Robert W. Ritzihttps://works.bepress.com/robert_ritzi/Recent works by Robert W. Ritzien-usCopyright (c) 2018 All rights reserved.Sun, 27 May 2018 01:29:58 -00003600Simulating the Heterogeneity in Braided Channel Belt Deposits: 1. A Geometric-Based Methodology and Codehttps://works.bepress.com/robert_ritzi/13/A geometric‐based simulation methodology was developed and incorporated into a computer code to model the hierarchical stratal architecture, and the corresponding spatial distribution of permeability, in braided channel belt deposits. The code creates digital models of these deposits as a three‐dimensional cubic lattice, which can be used directly in numerical aquifer or reservoir models for fluid flow. The digital models have stratal units defined from the kilometer scale to the centimeter scale. These synthetic deposits are intended to be used as high‐resolution base cases in various areas of computational research on multiscale flow and transport processes, including the testing of upscaling theories. The input parameters are primarily univariate statistics. These include the mean and variance for characteristic lengths of sedimentary unit types at each hierarchical level, and the mean and variance of log‐permeability for unit types defined at only the lowest level (smallest scale) of the hierarchy. The code has been written for both serial and parallel execution. The methodology is described in part 1 of this paper. In part 2 (Guin et al., 2010), models generated by the code are presented and evaluated.Ramya Ramanathan et al.https://works.bepress.com/robert_ritzi/13/ArticlesConservative Models: Parametric Entropy Vs. Temporal Entropy in Outcomeshttps://works.bepress.com/robert_ritzi/42/The geologic architecture in aquifer systems affects the behavior of fluid flow and the dispersion of mass. The spatial distribution and connectivity of higher-permeability facies play an important role. Models that represent this geologic structure have reduced entropy in the spatial distribution of permeability relative to models without structure. The literature shows that the stochastic model with the greatest variance in the distribution of predictions (i.e., the most conservative model) will not simply be the model representing maximum disorder in the permeability field. This principle is further explored using the Shannon entropy as a single metric to quantify and compare model parametric spatial disorder to the temporal distribution of mass residence times in model predictions. The principle is most pronounced when geologic structure manifests as preferential-flow pathways through the system via connected high-permeability sediments. As per percolation theory, at certain volume fractions the full connectivity of the high-permeability sediments will not be represented unless the model is three-dimensional. At these volume fractions, two-dimensional models can profoundly underrepresent the entropy in the real, three-dimensional, aquifer system. Thus to be conservative, stochastic models must be three-dimensional and include geologic structure.https://works.bepress.com/robert_ritzi/42/ArticlesCharacterization and Modeling of Spatial Variability in a Complex Alluvial Aquifer: Implications on Solute Transporthttps://works.bepress.com/robert_ritzi/27/Field investigations of stratified alluvial deposits suggest that they can give rise to a hierarchy of permeability modes across scales, corresponding to a hierarchy of sedimentary unit types and thus may lead to enhanced plume spread in such media. In this work, we model the sedimentary architecture of the alluvium deposits in Fortymile Wash, Nevada, using a hierarchical transition probability geostatistical approach. The alluvial aquifer comprises a segment of the groundwater flow pathway from the potential high-level nuclear waste repository at Yucca Mountain, Nevada to the downstream accessible environment and may be a natural barrier to radionuclide migration. Thus our main goal is to quantify the impact of spatial variability in the alluvium on solute transport. The alluvial aquifer is a gravel-dominated braid-belt deposit, having lower-permeability paleosols interstratified with higher-permeability gravel-bar deposits. A three-dimensional hierarchical hydrofacies model is developed through fusion of multiple geologic data types and sources. Markov chain models of transition probabilities are employed to represent complex patterns of spatial variability at each hierarchical level in a geostatistical fashion and to impose realistic constraints to such variations through conditioning on existing data. The link between the alluvium spatial variability and solute dispersion at different spatiotemporal scales is demonstrated using the stochastic-Lagrangian transport theory. We show that the longitudinal macrodispersivity can be on the order of hundreds to thousands of meters, and it may not reach its asymptotic value until after 1,000 years of traveltime.https://works.bepress.com/robert_ritzi/27/ArticlesIntroduction: Modeling Groundwater Flow and Reactive Transport in