The effect of natural flow of aquifers and associated dispersion on the onset of buoyancy‐driven convection in a saturated porous medium

Carbon dioxide injection into deep saline aquifers is an important option for managing CO₂ emissions. Injected CO₂ dissolves into formation brines from above, increasing brine density and creating an unstable hydrodynamic state favorable for natural convection. Long‐term buoyancy‐driven flow of high‐density CO₂‐saturated brine leads to faster trapping through improved dissolution and can reduce the risk of CO₂ leakage from storage sites. We investigate the role of natural flow of aquifers and associated dispersion on the onset of convection. A linear stability analysis of a transient concentration field in a laterally infinite, horizontal, and saturated porous layer with steady horizontal flow is presented. The layer is subjected to a sudden rise in CO₂ concentration from the top and is closed from the bottom. Solution of the stability equations is obtained using a Galerkin technique and the resulting equations are integrated numerically. We found simple scaling relationships that follow t_Dc～60(1 + Pe_T)Ra^‐2 for the onset time of convection and a～0.05Ra/(1 + Pe_T) for the wavenumber of the initial instabilities. Results reveal that transverse dispersion increases the time to onset of convection for the entire range of Ra. Furthermore, transverse dispersion decreases the critical wavenumber of the instabilities. These results facilitate screening candidate sites for geological CO₂ storage. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

Investigation of the pH effect of a typical host rock and buffer solution on CO2 sequestration in synthetic brines

Carbon dioxide sequestration in deep saline aquifers is a critical component of long-term storage options. It is suggested that the precipitation of mineral carbonates is mostly dependent on brine pH and is favoured above a basic pH of 9.0. However, brine pH will drop to acidic values once CO₂ is injected into the brine. Therefore, there is a need to raise brine pH and maintain it stable. Synthetic brines were used here instead of natural brines because of the difficulty in obtaining and storing natural brines. Therefore, experiments were conducted to prepare a series of synthetic brines and to compare their suitability to natural brines for carbon sequestration firstly. A typical host rock (Oriskany rock) and a buffer solution (NaCl/NaHCO₃) were selected to buffer brine pH. In a subsequent step, studies were conducted to correlate how brine samples respond in the presence of the host rock or the buffer solution at realistic reservoir temperatures (40 and 100 °C) and pressures (1160 and 1500 psi) for CO₂ storage. The results show that synthetic brines prepared can be used as analogues as natural brines for carbon sequestration studies in terms of chemical composition and pH response. Both XRD and SEM/EDS analyses confirmed the presence of mineral carbonates in the CO₂-rock-brine and the CO₂-buffer-brine experiments. However, the amount of carbonates precipitated from the CO₂-buffer-brine reactions is nearly 18 times larger than that formed from the CO₂-rock-brine experiments. ICP-MS studies also verified that there was only 4% reduction in Ca concentration in solution after the CO₂-rock-brine studies, while the concentrations of Ca and Sr decreased by 90% during the CO₂-buffer-brine experiments.     

Visualisation of mechanisms involved in Co2 injection and storage in hydrocarbon reservoirsand water-bearing aquifers

Storage of carbon dioxide (CO₂) in hydrocarbon reservoirs and saline aquifers is considered as one of the promising mitigation strategies to reduce the negative impact of this greenhouse gas. The static and dynamic behaviour of CO₂ in these storage sites which are located at various depths and geographical locations, affects the efficiency of this strategy. Understanding the impact of the conditions of these storage sites on mechanisms involved in CO₂ flow, displacement and trapping is also critical for the purpose of site selection and the design of CO₂ storage projects. In this paper we report the results of a series of CO₂ injection (CO₂I) flow visualisation (micromodel) experiments conducted using high-pressure transparent porous media representing various aquifer and depleted oil reservoirs storage conditions. The impact of pertinent parameters on the interaction between the stored CO₂ and the reservoir fluids were investigated. Both sub-critical and supercritical CO₂ were used to investigate the effect of pressure (depth) of the storage site on CO₂ trapping mechanisms. A faster CO₂ breakthrough (BT) was observed in the micromodel test simulating CO₂I into depleted oil reservoirs, compared to that into aquifers, reducing the sequestration capacity of the depleted oil reservoirs. Compared to the injection of supercritical CO₂, the BT of gaseous CO₂ happened faster, adversely affecting the CO₂ displacement performance. The results of these direct flow visualization experiments significantly improve our understanding of the complex mechanisms and interactions involved in CO₂I and storage in geological formations. This knowledge is essential for identifying storage conditions that would lead to maximising CO₂ storage capacity, for better understanding the ultimate fate of the stored CO₂ and the storage safety.     

