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In marine continental margins, high saturations of natural gas hydrate are observed within the gas hydrate stability zone, but the gas migration pathways to the hydrate reservoir are not often clear. We focus on a site in Green Canyon Block 955 (GC 955) in the Gulf of Mexico, where gas hydrate occurs in a 35 m-thick reservoir with saturations of 79-93% in numerous centimeter- to decimeter-thick sandy silt layers and little to no gas hydrate in the interbedded cm to decimeter-thick clayey silt layers. Different datasets (well logging, coring, and seismic data) suggest different types of potential methane migration mechanisms. For example, relatively high particulate organic carbon content in the reservoir and the microbial methane source indicated by gas chromatography and methane isotopes suggest gas might be made locally and migration could occur through dissolved methane sources or water advection. In contrast, the visible deeply rooted faults intersecting the gas hydrate reservoir and the gas pockets adjacent to the faults on seismic data suggest that free gas may migrate directly into the reservoir. We tested three different mechanisms to form gas hydrate at GC 955: short-range methane diffusion, short-range methane diffusion coupled with upward water advection, and free gas flow. The 1-D and 2-D numerical models show that both the short-range migration and the upward water advection are not enough to form high saturations gas hydrate at GC 955, and that free gas flow is required to form the high saturations of gas hydrate observed at GC 955.

This study developed a one-dimensional numerical model of sedimentation and compaction based on the equivalent isochrone framework to investigate the impact of creep on geopressure during burial. In this framework, the void ratio is a function of effective stress and strain rate; the change in void ratio is the same with each order of magnitude decrease in strain rate at a constant effective stress. We simulated lower void ratio and higher overpressure when creep was included compared to cases where no creep was present and void ratio is only a function of effective stress. Creep causes apparent overconsolidation. The apparent overconsolidation ratio is used to quantify the magnitude of creep; this is the vertical distance from the normal compression curve in a void ratio v. effective stress plot. The magnitude of creep depends on the loading rate, and increases with depth at sites with low sedimentation rates. These findings bridge the gap between laboratory and field observations on rock compression behaviours. For example, it provides one explanation why laboratory-derived compression curves have a higher void ratio at a given effective stress. In addition, it suggests under what conditions the rock will behave elastically. Thematic collection: This article is part of the Geopressure collection available at: https://www.lyellcollection.org/cc/geopressure

The University of Texas Hydrate Pressure Coring Expedition (UT-GOM2-1) recovered cores at near in situ formation pressures from a gas hydrate reservoir composed of sandy silt and clayey silt beds in Green Canyon Block 955 in the deep-water Gulf of Mexico. The expedition results are synthesized and linked to other detailed analyses presented in this volume. Millimeter- to meter-scale beds of sandy silt and clayey silt are interbedded on the levee of a turbidite channel. The hydrate saturation (the volume fraction of the pore space occupied by hydrate) in the sandy silts ranges from 79% to 93%, and there is little to no hydrate in the clayey silt. Gas from the hydrates is composed of nearly pure methane (99.99%) with less than 400 ppm of ethane or heavier hydrocarbons. The δ13C values from the methane are depleted (−60‰ to −65‰ Vienna Peedee belemnite), and it is interpreted that the gases were largely generated by primary microbial methanogenesis but that low concentrations of propane or heavier hydrocarbons record at least trace thermogenic components. The in situ pore-water salinity is very close to that of seawater. This suggests that the excess salinity generated during hydrate formation diffused away because the hydrate formed slowly or because it formed long ago. Because the sandy silt deposits have high hydrate concentration and high intrinsic permeability, they may represent a class of reservoir that can be economically developed. Results from this expedition will inform a new generation of reservoir simulation models that will illuminate how these reservoirs might be best produced.

