Xiang-Zhao Kong Publications

Dr. Xiang-Zhao Kong

Senior Assistant

kong_234x300

Mailing Address
Dr. Xiang-Zhao Kong
Geothermal Energy & Geofluids
Institute of Geophysics
NO F 57
Sonneggstrasse 5
CH-8092 Zurich Switzerland

Contact
Phone +41 44 632 55 86
Email xkong(at)ethz.ch

Administration
Dominique Ballarin Dolfin
Phone +41 44 632 3465
Email ballarin(at)ethz.ch

Publications

REFEREED PUBLICATIONS IN JOURNALS

25.  S.A. Galindo-Torres, T. Molebatsi, X.Z. Kong, A. Scheuermann, D. Bringemeier, L. Li Scaling solutions for connectivity and conductivity of continuous random networks, Physical Review E, Statistical, Nonlinear, and Soft Matter Physics, 92/4, pp. 041001, 2015. Abstract
Connectivity and conductivity of two-dimensional fracture networks (FNs), as an important type of continuous random networks, are examined systematically through Monte Carlo simulations under a variety of conditions, including different power law distributions of the fracture lengths and domain sizes. The simulation results are analyzed using analogies of the percolation theory for discrete random networks. With a characteristic length scale and conductivity scale introduced, we show that the connectivity and conductivity of FNs can be well described by universal scaling solutions. These solutions shed light on previous observations of scale-dependent FN behavior and provide a powerful method for quantifying effective bulk properties of continuous random networks.
/ Download
24.  B.M. Tutolo, X.-Z. Kong, W.E. Seyfried Jr., M.O. Saar High performance reactive transport simulations examining the effects of thermal, hydraulic, and chemical (THC) gradients on fluid injectivity at carbonate CCUS reservoir scales, International Journal of Greenhouse Gas Control, 39, pp. 285-301, 2015. Abstract
Carbonate minerals and CO2 are both considerably more soluble at low temperatures than they are at elevated temperatures. This inverse solubility has led a number of researchers to hypothesize that injecting low-temperature (i.e., less than the background reservoir temperature) CO2 into deep, saline reservoirs for CO2 Capture, Utilization, and Storage (CCUS) will dissolve CO2 and carbonate minerals near the injection well and subsequently exsolve and re-precipitate these phases as the fluids flow into the geothermally warm portion of the reservoir. In this study, we utilize high performance computing to examine the coupled effects of cool CO2 injection and background hydraulic head gradients on reservoir-scale mineral volume changes. We employ the fully coupled reactive transport simulator PFLOTRAN with calculations distributed over up to 800 processors to test 21 scenarios designed to represent a range of reservoir depths, hydraulic head gradients, and CO2 injection rates and temperatures. In the default simulations, 50 °C CO2 is injected at a rate of 50 kg/s into a 200 bar, 100 °C calcite or dolomite reservoir. By comparing these simulations with others run at varying conditions, we show that the effect of cool CO2 injection on reservoir-scale mineral volume changes tends to be relatively minor. We conclude that the low heat capacity of CO2 effectively prevents low-temperature CO2 injection from decreasing the temperature across large portions of the simulated carbonate reservoirs. This small thermal perturbation, combined with the low relative permeability of brine within the supercritical CO2 plume, yields limited dissolution and precipitation effects directly attributable to cool CO2 injection. Finally, we calculate that relatively high water-to-rock ratios, which may occur over much longer CCUS reservoir lifetimes or in materials with sufficiently high brine relative permeability within the supercritical CO2 plume, would be required to substantially affect injectivity through thermally-induced mineral dissolution and precipitation. Importantly, this study shows the utility of reservoir scale-reactive transport simulators for testing hypotheses and placing laboratory-scale observations into a CCUS reservoir-scale context.
