Density-driven Natural Convection Research Articles

Effect of permeability and density difference on convective mixing and storage of CO2 in saline aquifers

The density-driven natural convection is believed to have a positive impact on CO2 storage in saline aquifers. In this regard, studying the parameters that influence the onset of convection is crucial for determining the optimal storage location. Here, we conducted a numerical analysis to investigate how the permeability of the porous medium and the density difference between the brine and CO2-brine mixture affect the mixing efficiency, speed, and development of convective fingers. Results showed that greater permeability and density difference result in a faster onset of convection; hence, fluid velocity and mixing efficiency will increase. Moreover, considering CO2 dissolution based solely on Rayleigh number is not entirely accurate. In addition, if there are different permeability and density difference values, even with a constant Rayleigh number, the mixing of CO2 and brine would not be the same.

Kinetics of CO[formula omitted] dissolution for underground applications

In the current energy transition context, underground storage of carbon dioxide (CO2) in salt caverns and aquifers has emerged as a promising solution for the Power-to-Gas process complications. However, mass exchange between the CO2 and the aqueous phase present in the storage site remains a major problem. This paper addresses the question of the kinetics of this mass exchange by performing both experimental and numerical investigations. A series of experiments under storage conditions were carried out in a PVT (Pressure–Volume–Temperature) cell, exposing a volume of CO2 to pure water or brine at low and high pressure values ranging between 10 and 16.8 MPa and at constant temperatures of 30 and 40 °C. In order to interpret the obtained experimental results and to characterize the transient mass exchange, two modeling approaches were developed: a simplified one based on pure diffusion, and a second combining pure diffusion with density-driven natural convection. Both modeling approaches have proven accurate in predicting the kinetics of CO2 dissolution. The scope of this study is limited to the laboratory scale but it can be used to quantify the CO2 dissolution on the storage site scale.

Accelerated mass transfer enhancement by density-driven natural convection

ABSTRACT The prolonged heat and mass transfer can be enhanced by orders of magnitude that originate from the natural convection. This work examines the mass transfer enhancement in the fluid motion inside a horizontal capillary. In the experimental setup, dye or polystyrene (PS) in aqueous sucrose is taken on one side, and water is on the other side. The main purpose of employing two different systems (i.e. dye and PS) is that it can confirm whether the enhanced mass transfer is with only nanoparticles or it can be possible without nanoparticles in the presence of solute concentration gradients in a fluid. It is demonstrated that the induced flow is due to natural convection attributed to the density-driven flow. The induced flow in the present setup may open up various additional applications for the systems where rapid mixing is important, for instance, rapid formation of drug carriers.

An Experimental Study of Nonequilibrium Carbon Dioxide/Oil Interactions

SummaryIn this study, we use a custom-designed visual cell to investigate nonequilibrium carbon dioxide (CO2)/oil interactions under high-pressure/high-temperature conditions. We visualize the CO2/oil interface and measure the visual-cell pressure over time. We perform five sets of visualization tests. The first three tests aim at investigating interactions of gaseous (g), liquid (l), and supercritical (sc) CO2 with a Montney (MTN) oil sample. In the fourth test, to visualize the interactions in the bulk oil phase, we replace the opaque MTN oil with a translucent Duvernay (DUV) light oil (LO). Finally, we conduct an N2(sc)/oil test to compare the results with those of CO2(sc)/oil test. We also compare the results of nonequilibrium CO2/oil interactions with those obtained from conventional pressure/volume/temperature (PVT) tests.Results of the first three tests show that oil immediately expands upon injection of CO2 into the visual cell. CO2(sc) leads to the maximum oil expansion followed by CO2(l) and CO2(g). Furthermore, the rate of oil expansion in the CO2(sc)/oil test is higher than that in CO2(l)/oil and CO2(g)/oil tests. We also observe extracting and condensing flows at the CO2(l)/oil and CO2(sc)/oil interfaces. Moreover, we observe density-driven fingers inside the LO phase because of the local increase in the density of LO. The results of PVT tests show that the density of the CO2/oil mixture is higher than that of the CO2-free oil, explaining the density-driven natural convection during CO2(sc) injection into the visual cell. We do not observe either extracting/condensing flows or density-driven mixing for the N2(sc)/oil test, explaining the low expansion of oil in this test. The results suggest that the combination of density-driven natural convection and extracting/condensing flows enhances CO2(sc) dissolution into the oil phase, leading to fast oil expansion after CO2(sc) injection into the visual cell.