We explore the petrophysical behavior of the two interbedded lithofacies (sandy silt and clayey silt) that constitute the Green Canyon Block 955 hydrate reservoir in the deep-water Gulf of Mexico by performing experiments on reconstituted samples of the reservoir material. Sandy silts reconstituted to the in situ porosity have a permeability of 11.8 md (1.18 × 10−14 m2), which is similar to the intrinsic permeabilities measured in intact cores from hydrate reservoirs of similar grain size offshore Japan (Nankai Trough) and offshore India. Reconstituted clayey silts have a much lower intrinsic permeability of 3.84 × 10−4 md (3.84 × 10−19 m2) at the in situ stress. The reconstituted sandy silt is less compressible than the clayey silt. Mercury injection capillary pressure measurements demonstrate that the largest pores with the clayey silt are still smaller than the pores remaining after 90% hydrate saturation in sandy silt. We interpret that the methane solubility in pores of clayey silt is always less than that necessary to form hydrate, which explains why no hydrate is present in the clayey silt. We upscale the reservoir properties to estimate the behavior of interbedded sandy silt and clayey silt. We find the upscaled intrinsic horizontal and vertical permeabilities for the entire reservoir interval are 8.6 md (8.6 × 10−15 m2) and 1.4 × 10−3 md (1.4 × 10−18 m2). We estimate that during reservoir production, a maximum vertical strain of approximately 12% will result. Ultimately, this study will inform reservoir simulation models with petrophysical properties at scales of both individual lithofacies and reservoir formation.

Submarine landslides are prevalent on the modern-day seafloor, yet an elusive problem is constraining the timing of slope failure. Herein, we present a novel technique for constraining the age of submarine landslides without sediment core dating. Underneath a submarine landslide in the Orca Basin, Gulf of Mexico, in 3D seismic data we map an irregular bottom simulating reflection (BSR), which mimics the geometry of the pre-slide seafloor rather than the modern bathymetry. Based on the observed BSR, we suggest that the gas hydrate stability zone (GHSZ) is currently adjusting to the post-slide sediment temperature perturbations. We apply transient conductive heat flow modeling to constrain the response of the GHSZ to the slope failure, which yields a most likely age of ~8 ka demonstrating that gas hydrate systems can respond to slope failures even on the millennia timescales. We also provide an analytical approach to rapidly determine the age of submarine slides at any location.

Slow (8–15 day), stepwise, degassing of synthesized methane hydrate-bearing sand packs was used to determine in situ salinity to within 0.5 wt%. Rebounds in pressure during shut-in intervals between degassing steps record gradual hydrate dissociation. During successive dissociation steps and pressure rebounds, the pressure decreases more than predicted for a sample with homogenous salinity and temperature at thermodynamic equilibrium. We interpret that hydrate dissociation is limited by local pore water freshening and endothermic cooling around the dissociating hydrate. These results suggest that during production, hydrate dissociation in a reservoir will occur along the freshwater phase boundary even though the bulk salinity of the system may be higher. Pore pressure rebound during shut-in is more rapid in freshwater experiments than in brine experiments, which indicates that salt diffusion plays a role in limiting recovery to homogenous bulk equilibrium during hydrate dissociation. We demonstrate that it is possible to interpret the in-situ salinity from the temperature and pressure at the onset of dissociation. This approach can be applied to actual hydrate-bearing pressure core samples and the results will assist our understanding of how these hydrates form. Depressurization experiments with controlled shut-in periods provide insight into the role of salt and heat diffusion on dissociation in coarse-grained sediments, particularly relevant to pauses in production and in regions of a hydrate reservoir near the hydrate phase boundary.

Highlights • Code comparisons build confidence in simulators to model interdependent processes. • International hydrate reservoir simulators are compared over five complex problems. • Geomechanical processes significantly impact response of gas hydrate reservoirs. • Simulators yielded comparable results, however many differences are noted. • Equivalent constitutive models are required to achieve agreement across simulators. Geologic reservoirs containing gas hydrate occur beneath permafrost environments and within marine continental slope sediments, representing a potentially vast natural gas source. Numerical simulators provide scientists and engineers with tools for understanding how production efficiency depends on the numerous, interdependent (coupled) processes associated with potential production strategies for these gas hydrate reservoirs. Confidence in the modeling and forecasting abilities of these gas hydrate reservoir simulators (GHRSs) grows with successful comparisons against laboratory and field test results, but such results are rare, particularly in natural settings. The hydrate community recognized another approach to building confidence in the GHRS: comparing simulation results between independently developed and executed computer codes on structured problems specifically tailored to the interdependent processes relevant for gas hydrate-bearing systems. The United States Department of Energy, National Energy Technology Laboratory, (DOE/NETL), sponsored the first international gas hydrate code comparison study, IGHCCS1, in the early 2000s. IGHCCS1 focused on coupled thermal and hydrologic processes associated with producing gas hydrates from geologic reservoirs via depressurization and thermal stimulation. Subsequently, GHRSs have advanced to model more complex production technologies and incorporate geomechanical processes into the existing framework of coupled thermal and hydrologic modeling. This paper contributes to the validation of these recent GHRS developments by providing results from a second GHRS code comparison study, IGHCCS2, also sponsored by DOE/NETL. IGHCCS2 includes participants from an international collection of universities, research institutes, industry, national laboratories, and national geologic surveys. Study participants developed a series of five benchmark problems principally involving gas hydrate processes with geomechanical components. The five problems range from simple geometries with analytical solutions to a representation of the world's first offshore production test of methane hydrates, which was conducted with the depressurization method off the coast of Japan. To identify strengths and limitations in the various GHRSs, study participants submitted solutions for the benchmark problems and discussed differing results via teleconferences. The GHRSs evolved over the course of IGHCCS2 as researchers modified their simulators to reflect new insights, lessons learned, and suggested performance enhancements. The five benchmark problems, final sample solutions, and lessons learned that are presented here document the study outcomes and serve as a reference guide for developing and testing gas hydrate reservoir simulators.

