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Biogeochemical reactions occurring in soil pore space underpin gaseous emissions measured at macroscopic scales but are difficult to quantify due to their complexity and heterogeneity. We develop a volumetric-average method to calculate aerobic respiration rates analytically from soil with microscopic soil structure represented explicitly. Soil water content in the model is the result of the volumetric-average of the microscopic processes, and it is nonlinearly coupled with temperature and other factors. Since many biogeochemical reactions are driven by oxygen (O 2 ) which must overcome various resistances before reaching reactive microsites from the atmosphere, the volumetric-average results in negative feedback between temperature and soil respiration, with the magnitude of the feedback increasing with soil water content and substrate quality. Comparisons with various experiments show the model reproduces the variation of carbon dioxide emission from soils under different water content and temperature gradients, indicating that it captures the key microscopic processes underpinning soil respiration. We show that alongside thermal microbial adaptation, substrate heterogeneity and microbial turnover and carbon use efficiency, O 2 dissolution and diffusion in water associated with soil pore space is another key explanation for the attenuated temperature response of soil respiration and should be considered in developing soil organic carbon models.

Exsolution and re-dissolution of CO2 gas within heterogeneous porous media are investigated using experimental data and mathematical modeling. In a set of bench-scale experiments, water saturated with CO2 under a given pressure is injected into a 2-D water-saturated porous media system, causing CO2 gas to exsolve and migrate upwards. A layer of fine sand mimicking a heterogeneity within a shallow aquifer is present in the tank to study accumulation and trapping of exsolved CO2. Then, clean water is injected into the system and the accumulated CO2 dissolves back into the flowing water. Simulated exsolution and dissolution mass transfer processes are studied using both near-equilibrium and kinetic approaches and compared to experimental data under conditions that do and do not include lateral background water flow. The mathematical model is based on the mixed hybrid finite element method that allows for accurate simulation of both advection- and diffusion-dominated processes. Ministry of Education, Youth and Sports of the Czech Republic under the OP RDE [CZ.02.1.01/0.0/0.0/16_019/0000778, LTAUSA19021] Published version The work reported in this paper was supported by the Ministry of Education, Youth and Sports of the Czech Republic under the OP RDE grant number CZ.02.1.01/0.0/0.0/16_019/0000778 Centre for Advanced Applied Sciences and Inter-Excellence grant no. LTAUSA19021. Any use of trade, firm, or product names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.

A key finding in soil carbon studies over the past decade is that soil organic carbon (SOC) stabilization is not controlled by its molecular complexity and clay content but by its physicochemical protections including occlusion in aggregates and sorption/precipitation with organo-mineral associations. The organo-mineral complexes and the adsorbed SOC could be dissolved microbially under anoxic conditions, which is an important pathway in the carbon cycle but has been overlooked by most carbon models. As it is reported that organo-mineral associations are formed in aerobic conditions and could be lost under anaerobic conditions, there should be a positive correlation between SOC and ability of the aggregates to transport dissolved oxygen. We test this using two long-term experiments with a SOC gradient at Rothamsted Research in the UK: One experiment compares the effects of different fertilizations on yield of winter wheat and the other experiment aims to study the consequence of cropping system change for SOC dynamics. Aggregates in samples taken from plots under different treatments on the two experiments were scanned using X-ray computed tomography at 1.5 μm resolution; the ability of each aggregate to transport oxygen was calculated based on the pore-scale lattice Boltzmann simulation assuming that the aggregate is saturated as this is the most anaerobic scenario. We compared porosity and diffusion coefficient of all aggregates and link them to soil carbon measured from different treatments on the two experiments. The results showed that the agronomic practice changes occurring 67 and 172 years ago substantially reshaped the intra-aggregate structure, and that the accrual of SOC is positively correlated with diffusion coefficient of the aggregates to transport oxygen. However, the diffusion coefficient increases with SOC asymptotically, plateauing when SOC exceeds a threshold value. We also found that diffusion coefficient of the aggregates in cropped soils chemically fertilized trended with their porosity approximately in the same way, deviating from those for other non-cropped treatments or fertilized with farmyard manure.

