and CO2 storage compared to competitive adsorption, resulting in better EGR and CO2 sequestration efficiency at higher water saturation.

Effect of Convective Mixing Process on Storage of CO2 in Saline Aquifers with Layered Permeability

Convective mixing of free-phase CO2 and brine in saline aquifers is an established technique to accelerate the CO2 dissolution process. Correct estimation of the convection onset time and rate of CO2 dissolution into brine are two crucial parameters regarding safety issues, as the timescale for dissolution corresponds to the same time over which the free-phase CO2 has a chance to leak out from the storage site. In real practice, underground formations are heterogeneous with a layered structure, but the convective mixing in heterogeneous porous media has received less attention than the homogeneous one. This study aims to develop a basic understanding of the role of layered permeability media (layered structure with variation in permeability vertically) on the behavior of convective mixing via well-controlled laboratory experiments. The effects of layering and layer properties on the rate of dissolution of CO2 in water and geometries of the formed convection fingers are studied using a precise experimental set-up with layered-permeability Hele-Shaw cell geometry. Qualitative (snapshots of convection fingers) and quantitative data (amount of the dissolved CO2 into water) are collected simultaneously for a better understanding of the process. The behavior of convection fingers (after the onset of convection) and the effects of model properties on this mixing process are also discussed.

Investigation of growth limitation by CO2 mass transfer and inorganic carbon source for the microalga Chlorella vulgaris in a dedicated photobioreactor

An experiment was set up using a well-controlled photobioreactor to characterize the CO2 transfer and assimilation by photosynthetic microorganisms. This lab-scale system enabled accurate monitoring of the carbon sources in liquid and gas phases for in-depth characterization of the CO2 physical transfer and biological consumption of dissolved carbon.This paper presents a validation of the set-up by assessing the carbon mass balance in various absorption/desorption experiments. This is represented by the Carbon Recovery Percentage, which was found to be equal to 1 ± 0.1 confirming the accuracy of the time monitoring for the various carbon fluxes. These carbon fluxes were modeled to determine on-line the gas-liquid mass transfer coefficient (kLa). This enabled investigation of some aspects of CO2 dissolution in water, such as the effect of the culture medium composition and its pH value. Finally, carbon assimilation and growth limitation by the carbon source were studied for the microalga Chlorella vulgaris.

Modeling CO2 Solubility in Water at High Pressure and Temperature Conditions

CO2 dissolution in water at different temperature and pressure conditions is of essential interest for various environmental, geochemical, and thermodynamic related problems. The topic is of special interest in studies of CO2 geological sequestration in brine-bearing aquifers. In this Article, four powerful machine learning (ML) techniques—Radial Basis Function Neural Network (RBFNN), Multilayer Perceptron (MLP), Least-Squares Support Vector Machine (LSSVM), and Gene Expression Programming (GEP)—are implemented to develop economical, rapid, and reliable models to predict the solubility of CO2 in water. To expand the prediction capability of the ML approaches, their control parameters are optimized by various techniques. To this end, four back-propagation algorithms are applied in the MLP learning phase, while Particle Swarm Optimization (PSO), Differential Evolution (DE), Genetic Algorithm (GA), and Firefly Algorithm (FFA) are used to optimize the RBFNN and LSSVM control parameters. A wide-ranged database including temperature and pressure as inputs and CO2 solubility in pure water as output is utilized to develop the models, which are then compared with each other and also against existing models. The results demonstrate that the prediction performance of the proposed models is quite satisfactory. In addition, the comparison results reveal that the LSSVM-FFA is the best paradigm to estimate the solubility of CO2 in pure water, as it outperforms the other proposed ML techniques as well as prior models. The overall RMSE and R2 values for LSSVM-FFA are 0.3261 and 0.9930, respectively.

The mutual effects of injected fluid and rock during imbibition in the process of low and high salinity carbonated water injection into carbonate oil reservoirs

Abstract During water injection into oil reservoirs, interactions occur between injected water and the reservoir rock. The interactions are enhanced when dissolved carbon dioxide (CO2) is used in injectable fluid in enhanced oil recovery (EOR) process, especially in carbonate reservoirs. With the dissolution of CO2 in water, carbonic acid is formed. The formed acid reacts with the carbonate salts in the rock (calcium carbonate and magnesium carbonate) and dissolves them. The dissolution causes a change in the properties of the rock, including wettability, porosity and permeability. In this study, variations of porosity, permeability, rock mass and wettability during carbonated water imbibition were measured based on direct measurements of porosity, permeability, rock weight and contact angle. In addition, changes in the concentration of calcium, magnesium and bicarbonate ions from the dissolution of carbonate rocks in injected water were measured by sampling and titration of water. The experiments were carried out at a constant temperature and the pressure parameter was considered to be variable in order to investigate its effect on the cases discussed. The effect of base fluid salinity was investigated in two salinities equal to the initial salinity and salinity from the dilution of seawater. The carbonated water imbibition in oil-saturated plugs yields 48, 56, 69 and 75%, at pressures of 500, 1000, 1500, and 2000 psi, respectively, and low salinity resulted from the dilution of seawater as the base fluid. In addition to the dependence of changes in rock parameters on the pressure and salinity of the base fluid, the results show that porosity variations, permeability and, most importantly, wettability of the rock are so much that they can be effective in activating the process of the imbibition and increasing its power in oil production.