/ Download
23.  B.M. Tutolo, A.J. Luhmann, X.-Z. Kong, M.O. Saar, W.E. Seyfried Jr. CO2 sequestration in feldspar-rich sandstone: Coupled evolution of fluid chemistry, mineral reaction rates, and hydrogeochemical properties, Geochimica Et Cosmochimica Acta, 160, pp. 132-154, 2015. Abstract
To investigate CO2 Capture, Utilization, and Storage (CCUS) in sandstones, we performed three 150 °C flow-through experiments on K-feldspar-rich cores from the Eau Claire formation. By characterizing fluid and solid samples from these experiments using a suite of analytical techniques, we explored the coupled evolution of fluid chemistry, mineral reaction rates, and hydrogeochemical properties during CO2 sequestration in feldspar-rich sandstone. Overall, our results confirm predictions that the heightened acidity resulting from supercritical CO2 injection into feldspar-rich sandstone will dissolve primary feldspars and precipitate secondary aluminum minerals. A core through which CO2-rich deionized water was recycled for 52 days decreased in bulk permeability, exhibited generally low porosity associated with high surface area in post-experiment core sub-samples, and produced an Al hydroxide secondary mineral, such as boehmite. However, two samples subjected to ?3 day single-pass experiments run with CO2-rich, 0.94 mol/kg NaCl brines decreased in bulk permeability, showed generally elevated porosity associated with elevated surface area in post-experiment core sub-samples, and produced a phase with kaolinite-like stoichiometry. CO2-induced metal mobilization during the experiments was relatively minor and likely related to Ca mineral dissolution. Based on the relatively rapid approach to equilibrium, the relatively slow near-equilibrium reaction rates, and the minor magnitudes of permeability changes in these experiments, we conclude that CCUS systems with projected lifetimes of several decades are geochemically feasible in the feldspar-rich sandstone end-member examined here. Additionally, the observation that K-feldspar dissolution rates calculated from our whole-rock experiments are in good agreement with literature parameterizations suggests that the latter can be utilized to model CCUS in K-feldspar-rich sandstone. Finally, by performing a number of reactive transport modeling experiments to explore processes occurring during the flow-through experiments, we have found that the overall progress of feldspar hydrolysis is negligibly affected by quartz dissolution, but significantly impacted by the rates of secondary mineral precipitation and their effect on feldspar saturation state. The observations produced here are critical to the development of models of CCUS operations, yet more work, particularly in the quantification of coupled dissolution and precipitation processes, will be required in order to produce models that can accurately predict the behavior of these systems.
/ Download
22.  Y. Ma, X. -Z. Kong, A. Scheuermann, S. A. Galindo-Torres, D. Bringemeier, L. Li Microbubble transport in water-saturated porous media, Water Resources Research, 51/6, pp. 4359-4373, 2015. Abstract
Laboratory experiments were conducted to investigate flow of discrete microbubbles through a water-saturated porous medium. During the experiments, bubbles, released from a diffuser, moved upward through a quasi-2-D flume filled with transparent water-based gelbeads and formed a distinct plume that could be well registered by a calibrated camera. Outflowing bubbles were collected on the top of the flume using volumetric burettes for flux measurements. We quantified the scaling behaviors between the gas (bubble) release rates and various characteristic parameters of the bubble plume, including plume tip velocity, plume width, and breakthrough time of the plume front. The experiments also revealed circulations of ambient pore water induced by the bubble flow. Based on a simple momentum exchange model, we showed that the relationship between the mean pore water velocity and gas release rate is consistent with the scaling solution for the bubble plume. These findings have important implications for studies of natural gas emission and air sparging, as well as fundamental research on bubble transport in porous media.
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21.  B.M. Tutolo, A.J. Luhmann, X.-Z. Kong, M.O. Saar, W.E. Seyfried Jr. Experimental observation of permeability changes in dolomite at CO2 sequestration conditions, Environmental Science and Technology, 48/4, pp. 2445-2452, 2014. Abstract
Injection of cool CO2 into geothermally warm carbonate reservoirs for storage or geothermal energy production may lower near-well temperature and lead to mass transfer along flow paths leading away from the well. To investigate this process, a dolomite core was subjected to a 650 h, high pressure, CO2 saturated, flow-through experiment. Permeability increased from 10–15.9 to 10–15.2 m2 over the initial 216 h at 21 °C, decreased to 10–16.2 m2 over 289 h at 50 °C, largely due to thermally driven CO2 exsolution, and reached a final value of 10–16.4 m2 after 145 h at 100 °C due to continued exsolution and the onset of dolomite precipitation. Theoretical calculations show that CO2 exsolution results in a maximum pore space CO2 saturation of 0.5, and steady state relative permeabilities of CO2 and water on the order of 0.0065 and 0.1, respectively. Post-experiment imagery reveals matrix dissolution at low temperatures, and subsequent filling-in of flow passages at elevated temperature. Geochemical calculations indicate that reservoir fluids subjected to a thermal gradient may exsolve and precipitate up to 200 cm3 CO2 and 1.5 cm3 dolomite per kg of water, respectively, resulting in substantial porosity and permeability redistribution.
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20.  A.J. Luhmann, X.-Z. Kong, B.M. Tutolo, N. Garapati, B.C. Bagley, M.O. Saar, W.E. Seyfried Jr. Experimental dissolution of dolomite by CO2-charged brine at 100oC and 150 bar: Evolution of porosity, permeability, and reactive surface area, Chemical Geology, 380, pp. 145-160, 2014. Abstract
Hydrothermal flow experiments of single-pass injection of CO2-charged brine were conducted on nine dolomite cores to examine fluid–rock reactions in dolomite reservoirs under geologic carbon sequestration conditions. Post-experimental X-ray computed tomography (XRCT) analysis illustrates a range of dissolution patterns, and significant increases in core bulk permeability were measured as the dolomite dissolved. Outflow fluids were below dolomite saturation, and cation concentrations decreased with time due to reductions in reactive surface area with reaction progress. To determine changes in reactive surface area, we employ a power-law relationship between reactive surface area and porosity (Luquot and Gouze, 2009). The exponent in this relationship is interpreted to be a geometrical parameter that controls the degree of surface area change per change in core porosity. Combined with XRCT reconstructions of dissolution patterns, we demonstrate that this exponent is inversely related to both the flow path diameter and tortuosity of the dissolution channel. Even though XRCT reconstructions illustrate dissolution at selected regions within each core, relatively high Ba and Mn recoveries in fluid samples suggest that dissolution occurred along the core’s entire length and width. Analysis of porosity–permeability data indicates an increase in the rate of permeability enhancement per increase in porosity with reaction progress as dissolution channels lengthen along the core. Finally, we incorporate the surface area–porosity model of Luquot and Gouze (2009) with our experimentally fit parameters into TOUGHREACT to simulate experimental observations.
