• Energy Research

Abstract 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 .

Abstract 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.

Numerical modeling was performed to analyze the impacts of potential multiphase conditions on long-term subsurface pressure evolution in subsurface systems. An example site on the Bruce Peninsula in Southern Ontario, Canada was selected due to the large amount of available, high-quality data showing significantly underpressured water and the possible presence of gas phase methane. The system was represented by a 1-D model in which multiphase flow and hydromechanical coupling during the last glacial loading and unloading cycle were simulated. Single-phase flow simulations were performed with the USGS single-phase flow simulator SUTRA, and then both single- and multiphase simulations were performed with the multiphase simulator iTOUGH2-EOS7C from Lawrence Berkeley National Laboratory. This model archive data release contains all the input and output files for the simulations and is intended to accompany an article in the Journal of Geophysical Research: Solid Earth. Descriptions of the data in each subdirectory are given to facilitate understanding of this model archive. File descriptions are provided for select files to provide additional information for understanding this model archive. Support is provided for correcting errors in the data release and clarification of the modeling conducted by the U.S. Geological Survey. Users are encouraged to review the complete journal article to understand the purpose, documentation report construction, and limitations of this model.

AbstractThe goal of this study is to quantify the effects of geologic heterogeneity on the dynamic process of CO2 attenuation in shallow aquifers through the use of multi-scale laboratory experimentation and numerical modelling. First, an experiment was conducted in an intermediate scale two-dimensional cell that was packed with water-saturated porous media in a simple heterogeneous configuration. Constant-head boundary conditions were applied to the system, and then CO2-saturated water was injected through it. As the CO2-saturated water migrated through the system, gas phase CO2 evolved within the porous media, as observed by dielectric saturation sensors. The outflow of CO2 gas from the top of the system was monitored by a gas flow meter, and the outflow of water from the side of the tank was monitored via collection into a container that was placed on a computer-interfaced scale. After the multiphase CO2 evolution appeared to have reached steady state, clean water was injected through the cell until all of the CO2 was expelled from the system. In addition to the experiments, numerical models were performed using the Finite Element Heat and Mass transfer (FEHM) code as a first step toward planning larger scale experiments. Results indicate that the presence of geologic heterogeneity, which is ubiquitous in natural environments, can significantly hinder the migration of CO2 in the shallow subsurface.

To better understand the possible risks posed to shallow groundwater resources by geologic carbon sequestration (GCS), a multi-scale numerical modeling approach was invoked using the TOUGHREACT code from Lawrence Berkeley National Laboratory. The code solves coupled equations representing conservation of mass and energy on a finite difference grid to simulate multiphase, multicomponent, non-isothermal heat and mass transport in porous media. Two different two-dimensional cross-section modeling domains were constructed to improve understanding of groundwater flow and contaminant transport processes at a field site in soutwestern Utah. The site represents a natural analogue for leakage from a GCS site because water with elevated concentrations of salt and CO2 are migrating upward into a shallow aquifer system. The first modeling domain was designed to improve understanding of long-term hydrological leakage and conservative transport processes at the regional scale, while the second was designed to investigate site-scale reactive transport processes that could occur if the leaking fluids, or those with higher carbon dioxide (CO2) concentrations, were to interact with a potential source of contamination along their flow path toward the ground surface. Lead (Pb+2) was used as an example heavy metal contaminant and was incorporated into a region of iron oxide representing concretion zones that are common in the geographic area. Results indicate that the initial state of potential heavy metal contaminant (i.e., sorbed onto the surface of the iron oxide or in an oxide mineral assemblage as PbO, litharge), exerts strong control on the amount of contamination that may occur, and that precipitation readily sequesters mobilized Pb+2. This USGS data release contains all of the input and output files for the simulations described in the associated journal article. Descriptions of the data in each subdirectory are given to facilitate understanding of this model archive. File descriptions are provided for select files to provide additional information that may be of use for understanding this model archive. Support is provided for correcting errors in the data release and clarification of the modeling conducted by the U.S. Geological Survey. Users are encouraged to review the complete journal article (See 'Related External Resources' section below) to understand the purpose, report construction, and limitations of this model.

This data release serves as a model archive associated with an upcoming journal article. It contains all the input and output files from, as well as explanatory information about, numerical simulations that were conducted as part of a study of a site in Kemper County, Mississippi where deep geologic injection of CO2 has been proposed. The simulations represent groundwater flow within a portion of the deepest potential underground source of drinking water (USDW) at the site; the Eutaw aquifer. Simulations tested various hypothetical leakage rates from one of the two proposed CO2 injection wells, and analyzed detection of the dissolved CO2 plume at the other wells at the site. Simulations also tested the effect of hydrological plume management by water extraction from one of the wells.

