R-3 Repository :: Browsing by Author "Dugan, Brandon"

Browsing by Author "Dugan, Brandon"

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Biochar particle size, shape, and porosity act together to influence soil water properties
(Public Library of Science, 2017) Liu, Zuolin; Dugan, Brandon; Masiello, Caroline A.; Gonnermann, Helge M.
Many studies report that, under some circumstances, amending soil with biochar can improve field capacity and plant-available water. However, little is known about the mechanisms that control these improvements, making it challenging to predict when biochar will improve soil water properties. To develop a conceptual model explaining biochar’s effects on soil hydrologic processes, we conducted a series of well constrained laboratory experiments using a sand matrix to test the effects of biochar particle size and porosity on soil water retention curves. We showed that biochar particle size affects soil water storage through changing pore space between particles (interpores) and by adding pores that are part of the biochar (intrapores). We used these experimental results to better understand how biochar intrapores and biochar particle shape control the observed changes in water retention when capillary pressure is the main component of soil water potential. We propose that biochar’s intrapores increase water content of biochar-sand mixtures when soils are drier. When biochar-sand mixtures are wetter, biochar particles’ elongated shape disrupts the packing of grains in the sandy matrix, increasing the volume between grains (interpores) available for water storage. These results imply that biochars with a high intraporosity and irregular shapes will most effectively increase water storage in coarse soils.
Biochar-Induced Changes in Soil Hydraulic Conductivity and Dissolved Nutrient Fluxes Constrained by Laboratory Experiments
(Public Library of Science, 2014) Barnes, Rebecca T.; Gallagher, Morgan E.; Masiello, Caroline A.; Liu, Zuolin; Dugan, Brandon
The addition of charcoal (or biochar) to soil has significant carbon sequestration and agronomic potential, making it important to determine how this potentially large anthropogenic carbon influx will alter ecosystem functions. We used column experiments to quantify how hydrologic and nutrient-retention characteristics of three soil materials differed with biochar amendment. We compared three homogeneous soil materials (sand, organic-rich topsoil, and clay-rich Hapludert) to provide a basic understanding of biochar-soil-water interactions. On average, biochar amendment decreased saturated hydraulic conductivity (K) by 92% in sand and 67% in organic soil, but increased K by 328% in clay-rich soil. The change in K for sand was not predicted by the accompanying physical changes to the soil mixture; the sand-biochar mixture was less dense and more porous than sand without biochar. We propose two hydrologic pathways that are potential drivers for this behavior: one through the interstitial biochar-sand space and a second through pores within the biochar grains themselves. This second pathway adds to the porosity of the soil mixture; however, it likely does not add to the effective soil K due to its tortuosity and smaller pore size. Therefore, the addition of biochar can increase or decrease soil drainage, and suggests that any potential improvement of water delivery to plants is dependent on soil type, biochar amendment rate, and biochar properties. Changes in dissolved carbon (C) and nitrogen (N) fluxes also differed; with biochar increasing the C flux from organic-poor sand, decreasing it from organic-rich soils, and retaining small amounts of soil-derived N. The aromaticity of C lost from sand and clay increased, suggesting lost C was biochar-derived; though the loss accounts for only 0.05% of added biochar-C. Thus, the direction and magnitude of hydraulic, C, and N changes associated with biochar amendments are soil type (composition and particle size) dependent.
Development and Application of Stochastic Methods for Radiation Belt Simulations
(2015-09-04) Zheng, Liheng; Chan, Anthony A; Wolf, Richard A; Dugan, Brandon
This thesis describes a method for modeling radiation belt electron diffusion, which solves the radiation belt Fokker-Planck equation using its equivalent stochastic differential equations, and presents applications of this method to investigating drift shell splitting effects on radiation belt electron phase space density. The theory of the stochastic differential equation method of solving Fokker-Planck equations is formulated in this thesis, in the context of the radiation belt electron diffusion problem, and is generalized to curvilinear coordinates to enable calculation of the electron phase space density as a function of adiabatic invariants M, K and L. Based on this theory, a three-dimensional radiation belt electron model in adiabatic invariant coordinates, named REM (for Radbelt Electron Model), is constructed and validated against both known results from other methods and spacecraft measurements. Mathematical derivations and the essential numerical algorithms that constitute REM are presented in this thesis. As the only model to date that can solve the fully three-dimensional diffusion problem, REM is used to study the effects of drift shell splitting, which gives rise to M-L and K-L off-diagonal terms in the radiation belt diffusion tensor. REM simulation results suggest that drift shell splitting reduces outer radiation belt electron phase space density enhancements during electron injection events. Plots of the phase space density sources, which are unique products of the stochastic differential equation method, and theoretical analysis further reveal that this reduction effect is caused by a change of the phase space location of the source to smaller $L$ shells, and has a limit corresponding to two-dimensional local diffusion on a curved surface in the (M,K,L) phase space.