Flow and mixing efficiency characterisation in a CO2-assisted single-screw extrusion process by residence time distribution using Raman spectroscopy

Hot-melt extrusion of a bio-sourced polyamide has been implemented in a single-screw extruder with supercritical carbon dioxide injection. CO₂ acts as a plasticiser in the extruder barrel and as a physical blowing agent at the die. To insure a better mixing and dissolution of the CO₂ into the polymer melt, addition of a static mixer between the screw tip and the die was tested. The effect of both the static mixing element and the CO₂ injection on the melt flow behaviour has been elucidated. A recent technique of in-line Raman spectroscopy was implemented to make a residence time distribution study, using titanium dioxide as a tracer. The use of a static mixer exerts a major modification on the flow behaviour: it improves mixing by enhancing dispersion. In addition, the structure of the manufactured products was studied: the static mixer led to more homogeneous porous structure. The broad range of CO₂ incorporation (up to 25%, w/w) into the melt led to the manufacture of foams with adjustable porosity from 15 to 70%.     

Estimation of Interfacial Tension for Geological CO2 Storage

The effective CO₂ sequestration in saline aquifers as a climate change‐lessening solution is mainly governed by the interfacial tension (IFT) behavior between CO₂ and brine. An innovative and competent decision tree‐based approach called stochastic gradient boosting (SGB) tree algorithm was applied to predict the CO₂‐aquifer brine IFT as a function of temperature, pressure, and brine salinities. The produced results were compared with the previously reported outcomes of other machine learning models, namely, radial basis function networks, multilayer perceptron networks, least squares support vector machine, and adaptive neuro fuzzy inference system. Amongst all models, the developed SGB tree algorithm provided superior outputs and turned out to be the most accurate tool.     

A numerical study of capillary pressure–saturation relationship for supercritical carbon dioxide (CO2) injection in deep saline aquifer

Carbon capture and sequestration (CCS) is expected to play a major role in reducing greenhouse gas in the atmosphere. It is applied using different methods including geological, oceanic and mineral sequestration. Geological sequestration refers to storing of CO₂ in underground geological formations including deep saline aquifers (DSAs). This process induces multiphase fluid flow and solute transport behaviour besides some geochemical reactions between the fluids and minerals in the geological formation. In this work, a series of numerical simulations are carried out to investigate the injection and transport behaviour of supercritical CO₂ in DSAs as a two-phase flow in porous media in addition to studying the influence of different parameters such as time scale, temperature, pressure, permeability and geochemical condition on the supercritical CO₂ injection in underground domains. In contrast to most works which are focussed on determining mass fraction of CO₂, this paper focuses on determining CO₂ gas saturation (i.e., volume fraction) at various time scales, temperatures and pressure conditions taking into consideration the effects of porosity/permeability, heterogeneity and capillarity for CO₂–water system. A series of numerical simulations is carried out to illustrate how the saturation, capillary pressure and the amount of dissolved CO₂ change with the change of injection process, hydrostatic pressure and geothermal gradient. For example, the obtained results are used to correlate how increase in the mean permeability of the geological formation allows greater injectivity and mobility of CO₂ which should lead to increase in CO₂ dissolution into the resident brine in the subsurface.     

