Mechanisms Leading to Co-existence of Gas and Hydrate in Ocean Sediments

Project Number

DE-FC26-06NT43067

Last Reviewed Dated

Tue, 05/15/2012 - 12:00

Goal

The goal of this project is to quantitatively describe and understand the manner in which methane is transported within the Hydrate Stability Zone (HSZ) and consequently, the growth behavior of methane hydrates at both the grain scale and bed scale.

Schematic conceptual diagram showing fault pathways for thermal gas to supply hydrate accumulations and free gas accumulations

Performer(s)

University of Texas at Austin, Austin, TX 78712-0228
Massachusetts Institute of Technology, Cambridge, MA 02139

Background

The mass of carbon held in sediments below the sea-floor is a significant element of the earth’s carbon cycle. The amount currently in place may be large enough to implicate methane hydrates in global warming events in the geological past and also to raise the prospect of a vast energy resource. However, estimates of this mass and the rate at which it can accumulate in or dissipate from sediments vary widely. One reason for this is the difficulty in ascertaining the form and spatial distribution of methane within the HSZ. This project will quantitatively describe the manner in which methane is transported within the HSZ and will seek to prove or disprove the following hypothesis: The coupling among geomechanics, the dynamics of gas/water interfaces, and the phase behavior of gas/brine/hydrate systems, make co-existence of free gas and hydrate in the HSZ inevitable.

Schematic diagram of the two modes of methane gas invading a sediment. LEFT: invasion will occur if the capillary pressure (the difference between gas pressure and water pressure) exceeds the capillary entry pressure, which is inversely proportional to the pore diameter. RIGHT: invasion by fracture opening; if the exerted pressure is sufficient to overcome compression and friction at grain contacts, a fracture will form. In a multiphase environment, due to surface tension effects, the gas pressure will not dissipate quickly through the porous medium, and water at grain contacts will increase cohesion.

Impact

The successful results of this study will provide a mechanistic basis for observations of co-existing gas and hydrate in the HSZ. The model developed will have implications for interpretation of seismic and borehole log data and thus for the development of accurate estimates of the volume of carbon held in the HSZ. A validated model will be able to explain observations of lateral and vertical variability in hydrate saturation (e.g., the preferential occurrence of hydrates in coarse-grained material above and below a fine-grained layer). Finally, the model will be a step toward explaining active and passive hydrate accumulations using a single set of mechanisms.

Accomplishments (most recent listed first)

Development and validation of a coupled, fluid-micromechanical model for simple porous media have been completed. The first step was the generation of model sediments by settling and compaction, which reproduce the grain size distribution of granular materials of interest. The sediments were modeled by dense, random packings of spheres and represent coarse-grained hydrate-bearing formations such as the Mackenzie Delta. Using a Discrete Element Model (DEM), laboratory experiments for dry sediment samples were reproduced and the model successfully calibrated for micromechanical parameters such that they match stress/strain curves (a significant improvement compared to preliminary results). Sediments containing sand, such as from the Mackenzie Delta, correspond closely to the models exhibiting similar porosities (30% to 40%). As sand is replaced by silt, sediment porosity increases, and sediments dominated by clay may exhibit porosities greater than 70%. Though this project is focused on “sand-dominated” model sediments—which permit the simplest assessment of the pore-pressure induced coupling between fracture initiation and drainage of gas into sediment—physically reasonable methods for building models of silt- and clay-dominated sediments will be evaluated.

Computationally, the completed model consists of two overlapping and interacting networks: the grain network and the fluid network. This is a fully-coupled model with two-way coupling: (1) the pore fluid pressure exerts forces on solid grains, contributing to the deformation of the medium, and (2) grain rearrangements cause changes in the volume of individual pores which, in turn, yield pore-pressure changes. Moreover, the hydraulic properties of the medium will change as a result of deformation, which is reflected in the dependence of the pore-to-pore conductance on the distance between grains.

Conceptual picture of the fluid-solid interaction model at the pore scale

Following the completion of the fluid-micromechanical model, investigation of the poromechanical behavior of model sediments under a variety of scenarios, including single-fluid and two-fluid systems, took place. Of particular interest was the influence of bonding between grains and the dependence of mechanical deformation on the strength of those bonds. The model allows one to investigate under which conditions the material will “fracture” (the fluid pressure is sufficiently high that bonds will break under tension). By resolving the dynamics of the flow and the texture (layering) of the sediment, one can also investigate whether failure will be isotropic or in a preferential direction.