Effect of freeze-thaw cycling on grain size of biochar
(Public Library of Science, 2018) Liu, Zuolin; Dugan, Brandon; Masiello, Caroline A.; Wahab, Leila M.; Gonnermann, Helge M.; Nittrouer, Jeffrey A.
Biochar may improve soil hydrology by altering soil porosity, density, hydraulic conductivity, and water-holding capacity. These properties are associated with the grain size distributions of both soil and biochar, and therefore may change as biochar weathers. Here we report how freeze-thaw (F-T) cycling impacts the grain size of pine, mesquite, miscanthus, and sewage waste biochars under two drainage conditions: undrained (all biochars) and a gravity-drained experiment (mesquite biochar only). In the undrained experiment plant biochars showed a decrease in median grain size and a change in grain-size distribution consistent with the flaking off of thin layers from the biochar surface. Biochar grain size distribution changed from unimodal to bimodal, with lower peaks and wider distributions. For plant biochars the median grain size decreased by up to 45.8% and the grain aspect ratio increased by up to 22.4% after 20 F-T cycles. F-T cycling did not change the grain size or aspect ratio of sewage waste biochar. We also observed changes in the skeletal density of biochars (maximum increase of 1.3%), envelope density (maximum decrease of 12.2%), and intraporosity (porosity inside particles, maximum increase of 3.2%). In the drained experiment, mesquite biochar exhibited a decrease of median grain size (up to 4.2%) and no change of aspect ratio after 10 F-T cycles. We also document a positive relationship between grain size decrease and initial water content, suggesting that, biochar properties that increase water content, like high intraporosity and pore connectivity large intrapores, and hydrophilicity, combined with undrained conditions and frequent F-T cycles may increase biochar breakdown. The observed changes in biochar particle size and shape can be expected to alter hydrologic properties, and thus may impact both plant growth and the hydrologic cycle.
Effects of Stress on Failure Behavior of Shallow, Marine Muds from the Northern Gulf of Mexico
(2014-06-18) Zhao, Xin; Dugan, Brandon; Morgan, Julia K.; Sawyer, Dale S.
Direct simple shear (DSS) experiments on mud samples from 4.3-13.4 meters below sea floor (mbsf) document that stress impacts soil strength and pore pressure genesis during failure. As burial depth increases from 7.3 to 13.4 mbsf, cohesion decreases from 12.3 to 6.5 kPa and internal friction angle increases from 18° to 21°. For the same depth increase, peak shear strength increases from 30 to 63 kPa. For a specimen from 11.75 mbsf, an increase in maximum consolidation stress from 45 to 179 kPa results in an increase in the shear-induced pore pressure from 29 to 150 kPa. The normalized shear strength at peak shear, however, decreases from 0.37 to 0.25 over this consolidation range. Our results indicate that compaction induces a positive feedback on pore pressure genesis. This feedback suggests an increase in failure potential during burial at shallow depth. To further understand the physical controls on this behavior, we complete DSS experiments on resedimented samples to erase stress history and sediment fabric. For the resedimented samples, cohesion is 3.2 kPa and internal friction angle is 24°. An increase in maximum consolidation stress from 40 to 254 kPa results in an increase in the peak shear strength from 14 to 91 kPa and an increase in the shear-induced pore pressure from 22 to 203 kPa; however, the normalized shear strength at peak shear decreases from 0.32 to 0.28. Resedimented samples show similar strength and failure behavior to intact samples. By constraining pore pressure and strength response to initial stress state and fabric, we are beginning to gain better insight on slope failure dynamics. Thus, this study may provide constraints on submarine landslide risks by investigating impact of stress and sedimentary fabric on soil strength and pore pressure genesis during shear failure.
Evolution of Glacially Derived Freshwater and Overpressure in the Massachusetts Shelf: An Integration of Geophysical and Numerical Methods
(2013-10-24) Siegel, Jacob; Dugan, Brandon; Gonnermann, Helge M.; Anderson, John B.; Bedient, Philip B.