Abstract In order to assess the risk of CO 2 leakage affecting the groundwater quality in aquifers, it is important to understand the mechanisms of CO 2 gas release when brine carrying dissolved CO 2 migrates to the shallow subsurface from sequestrated zones of deep geologic formations. As the brine with dissolved CO 2 elevates where the water pressure is lower, development of a gas phase starts with evolution of gas out of liquid, followed by gas phase growth and movement. However, conditions under which CO 2 gas evolution is triggered, how the gaseous phase CO 2 migrates and/or gets entrapped in the naturally heterogeneous formations are not well understood due to the difficulties involved with obtaining detailed experimental data. In this study, our goal was to identify the conditions under which dissolved CO 2 forms a gas phase and to understand how the formed CO 2 gas migrates through the saturated soil formation. In particular, we put emphasis on the critical gas saturation (at which the onset of gas phase migration occurs) and how this is explained by the theoretical and modeling studies reported in the literature. We have performed a series of experiments in the laboratory using a highly instrumented long column under highly controlled conditions that are not feasible in field settings. The 4.5 m-long vertical column setup was instrumented with automated sensors to continuously monitor phase saturation, electrical conductivity (EC), temperature, and water pressure distribution along the column length as well as the rates of water and gas outflow at the upstream end of the column. The observations showed that (1) concentration of dissolved CO 2 influenced the vertical extent of the gas phase formation, (2) the gas formation pattern was different if the saturation pressure was lower or higher than the static water pressure at the injection port which results largely from the gravity and viscous forces somewhat competing under the conditions in the experiments, (3) the mass transfer-dominant period where bubbles grew and water outflow increased was relatively short, (4) gas outflow was detected at the column outlet only after a continuous gas phase was formed and breakthrough had occurred, (5) the critical gas phase saturation at which the generated gas phase gets mobilized was always about 0.3–0.4 in homogeneous cases, (6) for the heterogeneous cases, a gas saturation higher than the critical gas saturation was observed due to accumulation of gas phase under a finer layer, (7) the injection rate did not affect the gas formation behavior whereas the temperature variation did, and (8) in some cases formation of gas appeared to be triggered by heterogeneities. These observations are expected to improve our understanding of gas evolution for better conceptualization and model development.

Abstract A concern for geologic carbon sequestration is the potential for stored CO2 to leak upward into valuable shallow aquifers where it can cause potentially detrimental impacts to groundwater resources. Understanding the mechanisms of CO2 migration and predicting its movement in shallow aquifers is a critical part of determining those impacts. During leakage, CO2 dissolved in brines may travel upward, potentially causing the gas to be released from solution (exsolve). Exsolved gas may accumulate at soil layer transitions, or flow into the vadose zone and ultimately the atmosphere. For this study, a series of intermediate-scale laboratory experiments were conducted to observe CO2 gas evolution in heterogeneous porous media. Results indicate that: (1) heterogeneous interfaces as well as flow constrictions through discontinuities in low-permeability layers enhance the evolution of gas phase, provided the water pressure at those layers is less than the pressure at which the flowing water was saturated with CO2, (2) higher contrast between the sands in a 1-D heterogeneous system leads to faster gas evolution, and (3) the effects of water flow rate on the evolution of the gas phase are sensitive to two-dimensional water flow pattern fluctuations.

AbstractIn this work we conducted 1-D and 2-D simulations of CO2 flow and transport aimed at improving our fundamental understanding of processes associated with potential CO2 leakage into shallow aquifers resulting from carbon sequestration operations. In the 1-D simulations we optimized capillary pressure parameters and conducted sensitivity analysis to investigate the size of CO2 gas accumulation zones as the permeability in heterogeneous layers decreased. Our results indicate that accumulation zones do not necessarily increase as permeability decreases, and differences between capillary pressures for each layer play an important role. In the 2-D simulations we investigated the role of horizontal background flow, and its magnitude, on the formation of CO2 gas in the system. We showed that the formation of CO2 gas in the 2-D tank can be predicted with a simple 1-D expression when the horizontal background flow is not dominating the flow field near the leak. In addition, we showed that the location of the heterogeneous layer relative to the leak can have profound effects on the size and shape of the CO2 gas plume.