/ Download
19.  B.M. Tutolo, X.-Z. Kong, W.E. Seyfried Jr., M.O. Saar Internal consistency in aqueous geochemical data revisited: Applications to the aluminum system, Geochimica et Cosmochimica Acta, 133, pp. 216-234, 2014. Abstract
Internal consistency of thermodynamic data has long been considered vital for confident calculations of aqueous geochemical processes. However, an internally consistent mineral thermodynamic data set is not necessarily consistent with calculations of aqueous species thermodynamic properties due, potentially, to improper or inconsistent constraints used in the derivation process. In this study, we attempt to accommodate the need for a mineral thermodynamic data set that is internally consistent with respect to aqueous species thermodynamic properties by adapting the least squares optimization methods of Powell and Holland (1985). This adapted method allows for both the derivation of mineral thermodynamic properties from fluid chemistry measurements of solutions in equilibrium with mineral assemblages, as well as estimates of the uncertainty on the derived results. Using a large number of phase equilibria, solubility, and calorimetric measurements, we have developed a thermodynamic data set of 12 key aluminum-bearing mineral phases. These data are derived to be consistent with Na+ and K+ speciation data presented by Shock and Helgeson (1988), H4SiO4(aq) data presented by Stefánsson (2001), and the Al speciation data set presented by Tagirov and Schott (2001). Many of the constraining phase equilibrium measurements are exactly the same as those used to develop other thermodynamic data, yet our derived values tend to be quite different than some of the others’ due to our choices of reference data. The differing values of mineral thermodynamic properties have implications for calculations of Al mineral solubilities; specifically, kaolinite solubilities calculated with the developed data set are as much as 6.75 times lower and 73% greater than those calculated with Helgeson et al. (1978) and Holland and Powell (2011) data, respectively. Where possible, calculations and experimental data are compared at low T, and the disagreement between the two sources reiterates the common assertion that low-T measurements of phase equilibria and mineral solubilities in the aluminum system rarely represent equilibrium between water and well-crystallized, aluminum-bearing minerals. As an ancillary benefit of the derived data, we show that it may be combined with high precision measurements of aqueous complex association constants to derive neutral species activity coefficients in supercritical fluids. Although this contribution is specific to the aluminum system, the methods and concepts developed here can help to improve the calculation of water–rock interactions in a broad range of earth systems.
/ Download
18.  Y. Ma, A. Scheuermann, D. Bringemeier, X.-Z. Kong, L. Li Size distribution measurement for densely binding bubbles via image analysis, Experiments in Fluids, 55/1860, 2014. Download
17.  X.-Z. Kong, M.O. Saar DBCreate: A SUPCRT92-based program for producing EQ3/6, TOUGHREACT, and GWB thermodynamic databases at user-defined T and P, Computers and Geosciences, 51, pp. 415-417, 2013. Abstract
SUPCRT92 is a widely used software package for calculating the standard thermodynamic properties of minerals, gases, aqueous species, and reactions. However, it is labor-intensive and error-prone to use it directly to produce databases for geochemical modeling programs such as EQ3/6, the Geochemist’s Workbench, and TOUGHREACT. DBCreate is a SUPCRT92-based software program written in FORTRAN90/95 and was developed in order to produce the required databases for these programs in a rapid and convenient way. This paper describes the overall structure of the program and provides detailed usage instructions.
/ Download
16.  X.-Z. Kong, M.O. Saar Numerical study of the effects of permeability heterogeneity on density-driven convective mixing during CO2 dissolution storage, Int. J. Greenhouse Gas Control, 19, pp. 160-173, 2013. Abstract
Permanence and security of carbon dioxide (CO2) in geologic formations requires dissolution of CO2 into brine, which slightly increases the brine density. Previous studies have shown that this small increase in brine density induces convective currents, which greatly enhances the mixing efficiency and thus CO2 storage capacity and rate in the brine. Density-driven convection, in turn, is known to be largely dominated by permeability heterogeneity. This study explores the relationship between the process of density-driven convection and the permeability heterogeneity of an aquifer during CO2 dissolution storage, using high-resolution numerical simulations. While the porosity is kept constant, the heterogeneity of the aquifer is introduced through a spatially varying permeability field, characterized by the Dykstra-Parsons coefficient and the correlation length. Depending on the concentration profile of dissolved CO2, we classify the convective finger patterns as dispersive, preferential, and unbiased fingering. Our results indicate that the transition between unbiased and both preferential and dispersive fingering is mainly governed by the Dykstra-Parsons coefficient, whereas the transition between preferential and dispersive fingering is controlled by the permeability correlation length. Furthermore, we find that the CO2 dissolution flux at the top boundary will reach a time-independent steady state. Although this flux strongly correlates with permeability distribution, it generally increases with the permeability heterogeneity when the correlation length is less than the system size.