Physically and Chemically Heterogeneous Mediahttps://works.bepress.com/robert_ritzi/26/https://works.bepress.com/robert_ritzi/26/ArticlesAquitard Distribution in a Northern Reach of the Miami Valley Aquifer, Ohio, USA: Part 1. Three-Dimensional Geostatistical Evaluation of Physical Heterogeneityhttps://works.bepress.com/robert_ritzi/24/In the glacially-filled valleys of the western and central glaciated plains of North America, assemblages of sand-and-gravel outwash form large, productive regional aquifer systems. In these buried-valley aquifers, low-permeability facies, including till or lacustrine clay, are commonly juxtaposed with the permeable outwash. The geometry of the boundary between these facies must be defined in order to address contaminant problems. Although the exact geometry of the boundary between high- and low-permeability facies can only be determined by a prohibitive amount of subsurface information, indicator geostatistics can be used to evaluate the uncertainty at points between actual observations. As an example, the heterogeneity in the Miami Valley aquifer system, which extends 150 km through southwestern Ohio, USA, was examined. In a northern reach, a three-dimensional lithological data base with 30,094 observations was developed by coding all available logs with a binary indicator system, indicating the presence of either high-permeability facies (aquifer) or low-permeability facies (aquitard). This reach of the aquifer system has 36 percent by volume aquitard material, based upon the declustered global mean of the data. The aquitard material is stratified, and the elevation between 204-213 m has the most aquitard material. An interval 6.1 m thick within this zone was studied in more detail to determine the spatial correlation and areal distribution of the aquitard material. Within this interval, the aquitard facies occupies 45 percent of the volume. Based upon variography, the spatial correlation of the facies is greatest in the N50°E direction with a range of about 700 m. The min/max anisotropy ratio is 0.4. The probability of aquitard material existing at any given location was examined with indicator point kriging. Locations in the subsurface having a higher probability for the existence of aquitard material occur more along the valley margins, and locations with a lower probability occur more in the valley interior.https://works.bepress.com/robert_ritzi/24/ArticlesDesigning a Nitrate Monitoring Program in a Heterogeneous, Carbonate Aquiferhttps://works.bepress.com/robert_ritzi/10/Multilevel, double-packer slug tests were performed in open core holes to characterize heterogeneity within a flat-lying, fractured carbonate aquifer. Hydraulic conductivity was measured to vary by approximately three orders of magnitude in the vertical direction and by as much as two orders of magnitude in the horizontal direction. In general, hydraulic conductivity decreased with distance from the bedrock free-surface, as the degree of fracturing decreased. The results of the multilevel slug tests provided the basis for hydraulic conductivity zonation within a two-dimensional finite-element model, which simulated nitrate transport in cross section. In simulation of steady-state flow, several shallow, local ground-water flow systems occurred with exit points that are consistent with springs observable in the field. Yearly rotating crops of corn and soybeans were simulated with nitrate source concentrations based on pore water measurements in the lower soil zones of the field site. The majority of simulated nitrate movement occurred within shallow, localized ground-water flow systems, where much of the nitrate exited the system relatively soon after entering, without appreciable accumulation of nitrate through the years. Based on these results, a monitoring program with a relatively high spatial and temporal resolution in the shallow, localized ground-water flow systems was required in order to characterize nitrate concentrations in the ground water. Regional water quality surveys that involve a random sampling of existing, widely spaced wells would likely miss detecting the locations and times of high nitrate concentrations in ground water.https://works.bepress.com/robert_ritzi/10/ArticlesCapture Zone Geometry in a Fractured Carbonate Aquiferhttps://works.bepress.com/robert_ritzi/25/This study examined a fractured carbonate aquifer that has a transition from porous media type (continuum) flow near the bedrock surface to discrete fracture (non continuum) flow at depth. Three depth zones were delineated using a borehole flowmeter, borehole video logs, and pumping tests. The upper zone is fractured to the degree where it behaves hydraulically as a continuum, the middle zone is less fractured and behaves as a discretely fractured aquifer, and the lower zone is least fractured, has no measurable fracture interconnection, and behaves as an aquitard. These zones were not related to lithologic boundaries, showing that monitoring well design based solely on lithology may be inappropriate in some fractured systems.