A new generation of models, laboratory, and field studies is helping scientists answer important questions about this mysterious substance.

We show with a comprehensive hydrate formation model that diffusion of dissolved methane along the local concentration gradient alone cannot transport enough methane to form the concentrated methane hydrates in thick (>= several m) marine sand reservoirs. In this model, methane is generated locally by biodegradation of organic matter in mudstones and diffused into the bounding sandstone where the methane hydrate solubility decreases abruptly and hydrate precipitates from the extra methane in water. Hydrate formed by local diffusion has peak concentrations at the top and base of the sandstone, and it decreases sharply toward the center; salinity is slightly elevated relative to seawater, and its gradient is negligible across the sandstone and adjacent mudstones; dissolved methane concentration decreases from the mudstones to the sandstone. Overpressure is generated in the sediment due to reduction of sediment permeability and volume expansion when hydrate replaces pore water. There is a shallow zone where pore pressure converges toward the lithostatic pressure and fracturing is possible. This study provides further insights into hydrate formation by local diffusion of dissolved methane. In addition, the numerical model we developed in this study fully couples the hydrate-liquid capillary effect, dynamic pore size change, and transient pore pressure-dependent sediment compaction with the multiphase (gas, liquid and hydrate) flow and multicomponent (methane, water and salt) transport. Our model not only predicts pore pressure, but also incorporates the effect of hydrate formation on overpressure generation and subsequent dissipation. These modeling approaches are now at a level of sophistication that we can start to distinguish what physical processes, of the several proposed, are viable explanations for observed hydrate deposits.

We extend a one-dimensional analytical solution by including buoyancy-driven flow to explore the impact of gravity on hydrate formation from gas injection into brine-saturated sediments within the hydrate stability zone. This solution includes the fully coupled gas and liquid phase flow and the associated advective transport in a homogeneous system. We obtain the saturations assuming Darcy flow under combined pressure and gravity gradients; capillary forces are neglected. At a high gas supply rate, the overpressure gradient (gradient of water pressure deviation from the hydrostatic pressure) dominates the gas flow, and the hydrate saturation is independent of the flow rate and flow direction. At a low gas supply rate, the buoyancy (the drive for gas flow induced by the density difference between gas and liquid) dominates the gas flow, and the hydrate saturation depends on the flow rate and flow direction. Hydrate saturation is highest for upward flow, and lowest for downward flow. Hydrate saturation decreases with flow rate for upward flow, and increases with flow rate for downward flow. In all cases, hydrate saturation is constant behind the hydrate solidification front. Gas saturation is homogeneous and close to the residual value for upward flow at a low rate; gas flows at the rate it is supplied. Gas saturation is much greater than the residual value, and decreases from the gas inlet to the hydrate solidification front for downward flow at a very low rate. The effect of gravity is usually negligible in laboratory experiments, yet is significant in natural hydrate systems.