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15.  A.J. Luhmann, X.-Z. Kong, B.M. Tutolo, K. Ding, M.O. Saar, W.E. Seyfried Jr. Permeability reduction produced by grain reorganization and accumulation of exsolved CO2 during geologic carbon sequestration: A new CO2 trapping mechanism, Environmental Science and Technlogy, 47/1, pp. 242-251, 2013. Abstract
Carbon sequestration experiments were conducted on uncemented sediment and lithified rock from the Eau Claire Formation, which consisted primarily of K-feldspar and quartz. Cores were heated to accentuate reactivity between fluid and mineral grains and to force CO2 exsolution. Measured permeability of one sediment core ultimately reduced by 4 orders of magnitude as it was incrementally heated from 21 to 150 °C. Water-rock interaction produced some alteration, yielding sub-?m clay precipitation on K-feldspar grains in the core’s upstream end. Experimental results also revealed abundant newly formed pore space in regions of the core, and in some cases pores that were several times larger than the average grain size of the sediment. These large pores likely formed from elevated localized pressure caused by rapid CO2 exsolution within the core and/or an accumulating CO2 phase capable of pushing out surrounding sediment. CO2 filled the pores and blocked flow pathways. Comparison with a similar experiment using a solid arkose core indicates that CO2 accumulation and grain reorganization mainly contributed to permeability reduction during the heated sediment core experiment. This suggests that CO2 injection into sediments may store more CO2 and cause additional permeability reduction than is possible in lithified rock due to grain reorganization.
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14.  X.-Z. Kong, M. Holzner, F. Stauffer, W. Kinzelbach Time-resolved 3D visualization of air injection in a liquidsaturated refractive-index-matched porous medium, Exp. Fluids, 50, pp. 1659-1670, 2011. Download
13.  X.-Z. Kong, W. Kinzelbach Morphodynamics during air injection into water-saturated movable spherical granulates, Chem. Eng. Sci., Chem.Eng.Sci., 65, pp. 4652-4660, 2010. Download
12.  X.-Z. Kong, W. Kinzelbach Compaction and size segregation in a liquid-saturated grain packing due to pulsation effect during air injection, Chem.Eng.Sci., 65, pp. 2680-2688, 2010. Download
11.  F. Stauffer, X.-Z. Kong, W. Kinzelbach A stochastic model for air injection into saturated porous media, Water Resour, 32, pp. 1180-1186, 2009. Download
10.  X.-Z. Kong, W. Kinzelbach Migration of air channels: an instability of air flow in mobile saturated porous media, Chem.Eng.Sci., 64, pp. 1528-1535, 2009. Download
9.  W.-J. Wang, X.-Z. Kong, Z.-G. Zhu Friction and relative energy dissipation in sheared granular materials, Phys.Rev. E, 75/041302, 2007. Download
8.  X.-Z. Kong, M.-B. Hu, Q.-S. Wu, Y.-H. Wu Effects of vibration frequency on intruders’ position in granular bed, Phys. Lett A, 356, pp. 267-271, 2006. Download
7.  X.-Z. Kong, M.-B. Hu, Q.-S. Wu, Y.-H. Wu Effects of bottleneck on granular convection cells and segregation, Granular Matter, 8, pp. 119-124, 2006. Download
6.  X.-Z. Kong, M.-B. Hu, Q.-S. Wu, Y.-H. Wu Kinetic energy sandpile model for conical sandpile development by evolving rivers, Phys. Lett A, 348, pp. 77-81, 2006. Download
5.  M.-B. Hu, X.-Z. Kong, Q.-S. Wu, Y.-H. Wu Effects of container geometry on granular segregation pattern, Chinese Physics, 14, pp. 1793-1800, 2005. Download
4.  M.-B. Hu, X.-Z. Kong, Q.-S. Wu, Y.-H. Wu Granular segregation in a multi-bottleneck container: Mobility effect, Int. J. Mod. Phys. B, 19, pp. 1793-1800, 2005. Download
3.  X.-Z. Kong, M.-B. Hu, Q.-S. Wu, Y.-H. Wu Ring-like size segregation in vibrated cylinder with a bottleneck, Phys. Lett A, 341, pp. 278-284, 2005. Download
2.  M.-B. Hu, Q.-S. Wu, X.-Z. Kong, Y.-H. Wu Discharge oscillation of particles from a vertical Pipe with capillary outlet, Chinese Science Bulletin, 50, pp. 1076-1078, 2005. Download
1.  M.-B. Hu, X.-Z. Kong, Q.-S. Wu, Z.-G. Zhu Experimental study of energy absorption properties of granular materials under low frequency vibrations, Int. J. Mod. Phys. B., 18, pp. 2708-2712, 2004. Download

show/hide list of publications

REFEREED PUBLICATIONS IN JOURNALS

25.  S.A. Galindo-Torres, T. Molebatsi, X.Z. Kong, A. Scheuermann, D. Bringemeier, L. Li Scaling solutions for connectivity and conductivity of continuous random networks, Physical Review E, Statistical, Nonlinear, and Soft Matter Physics, 92/4, pp. 041001, 2015. Abstract
Connectivity and conductivity of two-dimensional fracture networks (FNs), as an important type of continuous random networks, are examined systematically through Monte Carlo simulations under a variety of conditions, including different power law distributions of the fracture lengths and domain sizes. The simulation results are analyzed using analogies of the percolation theory for discrete random networks. With a characteristic length scale and conductivity scale introduced, we show that the connectivity and conductivity of FNs can be well described by universal scaling solutions. These solutions shed light on previous observations of scale-dependent FN behavior and provide a powerful method for quantifying effective bulk properties of continuous random networks.
