Andrew V. Wolfsberg

Rock Fractures and Fluid Flow: Contemporary Understanding and Applications

How can fractures that are significant hydraulic conductors or barriers be located, identified, and characterized? How do flow and transport occur in fracture systems? How can changes in fracture systems be predicted and controlled? With a clear, well-stated focus, Rock Fractures and Fluid Flow seeks to address these questions by combining into one work the interdependencies of the disciplines needed to get at the answers.

Mesoscale Carbon Sequestration Site Screening and CCS Infrastructure Analysis

We explore carbon capture and sequestration (CCS) at the meso-scale, a level of study between regional carbon accounting and highly detailed reservoir models for individual sites. We develop an approach to CO(2) sequestration site screening for industries or energy development policies that involves identification of appropriate sequestration basin, analysis of geologic formations, definition of surface sites, design of infrastructure, and analysis of CO(2) transport and storage costs. Our case study involves carbon management for potential oil shale development in the Piceance-Uinta Basin, CO and UT. This study uses new capabilities of the CO(2)-PENS model for site screening, including reservoir capacity, injectivity, and cost calculations for simple reservoirs at multiple sites. We couple this with a model of optimized source-sink-network infrastructure (SimCCS) to design pipeline networks and minimize CCS cost for a given industry or region. The CLEAR(uff) dynamical assessment model calculates the CO(2) source term for various oil production levels. Nine sites in a 13,300 km(2) area have the capacity to store 6.5 GtCO(2), corresponding to shale-oil production of 1.3 Mbbl/day for 50 years (about 1/4 of U.S. crude oil production). Our results highlight the complex, nonlinear relationship between the spatial deployment of CCS infrastructure and the oil-shale production rate.

Aquifer structure identification using stochastic inversion

This study presents a stochastic inverse method for aquifer structure identification using sparse geophysical and hydraulic response data. The method is based on updating structure parameters from a transition probability model to iteratively modify the aquifer structure and parameter zonation. The method is extended to the adaptive parameterization of facies hydraulic parameters by including these parameters as optimization variables. The stochastic nature of the statistical structure parameters leads to nonconvex objective functions. A multi-method genetically adaptive evolutionary approach (AMALGAM-SO) was selected to perform the inversion given its search capabilities. Results are obtained as a probabilistic assessment of facies distribution based on indicator cokriging simulation of the optimized structural parameters. The method is illustrated by estimating the structure and facies hydraulic parameters of a synthetic example with a transient hydraulic response.

Upscaling of reactive mass transport in fractured rocks with multimodal reactive mineral facies.

[1] This paper presents a methodology for upscaling matrix‐material transport parameters in fractured‐flow dominated systems with multimodal reactive mineral facies. The upscaling method provides a theoretical and practical link between controlled experimental results at the laboratory/bench scale and multikilometer field scales at which contaminant remediation and risk assessment are actually conducted. As sorption reactions in matrix are in part determined by mineral properties, a new conceptual model is developed to reflect the hierarchical structure of reactive mineral facies at the microform, mesoform, and macroform scales. The conceptual model of hierarchical reactive matrix mineral facies is integrated with a dual‐porosity model for simulating diffusion of solutes out of fractures and sorption onto the matrix minerals. By assuming that sorption reactions primarily occur in the rock matrix, we develop a multimodal spatial random function for characterizing both the tortuosity (physical heterogeneity) and sorption coefficient (chemical heterogeneity) at different scales in the rock matrix. The effective tortuosity at the field scale is derived by volume averaging of mass transfer coefficient for a conservative species. Subsequently, using a sorbing species (e.g., uranium), we derive the equations for upscaling the sorption coefficients in a saturated, fractured‐rock system for field‐scale simulations. The effective field‐scale tortuosity and sorption coefficient are related to their mean, variance, integral scale, and domain size along a pathway through a three‐ dimensional flow field. The upscaled values increase with the integral scale and are larger than their geometric mean. Simulations conducted with upscaled sorption coefficients and tortuousities are compared very well with high‐resolution Monte Carlo simulations capturing the parameter spatial variations. Results of this study can be extended to explore scaledependenceofotherimportant transportparametersforfractured‐rocksolutetransport. Citation: Deng, H., Z. Dai, A. Wolfsberg, Z. Lu, M. Ye, and P. Reimus (2010), Upscaling of reactive mass transport in fractured rocks with multimodal reactive mineral facies, Water Resour. Res., 46, W06501, doi:10.1029/2009WR008363.