Qualitative and quantitative experimental study of convective mixing process during storage of CO2 in heterogeneous saline aquifers

CO2 dissolution is considered as one of the most promising mechanisms for trapping of free-phase CO2 into brine. It causes an increased density of the brine and initiation of gravitational instability that eventually leads to density-driven natural convection in saline aquifers. Correct estimation of the onset time for convection and the rate of dissolution of CO2 into brine is important because the timescale for dissolution corresponds to the timescale over which free-phase CO2 has a chance to leak out. The gravitational instability of a diffusive boundary layer in porous media has been studied in several papers in recent years, but there are few works about the behavior of density-driven natural convection mechanism in heterogeneous saline aquifers. Barriers such as shales and calcites layers are common types of heterogeneities in geological formations that are important in the fluid flow. Despite the recognized importance of convective dissolution in these heterogeneous geological formations, there is no experimental data available for studying the accelerated mass transfer rate of CO2 into these media.In this paper, we investigated the effect of the regular distribution of barriers on the rate of dissolution of CO2 into water and geometries of convection fingers. A series of experiments were performed using a precise experimental set-up with barrier heterogeneous Hele-Shaw cell geometries and by using CO2 and water. The approach and procedure for performing the experiments give us this opportunity to have both qualitative (images and movies) and quantitative (amount of the dissolved CO2 into water) data at the same time. The behavior of convection pattern after onset time and the effect of system properties on the behavior of convective mixing process will be presented and discussed. Moreover, some speeded-up movies from the experiments that are suitable for improving public awareness of the problem have been uploaded on the internet platform. Lastly, the relationships between dissolution flux after onset time for convection and barrier properties are discussed.

Effect of diffusing layer thickness on the density-driven natural convection of miscible fluids in porous media: Modeling of mass transport

Effect of diffusing layer thickness on the density-driven natural convection of miscible fluids in porous media: Modeling of mass transport

Qualitative and quantitative experimental study of convective mixing process during storage of CO2 in homogeneous saline aquifers

The storage of CO2 in deep formations like saline aquifers is being actively considered in order to support the reduction of greenhouse gas emissions. It has been observed that dissolution of CO2 into brine causes an increased density in the mixture and if the corresponding Rayleigh number of the porous medium is high enough to initiate convection flows, then density-driven natural convection occurs and the rate of dissolution increases. As such, an increased contribution by the dissolution mechanism for trapping of CO2 decreases the risk of leakage.However, despite the recognized importance of convective dissolution in geological CO2 storage, there is insufficient experimental data available for studying the accelerated mass transfer rate of CO2 into saline aquifers. In this paper, we focus on performing a series of novel experiments about the density-driven natural convection mechanism in a precise experimental set-up with homogeneous Hele-Shaw cell geometries and by using CO2 and water. Our new approach and procedure for performing the experiments give us an opportunity to have both qualitative (images and video) and quantitative (amount of dissolved CO2 into water) data at the same time. This study examines onset time for convection, critical wavelength of convection fingers and dissolution flux of CO2 into water as objective parameters. The growth and progress of convection fingers after onset time for convection and the effect of model properties on the behavior of the convective mixing process are presented and discussed. Moreover, there are some speeded-up videos from the experiments that are suitable for improving public awareness of the problem facing society. Based on the performed experiments, the calculated dimensionless onset times for convection are 194.90 and 227.80 from quantitative and qualitative measurements respectively and the calculated critical wavelength of convection fingers is 2π/0.0524. Additionally, the dissolution flux of CO2 into water after onset time for convection is related linearly to Δρ·g·cos(θ)·k·C0/μ by a prefactor of 0.021. These results are comparable with the available theoretical and numerical works in the literature. Comparison of the results of the experimental models and their equivalent numerical models shows that while the numerical models are good tools for prediction of growth mechanism, shapes and positions of convection fingers, their effectiveness in the prediction of the onset times for convection and the dissolution fluxes is uncertain.