The geometries of capture zones in this aquifer were determined by combining the field observations with numerical modeling. The capture zone geometries are very complex, containing thin a really extensive features around fractures in the middle zone which extend over an area 17.5 times greater than the capture area in the upper continuum zone. A capture zone computed with lumped aquifer parameters leads to inaccurate conceptualization of capture zone geometry at this site.
The presence of open core hole monitoring wells affected the flow regime under both ambient and pumping conditions. The wells act as short circuits between otherwise isolated fractures and fracture zones. By connecting the continuum to the non continuum flow regime with the wells, ambient flow in the non continuum regime was increased by a factor of 20. Under pumping conditions, the presence of the monitoring wells alters the capture zone of the pumping well. Discrete fractures provide a connection between the pumping and observation wells at depth that causes separate cones of depression to be formed around observation wells in the upper aquifer, and thus, the capture zone in the near-surface aquifer may include multiple, isolated areas around monitoring wells.Robert K. Podgorney et al.https://works.bepress.com/robert_ritzi/25/ArticlesExplaining the Thinness of Fresh Water Lenses in the Pleistocene Carbonate Aquifer on Andros Island, Bahamashttps://works.bepress.com/robert_ritzi/7/Two hypotheses are explored to explain the thinness of the fresh water lens on Andros Island, Bahamas. The lens is an order of magnitude thinner than predicted by the Ghyben-Herzberg theory. One hypothesis previously posed in the literature is that the base of the lens is governed by the contact between the Lucayan Formation and the pre-Lucayan limestones. An alternate hypothesis, posed here, is that thinning is caused by the hydraulic influence of low-permeability paleosols in the Lucayan Formation. These hypotheses were explored, along with the influence of recharge and other factors, using a numerical model for variable-density flow and salt transport. In the layered system of grainstones/packstones and paleosols within the Lucayan Formation itself, the velocity vectors are nearly horizontal in the grainstones/packstones, and they are nearly vertical in the paleosols. These strata above the pre-Lucayan, all lower than it in permeability, draw the base of the lens upward to a position well above the contact by significantly reducing pressure within and below the strata relative to hydrostatic pressure. It is the loss of pressure across the paleosols that dramatically thins the fresh water lens, and thus the predominant hydraulic control on lens thickness arises from the existence of paleosols.Robert W. Ritzi et al.https://works.bepress.com/robert_ritzi/7/ArticlesInfluence of Small-Scale Fluvial Architecture on CO<sub>2</sub> Trapping Processes in Deep Brine Reservoirshttps://works.bepress.com/robert_ritzi/47/A number of important candidate CO2 reservoirs exhibit sedimentary architecture reflecting fluvial deposition. Recent studies have led to new conceptual and quantitative models for sedimentary architecture in fluvial deposits over a range of scales that are relevant to CO2 injection and storage. We used a geocellular modeling approach to represent this multiscaled and hierarchical sedimentary architecture. With this model, we investigated the dynamics of CO2plumes, during and after injection, in such reservoirs. The physical mechanism of CO2 trapping by capillary trapping incorporates a number of related processes, i.e., residual trapping, trapping due to hysteresis of the relative permeability, and trapping due to hysteresis of the capillary pressure. Additionally, CO2 may be trapped due to differences in capillary entry pressure for different textural sedimentary facies (e.g., coarser-grained versus finer-grained cross sets). The amount of CO2 trapped by these processes depends upon a complex system of nonlinear and hysteretic characteristic relationships including how relative permeability and capillary pressure vary with brine and CO2 saturation. The results strongly suggest that representing small-scale features (decimeter to meter), including their organization within a hierarchy of larger-scale features, and representing their differences in characteristic relationships can all be critical to understanding trapping processes in some important candidate CO2 reservoirs.https://works.bepress.com/robert_ritzi/47/ArticlesHow Does the Connectivity of Open-Framework Conglomerates within Multi-Scale Hierarchical Fluvial Architecture Affect Oil-Sweep Efficiency in Waterflooding?https://works.bepress.com/robert_ritzi/46/Understanding multi-phase fluid flow and transport processes within aquifers, candidate reservoirs for CO2 sequestration, and petroleum reservoirs requires understanding a diverse set of geologic properties of the aquifer or reservoir, over a wide range of spatial and temporal scales. We focus on multiphase flow dynamics with wetting (e.g., water) and non-wetting (e.g., gas or oil) fluids, with one invading another.https://works.bepress.com/robert_ritzi/46/Articles