Study of the CO2 adsorption isotherms on El Hicha clay by statistical physics treatment: microscopic and macroscopic investigation

ABSTRACT

CO₂ introduction in deep aquifers based on adsorption phenomena represents geological tanks that reduce CO₂ emission. Thus, investigating carbon dioxide adsorption on rocks is becoming more interesting. In our work, carbon dioxide adsorption on El Hicha clay is extensively studied. Experimental data for CO₂ adsorption on this clay are given for the first time. All the corresponding parameters are simulated and interpreted using the multilayer model with two interaction energies. The effect of the key parameters involved in the adequate model on the isotherm curves are thus elucidated and interpreted. The formulation of this model is based on statistical physic formalism. Several hypotheses involving some physicochemical parameters which describe perfectly the adsorption process are used.

The characteristic parameters of the adsorption isotherm such as the number of carbon dioxide molecules per site (n), the receptor site densities (N_M), the number of adsorbed layers (N_L) and the energetic parameters (-ε₁) and (-ε₂) are estimated for the studied systems by a nonlinear least square regression. These parameters are discussed and interpreted considering their temperature dependence. In order to provide new macroscopic interpretations of adsorption mechanisms, three thermodynamic functions are also determined such as the entropy, the internal energy and the free enthalpy of Gibbs from experimental data. Thus, we prove theoretically and experimentally that CO₂ adsorption on El Hicha clay is feasible, spontaneous and exothermic in nature.     

Use of tracers to characterize the effects of a CO2-saturated brine on the petrophysical properties of a low permeability carbonate caprock

To assess the long-term safety of a geological carbon dioxide storage site, the confining properties of the rocks sealing an underground reservoir (caprocks) and their evolution inthe presence of CO₂ must be characterized. The present study consists in the measurement of the transport parameters of dissolved CO₂ through low permeability carbonate-rich caprocks. The properties of interest are the effective permeability and the diffusion coefficients of carbon dioxide dissolution products in water. The impact of carbon dioxide has been evaluated when altering rock samples by diffusion of a CO₂-saturated brine under reservoir thermodynamic conditions, and by comparison of the pre- and post-alteration measured values. Permeability was measured by a gas-tracing method and, to study diffusion, radioactive isotopes of carbon (¹⁴C) and hydrogen (³H) were used. Despite a porosity increase observed for all the studied samples, the low values of transport parameters, measured initially, were also measured after alteration, showing a non-catastrophic alteration of the material.     

An in situ electrochemical impedance spectroscopy/synchrotron radiation grazing incidence X-ray diffraction study of the influence of acetate on the carbon dioxide corrosion of mild steel

A combination of electrochemical impedance spectroscopy (EIS) and in situ synchrotron radiation grazing incidence X-ray diffraction (SR-GIXRD) has been used to study the influence of acetate on the carbon dioxide corrosion of mild steel. The SR-GIXRD data demonstrated that normal corrosion - in a carbon dioxide saturated brine - induced the formation of a thick corrosion scale of Fe₂(OH)₂CO₃ and Fe₂O₂CO₃, and this totally obscured the α-Fe diffraction peaks of the underlying steel substrate after 24 h. On the other hand, the carbon dioxide corrosion of mild steel in the presence of acetate also detected the Bragg diffraction peaks for Fe₂(OH)₂CO₃ and Fe₂O₂CO₃; however, the α-Fe diffraction peaks of the underlying steel substrate were not extinguished with time, and there was a reversal in the pattern of evolution of the intensities of the Fe₂(OH)₂CO₃ and Fe₂O₂CO₃ phases in acetate. Accordingly, the EIS data showed a poorly defined medium frequency time constant for the corroded steel specimen in brine spiked with acetate, and this medium frequency time constant was extinguished as a function of time. Alternatively, EIS of the corroded specimen also revealed a medium frequency time constant after 24 h. In addition, EIS complex-plane impedance plots showed that the corroded electrode had become passivated in an acetate-spiked brine, as evidenced by a three-fold enhancement in the charge transfer resistance at low frequency. These EIS/SR-GIXRD outcomes suggest that acetate affects the crystallization chemistry of the Fe₂(OH)₂CO₃/Fe₂O₂CO₃ corrosion scale, and this causes a mild passivation of the corroded steel surface.     