Understanding the soil-gas migration in unsaturated soil is important in a number of problems that include carbon loading to the atmosphere from the bio-geochemical activity and leakage of gases from subsurface sources from carbon storage unconventional energy development. The soil water dynamics in the vadose zone control the soil-gas pathway development and, hence, the gas flux's spatial and temporal distribution at the soil surface. The spatial distribution of soil-water content depends on soil water characteristics. The dynamics are controlled by the water flux at the land surface and water table fluctuations. Physical properties of soil give a better understanding of the soil gas dynamics and migration from greater soil depths. The fundamental process of soil gas migration under dynamic water content was investigated in the laboratory using an intermediate-scale test system under controlled conditions that is not possible in the field. The experiments focus on observing the methane gas migration in relation to the physical properties of soil and the soil moisture patterns. A 2D soil tank with dimensions of 60 cm × 90 cm × 5.6 cm (height × length × width) was used. The tank was heterogeneously packed with sandy soil along with a distributed network of soil moisture, temperature, and electrical conductivity sensors. The heterogeneous soil configuration was designed using nine uniform silica sands with the effective sieve numbers #16, #70, #8, #40/50, #110, #30/40, #50, and #20/30 (Accusands, Unimin Corp., Ottawa, MN), and a porosity ranging in values from 0.31 to 0.42. Four methane infrared gas sensors and a Flame Ionization detector (HFR400 Fast FID) were used for the soil gas sampling at different depths within the soil profiles and at the land surface. A complex transient soil moisture distribution and soil gas migration patterns were observed in the 2D tank. These processes were successfully captured by the sensors. These preliminary experiments helped us to understand the mechanism of soil moisture sensor response and methane gas migration into a heterogeneous sandy soil with a view to developing a large-scale test in a 3D tank (4.87 m × 2.44 m × 0.40 m) and finally transition to field deployment.

AGU Advances, 2 (1), ISSN:2576-604X

A laboratory study was conducted to evaluate the behavioral diversity of partitioning, interfacial, and conservative tracers in water-saturated subsurface systems where NAPLs were entrapped. Existing theoretical assumptions and computational methods, which rely on instantaneous equilibrium partitioning, were employed to test their validity and relative error when applied to complexly distributed NAPLs, such as the pooled occurrence. Results show assumption of reversible equilibrium behavior may lead to gross underestimation of NAPL amount and incorrect prediction of NAPL distribution.

The carbon storage and energy development activities in deep geologic zones that potentially affect water quality in shallow aquifers are of central importance in the energy-water nexus. The extraction of natural gas involves hydraulically fracturing deep shale formations. The storing of carbon dioxide in deep geologic formations is pursued to mitigate global warming. Both these activities have the potential to contaminate the shallow aquifers used for potable water and return the greenhouse gases to the atmosphere. In the case of carbon storage, both during and post-injection phases, it is possible for the CO2 and formation brine to leak through natural faults, pressure-induced fractures, or failed well casings. Two scientific challenges have to be addressed to safely store the carbon in the deep formation while protecting the shallow aquifers. The first, characterizing the affected geologic formations, and the second is monitoring the leakage. Monitoring involves determining leakage locations and tracking of the gas and brine plume through the geologic formation between the deep confining layer used for storage and the shallow aquifer. Challenges to the characterization derive from the limitations and sparsity of observational data in deep formations. Effective monitoring poses both scientific and engineering challenges as the leakage locations not known, and the resulting pathways cannot be predicted easily. This paper presents two studies where intermediate-scale testing systems were used to understand the processes that occur during the leakage of stored supercritical CO2. The focus of the first study was to better understand the process of CO2 gas exsolution after a leak from the deep confining formation. The second study addresses the issue of monitoring brine leakage from the confined formation where supercritical CO2 is stored. The improved understanding of these leakage processes will help to develop assessment and monitoring systems for storage permeance and protecting shallow sources of potable groundwater. It is not feasible to conduct experiments in the field to obtain both the fundamental process understanding and test and validate developed modeling and monitoring tools due to lack of control of boundary and initial conditions and expense in fully characterizing deep formations. Tests in intermediate scale synthetic aquifers where highly controlled experiments can be conducted to obtain accurate data provide an alternative to overcome this challenge. However, designing test systems and performing tests under ambient laboratory conditions different types of challenges. This paper will present some of the challenges and how they were overcome. The results on new process insights, how the data was used to assess the natural capacity of the aquifer attenuation of leaking gas, and validating inversion methods for site characterization and leakage detection will be presented. Even though this study focused on CO2 leakage, the results will be of value in problems of natural has leakage during hydraulic fracturing in alternate energy development.

