Brandon Dugan

Generalization of gas hydrate distribution and saturation in marine sediments by scaling of thermodynamic and transport processes

Gas hydrates dominated by methane naturally occur in deep marine sediment along continental margins. These compounds form in pore space between the seafloor and a sub-bottom depth where appropriate stability conditions prevail. However, the amount and distribution of gas hydrate within this zone, and free gas below, can vary significantly at different locations. To understand this variability, we develop a one-dimensional numerical model that simulates the accumulation of gas hydrates in marine sediments due to upward and downward fluxes of methane over time. The model contains rigorous thermodynamic and component mass balance equations that are solved using expressions for fluid flow in compacting sediments. The effect of salinity on gas hydrate distribution is also included. The simulations delineate basic modes of gas hydrate distribution in marine sediment, including systems with no gas hydrate, gas hydrate without underlying free gas, and gas hydrate with underlying free gas below the gas hydrate stability zone, for various methane sources. The results are scaled using combinations of dimensionless variables, particularly the Peclet number and Damkohler number, such that the dependence of average hydrate saturation on numerous parameters can be summarized using two contour maps, one for a biogenic source and one for upward flux from a deeper source. Simulations also predict that for systems at steady state, large differences in parameters like seafloor depth, seafloor temperature and geothermal gradient cause only small differences in average hydrate saturation when examined with scaled variables, although important caveats exist. Our model presents a unified picture of hydrate accumulations that can be used to understand well-characterized gas hydrate systems or to predict steady-state average hydrate saturation and distribution at locations for which seismic or core data are not available.

Biochar-induced changes in soil hydraulic conductivity and dissolved nutrient fluxes constrained by laboratory experiments.

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.

Pleistocene hydrogeology of the Atlantic continental shelf, New England

Salinity data from the Atlantic continental shelf off New England indicate that the freshwater/saltwater interface is far out of equilibrium with modern sea-level conditions. More than 150 km offshore of Long Island, New York. aquifer salinity levels are less than 5 parts per thousand (5 ppt). Salinity levels within confining units beneath Nantucket Island, Massachusetts, are 30%. 70% of seawater levels and exhibit a par abolic profile consistent with ongoing vertical diffusion. Here, we evaluate two fluid-flow-inducing mechanisms that could explain the apparent flushing of these coastal-plain aquifers: (1) meteoric recharge during Pleistocene sea-level low stands, and (2) subglacial recharge from the Laurentide Ice Sheet. Analytical models of vertical solute diffusion for the Nantucket confining units suggest that flushing of aquifers beneath Nantucket began in the late Pleistocene between ca. 195 and 21 ka; the models assume a diffusion coefficient of 3.0 × 10 - 1 1 m 2 /s. Cross-sectional numerical models of variable-density groundwater flow, heat, and solute transport could not reproduce the relatively low-salinity groundwaters observed off Long Island by applying boundary conditions consistent with Pleistocene seal-level fluctuations. Observed salinity conditions were most closely matched in the models by also including the effects of sub-glacial recharge from the Laurentide Ice Sheet and allowing groundwater to discharge from Miocene aquifers along submarine canyons near the continental slope. Simulated recharge induced by Laurentide Ice Sheet meltwater was probably short lived hut, on average, about two to ten times greater than modern subaerial levels. A sensitivity analysis performed using our cross-sectional model suggests that a narrow range of hydrologic conditions can drive fresh water long distances offshore across the continental shelf.

Origin and Extent of Fresh Paleowaters on the Atlantic Continental Shelf, USA

While the existence of relatively fresh groundwater sequestered within permeable, porous sediments beneath the Atlantic continental shelf of North and South America has been known for some time, these waters have never been assessed as a potential resource. This fresh water was likely emplaced during Pleistocene sea-level low stands when the shelf was exposed to meteoric recharge and by elevated recharge in areas overrun by the Laurentide ice sheet at high latitudes. To test this hypothesis, we present results from a high-resolution paleohydrologic model of groundwater flow, heat and solute transport, ice sheet loading, and sea level fluctuations for the continental shelf from New Jersey to Maine over the last 2 million years. Our analysis suggests that the presence of fresh to brackish water within shallow Miocene sands more than 100 km offshore of New Jersey was facilitated by discharge of submarine springs along Baltimore and Hudson Canyons where these shallow aquifers crop out. Recharge rates four times modern levels were computed for portions of New Englands continental shelf that were overrun by the Laurentide ice sheet during the last glacial maximum. We estimate the volume of emplaced Pleistocene continental shelf fresh water (less than 1 ppt) to be 1300 km(3) in New England. We also present estimates of continental shelf fresh water resources for the U.S. Atlantic eastern seaboard (10(4) km(3)) and passive margins globally (3 x 10(5) km(3)). The simulation results support the hypothesis that offshore fresh water is a potentially valuable, albeit nonrenewable resource for coastal megacities faced with growing water shortages.

Analytical theory relating the depth of the sulfate‐methane transition to gas hydrate distribution and saturation

[1] We develop a theory that relates gas hydrate saturation in marine sediments to the depth of the sulfate‐ methane transition (SMT) zone below the seafloor using steady state, analytical expressi ...