Nano-mixing of dipyridamole drug and excipient nanoparticles by sonication in liquid CO2

Nanoparticles (about 200 nm thick and 600–12000 nm long flakes) of dipyridamole, a poorly water-soluble anti-thrombosis drug, are produced by supercritical antisolvent solvent with enhanced mass transfer method. Applicability of sonication in liquid CO₂ for mixing of drug and excipient nanoparticles is demonstrated for several binary mixtures of drug and excipient. The drug particles are mixed with three different excipients: silica nanoparticles, lactose microparticles, and polyvinylpyrrolidone nanoparticles. To intimately mix at nanoscale, macro mixtures of dipyridamole and excipient particles are sonicated in liquid carbon dioxide. The effects of ultrasonic energy, amplitude, and component weight ratio are studied for the binary mixtures. Characterization of mixing is done using several methods. Scanning electron microscopy is used as a primary method for microscopic analysis. Two macroscopic effects, drug dissolution and blend homogeneity (relative standard deviation), are used to characterize mixing quality of drug/lactose mixture. Results of drug dissolution and blend homogeneity show effectiveness of the proposed mixing method for fine size particles. Material handling properties of drug/silica and lactose/silica mixtures were examined. Upon mixing, the handling properties are significantly improved as measured by compressibility index and Hausner ratio. Liquid CO₂ offers an environmentally benign media for mixing. In addition, the mixture obtained does not contain any residual solvent as compared to the sonication in organic liquids. Upon depressurization, CO₂ is easily removed from the mixture providing a facile recovery of the product.     

Notes on the dissolution rate of gas hydrates in undersaturated water

An elementary model for the dissolution of pure hydrate in undersaturated water is proposed that combines intrinsic decomposition within a desorption film and the subsequent diffusion of the released hydrate guest species into bulk water. Applying the proposed approach to recently published measurements of the decomposition rates of methane (CH₄) and carbon dioxide (CO₂) hydrates in deep seawater suggests that the concentration of the hydrate guest species at the interface between desorption film and diffusive boundary layer may be much lower than ambient solubility. Calculations, however, fail to account for the observed proportionality of decomposition rate with solubility for both CH₄ and CO₂ hydrates. This may indicate a limitation in the range of applicability of published formulas for intrinsic hydrate decomposition rates.     

Comparison of numerical codes for geochemical modelling of CO2 storage in target sandstone reservoirs

Long-term containment of CO₂ and storage security depends mainly on physical and geochemical trapping mechanisms. Injected CO₂ dissolves in the formation fluid and causes a sharp decrease in pH which in turn drives the dissolution and precipitation of minerals. Geochemical modelling is an important tool to understand and predict the behaviour of CO₂ reactivity. In this paper three numerical codes, PHREEQC, GEM and TOUGHREACT, are compared with respect to brine - CO₂ - rock reactions. Formation water compositions and mineralogies of three sandstone core samples from target CO₂-storage formations were used as input for kinetic models. Mineral replacement reactions were observed when CO₂ injection was initiated in all scenarios. While PHREEQC and GEM were generally in good agreement, TOUGHREACT gave diverging predictions on two models. It is considered that the discrepancies are caused primarily by the differences in the thermodynamic databases and activity models. The uncertainties in these calculations suggest that appropriate experimental tests should be performed to validate the models, so that they can be used to make predictions at the field scale.     