The continental shelf offshore Massachusetts, USA experienced repeated glaciations throughout the late Pleistocene that emplaced freshwater and generated overpressure in the shelf sediments that still remains offshore. To show this, I processed and interpreted high-resolution, multi-channel seismic data that was collected offshore Massachusetts to infer the glacial history and to incorporate the glacial history into numerical modeling. Interpretations of the seismic data reveal the shelf stratigraphy and the location of a late Pleistocene (Marine Oxygen Isotope Stage 12) ice sheet. The ice sheet extended 100 km farther onto the shelf compared to the Laurentide ice sheet during the Last Glacial Maximum (LGM). It also contained an ice stream that was likely sourced from the Gulf of Maine. I show that the late Pleistocene ice sheet influenced the shelf hydrogeology by generating overpressure and emplacing freshwater into the shelf sediments. Overpressure is modeled in 1D from high-resolution, full-waveform inversion p-wave velocities obtained from the seismic data and from a finite-difference fluid flow model that accounts for sedimentation and ice sheet loading. The results demonstrate how loading from the late Pleistocene ice sheet caused focused fluid flow that created localized zones of overpressure nearly 1-2 MPa in offshore sediments. Freshwater emplacement into shelf sediments is estimated with a finite-element, variable-density model of fluid flow and heat and solute transport that accounts for ice-sheet loading and sea-level change. The model helps explain how the late Pleistocene ice sheet emplaced nearly 100 km3 of freshwater into the sediments. Our results thus integrate seismic interpretations of ice sheet history with numerical techniques of fluid flow modeling to show how the past glacial history influenced the present freshwater distribution.
Fluid relationships in the Northern Gulf of Mexico using dissolved ion concentrations and strontium isotopes
(2008) Hubbard, L. Ashley; Dugan, Brandon; Dickens, Gerald R.
Pore fluids from the slope in the Gulf of Mexico demonstrate specific ion enrichment and a range in concentration. Dissolved metal and halogen fluid concentrations and strontium isotope ratios from ten sites on the Gulf of Mexico (GoM) continental slope were compared to identify the variations in chemistry. Geochemical discrepancies are interpreted as coming from chloride sources, fluid mixing and fluid diagenesis at depth. We have adapted a grouping scheme developed by Fu and Aharon (1998) in order to highlight seep fluid relationships, including seep fluid from six additional locations. Chloride sources were assessed based on bromide to chloride and sodium to chloride trends and strontium isotope ratios. Chloride source end members include connate seawater, dissolved salt, and ancient evaporated seawater. Out of the sites examined, three sites are classified as having a chloride signal dominated by salt dissolution. Bromide to chloride ratios fall between 0.23 x 10 -3 and seawater (1.5 x 10 -3 ). Sodium to chloride ratios fall between 1.16 and seawater (0.85) and strontium ratios have a large distribution (0.707911-0.709220). Evidence of subaerially evaporated seawater is preserved in fluids from at least two or three sites. Bromide to chloride ratios fall between 2.47 x 10 -3 and seawater. Sodium to chloride ratios fall between 0.75 and seawater and strontium ratios demonstrate a narrow range of values in the least altered fluids (0.708662-0.709172). The majority of saline vent fluids demonstrate mixing between chloride source end members and a wide range of dissolved ion concentrations. Diverse ion enrichment behavior clearly indicates that the processes controlling GoM seep fluid chemistries are complicated and often site dependent resulting from differences in flux, early brine generation and fluid/sediment interactions.
Glacially generated overpressure on the New England continental shelf: Integration of full-waveform inversion and overpressure modeling
(Wiley, 2014) Siegel, Jacob; Lizarralde, Daniel; Dugan, Brandon; Person, Mark
Localized zones of high-amplitude, discontinuous seismic reflections 100 km off the coast of Massachusetts, USA, have P wave velocities up to 190 m/s lower than those of adjacent sediments of equal depth (250m below the sea floor). To investigate the origin of these low-velocity zones, we compare the detailed velocity structure across high-amplitude regions to adjacent, undisturbed regions through full-waveform inversion. We relate the full-waveform inversion velocities to effective stress and overpressure with a power law model. This model predicts localized overpressures up to 2.2MPa associated with the high-amplitude reflections. To help understand the overpressure source, we model overpressure due to erosion, glacial loading, and sedimentation in one dimension. The modeling results show that ice loading from a late Pleistocene glaciation, ice loading from the Last Glacial Maximum, and rapid sedimentation contributed to the overpressure. Localized overpressure, however, is likely the result of focused fluid flow through a high-permeability layer below the region characterized by the high-amplitude reflections. These high overpressures may have also caused localized sediment deformation. Our forward models predict maximum overpressure during the Last Glacial Maximum due to loading by glaciers and rapid sedimentation, but these overpressures are dissipating in the modern, low sedimentation rate environment. This has important implications for our understanding continental shelf morphology, fluid flow, and submarine groundwater discharge off Massachusetts, as we show a mechanism related to Pleistocene ice sheets that may have created regions of anomalously high overpressure.