Author(s): Meyer, DW; Flemings, PB; DiCarlo, D; You, K; Phillips, SC; Kneafsey, TJ | Abstract: We form methane hydrate by injecting methane gas into a brine-saturated, coarse-grained sample under hydrate-stable thermodynamic conditions. Hydrate forms to a saturation of 11%, which is much lower than that predicted assuming three-phase (gas-hydrate-brine) thermodynamic equilibrium (67%). During hydrate formation, there are temporary flow blockages. We interpret that a hydrate skin forms a physical barrier at the gas-brine interface. The skin fails periodically when the pressure differential exceeds the skin strength. Once the skin is present, further hydrate formation is limited by the rate that methane can diffuse through the solid skin. This process produces distinct thermodynamic states on either side of the skin that allows gas to flow through the sample. This study illuminates how gas can be transported through the hydrate stability zone and thus provides a mechanism for the formation of concentrated hydrate deposits in sand reservoirs. It also illustrates that models that assume local equilibrium at the core-scale and larger may not capture the fundamental behaviors of these gas flow and hydrate formation processes.

We develop a one-dimensional analytical solution based on the method of characteristics to explore hydrate formation from gas injection into brine-saturated sediments within the hydrate stability zone. Our solution includes fully-coupled multiphase and multicomponent flow and the associated advective transport in a homogeneous system. Our solution shows that hydrate saturation is controlled by the initial thermodynamic state of the system, and changed by the gas fractional flow. Hydrate saturation in gas-rich systems can be estimated by 1−cl0clewhen Darcy flow dominates, where cl0 is the initial mass fraction of salt in brine, and cle is the mass fraction of salt in brine at three-phase (gas, liquid, and hydrate) equilibrium. Hydrate saturation is constant, gas saturation and gas flux decrease, and liquid saturation and liquid flux increase with the distance from the gas inlet to the hydrate solidification front. The total gas and liquid flux is constant from the gas inlet to the hydrate solidification front, and decreases abruptly at the hydrate solidification front due to gas inclusion into the hydrate phase. The advancing velocity of the hydrate solidification front decreases with hydrate saturation at a fixed gas inflow rate. This analytical solution illuminates how hydrate is formed by gas injection (methane, CO2, ethane, propane) at both the laboratory and field scales.

Methane hydrate formation and dissociation are buffered by salinity in a closed system. During hydrate formation, salt excluded from hydrate increases salinity, drives the system to three-phase (gas, water, and hydrate phases) equilibrium, and limits further hydrate formation and dissociation. We developed a zero-dimensional local thermodynamic equilibrium-based model to explain this concept. We demonstrated this concept by forming and melting methane hydrate from a partially brine-saturated sand sample in a controlled laboratory experiment by holding pressure constant (6.94 MPa) and changing temperature stepwise. The modeled methane gas consumptions and hydrate saturations agreed well with the experimental measurements after hydrate nucleation. Hydrate dissociation occurred synchronously with temperature increase. The exception to this behavior is that substantial subcooling (6.4°C in this study) was observed for hydrate nucleation. X-ray computed tomography scanning images showed that core-scale hydrate distribution was heterogeneous. This implied core-scale water and salt transport induced by hydrate formation. Bulk resistivity increased sharply with initial hydrate formation and then decreased as the hydrate ripened. This study reproduced the salinity-buffered hydrate behavior interpreted for natural gas-rich hydrate systems by allowing methane gas to freely enter/leave the sample in response to volume changes associated with hydrate formation and dissociation. It provides insights into observations made at the core scale and log scale of salinity elevation to three-phase equilibrium in natural hydrate systems.

Summary Dispersivity is widely accepted to be scale-dependent. Numerous studies have been published for solute transport in porous media with distance-dependent dispersivities in infinite or semi-infinite domains which set the effluent boundaries to be infinitely far away. However, discussion of finite domains is often overlooked. In this study, two new semi-analytical solutions for solute transport in a finite column are developed with linear-asymptotic (LAF solution) or exponential distance-dependent (EF solution) dispersivities and time-dependent sources. These two solutions are compared with the corresponding solutions for semi-infinite domains (LAI and EI solutions). Results show that breakthrough curves calculated by the LAF/EF solution change faster and possess higher peak values than those calculated by the LAI/EI solution, and the discrepancies increase with the growth rate of the dispersivity and the asymptotic dispersivity. At a specific time, the concentrations calculated by the LAF/EF and the LAI/EI solutions are the same close to the solute source. However, the concentrations calculated by the LAF/EF solution are higher than those calculated by the LAI/EI solution after a certain distance from the source. Such distances decrease with the growth rate of the dispersivity and the asymptotic dispersivity. The performances of the LAF, EF, LAI and EF solutions do not differ much when they are applied to interpret the laboratory column tests conducted in both homogeneous and heterogeneous soil columns.