Influence of Alkaline Additives and Buffers on Mineral Trapping of CO2 under Mild Conditions

Carbon dioxide (CO₂) gas is the main contributor to climate change. CO₂ storage in underground brines and oil‐field brines by mineral trapping has been considered as a promising alternative in order to reduce CO₂ emissions. However, permanent storage of CO₂ in stable carbonate minerals is greatly dependent on brine pH, being favored over an alkaline pH. The effect of alkaline additives (NaOH, KOH, CaO) and buffer solutions (NaHCO₃/NaOH, Na₂HPO₄/NaOH, NH₄Cl/NH₄OH) on the mineral trapping of CO₂ under mild conditions using a synthetic brine is investigated. The results indicate that both NaOH+NH₄Cl/NH₄OH and KOH+NH₄Cl/NH₄OH mixtures promote precipitation mainly of calcium carbonate (CaCO₃).     

Verfahrenstechnische Möglichkeiten zur Verringerung des Anstiegs von Kohlendioxid in der Luft

Engineering Solutions for Limiting the Increase of Carbon Dioxide in Air This article describes engineering solutions for limiting the increase of carbon dioxide in air. Fossile power plants are taken as a model for the source of CO₂. The global mass balance shows that the oceans play a most important role in the storage of the CO₂. The hypothesis is that it is not the absolute value of carbon dioxide concentration that is the real problem but rather its change. Keeping this in mind the present emissions should not be converted but stored for future times. This strategy is called ?hiding the CO₂“. The reduction of the emission is not very likely. It is believed that present actions to reduce the private power consumption will not really change the situation. A number of strategies for the sequestration of CO₂ are reported in the contribution. One proposal is to use shallow waters which form a thermohaline current for the sequestration. In this case, the injection of CO₂ is quite simple but the carbon dioxide travels hundreds of years in a deep sea current. Several scenarios are discussed for the fate of this CO₂‐enriched current. The environmental impact is briefly reported. This contribution describes the actual research needs, taking into account that similar research in Japan and in the U.S. is much more developed.     

High-pressure CO2-enhanced polymer interdiffusion and dissolution studied with in situ ATR-FTIR spectroscopic imaging

A novel application of attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopic imaging to study polymer interdiffusion and dissolution under high-pressure or supercritical carbon dioxide environments has been demonstrated. Miscible systems of polyvinylpyrrolidone (PVP) and poly(ethylene glycol) (PEG) of different molecular weights have been chosen for this investigation. These systems were subjected to a controlled pressure of CO₂ and the interfacial area of contact between the two polymers was studied by ATR-FTIR imaging in situ. Using this spectroscopic imaging approach, the phenomenon of polymer interdiffusion enhanced by CO₂ dissolved in both polymers was investigated as a function of time. The evolution of spatially resolved images as a function of time was studied with FTIR imaging and the corresponding concentration profiles for both polymers were obtained. The chemical specificity of FTIR imaging also allowed us to measure the amount of CO₂ dissolved in each domain of the polymer system. Effects of PEG molecular weight and pressure of CO₂ on the mechanism and the rate of polymer interdiffusion was investigated. This approach has not only shown the ability to visualise the process of interdiffusion but also demonstrated the ability of high-pressure CO₂ to ‘tune’ the rate of interdiffusion, this information is important for a better understanding of CO₂-induced mixing of polymeric materials.     

Gas-Liquid mass transfer in fixed beds with two-phase cocurrent downflow: Gas/newtonian and non-newtonian liquid systems

New data of gas-liquid mass transfer for cocurrent downflow through packed beds of porous and non-porous particles are presented. Mass transfer parameters for air/carbon dioxide/water, air/carbon dioxide/carboxymethylcellulose solution and air/carbon dioxide/sodium hydroxide systems were evaluated by least square fit of the calculated CO₂ concentration profiles in gas phase to the experimental values. The volumetric liquid-side mass transfer coefficient increases with the increase of the flow consistency index of the liquid. A comparison of the volumetric liquid-side mass transfer coefficient values evaluated with and without taking into account the axial dispersion shows that the influence of the liquid axial dispersion is significant at low liquid velocity and high CMC concentrations, and the influence of the gas axial dispersion is insignificant.     