The impact of lithologic heterogeneity and focused fluid flow upon gas hydrate distribution in marine sediments
(American Geophysical Union, 2014) Chatterjee, Sayantan; Bhatnagar, Gaurav; Dugan, Brandon; Dickens, Gerald R.; Chapman, Walter G.; Hirasaki, George J.
Gas hydrate and free gas accumulation in heterogeneous marine sediment is simulated using a two-dimensional (2-D) numerical model that accounts for mass transfer over geological timescales. The model extends a previously documented one-dimensional (1-D) model such that lateral variations in permeability (k) become important. Various simulations quantitatively demonstrate how focused fluid flow through high-permeability zones affects local hydrate accumulation and saturation. Simulations that approximate a vertical fracture network isolated in a lower permeability shale (kfracture >> kshale) show that focused fluid flow through the gas hydrate stability zone (GHSZ) produces higher saturations of gas hydrate (25–70%) and free gas (30–60%) within the fracture network compared to surrounding shale. Simulations with a dipping, high-permeability sand layer also result in elevated saturations of gas hydrate (60%) and free gas (40%) within the sand because of focused fluid flow through the GHSZ. Increased fluid flux, a deep methane source, or both together increase the effect of flow focusing upon hydrate and free gas distribution and enhance hydrate and free gas concentrations along the high-permeability zones. Permeability anisotropy, with a vertical to horizontal permeability ratio on the order of 10−2, enhances transport of methane-charged fluid to high-permeability conduits. As a result, gas hydrate concentrations are enhanced within these high-permeability zones. The dip angle of these high-permeability structures affects hydrate distribution because the vertical component of fluid flux dominates focusing effects. Hydrate and free gas saturations can be characterized by a local Peclet number (localized, vertical, focused, and advective flux relative to diffusion) relative to the methane solubility gradient, somewhat analogous to such characterization in 1-D systems. Even in lithologically complex systems, local hydrate and free gas saturations might be characterized by basic parameters (local flux and diffusivity).
Impacts of biochar concentration and particle size on hydraulic conductivity and DOC leaching of biochar-sand mixtures
(Elsevier, 2016) Liu, Zuolin; Dugan, Brandon; Masiello, Caroline A.; Barnes, Rebecca T.; Gallagher, Morgan E.; Gonnermann, Helge
The amendment of soil with biochar can sequester carbon and alter hydrologic properties by changing physical and chemical characteristics of soil. To understand the effect of biochar amendment on soil hydrology, we measured the hydraulic conductivity (K) of biochar–sand mixtures as well as dissolved organic carbon (DOC) in leachate. Specifically, we assessed the effects of biochar concentration and particle size on K and amount of DOC in the soil leachate. To better understand how physical properties influenced K, we also measured the skeletal density of biochars and sand, and the bulk density, the water saturation, and the porosity of biochar–sand mixtures. Our model soil was sand (0.251–0.853 mm) with biochar rates from 2 to 10 wt% (g biochar/g total soil × 100%). As biochar (<0.853 mm) concentration increased from 0 to 10 wt%, K decreased by 72 ± 3%. When biochar particle size was equal to, greater than, and less than particle size of sand, we found that biochar in different particle sizes have different effects on K. For a 2 wt% biochar rate, K decreased by 72 ± 2% when biochar particles were finer than sand particles, and decreased by 15 ± 2% when biochar particles were coarser than sand particles. When biochar and sand particle size were comparable, we observed no significant effect on K. We propose that the decrease of K through the addition of fine biochar was because finer biochar particles filled spaces between sand particles, which increased tortuosity and reduced pore throat size of the mixture. The decrease of K associated with coarser biochar was caused by the bimodal particle size distribution, resulting in more compact packing and increased tortuosity. The loss of biochar C as DOC was related to both biochar rate and particle size. The cumulative DOC loss was 1350% higher from 10 wt% biochar compared to pure sand. This large increase reflected the very small DOC yield from pure sand. In addition, DOC in the leachate decreased as biochar particle size increased. For all treatments, the fraction of carbon lost as DOC ranged from 0.06 to 0.18 wt% of biochar. These experiments suggest that mixing sandy soils with biochar is likely to reduce infiltration rates, holding water near the surface longer with little loss of biochar-derived carbon to groundwater and streams.
