Convection Of CO Research Articles

Resolving Convection of CO2 Ice Clouds in the Martian Polar Nights

AbstractMartian CO2 ice clouds are intriguing features, representing a rare occurrence of atmospheric condensation of a major component. These clouds play a crucial role due to their radiative properties, interactions with surface, and coupling with microphysical cycles of aerosols. Observations have been limited, prompting modeling studies to understand their formation and dynamics. Here, we present the first high‐resolution 3D simulations of CO2 ice clouds using a Large‐Eddy Simulation (LES) model incorporating CO2 microphysics. We investigate cloud formation in idealized temperature perturbations in the polar night. A reference simulation with a −2K perturbation demonstrates that the formed CO2 ice cloud possesses a convective potential, leading to its ascent in the troposphere. We determine the timescales and orders of magnitude of various phenomena involved in the lifecycle of a CO2 ice cloud. Sensitivity tests show that convection can be inhibited or intensified by the thermodynamic and microphysical conditions of the simulated environment.

Recovery of Ventilatory and Metabolic Responses to Hypoxia after Chronic Hypoxia in Neonatal Rats

Chronic hypoxia (CH) from birth blunts the hypoxic ventilatory response (HVR) in newborn rats, but there is conflicting evidence as to whether (or how quickly) the HVR recovers after these rats are returned to room air. In this study, Sprague-Dawley rats were exposed to either 21% O2 (Control) or 12% O2 (CH) for the first 3 postnatal days. Breathing and metabolism were studied using head-body plethysmography and respirometry immediately after the 3 days of CH (i.e., at P3) or after 1, 4-5, or 7 days of recovery in room air (i.e., at P4, P7-8, or P10). The early phase of the HVR was blunted in CH rats at P3 (13.7± 2.3 vs. 30.6±3.4 % increase from baseline during 12% O2), and the HVR was more biphasic in CH rats after 1 day of recovery. However, no significant differences in the HVR were observed between treatment groups after 3-4 or 7 days of recovery. Males and females showed similar patterns. Changes in ventilation during hypoxia were strongly biphasic in the younger age groups: ventilation increased initially, but this was followed by a gradual decline in ventilation towards or below baseline levels. However, when accounting for metabolic depression, all age groups continued to hyperventilate during the late phase of the HVR [i.e., the ratio of ventilation to CO2 production (CO2 convection requirement) remained elevated relative to baseline values]. The CO2 convection requirement response to hypoxia did not differ between treatment groups during the late phase of the HVR at any age, but metabolism and CO2 convection requirement could not be determined during the early phase of the HVR due to non-steady state conditions. In conclusion, 3 days of CH produces only a transient blunting of the HVR in newborn rats. Longer CH exposures may be required to elicit persistent changes in the HVR or, alternatively, this plasticity may not be revealed until after puberty. Supported by NIH grant P20 GM-103423 (Maine INBRE). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Heat Production Performance from an Enhanced Geothermal System (EGS) Using CO2 as the Working Fluid

CO2-based enhanced geothermal systems (CO2-EGS) are greatly attractive in geothermal energy production due to their high flow rates and the additional benefit of CO2 geological storage. In this work, a CO2-EGS model is built based on the available geological data in the Gonghe Basin, Northwest China. In our model, the wellbore flow is considered and coupled with a geothermal reservoir to better simulate the complex CO2 flow and heat production behavior. Based on the fractured geothermal reservoir at depths between 2900 m and 3300 m, the long-term (30-year) heat production performance is predicted using CO2 as the working fluid with fixed wellhead pressure. The results indicate that the proposed CO2-EGS will obtain an ascending heat extraction rate in the first 9 years, followed by a slight decrease in the following 21 years. Due to the significant natural convection of CO2 (e.g., low viscosity and density) in the geothermal reservoir, the mass production rate of the CO2-EGS will reach 150 kg/s. The heat extraction rates will be greater than 32 MW throughout the 30-year production period, showing a significant production performance. However, the Joule–Thomson effect in the wellbore will result in a drastic decrease in production temperature (e.g., a 62.6 °C decrease in the production well). This means that the pre-optimization analyses and physical material treatments are required during geothermal production using CO2 as the working fluid.