/ Download
24.  B.M. Tutolo, X.-Z. Kong, W.E. Seyfried Jr., M.O. Saar High performance reactive transport simulations examining the effects of thermal, hydraulic, and chemical (THC) gradients on fluid injectivity at carbonate CCUS reservoir scales, International Journal of Greenhouse Gas Control, 39, pp. 285-301, 2015. Abstract
Carbonate minerals and CO2 are both considerably more soluble at low temperatures than they are at elevated temperatures. This inverse solubility has led a number of researchers to hypothesize that injecting low-temperature (i.e., less than the background reservoir temperature) CO2 into deep, saline reservoirs for CO2 Capture, Utilization, and Storage (CCUS) will dissolve CO2 and carbonate minerals near the injection well and subsequently exsolve and re-precipitate these phases as the fluids flow into the geothermally warm portion of the reservoir. In this study, we utilize high performance computing to examine the coupled effects of cool CO2 injection and background hydraulic head gradients on reservoir-scale mineral volume changes. We employ the fully coupled reactive transport simulator PFLOTRAN with calculations distributed over up to 800 processors to test 21 scenarios designed to represent a range of reservoir depths, hydraulic head gradients, and CO2 injection rates and temperatures. In the default simulations, 50 °C CO2 is injected at a rate of 50 kg/s into a 200 bar, 100 °C calcite or dolomite reservoir. By comparing these simulations with others run at varying conditions, we show that the effect of cool CO2 injection on reservoir-scale mineral volume changes tends to be relatively minor. We conclude that the low heat capacity of CO2 effectively prevents low-temperature CO2 injection from decreasing the temperature across large portions of the simulated carbonate reservoirs. This small thermal perturbation, combined with the low relative permeability of brine within the supercritical CO2 plume, yields limited dissolution and precipitation effects directly attributable to cool CO2 injection. Finally, we calculate that relatively high water-to-rock ratios, which may occur over much longer CCUS reservoir lifetimes or in materials with sufficiently high brine relative permeability within the supercritical CO2 plume, would be required to substantially affect injectivity through thermally-induced mineral dissolution and precipitation. Importantly, this study shows the utility of reservoir scale-reactive transport simulators for testing hypotheses and placing laboratory-scale observations into a CCUS reservoir-scale context.
/ Download
23.  B.M. Tutolo, A.J. Luhmann, X.-Z. Kong, M.O. Saar, W.E. Seyfried Jr. CO2 sequestration in feldspar-rich sandstone: Coupled evolution of fluid chemistry, mineral reaction rates, and hydrogeochemical properties, Geochimica Et Cosmochimica Acta, 160, pp. 132-154, 2015. Abstract
To investigate CO2 Capture, Utilization, and Storage (CCUS) in sandstones, we performed three 150 °C flow-through experiments on K-feldspar-rich cores from the Eau Claire formation. By characterizing fluid and solid samples from these experiments using a suite of analytical techniques, we explored the coupled evolution of fluid chemistry, mineral reaction rates, and hydrogeochemical properties during CO2 sequestration in feldspar-rich sandstone. Overall, our results confirm predictions that the heightened acidity resulting from supercritical CO2 injection into feldspar-rich sandstone will dissolve primary feldspars and precipitate secondary aluminum minerals. A core through which CO2-rich deionized water was recycled for 52 days decreased in bulk permeability, exhibited generally low porosity associated with high surface area in post-experiment core sub-samples, and produced an Al hydroxide secondary mineral, such as boehmite. However, two samples subjected to ?3 day single-pass experiments run with CO2-rich, 0.94 mol/kg NaCl brines decreased in bulk permeability, showed generally elevated porosity associated with elevated surface area in post-experiment core sub-samples, and produced a phase with kaolinite-like stoichiometry. CO2-induced metal mobilization during the experiments was relatively minor and likely related to Ca mineral dissolution. Based on the relatively rapid approach to equilibrium, the relatively slow near-equilibrium reaction rates, and the minor magnitudes of permeability changes in these experiments, we conclude that CCUS systems with projected lifetimes of several decades are geochemically feasible in the feldspar-rich sandstone end-member examined here. Additionally, the observation that K-feldspar dissolution rates calculated from our whole-rock experiments are in good agreement with literature parameterizations suggests that the latter can be utilized to model CCUS in K-feldspar-rich sandstone. Finally, by performing a number of reactive transport modeling experiments to explore processes occurring during the flow-through experiments, we have found that the overall progress of feldspar hydrolysis is negligibly affected by quartz dissolution, but significantly impacted by the rates of secondary mineral precipitation and their effect on feldspar saturation state. The observations produced here are critical to the development of models of CCUS operations, yet more work, particularly in the quantification of coupled dissolution and precipitation processes, will be required in order to produce models that can accurately predict the behavior of these systems.