Influence of late Pleistocene glaciations on the hydrogeology of the continental shelf offshore Massachusetts, USA
(American Geophysical Union, 2014) Siegel, Jacob; Person, Mark; Dugan, Brandon; Cohen, Denis; Lizarralde, Daniel; Gable, Carl
Multiple late Pleistocene glaciations that extended onto the continental shelf offshore Massachusetts, USA, may have emplaced as much as 100 km3 of freshwater (salinity <5 ppt) in continental shelf sediments. To estimate the volume and extent of offshore freshwater, we developed a three-dimensional, variable-density model that couples fluid flow and heat and solute transport for the continental shelf offshore Massachusetts. The stratigraphy for our model is based on high-resolution, multichannel seismic data. The model incorporates the last 3 Ma of climate history by prescribing boundary conditions of sea level change and ice sheet extent and thickness. We incorporate new estimates of the maximum extent of a late Pleistocene ice sheet to near the shelf-slope break. Model results indicate that this late Pleistocene ice sheet was responsible for much of the emplaced freshwater. We predict that the current freshwater distribution may reach depths up to 500 meters below sea level and up to 30 km beyond Martha's Vineyard. The freshwater distribution is strongly dependent on the three-dimensional stratigraphy and ice sheet history. Our predictions improve our understanding of the distribution of offshore freshwater, a potential nonrenewable resource for coastal communities along recently glaciated margins.
Interactions between Biochar, Soil, and Water
(2016-09-08) Liu, Zuolin; Dugan, Brandon; Masiello, Caroline A
Biochar has been proposed as an approach for carbon sequestration and soil amendment. Biochar impacts soil carbon cycling, and in addition, the interaction between biochar and soil water can affect the soil hydrologic cycle as well as plant growth. For instance, after being applied into soil, biochar releases dissolved organic carbon (DOC) through leachate into ground water. This may influence the global carbon cycle. In addition, Biochar’s grain size and shape evolve due to natural processes which will then have feedback on the hydrologic properties of soil. Therefore, it is important to understand the effect of biochar on soil hydrologic properties as well as how the properties of biochar change in soil. Through laboratory experiments and numerical modeling, I investigated the effect of biochar on soil hydraulic conductivity and soil water retention. Meanwhile, I studied change of biochar in soil. For example, I measured DOC as a way of biochar carbon transport in the leachate of biochar-sand mixtures. Also, I tested how biochar’s grain size was altered by freeze and thaw cycling. Coupled with the mechanisms driving these changes, we can better understand the effect of biochar amendment to soil on our living environment. My results show that biochar’s effect on soil hydraulic conductivity (K), soil water retention, and DOC release vary with biochar grain size. Fine mesquite biochar changed K the most (72 ± 2% decrease), medium mesquite biochar did not cause significant change of K, and coarse mesquite biochar decreased K decreased by 15 ± 2%. Hydraulic conductivity also decreased with biochar concentration increase, by up to 72 ± 3% from 0-10 wt% mesquite biochar addition. Fine mesquite biochar did not affect plant available water significantly. The addition of medium and coarse mesquite biochar, however, increased plant available water by 75% and 125%, respectively. DOC in the leachate decreased as mesquite biochar particle size increased. The fraction of carbon lost as DOC ranged from 0.06 to 0.18 wt% of mesquite biochar. I propose that the decrease of K through the addition of fine biochar was because finer biochar particles filled spaces between sand particles which increased tortuosity and reduced pore throat size of the mixture. The decrease of K associated with coarser biochar was caused by the bimodal particle size distribution of biochar-sand mixture, resulting in more compact packing and increased tortuosity. The volume of pores inside biochar (intraporosity) and the shape of biochar particles control the observed changes in water retention. Intraporosity drives the increase in water retention of biochar-amended soils at more negative soil water potential values. At less negative soil water potential values, biochar particles’ elongated shape increases water retention by reducing the efficiency of particle packing, creating large gaps where water can be stored. My results show that biochar grain size plays an important role in controlling soil hydrologic properties and DOC leaching. However, the effect may not stay the same over a long term because biochar grain size can be changed by freeze and thaw cycling. The effect of freeze and thaw cycling on biochar grain size varies with biochar feedstock type. For instance, median grain diameter of pine biochar decreased by up to 28.8%, median grain diameter of miscanthus biochar decreased by up to 45.8%, and median grain diameter of mesquite biochar decreased by up to 32% from 0 to 20 freeze and thaw cycles. However, there was no significant change in grain size observed for sewage waste biochar after five freeze and thaw cycles. These results suggest that mixing sandy soils with biochar is likely to reduce infiltration rates, holding water near the surface longer, increase soil water storage with little loss of biochar-derived carbon to groundwater and streams. However, the reduction of biochar grain size by freeze and thaw cycling will drive changes in soil properties such as hydraulic conductivity and soil water retention.
