CO2 sequestration in deep aquifers

Journal ArticleDisposal and long-term sequestration of anthropogenic "greenhouse gases" such as CO2 is a proposed approach to reducing global warming. Deep, regional-scale aquifers in sedimentary basins are possible sites for sequestration, given their ubiquitous nature. We used a mathematical sedimentary basin model, including multiphase flow of CO2, groundwater, and brine, to evaluate residence times in possible aquifer storage sites and migration patterns and rates away from such sites in the Powder River Basin of Wyoming. We also used the model to simulate CO2 flow through fractures, to evaluate partitioning between fracture and rock matrix. These simulations provide insight regarding the ultimate propensity of permeability reductions versus permeability increases in the fracture zone associated with carbonate reactions. Regional-scale hydrologic properties, including the presence of fracture zones, were calibrated using surface heat flow data. Our initial results suggest that, in general, long-term (~1000 years or more) sequestration in deep aquifers is possible, if subsurface structure and permeability are well characterized. However, additional risks are involved. In addition to CO2 escape from sequestration aquifers into other aquifers or to the land surface, another environmental threat posed by subsurface sequestration is contamination by brines. We evaluated the potential for such unintended aquifer contamination by displacement of brines out of adjacent sealing layers such as marine shales. Results suggest that sustained injection of CO2 may incur wide-scale brine displacement out of adjacent sealing layers, depending on the injection history, initial brine composition, and hydrologic properties of both aquifers and seals

Regional-scale permeability by heat flow calibration in the Powder River Basin, Wyoming

Journal ArticleAbstract. Forward modeling of coupled fluid and heat flow in the Powder River basin, Wyoming, is used to explain anomalously high heat flow values observed in the southern portion of the basin. Effective basin-scale permeabilities of selected Powder River basin aquifers and aquitards were calibrated by matching surface heat flow measurements to simulation results. Fractures associated with a large anticline in the southwestern part of the basin were found to play a major role in the basin's thermal regime. While the model results are non-unique, they demonstrate that regional structural features play an important role in a basin's overall energy budget and fluid flow regime. With the results of the basin-scale model it is possible to evaluate regional-scale flow and transport processe

Mineralogic Residence and Desorption Rates of Sorbed 90Sr in Contaminated Subsurface Sediments: Implications to Future Behavior and In-Ground Stability

90Sr desorption process will be quantified in coarse-textured Hanford sediments contaminated by different waste types and a reaction-based reactive transport model developed to forecast 90Sr concentration dynamics in Hanford's 100-N plume. Previous research has addressed 137Cs desorption from HLW-contaminated sediment providing results critical for HLW tank farm closure decisions. This renewal focuses on 90Sr with the objective of providing fundamental knowledge to predict future in-ground behavior as required for sound remedial decisions. Preliminary observations that suggest that 10-y sorbed 90Sr in coarse-textured sediment resides in the interiors of basaltic lithic fragments. This intraparticle retention defines a new conceptual model for 90Sr retardation that is tentatively attributed to internal domains of phyllosilicates formed from the weathering of basaltic glass. Research will characterize the spatial locations, composition, and reactivity of these intragrain phyllosilicate domains using spectroscopic, microscopic, and wet chemical methods. Intragrain porosity, diffusivity, and tortuosity will be estimated using emersion experiments coupled with particle imaging (using electron, X-ray, and NMR techniques). Desorption rates and extent will be measured from contaminated Hanford sediments of different waste impact in electrolytes that promote isotopic exchange, ion exchange, and/or dissolution. Desorption results will be interpreted with a geochemical-physical model that incorporates aqueous speciation, mass transfer, and other important factors. Batch and column experiments will be performed with sediments from Hanfords 100-N plume to quantify factors controlling long-term release rates and river stage effects. Newfound understanding and geochemical parameters will be incorporated into the FLOTRAN reactive transport code for simulation of 100-N plume dynamics

Subsurface Multiphase Flow and Multicomponent Reactive Transport Modeling using High-Performance Computing

Numerical modeling has become a critical tool to the Department of Energy for evaluating the environmental impact of alternative energy sources and remediation strategies for legacy waste sites. Unfortunately, the physical and chemical complexity of many sites overwhelms the capabilities of even most “state of the art” groundwater models. Of particular concern are the representation of highly-heterogeneous stratified rock/soil layers in the subsurface and the biological and geochemical interactions of chemical species within multiple fluid phases. Clearly, there is a need for higher-resolution modeling (i.e. more spatial, temporal, and chemical degrees of freedom) and increasingly mechanistic descriptions of subsurface physicochemical processes. We present research being performed in the development of PFLOTRAN, a parallel multiphase flow and multicomponent reactive transport model. Written in Fortran90, PFLOTRAN is founded upon PETSc data structures and solvers and has exhibited impressive strong scalability on up to 4000 processors on the ORNL Cray XT3. We are employing PFLOTRAN in the simulation of uranium transport at the Hanford 300 Area, a contaminated site of major concern to the Department of Energy, the State of Washington, and other government agencies where overly-simplistic historical modeling erroneously predicted decade removal times for uranium by ambient groundwater flow. By leveraging the billions of degrees of freedom available through high-performance computation using tens of thousands of processors, we can better characterize the release of uranium into groundwater and its subsequent transport to the Columbia River, and thereby better understand and evaluate the effectiveness of various proposed remediation strategies

Pore scale modeling of reactive transport involved in geologic CO2 sequestration

We apply a multi-component reactive transport lattice Boltzmann model developed in previolls studies to modeling the injection of a C02 saturated brine into various porous media structures at temperature T=25 and 80 C. The porous media are originally consisted of calcite. A chemical system consisting of Na+, Ca2+, Mg2+, H+, CO2(aq), and CI-is considered. The fluid flow, advection and diHusion of aqueous species, homogeneous reactions occurring in the bulk fluid, as weB as the dissolution of calcite and precipitation of dolomite are simulated at the pore scale. The effects of porous media structure on reactive transport are investigated. The results are compared with continuum scale modeling and the agreement and discrepancy are discussed. This work may shed some light on the fundamental physics occurring at the pore scale for reactive transport involved in geologic C02 sequestration

Colloid-Facilitated Transport of Radionuclides Through The Vadose Zone

The main purpose of this project was to advance the basic scientific understanding of colloid and colloid-facilitated Cs transport of radionuclides in the vadose zone. We focused our research on the hydrological and geochemical conditions beneath the leadking waste tanks at the USDOE Hanford reservation. Specific objectives were (1) to determine the lability and thermodynamic stability of colloidal materials, which form after reacting Hanford sediments with simulated Hanford Tank Waste, (2) to characterize the interactions between colloidal particles and contaminants, i.e., Cs and Eu, (3) to determine the potential of Hanford sediments for in situe mobilization of colloids, (4) to evaluate colloid-facilitated radionuclide transport through sediments under unsaturated flow, (5) to implement colloid-facilitated contaminant transport mechanisms into a transport model, and (6) to improve conceptual characterization of colloid-contaminant-soil interactions and colloid-facilitated transport for clean-up procedures and long-term risk assessment

Colloid-Facilitated Transport of Radionuclides through the Vadose Zone

This project seeks to improve the basic understanding of the role of colloids in facilitating the transport of contaminants in the vadose zone. We focus on three major thrusts: (1) thermodynamic stability and mobility of colloids formed by reactions of sediments with highly alkaline tank waste solutions, (2) colloid-contaminant interactions, and (3) in situ colloid mobilization and colloid-facilitated contaminant transport occurring in both contaminated and uncontaminated Hanford sediments