To assess the risks of Geologic Carbon Sequestration (GCS), it is crucial to understand the fundamental physicochemical processes that may occur if and when stored CO2 leaks upward from a deep storage reservoir into the shallow subsurface. Intermediate-scale experiments allow for improved understanding of the multiphase evolution processes that control CO2 migration behavior in the subsurface, because the boundary conditions, initial conditions, and porous media parameters can be better controlled and monitored in the laboratory than in field settings. For this study, a large experimental test bed was designed to mimic a cross section of a shallow aquifer with layered geologic heterogeneity. As water with aqueous CO2 was injected into the system to mimic a CO2-charged water leakage scenario, the spatiotemporal evolution of the multiphase CO2 plume was monitored. Similar experiments were performed with two different sand combinations to assess the relative effects of different types of geologic facies transitions on the CO2 evolution processes. Significant CO2 attenuation was observed in both scenarios, but by fundamentally different mechanisms. When the porous media layers had very different permeabilities, attenuation was caused by local accumulation (structural trapping) and slow redissolution of gas phase CO2. When the permeability difference between the layers was relatively small, on the other hand, gas phase continually evolved over widespread areas near the leading edge of the aqueous plume, which also attenuated CO2 migration. This improved process understanding will aid in the development of models that could be used for effective risk assessment and monitoring programs for GCS projects.

In this study we take advantage of direct observations of a buoyant CO2-surrogate plume migrating through a synthetic sand aquifer saturated with a brine-analog to evaluate the effects of a heterogeneous capillary pressure field. The experiment involves two injection stages characterized by capillary number Ca∼10-6 separated by two redistribution stages allowing the plume to reach hydrostatic equilibrium (10-9

The role of capillary forces during buoyant migration of CO2 is critical towards plume immobilization during post-injection of geological carbon sequestration. However, the inherent heterogeneity of most candidate reservoirs makes it very challenging to evaluate the effects of capillary forces on the storage capacity of these formations and to assess in-situ plume evolution. Given the technical challenges in characterizing deep reservoir's migration and trapping phenomena, the development of carefully controlled laboratory experiments allows recreating the interplay of governing forces that are expected in-situ. Visual observations, complemented by saturation measurements across homogeneous and heterogeneous sand packs, provide direct insight into capillary/buoyancy-dominated flow processes, allowing assessment of predictive ability of existing and new two-phase flow simulators.

Meaningful model-based predictions of water quality and quantity are imperative for the designed footprint of managed aquifer recharge installations. A two-dimensional (2D) synthetic MAR system equipped with automated sensors (temperature, water pressure, conductivity, soil moisture, oxidation-reduction potential) and embedded water sampling ports was used to test and model fundamental subsurface processes during surface spreading managed aquifer recharge operations under controlled flow and redox conditions at the meso-scale. The fate and transport of contaminants in the variably saturated synthetic aquifer were simulated using the finite element analysis model, FEFLOW. In general, the model concurred with travel times derived from contaminant breakthrough curves at individual sensor locations throughout the 2D tank. However, discrepancies between measured and simulated trace organic chemical concentrations (i.e., carbamazepine, sulfamethoxazole, tris (2-chloroethyl) phosphate, trimethoprim) were observed. While the FEFLOW simulation of breakthrough curves captured overall shapes of trace organic chemical concentrations well, the model struggled with matching individual data points, although compound-specific attenuation parameters were used. Interestingly, despite steady-state operation, oxidation-reduction potential measurements indicated temporal disturbances in hydraulic properties in the saturated zone of the 2D tank that affected water quality.