Evolution of fault-induced salt precipitation due to convection of CO2 and brine along fault during CO2 storage in multilayered saline aquifer-caprock

Till now, salt precipitation occurring in fault zone, named fault-induced salt precipitation, has not been reported for CO2 storage, although mineral precipitation has been reported occurring in Little Grand Wash Fault zone due to co-leakage of CO2 and water. Salt precipitation in fault zone can play critical roles to prevent CO2 leakage along fault, because the activated fault can be sealed by salt precipitation. In view of this, it would be investigated whether the salt precipitation can occur in fault and concluded for the evolution of fault-induced salt precipitation, especially considering convection of CO2 and brine along fault during CO2 storage in multilayered saline aquifer-caprock. In detail, the theoretical model was established to solve the solid saturation based on mass and energy conservation equations. By simplifying the discretized mass conservation equation, it could be found that salt precipitation was controlled by brine migration rate. Then, the physical model was proposed considering the convection of CO2 and brine along the fault, formed by CO2 migrating upwards and brine migrating downwards along fault. Furthermore, the numerical model was established to validate salt precipitation occurring in fault zone. The numerical results showed that: (i) the convection of CO2 migrating upwards and brine migrating downwards could be observed from velocity vectors of fluid migration; (ii) slow migration of brine could be found by comparing migration rates of CO2 and brine; (iii) salt precipitation indeed occurred in fault zone. By the numerical results, the evolution of salt precipitation was divided into three stages, namely, Stage I: convection of CO2 and brine; Stage II: slow migration of brine; Stage III: salt precipitation occurring. Finally, the parametric analyses were performed to investigate the effects of different parameters on salt precipitation. Results indicated that increasing in the injection rate and the fault height but decreasing in the injection time interval, the fault dip angle and the distance between fault and injection well were more beneficial for the salt precipitation in fault. The findings of this study can help for better understanding of the phenomenon of salt precipitation occurring in fault.

Modeling density-driven flow in porous media by physics-informed neural networks for CO2 sequestration

Accurate prediction of density-driven convection of CO2 overlaying brine in porous media is crucial to the applications of geological carbon sequestration. In this paper, we introduce the physics-informed neural network (PINN) method to investigate the mass transfer of CO2 injected into a homogeneous subsurface porous formation, where the concentration and the streamfunction are approximated by neural network representations. To mitigate the computational burden and training difficulties arising from learning the long-term diffusion and convection processes, we implement a time domain decomposition-based scheme so that the PINN solution can represent short-interval dynamics in a sequential manner. Furthermore, Fourier-type basis functions are encoded in the PINN architecture to map the spatial coordinates onto an embedding feature space characterized by various frequencies. The numerical results show that the use of Fourier feature embedding improves the training performance by handling the notorious spectral bias in deep neural networks and enhances the stability for long-term prediction of flow and transport. The proposed PINN method is applied to a benchmark problem to understand the effect of natural convection on the simulation performance. Compared to the high-fidelity stabilized FEM solution, the PINN approach presents agreeable statistical characteristics of CO2-rich brine fingers, demonstrating the effectiveness of PINNs in modeling density-driven flow.

Strength Failure of CO2 Injection Tubular Strings Considering CO2 Phase Transition

Compared with traditional injection tubular strings, the stresses on CO2 injection tubular strings are more complex. The results from field applications show that the phase transition of CO2 fluid in CO2 injection strings is an important factor in the calculation of temperature distribution and analysis of string mechanics. Therefore, we propose a strength analysis method for CO2 injection tubular strings that considers the CO2 phase transition. We selected four CO2 injection strings in an oil field in China as examples to evaluate their strength and safety. First, we established coupled differential equations for the temperature, pressure, and physical parameters of CO2 injection strings according to the theory of fluid flow and heat transfer. Then, we used an adaptive fuzzy neural network to construct the model for calculating the CO2 convection heat transfer coefficient and used this to obtain the high-precision convection heat transfer coefficients of tubular strings under conditions of CO2 flooding. We analyzed the injection-string deformations that resulted from the piston, spiral bending, expansion, friction and temperature effects according to the stress characteristics of the CO2 injection strings with packers under different working conditions. Finally, we performed mechanical analyses on the collapse resistance, internal pressure resistance, and tensile and triaxial stresses of the CO2 injection strings, and the results of these analyses provide a theoretical basis for the strength analysis of CO2 injection strings.