Physically representative network models of the pore space in several of the model sediments were extracted. It was found that these networks are irregular but periodic in all three directions, thereby rendering them “infinite-acting” in the sense that no intrinsic boundaries exist at the edges of the network. Simulation using the DEM coupled to a level-set method progressive quasi-static (LSMPQS) algorithm enabled the first simulations of drainage (gas invasion of the water-saturated sediment) that are free of boundary effects inherent in traditional grain-scale or finite network models. Though finite network simulations accurately reproduce the behavior observed in laboratory experiments, which are, of course, finite, it is proposed that the simulations in the infinite-acting networks are more representative of the situation in sediments in nature. The LSMPQS tool is publically available on the Internet at http://users.ices.utexas.edu/~masha/lsmpqs/index.html [external site].

Simulations using the infinite-acting network model invoke a natural and physically robust criterion for trapping the wetting phase. If the wetting phase in a pore is part of a percolating cluster of wetting phase within the network, then it can be displaced; otherwise, it is trapped. Analyses of these simulations have been conducted to determine the fraction of the wetting phase that is disconnected as drainage proceeds.

Nearly all the water is part of the percolating cluster (red curve) during the early stages of drainage. The total area of gas/water interfaces (blue curve) increases steadily during this period. Because both phases (gas and water) are well-connected, it is expected that hydrate growth will not be limited by the availability of methane (CH₄) or water (H₂O). Some other process, such as the dissipation of the heat of fusion of the hydrate, is likely to be limiting.

As drainage proceeds through an intermediate range of saturations, the water phase remains well connected. However, the percolating cluster is becoming increasingly ramified (dendritic), so that the pathway from a typical pore to the backbone of the cluster is becoming longer and more tortuous. The diffusive transport of excess chlorine (remains in the water phase as water molecules enter the hydrate phase) is slower along such paths. Thus, the continued growth of hydrate is likely to be limited by the buildup of chlorine in this region.

As drainage proceeds to smaller water saturations, there is a rapid decline in the fraction of wetting phase that is still connected to the percolating cluster. The total gas/water interfacial area begins to decrease, and the number of gas/water menisci associated with the percolating cluster of water phase decreases even faster. Thus, when invading gas has drained a sediment well, it is anticipated that hydrate growth at gas/water interfaces will be limited by water availability.

The microporomechanics of two-fluid systems and the conditions under which sediments will fracture due to invading methane gas have been investigated. Migration of a gas phase through a deformable medium may occur by two end-member mechanisms: (1) capillary invasion through a rigid medium, and (2) fracture opening. The model is able to predict which one of the two end-member mechanisms for methane transport (sediment fracturing or capillary invasion) is dominant. Findings indicate that the most sensitive factor in determining the favored mechanism is the grain size: fracturing is favored for fine-grained sediments, while capillary invasion is favored for coarse-grained sediments. Shown in the figure below are two snapshots of the evolution of the methane-water interface for a coarse-grain sediment of characteristic size rmin = 1 mm. During the invasion of methane gas, there is virtually no movement of the solid grains. (The sediment acts like a rigid skeleton and the network of grain contact compressive forces remains the same during the process.) Invasion of gas from pore to pore occurs when the gas pressure (minus the water pressure) exceeds the capillary entry pressure of the throat. The behavior is completely different when a much smaller grain size is used. Mechanical effects become dominant as methane gas migrates through sediments of size rmin = 1 µm and the solid skeleton no longer behaves like a rigid medium. A fracture is created and propagates vertically. This information is now being analyzed to provide useful estimates of when the fracturing regime will be dominant in natural settings. The two main variables controlling the behavior are the lateral earth stress (confining stress) and the grain size. For more information on two-fluid system poromechanical behavior, see the Technical Report, “Fracture Initiation and Propagation” under "Additional Information" below.

Case rmin = 1 mm. Methane invasion by capillary pressure.

Case rmin = 1 µm. Methane invasion by fracture opening.

Hydrate formation in the model depends on the location and geometry of the gas/water interface. The gas/water interface in turn depends on the competition between capillarity-controlled meniscus movement and grain-mechanics-controlled sediment displacement (sediment fracturing). To study this competition, a catalogue of critical curvatures of the meniscus for drainage in the throats of the model sediments has been developed (see Task 5.1 Technical Report).

The capillary pressures during drainage and imbibition as a function of water saturation were examined (see Task 5.2 Technical Report). The capillary-controlled configurations of gas and water in the rough-walled fracture were then coupled to critical curvature and capillary pressure curves to examine the magnitude of methane drainage from the fracture into different model sediments (see Task 5.3 Technical Report). The behavior during cycles of increasing and decreasing methane pressure was investigated. The simulations provide the detailed geometry of the gas/water interface at each step of drainage or imbibition.