Three-dimensional Finger Structure of Natural Convection in Homogeneous and Heterogeneous Porous Medium

In carbon dioxide (CO2) geological storage, dissolution of CO2 into brine aquifers has been regarded as one of the effective mechanisms for increasing the storage security against the potential risk of leakage due to buoyancy. In this study, density-driven natural convection between miscible fluids with a nonlinear density property in porous medium with homogeneous and heterogeneous layered structure is visualized three-dimensionally by means of X-ray computer tomography technology. The plastic resin particle with different average diameter has been packed stratified as a heterogeneous porous medium. When fingers pass through the interface of heterogeneous layered structure with an increasing permeability, the finger-extension velocity increases, the diameter of fingers decreases, and the concentration in fingers decreases because of the enhanced dispersion. On the other hand, when fingers pass through the interface with a decreasing permeability, the finger-extension velocity decreases, the finger diameter increases, but the concentration in fingers remains the same, which suggests that the concentration in the finger changes nonlinearly against the permeability. The development of natural convection in the layered heterogeneous structure becomes slower than that in the homogeneous structure.

多孔質の層状不均質性が密度差自然対流に与える影響

多孔質の層状不均質性が密度差自然対流に与える影響

Three-dimensional structure of natural convection in a porous medium: Effect of dispersion on finger structure

Density-driven natural convection in a porous medium is of great interest in the geoscience and geoengineering fields, especially for carbon dioxide capture and storage (CCS). In this study, we applied a novel experimental approach to three-dimensional (3D) imaging of density-driven natural convection in a porous medium. Miscible fluid pairs with nonlinear density properties (a sodium chloride solution and a mixture of methanol and ethylene glycol doped with sodium iodide) were used to model density-driven natural convection from 3D imaging captured by X-ray computed tomography (CT). Slight fluctuations that appeared at the interface grew into large “fingers,” which interacted and merged with the neighboring fingers. The fingers extended vertically downward without changing their locations, forming a columnar structure. The finger-extension velocity increased in line with the Rayleigh number and was correlated with the characteristic velocity. The 3D images captured the local concentration of fingers in the course of convective mixing. We found that the decrease in finger-number density was not only related to the Rayleigh number but was also affected by transverse dispersion between the downward- and upward-flow regions. There was an exponential relationship between the finger-number density and the transverse dispersion coefficient. Transverse dispersion caused broadening of the fingers and reduction in the finger-number density because of the interaction and merging of fingers. In carbon dioxide (CO2) geological storage applications, this transverse dispersion influences the long-term-dissolution process of CO2 injected into aquifers.

Three-Dimensional Visualization of Natural Convection in Porous Media

Carbon dioxide (CO2) geological storage (CGS) is a preferred technique to mitigate anthropogenic climate change. In terms of CGS, density-driven natural convection dominated dissolution of CO2 into formation brine results in a stable trapping increasing the storage security.A full utilization of a nonlinear density property of fluid system has been applied to mimic the natural convection in saline aquifers. Three-dimensional natural convection in porous medium has been observed by employing X-ray computer tomography (CT) technology. Since the X-ray attenuation depends on the concentration of sodium iodide, the distribution of local concentration can be evaluated during the downward extension based on the CT images.We successively visualized the process of formation and the development of Rayleigh-Bénard fingers. This process modelled natural convection between heavier CO2 loaded brine and brine with little (or no) CO2. High density fingers extend downward at the characteristic buoyancy velocity and coalescent with the neighbouring finger before reaching the bottom of the porous medium. The coalescence of fingers leads to a decrease in finger number and an increase in finger diameter. As the Rayleigh number increases, the finger tends to be fine and extends faster. The mass transfer rate increases with the Rayleigh number also.

Pore-scale study based on lattice Boltzmann method of density driven natural convection during CO2 injection project

Saline aquifers are chosen for geological storage of greenhouse gas CO2 because of their storage potential. In almost all cases of practical interest, CO2 is present on top of the liquid and CO2 dissolution leads to a small increase in the density of the aqueous phase. This situation results in the creation of negative buoyancy force for downward density-driven natural convection and consequently enhances CO2 sequestration. In order to study CO2 injection at pore-level, an isothermal Lattice Boltzmann Model (LBM) with two distribution functions is adopted to simulate density-driven natural convection in porous media with irregular geometry obtained by image treatment. The present analysis showed that after the onset of natural convection instability, the brine with a high CO2 concentration infringed into the underlying unaffected brine, in favor of the migration of CO2 into the pore structure. With low Rayleigh numbers, the instantaneous mass flux and total dissolved CO2 mass are very close to that derived from penetration theory (diffusion only), but the fluxes are significantly enhanced with high Ra number. The simulated results show that as the time increases, some chaotic and recirculation zones in the flow appear obviously, which promotes the renewal of interfacial liquid, and hence enhances dissolution of CO2 into brine. This study is focused on the scale of a few pores, but shows implications in enhanced oil/gas recovery with CO2 sequestration in aquifers.

