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Geological carbon storage represents a substantial challenge for the subsurface geosciences. Knowledge of the subsurface can be captured in a quantitative form using computational methods developed within petroleum production. However, to provide good estimates of the likely outcomes over thousands of years, traditional 3D simulation methods should be combined with other techniques developed specifically to study large-scale, long-term migration problems, e.g., in basin modeling. A number of such methods have been developed as a separate module in the open-source Matlab Reservoir Simulation Toolbox (MRST). In this paper, we present a set of tools provided by this module, consisting of geometrical and percolation type methods for computing structural traps and spill paths below a sealing caprock. Using concepts from water management, these tools can be applied on large-scale aquifer models to quickly estimate potential for structural trapping, determine spill paths from potential injection points, suggest optimal injection locations, etc. We demonstrate this by a series of examples applied on publicly available datasets. The corresponding source code is provided along with the examples. [ABSTRACT FROM AUTHOR]

Abstract: A simple experiment for capillary sealed trap shows that leakage would occur only from the weakest point in the trap. As an extens ion of this concept, the sequestrated and accumulated CO2 in the structural high may happen to leak from the unexpected weak point near the top of sealing layer. On the other hand, if CO2 were sequestrated into the flank of tilted aquifer, CO2 in separate phas e would migrate upward within aquifer with no CO2 leakage into the sealing layer above and leaving some residual amount behind. According to this concept, the best location for sequestration of CO2 could be the flank of the structure rather than the top. [Copyright &y& Elsevier]

Abstract: Deep-sea basalt aquifers have been identified for geological CO2 sequestration in worldwide locations and offer vast reservoir capacities, permanent geochemical trapping potential, and other deep-sea protections that significantly reduce the risk of post- injection leakage. Using site-specific criteria, we target basalt aquifers along seismic and aseismic oceanic ridges, and calculate the injection volume potential for each. Results indicate that these aquifers occur in many oceans and could sequester 2.3–11.5 Tt-C along seismic ridges and 5.9–29.6 Tt-C along aseismic ridges. The enormous size of these potential reservoirs warrants further research and pilot experiments in each geographic setting. [Copyright &y& Elsevier]

Abstract: We present a sharp-interface mathematical model of CO2 migration in saline aquifers, which accounts for gravity override, capillary trapping, natural groundwater flow, and the shape of the plume during the injection period. The model leads to a nonlinear advection–diffusion equation, where the diffusive term is due to buoyancy forces, not physical diffusion. For the case of interest in geological CO2 storage, in which the mobility ratio is very unfavorable, the mathematical model can be simplified to a hyperbolic equation. We present a complete analytical solution to the hyperbolic model. The main outcome is a closed-form expression that predicts the ultimate footprint on the CO2 plume, and the time scale required for complete trapping. The capillary trapping coefficient emerges as the key parameter in the assessment of CO2 storage in saline aquifers. The expressions derived here have immediate applicability to the risk assessment and capacity estimates of CO2 sequestration at the basin scale. In a companion paper [Szulczewski and Juanes, GHGT-9, Paper 463 (2008)] we apply the model to specific geologic basins. [Copyright &y& Elsevier]

Abstract: The effect of flow instabilities on capillary trapping mechanisms is a major source of uncertainty in CO2 sequestration in deep saline aquifers. Standard macroscopic models of multiphase flow in porous media are unable to explain and quantitatively predict the onset and structure of viscous-unstable flows, such as the displacement of brine by the injected CO2. We present the first step of a research effort aimed at the experimental characterization and mathematical (continuum) modeling of such flows. Existing continuum models of multiphase flow are unable to explain why preferential flow (fingering) occurs during infiltration into homogeneous, dry soil. We present a macroscopic model that reproduces the experimentally observed features of fingered flows. The proposed model is derived using a phase-field methodology and does not introduce new independent parameters. From a linear stability analysis, we predict that finger velocity and finger width both increase with infiltration rate, and the predictions are in quantitative agreement with experiments. [Copyright &y& Elsevier]