The next step (see Task 6 Technical Report) was to couple grain-mechanics-controlled sediment displacement with capillarity-controlled meniscus movement (drainage and imbibition) to determine when fracturing is favored over capillary invasion and to explore the emerging behavior when a source of methane gas exists at prescribed pressure. Model simulation results indicate that the gas pressure may be much higher than the water pressure. The difference between the two may not be sufficient to overcome the capillary entry pressure to invade a pore throat (locally), but the associated forces may be sufficient to open up fractures within the sediment. In addition, simulations revealed that the percolation behavior that characterizes capillary displacements is much less pronounced when grains can be moved by the difference between gas and water pressures. Consequently, gas is less likely to displace water down to residual saturation. This leads to a gas/water configuration more conducive to methane hydrate formation. This information will be used in future tasks to model the growth of methane hydrate at these interfaces.

Research was then conducted on the coupled dynamics of the gas/water/sediment interface in the presence of both rigid and fragile hydrate films. Investigation of rigid hydrate films is documented in the October 2009 (Task 7.1) Technical Report (see “Additional Information” below). This report describes modeling done at both the micro- and macro-scale level to investigate the premise that hydrates form from co-existing phases of gas and water in sediments. The research investigates hydrate formation under various brine- and gas-limiting saturations, and for both fine- and coarse-grained sediments. Initial findings show that the volume change associated with hydrate formation may lead to pressure changes which could result in fracturing, drainage, and imbibition. Instabilities at the gas-water interface lead to rupture of the rigid hydrate film, and subsequent imbibition events. Model simulations suggest that for coarse-grained sediments, multiple cycles of film growth, rupture, and imbibition result in disseminated hydrate distribution, whereas in fine-grained sediments the film remains stable and the gas will remain in its own phase for longer periods.

Mechanical instability of fragile hydrate shells was investigated (see Quarterly Progress Report for January – March 2010 under “Additional Information” below) as an alternative means by which the gas-water interface is re-established, resulting in hydrate growth. When gas concentrations are limited, instability of the hydrate film arises due to a pressure drop associated with the consumption of methane. As the pressure drops, the model predicts that the hydrate shell implodes. Afterwards, water imbibes, compressing the gas into a smaller volume thereby raising the pressure again and hydrate formation resumes. Eventually, the hydrate shell becomes stable and eventually all the gas is converted to hydrate. In situations where gas is recharged, i.e. unlimited gas, and under certain geologic boundary conditions, the gas can traverse the sediments, making its way to the water column.

Modeling work focused on the role of multiphase flow and sediment mechanics on the dissociation process during thermal stimulation has shown that rapid dissociation-induced overpressures are not possible. This is because an increase in the pore pressure hinders further dissociation and the pressure is constrained by the phase equilibrium curve during the entire dissociation process. Regardless of the kinetic rate of dissociation, heat supply, and sediment permeability, the overpressure is bounded by the pressure at thermodynamic equilibrium. These findings have important implications for the response of natural sediments to ocean warming and for future methane production from hydrate bearing sediments.

FracDEM

FracDEMLSM

FracDEM - click on image to view movie

FracDEMLSM - click on image to view movie

Development of a fracture in fine-grained media, as predicted by our grain-scale models. The grains, assumed to be spherical, are shown in yellow. The initial configuration is obtained by settling the particles and compacting them to a desired confining stress. Initially, the pack is filled with brine. At time 0, gas invades two pores at the bottom of the sample, in a small region marked with an orange rectangle. From the initial configuration, the gas pressure is allowed to increase. It can invade the pores either by overcoming the capillary entry pressure, or by moving the grains. Pores that have been invaded by gas are denoted with a blue circle. The maroon lines indicate compressive forces between grains. This network of compressive forces changes drastically with the evolution of the fracture. The green lines indicate tension between grains, caused by capillary forces that hold the grains together. The network of tension forces also changes with time, as the gas invades into the sediment.

Left: Simulation using a discrete element model (DEM). The model accounts rigorously for the grain mechanics, and the fluid-structure interaction, but relies on simplified rules for gas invasion into the pores—capillary entry pressure—and simplified computation of forces from the fluids onto the grains. A vertical fracture develops. The fracture is preceded by tension between the grains at the tip of the fracture.

Right: Simulation using a coupled discrete element model with a level-set method progressive quasi-static (LSMPQS) algorithm. The PQS algorithm provides an accurate simulation of how the gas-water interface evolves. This coupled DEM-LSM model, explained in Task 6, therefore combines accurate interface evolution and rigorous grain mechanics. The results of the simulation are similar, but not identical, to those of the previous model. Initially, a similar fracture develops but at some point during fracture propagation, it splits into two fractures.

In collaboration with the United States Geological Survey, UTA/MIT researchers have validated the capillary- and fracture-dominated migration models using data from the Mt. Elbert stratigraphic test well and Upper Mystic Lake in Massachusetts, respectively.

UTA/MIT researchers have completed validation of bed-scale models. In collaboration with the United States Geological Survey, bed-scale models of capillary-dominated migration and fracture-dominated migration were tested against Mt. Elbert data and data from Upper Mystic Lake in Massachusetts, respectively.