Laurentide ice sheet meltwater influences and millennial-scale climate oscillations on the northwestern slope of the Gulf of Mexico during Marine Isotope Stage 6 and Termination II
(2009) O'Hayer, Walter Werley; Droxler, Andre W.; Dugan, Brandon
Sub-Milankovitch climate oscillations are well documented phenomena in the Gulf of Mexico during Marine Isotope Stage (MIS) 3 and Termination I, however very little is known about equivalent events during older time intervals. Basin 4 is located on the northwest slope of the Gulf of Mexico and has provided a detailed record of late MIS 6 and Termination II. The results of this study show that the delta18O record of planktonic foraminifer G. ruber contains millennial-scale climate oscillations during MIS 6, a series of meltwater spikes, and a climate reversal during Termination II. Paired delta18O -- Mg/Ca data across these events reveal that the unusually large amplitudes in the delta 18O record cannot be explained by sea surface temperature (SST) or ice volume, but rather are a response to isotopically light glacial meltwater from the paleo-Mississippi river. This conclusion supports the studies of similar oscillations during Termination I and MIS 3.
Microstructural Evolution of Porosity and Stress During the Formation of Brittle Shear Fractures: A Discrete Element Model Study
(Wiley, 2018) LongJohn, Tamunoisoala; Morgan, Julia K.; Dugan, Brandon
Brittle fracturing in rocks is a progressive process involving changes in stress, strain, and porosity. Changes in these properties occur heterogeneously within a rock and are manifest at the grain scale, which is difficult to observe directly in the laboratory or the field. This study uses the discrete element method to show that fractures correspond to zones of generally lower stresses, higher porosity, and highly localized dilation and distortional strain. Using the discrete element method, we conducted numerical biaxial experiments at different confining pressures to probe the internal conditions of a low cohesive sandy sediment numerical analog. When compression begins, the stresses within the sandy sediment are relatively homogeneous with anastomosing stress chains. At yield stress, when the confining pressure is relatively low, multiple dilational bands start to open. At peak stress, high‐magnitude stress chains localize adjacent to the developing shear band and distortion is evident. Postpeak stress, through‐going shear fractures are fully developed. High stresses are transmitted across the fracture where porosity is low through a network of particles in contact. With increasing confining pressure, distortion is favored over dilation during deformation. Also, the number of particles that define the width of a stress chain across a shear fracture, and the steepness of the fracture, increases. Our results can be applied to understanding stress conditions of field samples, and in constraining rock property changes during reservoir modeling of fractured reservoirs.
Microstructural Evolution of Stress and Porosity During the Formation of Brittle Shear Fractures: A Discrete Element Model Study
(2017-08-11) LongJohn, Tamunoisoala; Dugan, Brandon; Morgan, Julia K.
Brittle fracturing in rocks is a progressive process involving changes in stress, strain, and porosity. Changes in these properties occur heterogeneously within a rock, and are manifest at the grain scale, which is difficult to observe directly in the laboratory or the field. This study uses the discrete element method (DEM) to show that fractures correspond to zones of generally lower stresses, higher porosity, and highly localized dilation and distortional strain. Using the DEM, we conducted numerical biaxial experiments at different confining pressures to probe the internal conditions of a sandstone numerical analog. When compression begins, the stresses within the sandstone are relatively homogeneous with anastomosing stress chains. At yield stress, when the confining pressure is relatively low, multiple dilational bands start to open. At peak stress, high magnitude stress chains localize adjacent to the developing shear band and distortion is evident. Post peak stress, through-going shear fractures are fully developed. High stresses are transmitted across the fracture where porosity is low through a network of particles in contact. With increasing confining pressure, distortion is favored over dilation during deformation, Also, the number of particles that define the width of a stress chain across a shear fracture, and the steepness of the fracture increases. Our results can be applied to understanding stress conditions of field samples, and in constraining rock property changes during reservoir modeling of fractured reservoirs.