Numerical Modeling of CO2, Water, Sodium Chloride, and Magnesium Carbonates Equilibrium to High Temperature and Pressure

In this work, a thermodynamic model of CO2-H2O-NaCl-MgCO3 systems is developed. The new model is applicable for 0–200 °C, 1–1000 bar and halite concentration up to saturation. The Pitzer model is used to calculate aqueous species activity coefficients and the Peng–Robinson model is used to calculate fugacity coefficients of gaseous phase species. Non-linear equations of chemical potentials, mass conservation, and charge conservation are solved by successive substitution method to achieve phase existence, species molality, pH of water, etc., at equilibrium conditions. From the calculated results of CO2-H2O-NaCl-MgCO3 systems with the new model, it can be concluded that (1) temperature effects are different for different MgCO3 minerals; landfordite solubility increases with temperature; with temperature increasing, nesquehonite solubility decreases first and then increases at given pressure; (2) CO2 dissolution in water can significantly enhance the dissolution of MgCO3 minerals, while MgCO3 influences on CO2 solubility can be ignored; (3) MgCO3 dissolution in water will buffer the pH reduction due to CO2 dissolution.

Molecular simulation of equal density temperature in CCS under geological sequestration conditions

AbstractStoring carbon dioxide (CO2) in geological reservoirs, principally saline aquifers, is one of the main methods proposed to address the issue of anthropogenic CO2 emissions. With the aim of ensuring long‐term CO2 storage in geological reservoirs, the density of the CO2–brine solution is an important property that controls brine buoyancy. In this study, molecular dynamics simulations were performed to investigate the density of CO2–water solutions at temperatures in the range 60–240°C, pressures in the range 10–100 MPa, varying CO2 mass fractions (1%, 2%, and 3%), and varying CO2–NaCl–water solution salinities (2 and 4 mol/kg), all of which are crucial parameters for CO2 geological sequestration. The optimal combination model of CO2(TraPPE)–NaCl(OPLS)–water(TIP4P/2005) was determined by comparing experimental values with simulate density under geological conditions. The simulation results reproduced the trends of the experimental values. The equal density temperature (Te) indicated that solution density decreases upon CO2 dissolution in water when the temperature increases to a certain point. According to the simulation results, Te increases with increasing pressure and decreases with increasing salinity. Further, the results of analysis of the structural properties of the solution to determine the causes of Te showed that, with increasing temperature, the distance between CO2 molecules and adjacent atoms increases significantly, which reduces the local density around the CO2 molecules. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.

Modification of fumarolic gases by the ice-covered edifice of Erebus volcano, Antarctica

The chemistry of gases measured in ice caves and from warm geothermal ground at Erebus volcano, Antarctica, shows that gas emissions are dominated by air, with varying amounts of added volcanic CO2. This suggests widespread circulation of air through the volcanic edifice, as well as spatially or temporally varying contributions from magmatic degassing.The resulting gases are further modified by two processes. The first is CO2 dissolution in water, resulting in fractionation from magmatic δ 13C-CO2 values, which are estimated to be around −4‰, to heavier values, up to −1‰. Assuming all magmatic CO2 is dissolved in neutral water as HCO3−, this requires hydrothermal temperatures of over 120 °C. However, other phases such as calcite may be present, implying even higher temperatures, while lower water pH values could result in similar isotope ratios at much lower temperatures, such as 60 °C at pH of 5.3. A large proportion of magmatic CO2 must be lost to this hydrothermal system or to mixing with air. The hydrothermal influence is localized to certain areas on the volcano, which may be associated with high velocity zones identified in previous studies by seismic tomography. Two sites with stronger magmatic signatures, by contrast, are above low velocity zones representing possible shallow magma storage.The second modification is the removal of oxygen from both deeply-sourced and air-derived gases. This is likely due to prevailing conditions in the subsurface, as it is independent of the original source of the gases and of hydrothermal modifications, and thus may affect sites with magmatic, air-like, or hydrothermal signatures.