An empirical theory for gravitationally unstable flow in porous media

In this paper, we follow a similar procedure as proposed by Koval (SPE J 3(2):145–154, 1963) to analytically model CO2 transfer between the overriding carbon dioxide layer and the brine layer below it. We show that a very thin diffusive layer on top separates the interface from a gravitationally unstable convective flow layer below it. Flow in the gravitationally unstable layer is described by the theory of Koval, a theory that is widely used and which describes miscible displacement as a pseudo two-phase flow problem. The pseudo two-phase flow problem provides the average concentration of CO2 in the brine as a function of distance. We find that downstream of the diffusive layer, the solution of the convective part of the model, is a rarefaction solution that starts at the saturation corresponding to the highest value of the fractional-flow function. The model uses two free parameters, viz., a dilution factor and a gravity fingering index. A comparison of the Koval model with the horizontally averaged concentrations obtained from 2-D numerical simulations provides a correlation for the two parameters with the Rayleigh number. The obtained scaling relations can be used in numerical simulators to account for the density-driven natural convection, which cannot be currently captured because the grid cells are typically orders of magnitude larger than the wavelength of the initial fingers. The method can be applied both for storage of greenhouse gases in aquifers and for EOR processes using carbon dioxide or other solvents.

Simulation Study of Density-Driven Natural Convection Mechanism in Isotropic and Anisotropic Brine Aquifers Using a Black Oil Reservoir Simulator

For simulation models of density-driven natural convection in brine aquifers gravitational instabilities are usually triggered by numerical round off errors. Using this method for initiating onset of convective flow, simulation results show that the onset time for convection is orders of magnitude larger than the ones predicted by theoretical stability analysis theory.In this study, a new approach for initializing of the model is developed based on available theoretical works. This initialization method is seen to give more accurate predictions of onset time for convection in the simulation models of isotropic and anisotropic brine aquifers.

Numerical Simulation of Natural Convection in Heterogeneous Porous media for CO2 Geological Storage

We report a modeling and numerical simulation study of density-driven natural convection during geological CO2 storage in heterogeneous formations. We consider an aquifer or depleted oilfield overlain by gaseous CO2, where the water density increases due to CO2 dissolution. The heterogeneity of the aquifer is represented by spatial variations of the permeability, generated using Sequential Gaussian Simulation method. The convective motion of the liquid with dissolved CO2 is investigated. Special attention is paid to instability characteristics of the CO2 concentration profiles, variation of mixing length, and average CO2 mass flux as a function of the heterogeneity characterized by the standard deviation and the correlation length of the log-normal permeability fields. The CO2 concentration profiles show different flow patterns of convective mixing such as gravity fingering, channeling, and dispersive based on the heterogeneity medium of the aquifer. The variation of mixing length with dimensionless time shows three separate regimes such as diffusion, convection, and second diffusion. The average CO2 mass flux at the top boundary decreases quickly at early times then it increases, reaching a constant value at later times for various heterogeneity parameters.

Nazari Moghaddam and Rostami’s Reply to My Comment on Their Paper “Quantification of Density-Driven Natural Convection for Dissolution Mechanism in CO2 Sequestration”

Nazari Moghaddam and Rostami’s Reply to My Comment on Their Paper “Quantification of Density-Driven Natural Convection for Dissolution Mechanism in CO2 Sequestration”

Comments on the Paper “Quantification of Density-Driven Natural Convection for Dissolution Mechanism in CO2 Sequestration” by R. Nazari Moghaddam et al. (2011)

Comments on the Paper “Quantification of Density-Driven Natural Convection for Dissolution Mechanism in CO2 Sequestration” by R. Nazari Moghaddam et al. (2011)

Reply to the “Comments on the Paper ‘Quantification of Density-Driven Natural Convection for Dissolution Mechanism in CO2 Sequestration’ by R. Nazari Moghaddam et al. (2011)”