Abstract: CO2 can be immobilized in an aquifer as a residual phase trapped by capillary forces. The “inject low and let rise” strategy of storing CO2 in d eep saline aquifers is one method to maximize residual trapping. The more uniform the saturation front during upward movement of CO2, the greater the amount of CO2 trapped in this form. Previous studies suggest that capillary pressure i s a secondary influence on the CO2 plume, and that the permeability field governs the major features of the rising plume. In this work we show that sp atially varying capillary entry pressure can cause qualitatively different plume behavior. To illustrate the behavior, we si mulate buoyancy-driven migration when supercritical CO2 is placed below brine in a heterogeneous domain. After generating geostatistical realizations of permeability, we apply the Leverett -function so that each grid block has a drainage curve ( vs ) physically consistent with its permeability. The behavior of the displacement front depends strongly on the correlation structure of the heterogeneity and upon the magnitude of entry pressure. In a relatively homogeneous domain, capillar ity is a second-order effect. It damps the tendency toward channeling of the rising CO2 front and increases the uniformity of the plume. As the heterogeneity of the aquifer increases, capillarity begins to dominate buoyancy. Regions with smaller permeability, through which single-phase flow would readily occur, can completely block rising CO2, simply because the capillary entry pressure in these regions is somewhat larger than in neighboring regions. These local capillary barriers prevent CO2 from rising and cause it to mov e laterally. The disruption can be so extreme that above-residual saturations of CO2 accumulate below these barriers. These local accumulations can remain undisturbed when the top seal of the aquifer is breached. Thus we distinguish them as a new mode of CO2 trapping, dubbed “local capillary trapping”. Overall, the displacement front is much less uniform in heterog eneous domains. Despite the nonuniformity, the extent of dissolution trapping remains significant, much more than would be expected considering the ramified structure of the displacement front. T he reduction in residual phase trapping is balanced by an increase in local capillary trapping. Thus the extent of immobilization is similar to that found in previous works. [Copyright &y& Elsevier]

Abstract: Safely sequestering CO2 in a deep saline aquifer requires calculating how much CO2 the aquifer can store. Since offsetting nationwide emissions requires sequestering large quantities of CO2, this calculation should apply at the large scale of geologic basins. The only method to calculate storage capacity at the basin scale, however, is not derived from multiphase flow dynamics, which play a critical role in CO2 storage. In this study, we explain a new model to calculate basin-scale storage capacity that is derived from flow dynamics and captures the dynamic phenomena of gravity override and capillary trapping. Despite the fact that the model is dynamic, it is simple since it is a closed form expression with few terms. We demonstrate how to apply it on the Fox Hills Sandstone in the Powder River Basin. [Copyright &y& Elsevier]

Abstract: Motivated by geological carbon dioxide (CO2) storage, we present a simple model for the migration of immiscible gravity currents with constant residual trapping in a two-dimensional confined and sloping aquifer. Residual trapping acts as a loss-term, it reduces the current volume as long as the CO2 plume continues to migrate. Comparison with numerical results shows that the hyperbolic limit of the governing equations is a good approximation for currents with large mobility. In gently sloping aquifers, the current evolution is divided into an initial stage, with power-law decrease of volume, and a later stage, characterized by a rapid decay of the plume volume. Our results suggest that the efficient residual trapping in dipping aquifers may allow CO2 storage in aquifers lacking structural closure, if CO2 is injected far enough from the outcrop of the aquifer. However, the poor vertical sweep may lead to very long migration distances that may limit the storage capacity of in such aquifers. All results presented here are based on [M.A. Hesse, F.M. Orr Jr, H.A. Tchelepi, Gravity currents with residual trapping, J. Fluid Mech. 611 (2008) 35–60.]. [Copyright &y& Elsevier]

Abstract: Numerical simulation using TOUGHREACT software has shown that more than 20Mt of CO2 can be stored for a period up to 10,000 years in a 2-D model system taking from the geology of the Tokyo Bay area. Carbonate precipitation occurs extensively in the front of enlarging plume, forming a shell enclosing CO2. The distribution of dawsonite is predicted to be dependent on the dissolution of plagioclase which presents abundantly in sandstones of the Tokyo Bay area as well as those in young sedimentary strata of Japanese Islands, suggesting its potential importance in the mineral trapping of CO2. [Copyright &y& Elsevier]

Abstract: Numerous research results have been published on the storage science and the technology of CO2 storage in saline aquifers to address the knowledge gaps identified by the IPCC Special Report on Carbon Capture and Storage in 2005. Significant progress has been made towards developing consistent methodologies for the classification and determination of CO2 storage capacity and towards comprehensive guidelines regarding site selection, site characterisation, risk assessment and monitoring and plans. Although advancements have been made with respect to geochemical processes and numerical simulations, a larger portfolio of CO2 storage operations is needed to provide realistic data for model calibration and substantiation of time frames for various trapping mechanisms. The experience from existing storage projects has shown that CO2 geological storage in saline aquifers is a technology that can be successfully and safely applied today. However, proper regulations for CO2 storage are needed to reduce the current uncertainty associated with the economics of CO2 storage and to accelerate the deployment of CCS technology. [Copyright &y& Elsevier]