Diffusive flux is traditionally treated as the dominant mechanism of gas transport in unsaturated zones under natural conditions, and advective flux is usually neglected. However, some researchers have found that pressure-driven and density-driven advective flux may also be significant under certain conditions. This article compares the diffusive, pressure-driven and density-driven advective fluxes of gas phase volatile organic compound (VOCs) in unsaturated zones under various natural conditions. The presence of a less or more permeable layer at ground surface in a layered unsaturated zone is investigated for its impact on the net contribution of advective and diffusive fluxes. Results show although the transient advective flux can be greater than the diffusive flux, under most of the field conditions the net contribution of the advective flux is one to three orders of magnitude less than the diffusive flux, and the influence of the density-driven flux is undetectable. The advective flux contributes comparably with the diffusive flux only when the gas-filled porosity is less than 0.05. The presence of a less permeable layer at ground surface slightly increases the total flux in the underlying layer, while the presence of a more permeable layer at ground surface significantly increases the total flux in it. When the magnitude of water table fluctuation is less than 1 cm, and the period is greater than 0.5 day, the fluctuation of the water table can be simulated by fixing the water table position and setting a fluctuating moving velocity at the water table.

The distribution of antimony (Sb) in topsoil and moss (Dicranum angustum) in disturbed and undisturbed areas, as well as coal and gangue, in Ny-Ålesund, Arctic was examined. Results show that the weathering of coal bed could not contribute to the increase of Sb concentrations in topsoil and moss in the study area. The distribution of Sb is partially associated with traffic and historical mining activities. The occurrence of the maximum Sb concentration is due to the contribution of human activities. In addition, the decrease of Sb content in topsoil near the coastline may be caused by the washing of seawater. Compared with topsoils, moss could be a useful tool for monitoring Sb in both highly and lightly polluted areas.

In this study, a solution to one-dimensional vertical airflow induced by the time-dependent atmospheric pressure fluctuations is developed in a three-layer unsaturated zone. The discrete atmospheric pressure data are transformed into a continuous boundary condition using the Fourier series analysis. The solution is applied to interpret the field air pressure data in a three-layer unsaturated zone reported in previous studies. The new solution improves the previous solution by reducing the required data measurement and input. The new solution is found to be accurate enough for the purpose of this study by comparing with a numerical solution developed in COMSOL Multiphysics. Given the necessary hydrogeological parameters, the new solution is capable of calculating the air permeability of each layer above a specified depth where the air pressures are known. Sensitivity analysis of the new solution shows that location, thickness, and air permeability of the less permeable layer impose large influence on the propagation of the atmospheric pressure fluctuations. Variations of air-filled porosity in soil layers in/below the less permeable layer may lead to greater amplitude attenuation and phase lag of air pressure than those in soil layers above the less permeable layer.

Solutions to soil vapor extraction (SVE) are indispensable to characterize the unsaturated zone and to optimize SVE. Most of the existing solutions neglect the fluctuations of atmospheric pressure and water table. This study presents a new semi-analytical solution for SVE by considering the atmospheric pressure and water table fluctuations. Comparisons between the new and previous solutions indicate that the water table effect is negligible but the atmospheric pressure effect is non-negligible for the interpretation of gas pressure in a non-coastal site where the daily water table fluctuation is in centimeters scale; both the water table and atmospheric pressure fluctuations need to be considered in a coastal site where the daily water table fluctuation is in tens of centimeters scale. Tidal-induced downward moving water table increases the depth-averaged radius of influence, which is insensitive to atmospheric pressure fluctuation. Less vertical gas permeability leads to greater atmospheric pressure and water table effect.