Effects of Impurity on the Density-Driven Convection of CO2 in Fractured and Rhythmic Saline Aquifers

Effects of Impurity on the Density-Driven Convection of CO2 in Fractured and Rhythmic Saline Aquifers

Effects of Impurity on the Density-Driven Convection of CO2 in Fractured and Rhythmic Saline Aquifers

Effects of Impurity on the Density-Driven Convection of CO2 in Fractured and Rhythmic Saline Aquifers

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

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

CO2 dissolution and convection in oil at realistic reservoir conditions: A visualization study

CO2 convective mixing has been extensively studied for CO2 dissolution in saline water but very limited with the presence of oil. The objective of this work was to visually study the supercritical CO2 (sCO2) dissolution and convective mixing into oil at realistic reservoir temperature and pressure conditions with and without the presence of porous media.A specially designed high-pressure 2D-cell was used to investigate the sCO2 mixing into oil phases. Schlieren imaging method was used as the visualization method. The experiments were carried out at 100 bar and 50 °C using n-octane, n-decane, and crude oil as the main oils. Porous media with different permeability was prepared using glass beads.Convective fingering was found to accelerate the mixing of CO2 with n-octane and n-decane. It was not possible to visualize the CO2 convective fingering in crude oil due to the low opacity of the oil phase. The CO2 dissolution into oil phases was quite instantaneous and fast without the presence of porous media. The swelling of oil was measured as 55%, 50% and 11% for n-decane, n-octane and crude oil respectively without the presence of porous media. Boundary effects were affecting the CO2 mixing due to the circular shape of the 2D-cell. Having a water layer below the oil layer tends to dampen the CO2 transport from the oil phase to the water phase. CO2 dissolution into oil saturated porous media was slower compared to that without the presence of porous media. The mixing of CO2 was faster at higher permeability than at lower permeability. Visualization of CO2 convective mixing/fingers inside oil-saturated porous media using a Hele-Shaw cell yet to be achieved experimentally.

Convective Mixing Driven by Non-monotonic Density

CO_text {2} injection for enhanced oil recovery (CO_text {2}-EOR) or for storage in depleted oil and gas reservoirs can be a means for disposing of anthropogenic CO_text {2} emissions to mitigate climate change. Fluid flow and mixing of CO_text {2} and hydrocarbons in such systems are governed by the underlying physics and thermodynamics. Gravity effects such as gravity override and convection are mechanisms that can alter fluid flow dynamics, impacting CO_text {2} migration, oil production and eventual CO_text {2} storage at the field scale. This study focuses on convection in a miscible setting caused by non-monotonicity in oil density when mixed with CO_text {2}, i.e., a maximum mixture density occurs at an intermediate CO_text {2} concentration. We perform high-resolution simulations to quantify the convective behavior in a simple box system where gravity effects are isolated. We show that convection of CO_text {2} in oil is dependent on whether CO_text {2} originates from above or below the oil zone. From above, convection follows classic convective mixing but is accelerated by viscosity decrease with increasing CO_text {2}. From below, convection flows upward due to CO_text {2} buoyancy, but is countered by downward convection due to the heavier mixture density. This convective system is significantly more complex and efficient than from above. We characterize the instabilities in both early- and late-time regimes and quantify mixing rates. For a 100 mD reservoir, convective fingers would be on the order of centimeters in width and mix over a meter length scale within days to a month, depending on the placement of CO_text {2}. The simulations are performed in non-dimensional form and thus can be rescaled to a different reservoir parameters. Our results give important insights into field-scale impacts of convective mixing and can guide future work in development of upscaled models and experimental design.

Unstable Density‐Driven Convection of CO2 in Homogeneous and Heterogeneous Porous Media With Implications for Deep Saline Aquifers

AbstractDensity‐driven convection mixing has been identified to enhance CO2 trapping in deep saline aquifers storage. In this study, the dynamics of fingering instability and associated mass transfer caused by convection mixing between the movements of miscible analogue fluid pairs are investigated by magnetic resonance imaging (MRI). Two kinds of homogeneous porous media and six kinds of heterogeneous porous media with varying permeability permutations were used at ambient pressure and temperature. In homogeneous porous media, the detail analyses on the data of fingering pattern, finger number and distribution identify that the large finger sizes and high sinking velocities can be developed when the permeability up to 3.3 × 10−11 m2. In heterogeneous porous media, the sand layer first touched by downward fingers plays a dominant role in finger evolution, channeling and fingering convection mechanisms were found among these cases. Moreover, the dissolution rate characterized by Sherwood number (Shm) (enhance or inhibit fluid mixing, depending on the layout of the sand layer) was found to be independent of the local convection dissolution, such as vortex flow and finger splitting at the sand boundary with mutation permeability (enhanced fluid mixing, promoting CO2 dissolution in carbon storage). Finally, the total CO2 dissolution rate Shm in all cases was compared, Shm are at a range of 970–5,079 in homogeneous porous media, while are 190–688 in heterogeneous porous media.