The atmosphere, soils, and vegetation near the land-atmosphere interface are in a state of continuous dynamic interaction via a myriad of complex interrelated feedback processes which collectively, remain poorly understood. Studying the fundamental nature and dynamics of such processes in atmospheric, ecological, and/or hydrological contexts in the field setting presents many challenges; current experimental approaches are an important factor given a general lack of control and high measurement uncertainty. In an effort to address these issues and reduce overall complexity, new experimental design considerations (two-dimensional intermediate-scale coupled wind tunnel-synthetic aquifer testing using synthetic plants) for studying soil-plant-atmosphere continuum soil moisture dynamics are introduced and tested in this study. Validation of these experimental considerations, particularly the adoption of synthetic plants, is required prior to their application in future research. A comparison of three experiments with bare soil surfaces or transplanted with a Stargazer lily/limestone block was used to evaluate the feasibility of the proposed approaches. Results demonstrate that coupled wind tunnel-porous media experimentation, used to simulate field conditions, reduces complexity, and enhances control while allowing fine spatial-temporal resolution measurements to be made using state-of-the-art technologies. Synthetic plants further help reduce system complexity (e.g., airflow) while preserving the basic hydrodynamic functions of plants (e.g., water uptake and transpiration). The trends and distributions of key measured atmospheric and subsurface spatial and temporal variables (e.g., soil moisture, relative humidity, temperature, air velocity) were comparable, showing that synthetic plants can be used as simple, idealized, nonbiological analogs for living vegetation in fundamental hydrodynamic studies.

Once the injection phase of a geological carbon sequestration operation ends, capillary forces are critical for hampering the upward migration of buoyant CO2 to shallower environments. Reservoir heterogeneity is also a key factor controlling plume spreading and trapping efficiency as well as a source of uncertainty about the deep subsurface. However, these two features are typically overlooked or overly simplified by numerical models that emphasize on viscous flow driven by pressure gradients. In particular, when considering far-field regions of storage reservoirs with vanishing pressure gradients, the physics governing the CO2 flow into brine-saturated rocks become dominated by capillary and buoyancy forces. Hence, there is currently a debate on whether numerical models founded on Darcy’s law can capture some of the migration phenomena that characterize long-term CO2 migration in saline formations. In this study, we consider experimental observations from a previous physical model that illustrates flow patterns of a CO2-surrogate plume migrating through a well-characterized sand aquifer saturated with a brine-analog. Then, we compare Monte Carlo sets of invasion percolation simulations with direct observations of plume distribution to evaluate the effects of a heterogeneous capillary pressure field. The observed plume distribution appears to be controlled by a combination of buoyant migration and trapping at capillary barriers under near-hydrostatic pressure conditions. Although these observations are gathered in a simplified and relatively small laboratory system, an analogous behavior has been documented at reservoir scale for the Sleipner storage site, where a series of shale layers inhibited the buoyant migration of the plume.

Members of the editorial board of AGU Advances express their appreciation to those who served as peer reviewers for the journal in its inaugural year.

In modeling transport within naturally heterogeneousaquifers, it is usually assumed that the transport equations valid at local scales can also be applied at larger scales. At larger scales, the heterogeneous domain is represented by an equivalenthomogeneous medium. Convergent-flow tracer tests constitute one of the most frequently used field tests to estimate effective input parameters of equivalent homogeneous aquifers. Traditionally, statistical approaches applied to groundwater flow andsolute transporthave provided tools to estimate these equivalent parameters. These approaches are based on a number of simplifications including the assumption that the pointtransmissivityvalues follow a multilog-normal random function. Several investigators have found that this assumption may not be valid in many field cases. In order to study the applicability of the equivalent homogeneous formulation in a nontraditional stochastic field, a number of experimental and numerical studies were conducted. The results are used to determine the apparent values of porosity anddispersivitythat would be obtained if convergent-flow tracer tests were conducted in a deterministically generated heterogeneous transmissivity field displaying anisotropy in the correlation structure. It is shown that in this particularheterogeneous media, apparent porosity strongly depends on connectivity rather than on transmissivity. This dependence on connectivity questions the theoretical results obtained in continuum equivalent fields to estimate effective porosity.