/ Download
22.  Y. Ma, X. -Z. Kong, A. Scheuermann, S. A. Galindo-Torres, D. Bringemeier, L. Li Microbubble transport in water-saturated porous media, Water Resources Research, 51/6, pp. 4359-4373, 2015. Abstract
Laboratory experiments were conducted to investigate flow of discrete microbubbles through a water-saturated porous medium. During the experiments, bubbles, released from a diffuser, moved upward through a quasi-2-D flume filled with transparent water-based gelbeads and formed a distinct plume that could be well registered by a calibrated camera. Outflowing bubbles were collected on the top of the flume using volumetric burettes for flux measurements. We quantified the scaling behaviors between the gas (bubble) release rates and various characteristic parameters of the bubble plume, including plume tip velocity, plume width, and breakthrough time of the plume front. The experiments also revealed circulations of ambient pore water induced by the bubble flow. Based on a simple momentum exchange model, we showed that the relationship between the mean pore water velocity and gas release rate is consistent with the scaling solution for the bubble plume. These findings have important implications for studies of natural gas emission and air sparging, as well as fundamental research on bubble transport in porous media.
/ Download
21.  B.M. Tutolo, A.J. Luhmann, X.-Z. Kong, M.O. Saar, W.E. Seyfried Jr. Experimental observation of permeability changes in dolomite at CO2 sequestration conditions, Environmental Science and Technology, 48/4, pp. 2445-2452, 2014. Abstract
Injection of cool CO2 into geothermally warm carbonate reservoirs for storage or geothermal energy production may lower near-well temperature and lead to mass transfer along flow paths leading away from the well. To investigate this process, a dolomite core was subjected to a 650 h, high pressure, CO2 saturated, flow-through experiment. Permeability increased from 10–15.9 to 10–15.2 m2 over the initial 216 h at 21 °C, decreased to 10–16.2 m2 over 289 h at 50 °C, largely due to thermally driven CO2 exsolution, and reached a final value of 10–16.4 m2 after 145 h at 100 °C due to continued exsolution and the onset of dolomite precipitation. Theoretical calculations show that CO2 exsolution results in a maximum pore space CO2 saturation of 0.5, and steady state relative permeabilities of CO2 and water on the order of 0.0065 and 0.1, respectively. Post-experiment imagery reveals matrix dissolution at low temperatures, and subsequent filling-in of flow passages at elevated temperature. Geochemical calculations indicate that reservoir fluids subjected to a thermal gradient may exsolve and precipitate up to 200 cm3 CO2 and 1.5 cm3 dolomite per kg of water, respectively, resulting in substantial porosity and permeability redistribution.
/ Download
20.  A.J. Luhmann, X.-Z. Kong, B.M. Tutolo, N. Garapati, B.C. Bagley, M.O. Saar, W.E. Seyfried Jr. Experimental dissolution of dolomite by CO2-charged brine at 100oC and 150 bar: Evolution of porosity, permeability, and reactive surface area, Chemical Geology, 380, pp. 145-160, 2014. Abstract
Hydrothermal flow experiments of single-pass injection of CO2-charged brine were conducted on nine dolomite cores to examine fluid–rock reactions in dolomite reservoirs under geologic carbon sequestration conditions. Post-experimental X-ray computed tomography (XRCT) analysis illustrates a range of dissolution patterns, and significant increases in core bulk permeability were measured as the dolomite dissolved. Outflow fluids were below dolomite saturation, and cation concentrations decreased with time due to reductions in reactive surface area with reaction progress. To determine changes in reactive surface area, we employ a power-law relationship between reactive surface area and porosity (Luquot and Gouze, 2009). The exponent in this relationship is interpreted to be a geometrical parameter that controls the degree of surface area change per change in core porosity. Combined with XRCT reconstructions of dissolution patterns, we demonstrate that this exponent is inversely related to both the flow path diameter and tortuosity of the dissolution channel. Even though XRCT reconstructions illustrate dissolution at selected regions within each core, relatively high Ba and Mn recoveries in fluid samples suggest that dissolution occurred along the core’s entire length and width. Analysis of porosity–permeability data indicates an increase in the rate of permeability enhancement per increase in porosity with reaction progress as dissolution channels lengthen along the core. Finally, we incorporate the surface area–porosity model of Luquot and Gouze (2009) with our experimentally fit parameters into TOUGHREACT to simulate experimental observations.