Visual investigation of CO2 dissolution and convection in heterogeneous porous media at reservoir temperature and pressure conditions

AbstractCO2 convective mixing in saline aquifers has been widely studied numerically and experimentally. Reservoir heterogeneity is significant for CO2 convective mixing and experimental studies are still limited. In this study, we have conducted a visualization of CO2 convective mixing experiments in heterogeneous porous media at reservoir conditions using CO2 and water. We have used a two‐dimension Hele–Shaw cell, different glass beads of different permeability at porous media, and water solution with pH indicator. Glass beads were packed in a different way (horizontally and vertically) to generate the heterogeneity inside the test cell. We have studied transport velocity deviation due to the heterogeneity and effects of permeability transition zone together with the effects of boundary conditions. It was found out that having a low permeable layer below a high permeable layer restructure the flow of CO2 fingers and dampens the CO2 transport velocity. With the vertical permeability zones, having a high permeability zone accelerates CO2 gravity transport through that zone which is a good representation for a fracture or a fault in the reservoir. CO2 convection onset is governed by the vertical high‐permeable layer. Boundary conditions have been dominant with the presence of high permeable zones. It also found out that the experimental results presented in this study match with the simulation studies that are available in the literature. © 2021 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons Ltd.

Quantitative study of density-driven convection mass transfer in porous media by MRI

CO2 geologic storage in deep saline aquifers is a promising technology to reduce greenhouse gas emissions and maintain the pace of economic development. The dissolution and convection of CO2 injected into brines are the primary trapping mechanisms to enhance storage efficiency and provide long-term storage security. The spatial mass transfer characteristics of convection in porous media remain poorly understood. In this study, two kinds of analogous fluid pairs were used as the equivalents of CO2-saturated brine and in situ brine. Density-driven convection experiments using magnetic resonance imaging (MRI) technology in heterogeneous porous media were performed at various stages, including the diffusion stage, the unstable trigger stage and the convection development stage. Combined with the spatial evolution of the fingers and the change in MRI mean intensity, the local fluid concentration and overall mass transfer behavior could be quantified. Physical quantities associated with mass transfer by convection, such as the mixing layer thickness and scalar dissipation rate at the diffusion stages, the onset time at the unstable trigger stage, the average velocity of the front finger, the finger number, the transverse concentration gradient of the finger, and the sweep area of the finger at the convection development stage, were also analyzed systematically. We found that convective mixing substantially promotes mass transfer over pure diffusion, and the dissolution rate can be further enhanced by improving the injection method at each stage. The range of Ra in this study covers most CO2 capture and storage (CCS) sites and can provide basic data for engineering applications.

Quartz halogen radiated fast and energy‐efficient convective vacuum drying of green tea fortified Himsagar mango: Optimization, kinetics, and quality assessment