The advection-dispersion equation (ADE) often fails to predict solute transport, in part due to incomplete mixing in the subsurface, which the development of non-local models has attempted to deal with. One such model is dual-domain mass transfer (DDMT); one parameter that exists within this model type is called immobile porosity. Here, we explore the complexity of estimating immobile porosity under varying flow rates and density dependencies in a large-scale heterogeneous system. Immobile porosity is estimated experimentally and using numerical models in 3-D flow systems, and is defined by domains of comparatively low advective velocity instead of truly immobile regions at the pore scale. Tracer experiments were conducted in a mesoscale 3-D tank system with embedded large impermeable zones and the generated data were analyzed using a numerical model. The impermeable zones were used to explore how large-scale structure and heterogeneity affect parameter estimation of immobile porosity, assuming a dual-porosity model, and resultant characterization of the aquifer system. Spatially and temporally co-located fluid electrical conductivity (σf) and bulk apparent electrical conductivity (σb)-using geophysical methods-were measured to estimate immobile porosity, and numerical modeling (i.e., SEAWAT and R3t) was conducted to explore controls of the immobile zones on the experimentally observed flow and transport. Results showed that density-dependent flow increased the hysteresis between measured fluid and bulk electrical conductivity, resulting in larger interpreted immobile pore-space estimates. Increasing the dispersivity in the model simulations decreased the estimated immobile porosity; flow rate had no impact. Overall, the results of this study highlight the difficulty faced in determining immobile porosity values in field settings, where hydrogeologic processes may vary temporally. Our results also highlight that immobile porosity is an effective parameter in an upscaled model whose physical meaning is not necessarily clear and that may not align with intuitive interpretations of a porosity.

Aquifers show troubling signs of irreversible depletion as climate change, population growth, and urbanization lead to reduced natural recharge rates and overuse. One strategy to sustain the groundwater supply is to recharge aquifers artificially with reclaimed water or stormwater via managed aquifer recharge and recovery (MAR) systems. Unfortunately, MAR systems remain wrought with operational challenges related to the quality and quantity of recharged and recovered water stemming from a lack of data-driven, real-time control. This paper presents a laboratory scale proof-of-concept study that demonstrates the capability of a real-time, simulation-based control optimization algorithm to ease the operational challenges of MAR systems. Central to the algorithm is a model that simulates water flow and transport of dissolved chemical constituents in the aquifer. The algorithm compensates for model parameter uncertainty by continually collecting data from a network of sensors embedded within the aquifer. At regular intervals the sensor data is fed into an inversion algorithm, which calibrates the uncertain parameters and generates the initial conditions required to model the system behavior. The calibrated model is then incorporated into a genetic algorithm that executes simulations and determines the best management action, for example, the optimal pumping policy for current aquifer management goals. Experiments to calibrate and validate the simulation-optimization algorithm were conducted in a small two-dimensional synthetic aquifer under both homogeneous and heterogeneous packing configurations. Results from initial experiments validated the feasibility of the approach and suggested that our system could improve the operation of full-scale MAR facilities.

The role of capillary forces during buoyant migration of CO2 is critical toward plume immobilization within the postinjection phase of a geological carbon sequestration operation. However, the inherent heterogeneity of the subsurface makes it very challenging to evaluate the effects of capillary forces on the storage capacity of these formations and to assess in situ plume evolution. To overcome the lack of accurate and continuous observations at the field scale and to mimic vertical migration and entrapment of realistic CO2 plumes in the presence of a background hydraulic gradient, we conducted two unique long-term experiments in a 2.44 m × 0.5 m tank. X-ray attenuation allowed measuring the evolution of a CO2-surrogate fluid saturation, thus providing direct insight into capillarity-dominated and buoyancy-dominated flow processes occurring under successive drainage and imbibition conditions. The comparison of saturation distributions between two experimental campaigns suggests that layered-type heterogeneity plays an important role on nonwetting phase (NWP) migration and trapping, because it leads to (i) longer displacement times (3.6 months versus 24 days) to reach stable trapping conditions, (ii) limited vertical migration of the plume (with center of mass at 39% versus 55% of aquifer thickness), and (iii) immobilization of a larger fraction of injected NWP mass (67.2% versus 51.5% of injected volume) as compared to the homogenous scenario. While these observations confirm once more the role of geological heterogeneity in controlling buoyant flows in the subsurface, they also highlight the importance of characterizing it at scales that are below seismic resolution (1–10 m).