Abstract: To utilize aquifers without proven seal structures as a CO2 storage reservoir, CO2 should be trapped by residual gas trapping and solubility trapping as well as physical trapping by cap-rock. We investigated the mechanism of the residual gas trapping from a microscopic point of view by means of micro-focused X-ray CT. First, we imaged trapped air bubbles in a Berea sandstone chip after spontaneous imbibition at atmospheric pressure. With the resolution of 6.52 m/pixel, 610 sheets of slice images of 608×608 pixels were reconstructed. Pore structure and trapped bubbles were clearly visualized. Next, distributions of trapped bubbles in Berea and Tako sandstone were imaged in core-flooding at the capillary number of 1.0×10−6. The trapped bubbles consist of two-type; one occupies the center of pore with a pore-scale size and the other has a pore-network scale size. In low porosity porous media such as sandstone, connected bubbles has high contribution to trapped gas saturation. The distributions of trapped bubbles were uniform throughout the core except for the heterogeneity due to sedimentary layers. With a packed bed of glass beads 600 μm in diameter, the effects of capillary number and the injected water volume were investigated. Even in the packed bed of glass beads, which has a high porosity of 38%, once the gas bubbles are trapped, they are stable against the water flow rate that corresponds to the capillary number of 1.0×10−4. Even at very high capillary number of 4.7×10−5, residual gas saturation is independent of volume of injected water up to 15 pore volumes. Third, distributions of the residual gas saturation in the core were measured at supercritical conditions on core scale. The effect of fracture on the residual gas saturation was also investigated. The fracture reduces the maximum CO2 saturation to about 20% at the end of CO2 injection. However, most of the CO2 were trapped after the water injection. The Land type relationship between trapped CO2 saturation and the maximum CO2 saturation was obtained for supercritical CO2. [Copyright &y& Elsevier]

Abstract: Ketzin, in the Northeast German Basin (NEGB), is the site for pilot injection of CO2 (CO2SINK project) into a saline aquifer (the Upper Triassic Stuttgart Formation) situated at a depth of about 630–700m. This paper reports the baseline characterization of the reservoir formation based on new core material and well-logs obtained from one injection well and two observations wells, drilled at a distance from 50m to 100m from each other. The reservoir is lithologically heterogeneous and made up by fluvial sandstones and siltstones interbedded with mudstones showing remarkable differences in porosity. The thickest sandstone units are associated with channel sandstone, whose thickness varies over short lateral distances. In-depth petrographic, mineralogical, mineral-chemical, and whole-rock geochemical analysis were performed focusing on the sandstone intervals, which display the best reservoir properties for CO2 injection. The dominantly fine-grained and well to moderately-well sorted, immature sandstones classify as feldspathic litharenites and lithic arkoses. Quartz (22–43wt.%), plagioclase (19–32wt.%), and K-feldspar (5–13wt.%) predominate mineralogically. Muscovite plus illite and mixed-layer minerals are omnipresent (4–13wt.%). Quartz, feldspar, as well as meta-sedimentary and volcanic rock fragments comprise the most abundant detrital components, which often are rimmed by thin, early diagenetic coatings of ferric oxides, and locally of clay minerals. Feldspar grains may be unaltered and optically clear, partially to completely dissolved, partially altered to sheet silicates (mainly illite), or albitized. Analcime and anhydrite constitute the most widespread, often spatially associated pore-filling cement minerals. Authigenic dolomite, barite, and coelestine is minor. The percentage of cements ranges in total from about 5vol.% to 32vol.%. Except of samples intensely cemented by anhydrite and analcime, total porosities of the sandstones range from 13% to 26%. The fraction of intergranular porosity varies between 12% and 21%. About 1–5% porosity has been generated by dissolution of detrital plagioclase, K-feldspar, and volcanic rock fragments. The comparatively large modal abundance of feldspars, micas, chlorite, clay minerals, Fe–Ti-oxides, and analcime account for the richness in Ti, Al, Fe, Mg, Na, and K, and the paucity in Si, of the Stuttgart sandstones relative to mature sandstones. Altogether, these sandstones are comparatively rich in minerals that may potentially react with the injected CO2. [Copyright &y& Elsevier]