AbstractThe nutritional attributes of Himsagar mango (HM; Mangifera indica L.) were enriched through fortification with green tea (GT; Camellia sinensis) extract followed by an energy‐proficient rapid drying technique. The HM fortified by GT (HMGT) was dehydrated employing vacuum‐drying under quartz halogen radiation (QHRVD) and the optimal process parameters were determined corresponding to a minimal moisture content of the final dehydrated HMGT deploying Taguchi orthogonal design. Additionally, an efficient periodic convective‐drying protocol was coupled with QHRVD (PQHRVD) to further escalate the dehydration of HMGT through cyclic injection of CO2 to stimulate forced diffusion, thus facilitating drying time reduction by approximately 50% compared to QHRVD. Faster drying rate and superior energy‐efficiency could be achieved in PQHRVD owing to higher effective moisture diffusivity (4.71 × 10−10 m2 s−1) and lower activation energy (2.13 kJ mol−1) in comparison with QHRVD (2.03 × 10−10 m2 s−1; 3.65 kJ mol−1). Thus, the periodic CO2 convection exhibited a remarkable reduction in processing time and energy consumption and produced a functionally enriched HM with significantly higher antioxidant properties, phenolics, ascorbic acid, and flavonoid contents.Practical applicationsHimsagar mango (HM), a tropical high‐demand fruit has been fortified with green tea (GT) (Camellia sinensis) extract for quality improvement. The amalgamated Himsagar mango was subsequently vacuum‐dried to improve the product shelf‐life and to augment the nutritional attributes. The energy‐efficient quartz halogen radiated vacuum‐drying (VD) technique was optimized to render a superior quality foodstuff. The VD operation was further intensified through periodic convection by CO2 to make the operation more beneficial. The cyclic CO2 convection resulted in significant augmentation in drying rate and rendered a lucrative foodstuff. The nutritionally enriched “HM fortified by GT” (HMGT) product displayed improved nutritional attributes (higher antioxidant properties, phenolics, ascorbic acid, and flavonoid contents) and enhanced shelf life for potential application in preparation of candy, cake, sweet, or dietary supplements. Therefore, the energy‐proficient and rapid drying method providing the functionally improved HMGT product can lay a prospective foundation for bakery and related food industries.

Assessment and modification of buoyancy criteria for supercritical pressure CO2 convection heat transfer in a horizontal tube

Buoyancy has significant effects on convection heat transfer of supercritical pressure fluids. This paper numerically investigates the asymmetric heat transfer characteristics of supercritical pressure CO2 in a horizontal tube with taking buoyancy effects into account. The applicability of three buoyancy criteria is validated by the computational results and new thresholds for buoyancy parameters are proposed. The software ANSYS FLUENT is adopted for present work. The computational variable is heat flux ranging from 1 kW/m2 to 22 kW/m2, the computational constants include tube diameter (22.14 mm), inlet temperature (288.55 K), outlet pressure (7.59 MPa) and mass flow rate (0.148 kg/s). The outlet temperatures for all cases are less than the pseudo-critical temperature. Results show that the asymmetric heat transfer does not change with the magnitude of buoyancy and the variation of overall heat transfer performance desynchronizes with the appearance of natural convection. The original thresholds of two buoyancy parameters are small to indicate the onset of buoyancy effect on overall heat transfer performance, the recommended critical values are given in this paper. The other buoyancy parameter is unsuitable to predict the buoyancy effect on overall heat transfer performance.

Large eddy simulations of high-heat-flux supercritical CO2 convection in microchannels: Mixed convection and non-uniform heating

Near-critical-point supercritical fluid convection is a promising solution for emerging high-flux thermal management needs because of the high fluid thermal conductivities and specific heats. Supercritical convection has been extensively studied for large diameter channels with uniform heating (4.1 < D < 22.7 mm, 0.05 < q'' < 330 W cm−2). However, limited information is available on transport processes to guide engineering of high-flux compact supercritical heat transfer equipment, which often have non-uniform heating distributions. To address this need, large eddy simulations (LES) are employed to study supercritical CO2 convection in microchannels (750μm×737μm cross-section). First, the simulation approach is validated with published experimental data. Studies are then conducted for horizontal isothermal heated channels at reduced pressure Pr=1.1, mass fluxes G=100-1000kgm-2s-1 (Re=3000-35,700), average wall heat fluxes q''=24-62Wcm-2, and bulk flow temperatures Tbulk=291-317K (inside and outside pseudocritical range). Results are used to assess the applicability of published supercritical convection correlations for microchannel heat exchangers. All available supercritical correlations are found to under-predict heat transfer coefficients at high heat fluxes (q''=58-62Wcm-2). The bottom-to-top wall heat transfer coefficient (HTC) ratios from these cases are used to determine a new criterion for the onset of significant mixed convection effects. At low mass fluxes (G=100kgm-2s-1), this HTC ratio is found to exceed 2.5×. Simulations indicate that, at these conditions, increased heat fluxes lead to reduced HTCs for low and high mass fluxes, but increased HTCs at intermediate mass flux values. Finally, an illustrative case is evaluated to assess the impact of conjugate heat transfer effects in microscale supercritical convection, and highlight limitations of conventional modeling approaches. This study quantifies the accuracy of convection correlations for high-heat-flux microchannel pseudocritical conditions, provides new criteria for predicting the onset of mixed convection in microchannel supercritical flows, and demonstrates the impact of conjugate heat transfer effects in microchannel supercritical heat exchangers.