CHARACTERIZATION OF FLUIDIZED BED REACTORS WITH GAS TRACER MEASUREMENTS

In a steady state bench scale fluidized bed the decomposition reaction of NaHCO₃ was carried out. The residence times distributions, DRT, of carbon dioxide (the gaseous product) and non adsorbing argon (the reference tracer) were mass spectroscopically measured as a function of the bed temperature. By means of single-, two- and three-phase dispersion models as well as by a cell model, the DRT's were evaluated on line by a computer.

The steady state transverse and longitudinal concentration profiles of these tracers upstream from the plane source were also measured and evaluated by a dispersion model as well as by a counter current back mixing model. Comparison of the steady state and nonsteady state dispersion coefficient measurements indicate that the longitudinal gas mixing is only partially due to backmixing. The experimentally determined wake fractions agree well with those published in the literature. Since the adsorption rate of CO₂ on the pore surface area of the particles in the dense phase is high no interphase transfer from the interstitial gas of the dense phase into the bubble phase takes place.

The desorption of CO₂ and its return into the interstitial gas and than into the gas phase occurs only slowly and with an initial time lag. The on-line DRT can be used as a diagnostical technique for investigation of the reactor during its operation, if operation disturbances or breakdowns occur.     

Heterogeneous porosity distribution in Portland cement exposed to CO2-rich fluids

Efficient and safe storage of injected supercritical carbon dioxide (CO₂) underground is now one potential solution for reducing CO₂ emissions in the atmosphere. Preventing any CO₂ leakage through a wellbore annulus after injection is a key to maintaining long-term wellbore integrity. Most wells in depleted oil and gas fields may be re-used to inject CO₂. These wells were mostly cemented with conventional Portland cement. It is thus crucial to study how such cement behaves at depth in CO₂-rich fluids.Set cement samples are exposed to CO₂ fluids under pressure and temperature to simulate downhole conditions. The degraded cement exhibits significant mineralogical changes and heterogeneous porosity distribution. The bulk porosity evolution, as well as local porosity gradients through the samples, is quantified using combined mercury porosimetry and back-scattered electron image analysis. Both techniques show an initial sealing stage related to calcium carbonate precipitation plugging the porosity, followed by a dissolution stage marked by a significant increase of porosity.     

Measurement and thermodynamic modeling of partition coefficients in N,N-dimethylacetamide-water-carbon dioxide system

The partition coefficients of N,N-dimethylacetamide (N,N-DMA) between the water and the supercritical and near-critical carbon dioxide (CO₂) phases were measured in the temperature range of 298.15-328.15 K and the pressure range of 8.3-24.1 MPa. The measurements were carried out in a 56 ml vessel by contacting the carbon dioxide and the aqueous phases. The partition coefficients of N,N-DMA increased from 0.05 to 0.150 with increasing pressure at a constant temperature and increased with temperature at a constant density. The bubble point pressures of N,N-DMA-CO₂ mixtures were measured at 298.15 K, 308.15 K and 318.15 K and were found to increase with increasing mole fraction of CO₂. The partition coefficients were modeled using the Peng-Robinson Equation of State (PREOS) combined with modified van der Waals mixing rule. The binary interaction parameters for the CO₂-H₂O pair were taken from the literature and were regressed for CO₂-N,N-DMA and H₂O-N,N-DMA pairs by fitting partition coefficients data. The binary interaction parameter for CO₂-N,N-DMA pair was found to depend linearly on temperature. The bubble point pressures of N,N-DMA and CO₂ were also measured and could be predicted fairly well using the regressed binary interaction parameters.