Incorporating hysteresis into models is important to accurately capture the two phase flow behavior when porous media systems undergo cycles of drainage and imbibition such as in the cases of injection and post-injection redistribution of CO2 during geological CO2 storage (GCS). In the traditional model of two-phase flow, existing constitutive models that parameterize the hysteresis associated with these processes are generally based on the empirical relationships. This manuscript presents development and testing of mathematical hysteretic capillary pressure—saturation—relative permeability models with the objective of more accurately representing the redistribution of the fluids after injection. The constitutive models are developed by relating macroscopic variables to basic physics of two-phase capillary displacements at pore-scale and void space distribution properties. The modeling approach with the developed constitutive models with and without hysteresis as input is tested against some intermediate-scale flow cell experiments to test the ability of the models to represent movement and capillary trapping of immiscible fluids under macroscopically homogeneous and heterogeneous conditions. The hysteretic two-phase flow model predicted the overall plume migration and distribution during and post injection reasonably well and represented the postinjection behavior of the plume more accurately than the nonhysteretic models. Based on the results in this study, neglecting hysteresis in the constitutive models of the traditional two-phase flow theory can seriously overpredict or underpredict the injected fluid distribution during post-injection under both homogeneous and heterogeneous conditions, depending on the selected value of the residual saturation in the nonhysteretic models.

The impact of geologic heterogeneity on capillary trapping of supercritical CO2 (scCO2) has been recognized and appraised through laboratory experimentation and modeling. However, how different injection strategies can be optimized to improve capillary trapping has not received adequate attention. We present a study based on stochastic analysis to show the impact of injection scheduling on capillary trapping of scCO2 in heterogeneous geological formations. Improvement of trapping efficiency consists of maximizing the trapped volume within a predefined secure zone of the storage formation with the goal of avoiding plume intersection with potential leakage pathways. Using knowledge acquired from physical experiments in intermediate-scale tanks with controlled heterogeneity, we conduct numerical simulations following a stochastic approach to extend the observed outcomes to a series of equally probable permeability scenarios. Our simulations involve the same combination of surrogate fluids that are used in the experiments to mimic viscosity and density contrasts between scCO2 and brine. To account for uncertainty of formation heterogeneity, several permeability fields with three different variances and two horizontal correlation lengths are generated. The same volume of scCO2 is emplaced using four different modes of injection scheduling. Results suggest that injection strategies aimed at enhancing capillary trapping of scCO2 and increasing the probability of constraining the plume within a secure zone are strictly related to the specific heterogeneity of the reservoir. This preliminary theoretical finding suggests the potential for maximizing secure capillary trapping through the proper selection of injection schedule to fit the formation heterogeneity. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd.

In general, analytical solutions serve a useful purpose to obtain better insights and to verify numerical codes. For flow of two incompressible and immiscible phases in homogeneous porous media without gravity, one such method that neglects capillary pressure in the solution was first developed by Buckley and Leverett (1942). Subsequently, McWhorter and Sunada (1990) derived an exact solution for the one and two dimensional cases that factored in capillary effects. This solution used a similarity transform that allowed to reduce the governing equations into a single ordinary differential equation (ODE) that can be further integrated into an equivalent integral equation. We present a revision to McWhorter and Sunada solution by extending the self-similar solution into a general multidimensional space. Inspired by the derivation proposed by McWhorter and Sunada (1990), we integrate the resulting ODE in the third and higher dimensions into a new integral equation that can be subsequently solved iteratively by means of numerical integration. We developed implementations of the iterative schemes for one- and higher dimensional cases that can be accessed online on the authors’ website.