Modeling Fluid Flow Effects on Shallow Pore Water Chemistry and Methane Hydrate Distribution in Heterogeneous Marine Sediment
(2012-09-05) Chatterjee, Sayantan; Hirasaki, George J.; Chapman, Walter G.; Zygourakis, Kyriacos; Dickens, Gerald R.; Dugan, Brandon
The depth of the sulfate-methane transition (SMT) above gas hydrate systems is a direct proxy to interpret upward methane flux and hydrate saturation. However, two competing reaction pathways can potentially form the SMT. Moreover, the pore water profiles across the SMT in shallow sediment show broad variability leading to different interpretations for how carbon, including CH4, cycles within gas-charged sediment sequences over time. The amount and distribution of marine gas hydrate impacts the chemistry of several other dissolved pore water species such as the dissolved inorganic carbon (DIC). A one-dimensional (1-D) numerical model is developed to account for downhole changes in pore water constituents, and transient and steady-state profiles are generated for three distinct hydrate settings. The model explains how an upward flux of CH4 consumes most SO42- at a shallow SMT implying that anaerobic oxidation of methane (AOM) is the dominant SO42- reduction pathway, and how a large flux of 13C-enriched DIC enters the SMT from depth impacting chemical changes across the SMT. Crucially, neither the concentration nor the d13C of DIC can be used to interpret the chemical reaction causing the SMT. The overall thesis objective is to develop generalized models building on this 1-D framework to understand the primary controls on gas hydrate occurrence. Existing 1-D models can provide first-order insights on hydrate occurrence, but do not capture the complexity and heterogeneity observed in natural gas hydrate systems. In this study, a two-dimensional (2-D) model is developed to simulate multiphase flow through porous media to account for heterogeneous lithologic structures (e.g., fractures, sand layers) and to show how focused fluid flow within these structures governs local hydrate accumulation. These simulations emphasize the importance of local, vertical, fluid flux on local hydrate accumulation and distribution. Through analysis of the fluid fluxes in 2-D systems, it is shown that a local Peclet number characterizes the local hydrate and free gas saturations, just as the Peclet number characterizes hydrate saturations in 1-D, homogeneous systems. Effects of salinity on phase equilibrium and co-existence of hydrate and gas phases can also be investigated using these models. Finally, infinite slope stability analysis assesses the model to identify for potential subsea slope failure and associated risks due to hydrate formation and free gas accumulation. These generalized models can be adapted to specific field examples to evaluate the amount and distribution of hydrate and free gas and to identify conditions favorable for economic gas production.
Numerical Modeling of the Formation and Evolution of Basement-Involved Structures in Wyoming
(2014-01-21) Zhang, Jie; Morgan, Julia K.; Anderson, John B.; Akin, John Edward.; Dugan, Brandon; Sawyer, Dale S.
The Wyoming foreland is composed of basement-involved structures and intermontane basins formed during the Laramide Orogeny. Based on their sizes, structures in this area can be categorized into primary uplifts and secondary folds. Tectonic models suggest the primary uplifts form by sliding the crustal slabs along a deep-seated, large-scale regional detachment in the lower crust, and rotating the basement wedges along listric primary faults. The secondary folds are located close to and trend sub-parallel to the adjacent primary structures, suggesting a causative or correlative relationship between the two, although this connection has not been firmly established through field and seismic investigations. I carry out numerical simulations using both the finite element method (FEM) and discrete element method (DEM) to explore the structural evolution of these secondary basement-involved structures. The first study investigates the Laramide-age Sheep Mountain anticline, located in the eastern Bighorn Basin of Wyoming, using comparative FEM and DEM simulations. The kinematic and mechanical results of the two simulations are similar, thus verifying the methodological comparison. Differences in the geometric details, however, provide important perspectives on the capabilities of the two methods. The mechanical properties defined through this comparative study are then employed in DEM simulations that investigate the relationships between primary and secondary structures during the displacement of large crustal slabs along primary thrust faults. My results show that the displacements and geometries of the primary faults have great impact on the distributions and throw values of the secondary faults. For shallow primary faults with limited regional shortening, the numbers and the displacements of secondary faults are evenly distributed across the basin, with no preference in dip direction. For steep primary faults with significant regional shortening, conjugate faults form early and subsequently cluster into groups. I also explore the influences of initial sedimentary thickness, sedimentary mechanical stratigraphy, and syn-tectonic sedimentation on the distribution of secondary faults. Thicker Pre-Laramide deposits allow more secondary faults to form early during deformation, absorbing the horizontal shortening within the sedimentary layer. The presence of weak shale layers in the sedimentary section allows numerous small faults to form, and limits the depth of all the faults. Syn-tectonic sedimentation reduces the number of secondary faults that form in the basinal area, and displacements along those faults are very small. In this case, most of the deformation is accommodated by the faults located above the ramp take-off location, at the edge of the syn-tectonic deposits.