Chlorine-36 data at Yucca Mountain: statistical tests of conceptual models for unsaturated-zone flow.

An extensive set of chlorine-36 (36Cl) data has been collected in the Exploratory Studies Facility (ESF), an 8-km-long tunnel at Yucca Mountain, Nevada, for the purpose of developing and testing conceptual models of flow and transport in the unsaturated zone (UZ) at this site. At several locations, the measured values of 36Cl/Cl ratios for salts leached from rock samples are high enough to provide strong evidence that at least a small component of bomb-pulse 36Cl, fallout from atmospheric testing of nuclear devices in the 1950s and 1960s, was measured, implying that some fraction of the water traveled from the ground surface through 200-300 m of unsaturated rock to the level of the ESF during the last 50 years. These data are analyzed here using a formal statistical approach based on log-linear models to evaluate alternative conceptual models for the distribution of such fast flow paths. The most significant determinant of the presence of bomb-pulse 36Cl in a sample from the welded Topopah Spring unit (TSw) is the structural setting from which the sample was collected. Our analysis generally supports the conceptual model that a fault that cuts through the nonwelded Paintbrush tuff unit (PTn) that overlies the TSw is required in order for bomb-pulse 36Cl to be transmitted to the sample depth in less than 50 years. Away from PTn-cutting faults, the ages of water samples at the ESF appear to be a strong function of the thickness of the nonwelded tuff between the ground surface and the ESF, due to slow matrix flow in that unit.

Groundwater flow and radionuclide transport calculations for a performance assessment of a low-level waste site☆

Abstract Predictions of subsurface radionuclide transport are used to support the groundwater pathway analysis for the performance assessment of the low-level, solid radioactive waste site at Los Alamos National Laboratory. Detailed process-based models rather than higher-level performance-assessment models are used to perform the transport calculations. The deterministic analyses predict the fate of the waste from its source, through the vadose zone, into the saturated zone and, finally, the potential dose to humans at the accessible environment. The calculations are run with the finite-element code FEHM, which simulates fluid flow, heat transport, and reactive, contaminant transport through porous and fractured media. The modeling approach for this study couples realistic source-term models with an unsaturated-zone flow and transport model, which is then linked to the saturated-zone flow and transport model. The three-dimensional unsaturated-zone flow and transport model describes the complex hydrology associated with the mesa-top and volcanic geology of the site. The continued migration of nuclides into the main aquifer is calculated using a three-dimensional, steady-flow, saturated-zone model that maintains the spatial and temporal distribution of nuclide flux from the vadose zone. Preliminary results for the aquifer-related dose assessments show that doses are well below relevant performance objectives for low-level waste sites. A general screening technique that compares the nuclides half-life to its unsaturated-zone travel time is described. This technique helps to decrease the number of transport calculations required at a site. In this case, over half the nuclides were eliminated from further consideration through this screening.

Upscaling retardation factor in hierarchical porous media with multimodal reactive mineral facies

Aquifer heterogeneity controls spatial and temporal variability of reactive transport parameters and has significant impacts on subsurface modeling of flow, transport, and remediation. Upscaling (or homogenization) is a process to replace a heterogeneous domain with a homogeneous one such that both reproduce the same response. To make reliable and accurate predictions of reactive transport for contaminant in chemically and physically heterogeneous porous media, subsurface reactive transport modeling needs upscaled parameters such as effective retardation factor to perform field-scale simulations. This paper develops a conceptual model of multimodal reactive mineral facies for upscaling reactive transport parameters of hierarchical heterogeneous porous media. Based on the conceptual model, covariance of hydraulic conductivity, sorption coefficient, flow velocity, retardation factor, and cross-covariance between flow velocity and retardation factor are derived from geostatistical characterizations of a three-dimensional unbounded aquifer system. Subsequently, using a Lagrangian approach the scale-dependent analytical expressions are derived to describe the scaling effect of effective retardation factors in temporal and spatial domains. When time and space scales become sufficiently large, the effective retardation factors approximate their composite arithmetic mean. Correlation between the hydraulic conductivity and the sorption coefficient can significantly affect the values of the effective retardation factor in temporal and spatial domains. When the temporal and spatial scales are relatively small, scaling effect of the effective retardation factors is relatively large. This study provides a practical methodology to develop effective transport parameters for field-scale modeling at which remediation and risk assessment is actually conducted. It does not only bridge the gap between bench-scale measurements to field-scale modeling, but also provide new insights into the influence of hierarchical mineral distribution on effective retardation factor.