Reply to the “Comments on the Paper ‘Quantification of Density-Driven Natural Convection for Dissolution Mechanism in CO2 Sequestration’ by R. Nazari Moghaddam et al. (2011)”

Quantification of Density-Driven Natural Convection for Dissolution Mechanism in CO2 Sequestration

Dissolution of CO2 into brine causes the density of the mixture to increase. The density gradient induces natural convection in the liquid phase, which is a favorable process of practical interest for CO2 storage. Correct estimation of the dissolution rate is important because the time scale for dissolution corresponds to the time scale over which free phase CO2 has a chance to leak out. However, for this estimation, the challenging simulation on the basis of convection–diffusion equation must be done. In this study, pseudo-diffusion coefficient is introduced which accounts for the rate of mass transferring by both convection and diffusion mechanisms. Experimental tests in fluid continuum and porous media were performed to measure the real rate of dissolution of CO2 into water during the time. The pseudo diffusion coefficient of CO2 into water was evaluated by the theory of pressure decay and this coefficient is used as a key parameter to quantify the natural convection and its effect on mass transfer of CO2. For each experiment, fraction of ultimate dissolution is calculated from measured pressure data and the results are compared with predicted values from analytical solution. Measured CO2 mass transfer rate from experiments are in reasonable agreement with values calculated from diffusion equation performed on the basis of pseudo-diffusion coefficient. It is suggested that solving diffusion equation with pseudo diffusion coefficient herein could be used as a simple and rapid tool to calculate the rate of mass transfer of CO2 in CCS projects.

Carbon Dioxide Storage Capacity of Organic-Rich Shales

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 134583, ’Carbon Dioxide Storage Capacity of Organic-Rich Shales,’ by S.M. Kang, SPE, E. Fathi, SPE, R.J. Ambrose, SPE, I.Y. Akkutlu, SPE, and R.F. Sigal, SPE, University of Oklahoma, prepared for the 2010 SPE Annual Technical Conference and Exhibition, Florence, Italy, 19-22 September. The paper has been peer reviewed for SPE Journal. An experimental study was performed on the ability of Barnett-shale core samples to store carbon dioxide (CO2). A new analytical methodology was developed that allows interpreting pressure/volume data in terms of measurements in total porosity and Langmuir parameters of core plugs. The method considers pore-volume compressibility and sorption effects and allows small gas-leakage adjustments at high pressures. Organic shale has the ability to store significant amounts of gas permanently by trapping the gas in an adsorbed state within its finely dispersed organic matter (i.e., kerogen). Introduction A primary consideration in subsurface sequestration of anthropogenic CO2 is knowledge of gas storability of the geological formations. At first thought, a high-pore-volume formation would seem to be a good candidate for the purpose. However, not all high-porosity formations are suitable for permanent storage of the gas. Some formations lack a physical mechanism for trapping gas. In the absence of a trapping mechanism, a free-gas cap is created in the formation artificially, which may not warrant long-term storage of the injected gas. Needed trapping is associated with fluid/fluid or fluid/solid interactions in porous media such as dissolution, physical adsorption, or some homogeneous and heterogeneous reactions. Depleted oil reservoirs, for example, could be considered acceptable targets. These reservoirs can trap CO2 that is not dissolved in reservoir brine to the extent that their seals for hydrocarbons also form a capillary seal for CO2. A second trapping mechanism is absorption of the injected gas by the immobile residual oil. This can be accomplished through multiple-contact (dynamic) miscibility, which involves simultaneous phase-change and mass-transport phenomena. Saline aquifers, on the other hand, allow aqueous-phase precipitation reactions as well as absorption by the formation water. The dissolved gas promotes density-driven natural convection of water and the related hydrodynamic instabilities. However, the injected gas could be transported and dispersed over large distances, leading to uncertainties concerning its fate. Coalbeds and naturally occurring gas-hydrate reservoirs have been proposed as economically feasible choices because the injected CO2 could be sequestered in sorbed states (adsorbed on microporous coal material surfaces and absorbed into organic macromolecular openings in coal and in water) and could aid enhanced recovery of natural gas by an in-situ molecular-swapping mechanism that promotes release and displacement of methane as a free gas. Introduction of greenhouse gases into these formations, however, is a difficult field operation with a significant or complete loss in well injectivity.