/ Download
19.  B.M. Tutolo, X.-Z. Kong, W.E. Seyfried Jr., M.O. Saar Internal consistency in aqueous geochemical data revisited: Applications to the aluminum system, Geochimica et Cosmochimica Acta, 133, pp. 216-234, 2014. Abstract
Internal consistency of thermodynamic data has long been considered vital for confident calculations of aqueous geochemical processes. However, an internally consistent mineral thermodynamic data set is not necessarily consistent with calculations of aqueous species thermodynamic properties due, potentially, to improper or inconsistent constraints used in the derivation process. In this study, we attempt to accommodate the need for a mineral thermodynamic data set that is internally consistent with respect to aqueous species thermodynamic properties by adapting the least squares optimization methods of Powell and Holland (1985). This adapted method allows for both the derivation of mineral thermodynamic properties from fluid chemistry measurements of solutions in equilibrium with mineral assemblages, as well as estimates of the uncertainty on the derived results. Using a large number of phase equilibria, solubility, and calorimetric measurements, we have developed a thermodynamic data set of 12 key aluminum-bearing mineral phases. These data are derived to be consistent with Na+ and K+ speciation data presented by Shock and Helgeson (1988), H4SiO4(aq) data presented by Stefánsson (2001), and the Al speciation data set presented by Tagirov and Schott (2001). Many of the constraining phase equilibrium measurements are exactly the same as those used to develop other thermodynamic data, yet our derived values tend to be quite different than some of the others’ due to our choices of reference data. The differing values of mineral thermodynamic properties have implications for calculations of Al mineral solubilities; specifically, kaolinite solubilities calculated with the developed data set are as much as 6.75 times lower and 73% greater than those calculated with Helgeson et al. (1978) and Holland and Powell (2011) data, respectively. Where possible, calculations and experimental data are compared at low T, and the disagreement between the two sources reiterates the common assertion that low-T measurements of phase equilibria and mineral solubilities in the aluminum system rarely represent equilibrium between water and well-crystallized, aluminum-bearing minerals. As an ancillary benefit of the derived data, we show that it may be combined with high precision measurements of aqueous complex association constants to derive neutral species activity coefficients in supercritical fluids. Although this contribution is specific to the aluminum system, the methods and concepts developed here can help to improve the calculation of water–rock interactions in a broad range of earth systems.
/ Download
18.  Y. Ma, A. Scheuermann, D. Bringemeier, X.-Z. Kong, L. Li Size distribution measurement for densely binding bubbles via image analysis, Experiments in Fluids, 55/1860, 2014. Download
17.  X.-Z. Kong, M.O. Saar DBCreate: A SUPCRT92-based program for producing EQ3/6, TOUGHREACT, and GWB thermodynamic databases at user-defined T and P, Computers and Geosciences, 51, pp. 415-417, 2013. Abstract
SUPCRT92 is a widely used software package for calculating the standard thermodynamic properties of minerals, gases, aqueous species, and reactions. However, it is labor-intensive and error-prone to use it directly to produce databases for geochemical modeling programs such as EQ3/6, the Geochemist’s Workbench, and TOUGHREACT. DBCreate is a SUPCRT92-based software program written in FORTRAN90/95 and was developed in order to produce the required databases for these programs in a rapid and convenient way. This paper describes the overall structure of the program and provides detailed usage instructions.
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16.  X.-Z. Kong, M.O. Saar Numerical study of the effects of permeability heterogeneity on density-driven convective mixing during CO2 dissolution storage, Int. J. Greenhouse Gas Control, 19, pp. 160-173, 2013. Abstract
Permanence and security of carbon dioxide (CO2) in geologic formations requires dissolution of CO2 into brine, which slightly increases the brine density. Previous studies have shown that this small increase in brine density induces convective currents, which greatly enhances the mixing efficiency and thus CO2 storage capacity and rate in the brine. Density-driven convection, in turn, is known to be largely dominated by permeability heterogeneity. This study explores the relationship between the process of density-driven convection and the permeability heterogeneity of an aquifer during CO2 dissolution storage, using high-resolution numerical simulations. While the porosity is kept constant, the heterogeneity of the aquifer is introduced through a spatially varying permeability field, characterized by the Dykstra-Parsons coefficient and the correlation length. Depending on the concentration profile of dissolved CO2, we classify the convective finger patterns as dispersive, preferential, and unbiased fingering. Our results indicate that the transition between unbiased and both preferential and dispersive fingering is mainly governed by the Dykstra-Parsons coefficient, whereas the transition between preferential and dispersive fingering is controlled by the permeability correlation length. Furthermore, we find that the CO2 dissolution flux at the top boundary will reach a time-independent steady state. Although this flux strongly correlates with permeability distribution, it generally increases with the permeability heterogeneity when the correlation length is less than the system size.