A modeling strategy to investigate carbonated water injection for EOR and CO2 sequestration

Carbonated water injection (CWI) has been well investigated to improve oil recovery when compared to other enhanced oil recovery (EOR) techniques both in the secondary and tertiary modes. Extra oil recovery and CO2 sequestration associated with CWI have been studied through several experimental works. There are not adequate number of modelling studies about the CWI operation in the open sources because of the complex multi-physics involved with the fluid-fluid and fluid-rock interactions during CWI processes. Hence, further experimental and modelling investigations are needed to be conducted on CWI to systematically capture/comprehend the governing physics and complex displacement mechanisms. This research work will focus on the analysis of vital aspects such as oil recovery amount (and mechanisms), fluids distribution, and effects of operational parameters and well placement on the performance of CWI for both EOR and CO2 sequestration purposes. To achieve these objectives, a 3-D heterogenous reservoir model is developed using the experimental data reported in the recent literature. A new approach of using the grid local pressure to model CWI is adopted where the moles of CO2/water are controlled by their injection rates. The dissolution of CO2 in water is modeled by the Henry’s law for each subsequent grid local pressure. In this research, it is found that through CWI, an additional oil recovery can be achieved when compared to plain (conventional) waterflooding (WF) in the secondary recovery mode. A subsequent increase in the injection pressure leads to more dissolution of CO2 and enhancement of the overall performance of CWI. There is an optimum injector rate, which ensures an effective mass transfer across phases. An optimal well orientation will also give a better recovery performance during CWI. The amount of CO2 stored is also illustrated in this work as an additional benefit achieved in the CWI processes.

Geochemistry of CO2-Rich Gases Venting From Submarine Volcanism: The Case of Kolumbo (Hellenic Volcanic Arc, Greece)

Studies of submarine hydrothermal systems in Mediterranean Sea are limited to the southern Italian volcanism, while are totally missing in the Aegean. Here we report on the geochemistry of high-temperature fluids (up to 220°C) venting at 500 m b.s.l. from the floor of Kolumbo submarine volcano (Hellenic Volcanic Arc, Greece), which is located 7 km northeast of Santorini Island. Despite the recent unrest at Santorini, Kolumbo submarine volcano is considered more active due to a higher seismicity. Rizzo et al. (2016) investigated the He-isotope composition of gases collected from seven chimneys and showed that are dominated by CO2 (>97%), with only a small air contamination. Here we provide more-complete chemical data and isotopic compositions of CO2 and CH4, and Hg(0) concentration. We show that the gases emitted from different vents are fractionated by the partial dissolution of CO2 in water. Fractionation is also evident in the C-isotope composition (13CCO2), which varies between –0.04‰ and 1.15‰. We modelled this process to reconstruct the chemistry and 13CCO2 of intact magmatic gases before fractionation. We argue that the CO2 prior to CO2 dissolution in water had 13C ~ –0.4‰ and CO2/3He ~ 1•1010. This model reveals that the gases emitted from Kolumbo originate from a homogeneous mantle contaminated with CO2, probably due to decarbonation of subducting limestone, which is similar to other Mediterranean arc volcanoes (e.g. Stromboli, Italy). The isotopic signature of CH4 (13C ~ –18‰ and D ~ –117‰) is within a range of values typically observed for hydrothermal gases (e.g. Panarea and Campi Flegrei, Italy), which is suggestive of mixing between thermogenic and abiotic CH4. We report that the concentrations of Hg(0) in Kolumbo fluids are particularly high (~61 to 1300 ng m–3) when compared to land-based fumaroles located on Santorini and worldwide aerial volcanic emissions. This finding may represent further evidence for the high level of magmatic activity at Kolumbo. Based on the geo-indicators of temperature and pressure, we calculate that the magmatic gases equilibrate within the Kolumbo hydrothermal system at about 270°C and at a depth of ~1 km b.s.l..

Experimental Study on Feasibility of Enhanced Gas Recovery through CO2 Flooding in Tight Sandstone Gas Reservoirs