Publisher's version (útgefin grein), This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through online media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focused on the process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come., We would like to thank the members of the IAHS, EGU, AGU and IAH for supporting this initiative. The LinkedIn group and overall secretariat was hosted by the IAHS, the Splinter meeting by EGU and the Vienna Catchment Science Symposium by the Vienna Doctoral Programme on Water Resource Systems (DK W1219-N28) funded by the Austrian Science Funds (FWF)., "Peer Reviewed"

There is a concern that underground stored CO 2 may potentially escape and intrude shallow aquifers that may lead to degradation of the water quality and possible mobilization of metals. Moreover migrating CO 2 gas may invade buildings causing lethal conditions for humans. A better understanding of gaseous CO 2 migration in heterogeneous aquifer systems at low pressure will help in risk assessment of Carbon Capture and Storage (CCS). We performed complementary 1D column and 2D tank experiments by injecting gaseous CO 2 at three different rates into constructed heterogeneous porous media in laboratory setups. Soil moisture sensors were installed in the test systems to monitor the movement of the gaseous phase. The generated experimental data was analyzed using the numerical multiphase modeling code T2VOC. The results confirmed that large-scale heterogeneity controls overall gaseous CO 2 migration in porous media while processes occurring at smaller scale are of significance for gas saturations. Monitoring the exact gaseous movement and concentrations is difficult even in a constructed heterogeneous medium and point measurements are not always capturing the full dynamics of the system. The numerical model provided good estimates of the general flow of the gaseous phase around larger heterogeneous features and estimated the total amount of gaseous CO 2 in the test system to a good approximation. However, the numerical model was not able to adequately describe the processes occurring at smaller scale including unstable gaseous movement at low flow rates and dissolution of CO 2 .

Multiphase flow models that simulate flow and entrapment behavior of supercritical CO 2 (scCO 2 ) can be used to design and evaluate CO 2 sequestration strategies in deep geologic formations. The accuracy of model predictions depends on the constitutive models that relate capillary pressure ( P c ) and relative permeability ( k r ) to phase saturations ( S w ). Any studies to evaluate various constitutive models remain a major challenge, as necessary data are not easy to obtain in field settings. As it is not feasible to create deep formation pressure conditions in the laboratory to keep CO 2 under supercritical conditions, surrogate fluids that mimic the behavior of scCO 2 and brine were used. The k r – S w relationship for the surrogate fluid system was measured independently using long-column and hydrostatic methods. The measured P c – S w data for the same surrogate fluid system was used to derive the k r – S w relationships using the van Genuchten–Mualem model. The relative permeability relationships obtained independently from experiments and derived from measured P c – S w data using the van Genuchten–Mualem constitutive model were then applied to a multiphase model to simulate an injection experiment in an intermediate scale test tank. The comparison demonstrated that the model simulation that used directly measured k r – S w relationships was able to match the experimental observations slightly better than the simulation performed using the empirically derived relative permeability functions. Even though this was a limited study using a homogeneous packing configuration, the results demonstrate the importance of using the correct relative permeability functions in multiphase models used in carbon storage studies.

The physicochemical processes associated with CO 2 leakage into shallow aquifer systems are complex and span multiple spatial and time scales. Continuum-scale numerical models that faithfully represent the underlying pore-scale physics are required to predict the long-term behavior and aid in risk analysis regarding regulatory and management decisions. This study focuses on benchmarking the numerical simulator, FEHM, with intermediate-scale column experiments of CO 2 gas evolution in homogeneous and heterogeneous sand configurations. Inverse modeling was conducted to calibrate model parameters and determine model sensitivity to the observed steady-state saturation profiles. It is shown that FEHM is a powerful tool that is capable of capturing the experimentally observed outflow rates and saturation profiles. Moreover, FEHM captures the transition from single- to multi-phase flow and CO 2 gas accumulation at interfaces separating sands. We also derive a simple expression, based on Darcy's law, for the pressure at which CO 2 free phase gas is observed and show that it reliably predicts the location at which single-phase flow transitions to multi-phase flow.