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15.  A.J. Luhmann, X.-Z. Kong, B.M. Tutolo, K. Ding, M.O. Saar, W.E. Seyfried Jr. Permeability reduction produced by grain reorganization and accumulation of exsolved CO2 during geologic carbon sequestration: A new CO2 trapping mechanism, Environmental Science and Technlogy, 47/1, pp. 242-251, 2013. Abstract
Carbon sequestration experiments were conducted on uncemented sediment and lithified rock from the Eau Claire Formation, which consisted primarily of K-feldspar and quartz. Cores were heated to accentuate reactivity between fluid and mineral grains and to force CO2 exsolution. Measured permeability of one sediment core ultimately reduced by 4 orders of magnitude as it was incrementally heated from 21 to 150 °C. Water-rock interaction produced some alteration, yielding sub-?m clay precipitation on K-feldspar grains in the core’s upstream end. Experimental results also revealed abundant newly formed pore space in regions of the core, and in some cases pores that were several times larger than the average grain size of the sediment. These large pores likely formed from elevated localized pressure caused by rapid CO2 exsolution within the core and/or an accumulating CO2 phase capable of pushing out surrounding sediment. CO2 filled the pores and blocked flow pathways. Comparison with a similar experiment using a solid arkose core indicates that CO2 accumulation and grain reorganization mainly contributed to permeability reduction during the heated sediment core experiment. This suggests that CO2 injection into sediments may store more CO2 and cause additional permeability reduction than is possible in lithified rock due to grain reorganization.
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14.  X.-Z. Kong, M. Holzner, F. Stauffer, W. Kinzelbach Time-resolved 3D visualization of air injection in a liquidsaturated refractive-index-matched porous medium, Exp. Fluids, 50, pp. 1659-1670, 2011. Download
13.  X.-Z. Kong, W. Kinzelbach Morphodynamics during air injection into water-saturated movable spherical granulates, Chem. Eng. Sci., Chem.Eng.Sci., 65, pp. 4652-4660, 2010. Download
12.  X.-Z. Kong, W. Kinzelbach Compaction and size segregation in a liquid-saturated grain packing due to pulsation effect during air injection, Chem.Eng.Sci., 65, pp. 2680-2688, 2010. Download
11.  F. Stauffer, X.-Z. Kong, W. Kinzelbach A stochastic model for air injection into saturated porous media, Water Resour, 32, pp. 1180-1186, 2009. Download
10.  X.-Z. Kong, W. Kinzelbach Migration of air channels: an instability of air flow in mobile saturated porous media, Chem.Eng.Sci., 64, pp. 1528-1535, 2009. Download
9.  W.-J. Wang, X.-Z. Kong, Z.-G. Zhu Friction and relative energy dissipation in sheared granular materials, Phys.Rev. E, 75/041302, 2007. Download
8.  X.-Z. Kong, M.-B. Hu, Q.-S. Wu, Y.-H. Wu Effects of vibration frequency on intruders’ position in granular bed, Phys. Lett A, 356, pp. 267-271, 2006. Download
7.  X.-Z. Kong, M.-B. Hu, Q.-S. Wu, Y.-H. Wu Effects of bottleneck on granular convection cells and segregation, Granular Matter, 8, pp. 119-124, 2006. Download
6.  X.-Z. Kong, M.-B. Hu, Q.-S. Wu, Y.-H. Wu Kinetic energy sandpile model for conical sandpile development by evolving rivers, Phys. Lett A, 348, pp. 77-81, 2006. Download
5.  M.-B. Hu, X.-Z. Kong, Q.-S. Wu, Y.-H. Wu Effects of container geometry on granular segregation pattern, Chinese Physics, 14, pp. 1793-1800, 2005. Download
4.  M.-B. Hu, X.-Z. Kong, Q.-S. Wu, Y.-H. Wu Granular segregation in a multi-bottleneck container: Mobility effect, Int. J. Mod. Phys. B, 19, pp. 1793-1800, 2005. Download
3.  X.-Z. Kong, M.-B. Hu, Q.-S. Wu, Y.-H. Wu Ring-like size segregation in vibrated cylinder with a bottleneck, Phys. Lett A, 341, pp. 278-284, 2005. Download
2.  M.-B. Hu, Q.-S. Wu, X.-Z. Kong, Y.-H. Wu Discharge oscillation of particles from a vertical Pipe with capillary outlet, Chinese Science Bulletin, 50, pp. 1076-1078, 2005. Download
1.  M.-B. Hu, X.-Z. Kong, Q.-S. Wu, Z.-G. Zhu Experimental study of energy absorption properties of granular materials under low frequency vibrations, Int. J. Mod. Phys. B., 18, pp. 2708-2712, 2004. Download