Biochar physico-chemical properties as affected by environmental exposure

To best use biochar as a sustainable soil management and carbon (C) sequestration technique, we must understand the effect of environmental exposure on its physical and chemical properties because they likely vary with time. These properties play an important role in biochars environmental behavior and delivery of ecosystem services. We measured biochar before amendment and four years after amendment to a commercial nectarine orchard at rates of 5, 15 and 30tha(-1). We combined two pycnometry techniques to measure skeletal (ρs) and envelope (ρe) density and to estimate the total pore volume of biochar particles. We also examined imbibition, which can provide information about soil hydraulic conductivity. Finally, we investigated the chemical properties, surface, inner layers atomic composition and C1s bonding state of biochar fragments through X-ray photoelectron spectroscopy (XPS). Ageing increased biochar skeletal density and reduced the water imbibition rate within fragments as a consequence of partial pore clogging. However, porosity and the volume of water stored in particles remained unchanged. Exposure reduced biochar pH, EC, and total C, but enhanced total N, nitrate-N, and ammonium-N. X-ray photoelectron spectroscopy analyses showed an increase of O, Si, N, Na, Al, Ca, Mn, and Fe surface (0-5nm) atomic composition (at%) and a reduction of C and K in aged particles, confirming the interactions of biochar with soil inorganic and organic phases. Oxidation of aged biochar fragments occurred mainly in the particle surface, and progressively decreased down to 75nm. Biochar surface chemistry changes included the development of carbonyl and carboxylate functional groups, again mainly on the particle surface. However, changes were noticeable down to 75nm, while no significant changes were measured in the deepest layer, up to 110nm. Results show unequivocal shifts in biochar physical and chemical properties/characteristics over short (~years) timescales.

Transient hydraulic fracturing and gas release in methane hydrate settings: A case study from southern Hydrate Ridge

Episodic seafloor methane venting is associated with focused fluid flow through fracture systems at many sites worldwide. We investigate the relationship between hydraulic fracturing and transient gas pressures at southern Hydrate Ridge, offshore Oregon, USA. Two colocated seismic surveys, acquired 8 years apart, at Hydrate Ridge show seismic amplitude variations interpreted as migration of free gas in a permeable conduit, Horizon A, feeding an active methane hydrate province. The geophysical surveys also reveal transients in gas venting to the water column. We propose that episodic gas migration and pressure fluctuations in the reservoir underlying the regional hydrate stability zone (RHSZ) at southern Hydrate Ridge influence methane supply to the RHSZ and are linked with periodic fracturing and seafloor methane venting. We model the effect of pore pressure variations within the deep methane source on fracturing behavior with a 1D model that couples multiphase flow, hydrate accumulation, and pore pressure buildup. As the reservoir pressure increases, fractures open when the pore pressure exceeds the hydrostatic vertical effective stress. Gas then flows through the fractures and vents at the seafloor while hydrate precipitates in the fracture system. We show that active seafloor gas venting occurs for approximately 30 years, and that the available methane reservoir is exhausted 30 to 55 years after the onset of pressure buildup. This provides important constraints on the time scale of transient fluid flow at southern Hydrate Ridge, and illustrates how pore pressure pulses affect fluid flow and fracturing behavior in active methane hydrate provinces.

Consolidation, effective stress, and fluid pressure of sediments from ODP Site 1073, US mid-Atlantic continental slope

Abstract Fluid pressure interpreted from uniaxial consolidation tests equals 80% of the overburden (total) stress in Pleistocene mudstones at Ocean Drilling Program (ODP) Site 1073 of the US mid-Atlantic continental slope, offshore New Jersey. The fluid overpressure decreases effective vertical stress and preserves void ratio ( e ) greater than 0.8 in Pleistocene sediments. Overpressure in Oligocene–Miocene sediments is lower even though e is high. Cementation preserves high void ratio of Oligocene–Miocene sediments and therefore the void ratio does not record overpressure and low effective vertical stress. Sedimentation-flow models simulate overpressure and high void ratio in the Pleistocene sediments when fluid flow is coupled with sediment loading at ODP Site 1073. Lateral fluid migration is interpreted to be focused in the Oligocene–Miocene sediments, which are 2.5 times more permeable than the overlying Pliocene–Pleistocene mudstones. The lateral flow provides fluids to ODP Site 1073 to maintain high void ratio, high overpressure, and low effective vertical stress.

The impact of lithologic heterogeneity and focused fluid flow upon gas hydrate distribution in marine sediments

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).

Seismic and Thermal Characterization of a Bottom-simulating Reflection in the Northern Gulf of Mexico

High-resolution multichannel seismic reflection data, exploration industry three-dimensional (3-D) seismic data, and heat-flow measurements collected on the southeast side of a minibasin (Casey basin) in the northern Gulf of Mexico continental slope have been used to characterize a bottom-simulating reflector (BSR). The BSR, which covers a small area of about 15 km2 (6 mi2), is identified by crosscutting relationships with seismic stratigraphy. Two mounds are identified. The larger Alpha mound is structurally formed at the junction of three arms of the structural high east of the minibasin. The smaller Beta mound may be a seep site. Conventional heat-flow measurements yield higher gradients (39–49 mK/m) to the northeast of the structural high and lower values (30–38 mK/m) to the south and west along the edge of the minibasin, which is separated from the structural high by the eastern Casey fault zone. When the near-sea-floor thermal gradients are extrapolated to the depth of the BSR, the resulting temperatures are generally too low if the BSR marks the base of the hydrate stability zone in a methane-only gas-hydrate system. Plausible changes in pore-water salinity or gas composition cannot account for this disparity, and thermal perturbations caused by fluid downwelling, mass wasting, or depth-dependent thermal conductivity variations might best explain the low predicted BSR temperatures. The recognition of a BSR in the study area provides geophysical evidence that a hydrate stability zone with trapped free gas at its base exists in the northern Gulf and that minibasins can be locations for finding subsurface hydrate-associated free gas and probable gas hydrate.