A pore-level model of hydrate growth at gas-brine interfaces has been developed to assess sub-permafrost hydrate accumulations. This model assumes that all hydrate films are mechanically robust( breaking when subject to a pressure difference). Depending on whether the rate of supply of water to the hydrate interface is slow or fast relative to the rate of hydrate growth, the model predicts a one-to-one replacement of gas-filled pores by hydrate-filled pores (slow water supply), or a mixture of water-filled pores enclosed by hydrate shells (fast water supply). The volume of phases in realistic granular material is readily accessible from the level-set method simulation of drainage and imbibition developed during earlier phases of the project. Thus these two limiting cases can constrain the ratio of volumes of gas and water that enter the domain. These ratios can then be used to predict modern hydrate saturations with results comparable to data from the Mt. Elbert test well.

Efforts focused on modeling methane transport in lake sediments have been tested against a four-month record of hydrostatic load and methane flux in Upper Mystic Lake, outside Boston, Massachusetts. The premise is that methane transport in these environments is controlled by dynamic conduits, which dilate and release gas as the falling hydrostatic pressure reduces the effective stress below the tensile strength of the sediments. The model is able to predict the timing and relative magnitude of the venting events, which suggests that it is capable of capturing the essential dynamics of methane ebullition (refer to Quarterly Progress Report for January through March, 2011).

In the last several months of the project, UTA/MIT continued modeling hydrate formation and stability at the bed-scale by investigating the episodic venting of methane into the water column under tidal influence. Focused ebullition has been observed at Hydrate Ridge, especially in a periodic pattern during low tides. The mechanism that allows methane gas to travel through the Hydrate Stability Zone (HSZ) and escape only during low tides is not well understood. Researchers have modeled a shallow, hydrate-bearing region in order to explore how fracture flow, tidal forcing, and hydrate formation interact. Preliminary findings suggested that the amount of gas released to the ocean is critically dependent on the vertical gas conductivity and kinetic hydrate formation rate (refer to the Quarterly Progress Report for July through September, 2011 listed below under “Additional Information”).

Current Status

(May 2012)
The project has been completed and the final scientific/technical report received (see “Additional Information” below). A major scientific contribution of this project is the identification of two broad regimes for gas displacing brine within sediments: a fracturing regime, which applies for low permeability sediments, and a fingering regime, which applies in higher permeability sediments. Validation has shown that the bed-scale models are consistent with actual data from the field.

Project Start

Sun, 10/01/2006 - 12:00

Project End

Sat, 12/31/2011 - 12:00

DOE Contribution

Phase 1: $590,875
Phase 2: $682,111
Planned Total Funding (if project continues through all project phases):
DOE Contribution: $1,272,986

Performer Contribution

Phase 1: $148,135
Phase 2: $171,378
Planned Total Funding (if project continues through all project phases):
Performer Contribution: $319,513

Contact Information

NETL - Robert Vagnetti (Robert.Vagnetti@netl.doe.gov or 304-285-1334)
University of Texas at Austin – Steven Bryant (steven_bryant@mail.utexas.edu or 512-471-3250)
Massachusetts Institute of Technology - Ruben Juanes (juanes@mit.edu or 617-253-7191)
If you are unable to reach the above personnel, please contact the content manager.

Additional Information

In addition to the information provided here, a full listing of project related publications and presentations as well as a listing of funded students can be found in the Methane Hydrate Program Bibliography [PDF].

Final Project Report [PDF-47.4MB] - April, 2012

Technology Status Assessment [PDF-1.01MB] - November 2006

Kick-off meeting presentation [PDF-622KB] - January 9, 2007

Technical Report (Task 3) - Sediment Model [PDF-5.63MB] - April 2007

Technical Report (Task 3) - Sediment Model Appendix [PDF-24.4MB] - April 2007

Technical Report (Task 4) - Fracture Initiation and Propagation [PDF-2.79MB] - May 2008

2008 OTC paper - Mechanisms by Which Methane Gas and Methane Hydrate Coexist In Ocean Sediments [PDF-5.97MB] - May 2008

Technical Report (Task 5.1) - Critical Curvatures [PDF-698KB] - June 2008

2008 ICGH Paper - Pore-scale Mechanistic Study of the Preferential Mode of Hydrate Formation in Sediments: Coupling of Multiphase Fluid Flow and Sediment Mechanics [PDF] - July, 2008

2008 ICGH Paper - Grain Scale Study of Hydrate Formation in Sediments from Methane Gas: Role of Capillarity [PDF] - July, 2008

Technical Report (Task 5.2) - Compute gas/water interface geometry during drainage and imbibition in model sediments [PDF-5.68MB] - October 2008

Technical Report (Task 5.3) - Compute gas/water interface geometry with fracture [PDF-1.41MB] - October 2008