Density‐Driven Convection in a Fractured Porous Media: Implications for Geological CO2 Storage

AbstractDissolution trapping is one of the primary mechanisms of carbon dioxide (CO2) storage in a geological formation. In this study, a numerical model was used to examine the impacts of single and multiple fractures on the transport of dissolved CO2 plumes in various geological settings. The effects of the fracture angle, fracture‐matrix permeability ratio, fracture intersection, and matrix heterogeneity on density‐driven CO2 convection were systematically investigated. The fractures were found to play time‐varying roles in both homogeneous and heterogeneous media by serving as preferential pathways for both CO2‐rich plumes (fingers) and CO2‐free water. The competition between the enhancement of convective mixing and the inhibition of finger growth by the upward flow of freshwater generated a complex flow system. The interaction between the strong upward flow of freshwater through the fractures and the falling CO2‐rich fingers through the porous matrix induced a positive feedback, resulting in accelerated domain‐scale circulation and CO2 dissolution. While the CO2‐rich fingers grew relatively evenly at the top boundary in the homogeneous media, they selectively developed through the high permeable zones in the heterogeneous media. Compared with homogeneous media, the heterogeneous media preserving fractures particularly generated a more dynamic fracture‐matrix mass transfer, resulting in more rapid CO2 dissolution. The findings of this study were extended to examine the effects of fracture connectivity on the enhancement of CO2 transport and dissolution on a field scale.

Solutal convection in porous media: Comparison between boundary conditions of constant concentration and constant flux

We numerically examine solutal convection in porous media, driven by the dissolution of carbon dioxide (CO2) into water---an effective mechanism for CO2 storage in saline aquifers. Dissolution is associated with slow diffusion of free-phase CO2 into the underlying aqueous phase followed by density-driven convective mixing of CO2 throughout the water-saturated layer. We study the fluid dynamics of CO2 convection in the single aqueous-phase region. A comparison is made between two different boundary conditions in the top of the formation: (i) a constant, maximum aqueous-phase concentration of CO2, and (ii) a constant, low injection-rate of CO2, such that all CO2 dissolves instantly and the system remains in single phase. The latter model is found to involve a nonlinear evolution of CO2 composition and associated aqueous-phase density, which depend on the formation permeability. We model the full nonlinear phase behavior of water-CO2 mixtures in a confined domain, consider dissolution and fluid compressibility, and relax the common Boussinesq approximation. We discover new flow regimes and present quantitative scaling relations for global characteristics of spreading, mixing, and a dissolution flux in two- and three-dimensional media for both boundary conditions. We also revisit the scaling behavior of Sherwood number (Sh) with Rayleigh number (Ra), which has been under debate for porous-media convection. Our measurements from the solutal convection in the range 1, 500<Ra<135, 000 show that the classical linear scaling Sh ~ Ra is attained asymptotically for the constant-concentration case. Similarly linear scaling is recovered for the constant-flux model problem.

Effects of Permeability Variations on CO2 Convection in Anisotropic and Heterogeneous Saline Formations

This study simulated the natural convection of dissolved carbon dioxide (CO_2) in a small-scale heterogeneous saline formation using the state module ECO2N equation in the TOUGHREACT model. A one-way downscaling approach that involves using a series of sub-models in simulation procedures was proposed to efficiently simulate problems with high-scale discrepancies. This study evaluated the effects of different degrees of small-scale permeability variations on the vertical migration of dissolved CO_2. The sequential Gaussian simulation model was used to generate unconditional random permeability fields for different natural logarithm of permeability (lnk) variations (i.e., lnk variances and correlations in x and z directions). The results showed an identical transition zone of dissolved CO_2 near the top boundary, where a constant CO_2 gas saturation was specified. The local permeability variations can trigger fingerings and enhance the vertical convection of the dissolved CO_2. The number of fingerings depends on the variations in permeability near the front interface of the dissolved CO_2 (i.e., the bottom edge of the transition zone for the dissolved CO_2). However, the fingering patterns and developments are constrained by the permeability variations along the fingering paths. At the same mean lnk permeability the convection fluxes increase with an increase in lnk variances. However, an increase in lateral correlations (i.e., increase in the correlation lengths in the x direction) can slightly reduce the convection fluxes at the same lnk variance. The highly variable flux rates of the dissolved CO_2 occur early and the variations in the flux rate decrease with time.