Coastal land reclamation is a common practice in many regions around the World. A reclaimed coastal land often has a two-layer system: a highly permeable layer consisting of crushed rock fragments underneath a less permeable layer. This study deals with vertical airflow in a reclaimed coastal land induced by the periodic variations of water table elevation due to tide fluctuations. The influence of atmospheric pressure fluctuations can be easily dealt with through a simple superposition procedure reported by Li and Jiao, thus is not considered here. A time-series Fourier analysis method is developed to determine air permeability based on a tide-induced airflow model reported by Li and Jiao for the reclaimed coastal land. This method employs the amplitude attenuations of subsurface air pressure to search for values of dimensionless air-leaking-resistance which can be subsequently used to calculate air permeability. Two types of data, the air pressure data in the upper less permeable layer and the tidal fluctuation data, are required to calculate the air permeability. Type curves relating to the amplitude attenuations and the relative depth can also be used to determine the values of air permeability. A field site in Hong Kong International Airport is used to demonstrate the applicability of this method for reclaimed coastal land consisting of a marine sand layer above a layer of crushed rock fragments. The application shows that the amplitude attenuations of diurnal and semi-diurnal components in the Fourier series are most reliable for determining the value of air permeability.

Tide-induced airflow is commonly seen in coastal lands and affects ground stability especially with a less permeable pavement on the ground surface. A tide-induced airflow model in a two-layered unsaturated zone consisting of a highly permeable layer underneath a less permeable layer was established by Li and Jiao [Li HL, JJ Jiao. One-dimensional airflow in unsaturated zone induced by periodic water table fluctuation. Water Resour Res 2005;41:W04007. doi:10.1029/2004WR003916] to describe the one-dimensional airflow with constant atmospheric pressure at the ground surface. In this study, we expand the Li and Jiao model by considering the realistic atmospheric pressure fluctuations and the initial condition. A new transient solution to the airflow model is developed for an initial boundary value problem (IBVP). The transient solution can be used not only to calculate the subsurface air pressure at a future time with a known initial condition, but also to evaluate the asymptotic air pressure variations when time becomes long. The amplitude ratio and phase lag of the subsurface air pressure relative to the tide-induced hydraulic head variations inside the unconfined aquifer below the unsaturated zone are investigated. The results reveal that effect on the subsurface pressure due to changes of atmospheric pressure amplitude depends on the configurations of air resistance in the less permeable layer and the air-filled porosity difference in the two layers. The introduction of atmospheric pressure fluctuations into the airflow model leads to insignificant influence on water table level. A field application of the new solution at Hong Kong International Airport in Hong Kong, China is demonstrated. It indicates that the new transient solution can be conveniently used to evaluate the subsurface air pressure with discrete atmospheric pressure data at the ground surface.

When an open well is installed in an unsaturated zone, gas will flow to/from the well depending on the pressure difference between the well and the surrounding media. This process is called barometric pumping and the well is called a barometric pumping well (BPW). Understanding subsurface gas pressure distribution and gas flow rate to/from a BPW is indispensable to optimize passive soil vapor extraction. This study presents a 2-D semi-analytical solution to calculate the subsurface gas pressure and gas flow rate to/from a BPW with and without a check valve. The problem is conceptualized as a mixed-type boundary value problem. The solution for pumping without a check valve is used to analyze the behavior of the radius of influence (ROI). Results show that ROI is time-dependent. It increases with radial gas permeability and decreases with vertical gas permeability. Field application of the solution without a check valve demonstrates the high accuracy of the developed solution.

[1] Accurate description of induced gas flow in an unsaturated zone is indispensable for characterizing the unsaturated zone. Hantush's approximation and a finite difference approximation of the leakage term are frequently employed in modeling the induced gas flow in an unsaturated zone underlying a leaky confining layer. However, no studies have been conducted on the errors that result from these approximations. This study presents a physically more rigorous model based on mass conservation in both the upper leaky confining layer and the lower permeable layer (the unsaturated zone). Flow in the upper layer is assumed to be vertical, and in the lower layer, it is two-dimensionally axisymmetric, with gas pressure and flux continuity at the interface of the two layers. A new steady state solution for a well with a finite radius and two new transient solutions for a well with an infinitely small radius and with a finite radius are developed for this rigorous model. In addition, another new transient solution is derived using the finite difference approximation of the leakage term for the same problem. The developed solutions are compared with previous approximate solutions. Results indicate that Hantush's approximation overestimates the gas pressure at shallower depths and underestimates it at deeper depths under the condition of gas injection. This error changes with the hydrogeological parameters and well configurations. The finite difference approximation works reasonably well under the usual field conditions. Neglecting the well radius for a small radius well does not induce much error.