Overpressure and Earthquake Initiated Slope Failure in the Ursa Region, Northern Gulf of Mexico
(2010) Stigall, Justin Lee; Dugan, Brandon; Morgan, Julia K.; Anderson, John B.
We use two-dimensional fluid flow and slope stability models to study the evolution of overpressure and slope stability in the Ursa region, northern Gulf of Mexico. Our model predictions match measured overpressures from Integrated Ocean Drilling Project Expedition 308 Site U1324 above 200 mbsf, but overpredicts deeper overpressures by 0.4-1.1 MPa. Slope stability models predict a slope failure at 61 ka on the eastern end of the Ursa region. This predicted failure corresponds to a mass transport deposit (MTD) that has been interpreted as a retrogressive failure initiated by high overpressure. Overpressure alone could not drive failure of a second MTD at ~27 ka. We predict that a magnitude 5 earthquake within 140 km of the Ursa region would initiate this failure. We conclude that overpressure could drive submarine slope failures and horizontal acceleration from earthquakes can further facilitate this process.
Pore size controls on the base of the methane hydrate stability zone in the Kumano Basin, offshore Japan
(American Geophysical Union, 2014) Daigle, Hugh; Dugan, Brandon
The base of the methane hydrate stability zone (MHSZ) in the Kumano Basin, offshore Japan, is marked by a bottom-simulating reflection (BSR) on seismic data. At Integrated Ocean Drilling Program Site C0002, which penetrates this BSR, the in situ temperature profile combined with bulk seawater methane equilibrium conditions suggest that the base of the MHSZ is 428 m below seafloor (bsf), which is 28 m deeper than the observed BSR (400 m bsf). We found that submicron pore sizes determined by mercury injection capillary pressure are sufficiently small to cause 64% of the observed uplift of the base of the MHSZ by the Gibbs-Thomson effect. This is the most thorough characterization of pore sizes within the MHSZ performed to date and illustrates the extent to which pore size can influence MHSZ thickness. Our results demonstrate the importance of considering lithology and pore structure when assessing methane hydrate stability conditions in marine sediments.
Pore-Scale Controls on Permeability, Fluid Flow, and Methane Hydrate Distribution in Fine-Grained Sediments
(2011) Daigle, Hugh Callahan; Dugan, Brandon
Permeability in fine-grained sediments is governed by the surface area exposed to fluid flow and tortuosity of the pore network. I modify an existing technique of computing permeability from nuclear magnetic resonance (NMR) data to extend its applicability beyond reservoir-quality rocks to the fine-grained sediments that comprise the majority of the sedimentary column. This modification involves correcting the NMR data to account for the large surface areas and disparate mineralogies typically exhibited by fine-grained sediments. Through measurements on resedimented samples composed of controlled mineralogies, I show that this modified NMR permeability algorithm accurately predicts permeability over 5 orders of magnitude. This work highlights the importance of pore system surface area and geometry in determining transport properties of porous media. I use these insights to probe the pore-scale controls on methane hydrate distribution and hydraulic fracturing behavior, both of which are controlled by flux and permeability. To do this I employ coupled poromechanical models of hydrate formation in marine sediments. Fracture-hosted methane hydrate deposits are found at many sites worldwide, and I investigate whether pore occlusion and permeability reduction due to hydrate formation can drive pore fluid pressures to the point at which the sediments iii fracture hydraulically. I find that hydraulic fractures may form in systems with high flux and/or low permeability; that low-permeability layers can influence the location of fracture initiation if they are thicker than a critical value that is a function of flux and layer permeability; that capillary-driven depression of the triple point of methane in finegained sediments causes hydrate to form preferentially in coarse-grained layers; that the relative fluxes of gas and water in multiphase systems controls hydrate distribution and the location of fracture initiation; and that methane hydrate systems are dynamic systems in which methane flux and hydrate formation cause changes in fluid flow on time scales of hundreds to thousands of years. My results illustrate how pore-scale processes affect macro scale properties of methane hydrate systems and generally affect fluid flow and transport from pore to basin scale.

Browsing by Author "Dugan, Brandon"

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