TYBO/BENHAM: Model Analysis of Groundwater Flow and Radionuclide Migration from Underground Nuclear Tests in Southwestern Pahute Mesa, Nevada

Recent field studies have led to the discovery of trace quantities of plutonium originating from the BENHAM underground nuclear test in two groundwater observation wells on Pahute Mesa at the Nevada Test Site. These observation wells are located 1.3 km from the BENHAM underground nuclear test and approximately 300 m from the TYBO underground nuclear test. In addition to plutonium, several other conservative (e.g. tritium) and reactive (e.g. cesium) radionuclides were found in both observation wells. The highest radionuclide concentrations were found in a well sampling a welded tuff aquifer more than 500m above the BENHAM emplacement depth. These measurements have prompted additional investigations to ascertain the mechanisms, processes, and conditions affecting subsurface radionuclide transport in Pahute Mesa groundwater. This report describes an integrated modeling approach used to simulate groundwater flow, radionuclide source release, and radionuclide transport near the BENHAM and TYBO underground nuclear tests on Pahute Mesa. The components of the model include a flow model at a scale large enough to encompass many wells for calibration, a source-term model capable of predicting radionuclide releases to aquifers following complex processes associated with nonisothermal flow and glass dissolution, and site-scale transport models that consider migration of solutes and colloids in fractured volcanic rock. Although multiple modeling components contribute to the methodology presented in this report, they are coupled and yield results consistent with laboratory and field observations. Additionally, sensitivity analyses are conducted to provide insight into the relative importance of uncertainty ranges in the transport parameters.

Colloid-Facilitated Plutonium Transport in Fractured Tuffaceous Rock

Colloids have the potential to enhance the mobility of strongly sorbing radionuclide contaminants in groundwater at underground nuclear test sites. This study presents an experimental and numerical investigation of colloid-facilitated plutonium transport in fractured porous media to identify plutonium reactive transport processes. The transport parameters for dispersion, diffusion, sorption, and filtration are estimated with inverse modeling by minimizing the least-squares objective function of multicomponent concentration data from multiple transport experiments with the shuffled complex evolution metropolis algorithm. Capitalizing on an unplanned experimental artifact that led to colloid formation, we adopt a stepwise strategy to first interpret the data from each experiment separately and then to incorporate multiple experiments simultaneously to identify a suite of plutonium-colloid transport processes. Nonequilibrium or kinetic attachment and detachment of plutonium-colloid in fractures were clearly demonstrated and captured in the inverted modeling parameters along with estimates of the source plutonium fraction that formed plutonium-colloids. The results from this study provide valuable insights for understanding the transport mechanisms and environmental impacts of plutonium in groundwater aquifers.

Integrated Vulnerability and Impacts Assessment for Natural and Engineered Water-Energy Systems in the Southwest and Southern Rocky Mountain Region

In the Southwest and Southern Rocky Mountains (SWSRM), energy production, energy resource extraction, and other high volume uses depend on water supply from systems that are highly vulnerable to extreme, coupled hydro-ecosystem-climate events including prolonged drought, flooding, degrading snow cover, forest die off, and wildfire. These vulnerabilities, which increase under climate change, present a challenge for energy and resource planners in the region with the highest population growth rate in the nation. Currently, analytical tools are designed to address individual aspects of these regional energy and water vulnerabilities. Further, these tools are not linked, severely limiting the effectiveness of each individual tool. Linking established tools, which have varying degrees of spatial and temporal resolution as well as modeling objectives, and developing next-generation capabilities where needed would provide a unique and replicable platform for regional analyses of climate-water-ecosystem-energy interactions, while leveraging prior investments and current expertise (both within DOE and across other Federal agencies).