The development of natural gas in tight sandstone gas reservoirs via CH4-CO2 replacement is promising for its advantages in enhanced gas recovery (EGR) and CO2 geologic sequestration. However, the degree of recovery and the influencing factors of CO2 flooding for enhanced gas recovery as well as the CO2 geological rate are not yet clear. In this study, the tight sandstone gas reservoir characteristics and the fluid properties of the Sulige Gasfield were chosen as the research platform. Tight sandstone gas long-core displacement experiments were performed to investigate (1) the extent to which CO2 injection enhanced gas recovery (CO2-EGR) and (2) the ability to achieve CO2 geological storage. Through modification of the injection rate, the water content of the core, and the formation dip angle, comparative studies were also carried out. The experimental results demonstrated that the gas recovery from CO2 flooding increased by 18.36% when compared to the depletion development method. At a lower injection rate, the diffusion of CO2 was dominant and the main seepage resistance was the viscous force, which resulted in an earlier CO2 breakthrough. The dissolution of CO2 in water postponed the breakthrough of CO2 while it was also favorable for improving the gas recovery and CO2 geological storage. However, the effects of these two factors were insignificant. A greater influence was observed from the presence of a dip angle in tight sandstone gas reservoirs. The effect of CO2 gravity separation and its higher viscosity were more conducive to stable displacement. Therefore, an additional gas recovery of 5% to 8% was obtained. Furthermore, the CO2 geological storage exceeded 60%. As a consequence, CO2-EGR was found to be feasible for a tight sandstone gas reservoir while also achieving the purpose of effective CO2 geological storage especially for a reservoir with a dip angle.

Effects of acidic gases and operation parameters on denitrification in oxy-fuel CO2 compression process

Oxy-fuel combustion technology has great potential as a technically feasible method to significantly reduce CO2 emissions from coal-fired power plants. In an oxy-fuel combustion system, gas impurities such as SO2 and NOx must be removed before CO2 recovery because they can cause potential corrosion risk to CO2 compression and purification units (CPU). An attractive strategy is to simultaneously remove SO2 and NOx in the existing CO2 CPU system in place of traditional flue gas treatment device. Since NOx contains more than 90% of insoluble NO, the removal of NO has attracted much attention. In this study, NO removal efficiency in CO2 compression process were experimentally investigated on a pressurized reaction system. Effects of acidic gases (SO2, CO2) and operation parameters (pressure, temperature, initial O2 concentration and residence time) were taken into consideration. NO removal was achieved in pressurized reaction system as a result of oxidation to NO2 in the presence of O2 and the absorption of NO2 in water. NO conversion to NO2 was the key step for NO removal, which was strongly and positively dependent on the pressure and negatively dependent on the temperature. With the addition of 1000 ppm SO2 in simulated gases, NO removal efficiency was reduced by 8.1% at 2.0 MPa. The gas phase and liquid phase reactions between SO2 and NO2 suggested that partial NO2 was converted to insoluble NO when reacting with SO2. It was also found that NO removal efficiency in O2/CO2 atmosphere was slightly higher than that in O2/N2 atmosphere. The dissolution of CO2 in water increased the residence time as well as the acidity of the aqueous solution, which could facilitate the oxidation of NO.

Formation and paleoenvironment of rhizoliths of Shiyang River Basin, Tengeri Desert, NW China

Rhizoliths found in bed of the late Quaternary paleolake Zhuyezhe, Minqin Basin, central Tengeri Desert, NW China were studied. Vegetation coverage is present at some locations in the paleolake area although, much of the bed area is covered with moving dunes. Rhizoliths are occasionally eroded out and exposed to the air. The morphology of the rhizoliths resembles singular or branching carbonated tubules which are hollow in their central part. The rhizoliths are black to grey, and are broken and scattered randomly on the sand surface of the lake bed. The lacustrine deposits are black greenish silt or silty clay with a large quantity of white petite lake snail shells. The possible plant types, sources and formation conditions of carbon dioxide and calcium, the sedimentary and diagenetic environments, and the process of rhizolith formation were discussed via examining rhizoliths macromorphology, studying the micromorphology and mineralogy by microscopy, cathodoluminescence and scanning electron microscope, and studying the chemical compositions of cementing minerals and fragments by energy dispersive X-ray spectra. The original roots of the rhizoliths belong to hydrophytes, such as Typha latifolia, Scirpus maritimus and Carex stenophylla. Lake snails and previous pollen data indicate that the rhizoliths formed in a sedimentary environment of shallow fresh water lake like marginal or palustrine areas during the Holocene. Shallow lake water disturbance by desert wind above the loose sandy sediment or soil was favoring the rhizoliths formation. A continuous supply of oxygen through water disturbance led to complete oxidation of roots and producing carbon dioxide. Dissolution of CO2 in water and so, carbonic acid production resulted in minerals weathering such as feldspar and primary carbonate particles and the release of K+ and Ca2+ ions. At presence of CO2 and Ca2+ saturated in the pore water around the roots of hydrophytes within the sediments or soil, carbonate precipitation occurred around the root channel and led to rhizoliths formation as tubules. The petrifaction process therefore, has happened in margin of the paleolake with shallow water and weak redox condition, which implies the suitability of rhizolith for reconstructing paleoenvironment.