DE-FC26-06NT43067

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

Performers
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 seafloor is a significant element of the Earth’s carbon cycle. The amount currently in place may be very 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.

Potential Impacts
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 with a single set of mechanisms.

Accomplishments
Development and validation of a coupled fluid-micromechanical model for simple porous media has 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. The Discrete Element Model (DEM) has reproduced laboratory experiments for dry sediment samples. In addition, the models have been successfully calibrated for micromechanical parameters such that they match stress/strain curves, which is a significant improvement to preliminary results. Sediments containing sand, such as 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 permits 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 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. This 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 the 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. This 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.

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. Analysis of these simulations have been conducted to determine the fraction of the wetting phase that is disconnected as drainage proceeds. A qualitative depiction of the results are shown below.

A qualitative depiction of the results to date

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 availability of CH4 nor of 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 (red curve). 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 Cl– (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 chlorinity 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 (red curve). The total gas/water interfacial area begins to decrease (blue curve), and the number of gas/water menisci associated with the percolating cluster of water phase decreases even faster (broken black line). Thus, when invading gas has drained a sediment well, it is anticipate that hydrate growth at gas/water interfaces will be limited by water availability.

Investigation of the microporomechanics of two-fluid systems and on 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 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 Task 4 Technical Report - Fracture Initiation and Propagation under "Additional Information" below.

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

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

Current Status
Hydrate formation in the model depends upon 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). A catalog of critical curvatures for model sediments based on the results of the Task 5.1 work has been generated and is available for downloading via the following ftp site [ ftp://ftp.netl.doe.gov/pub/HydrateProject43067/ ].

In Task 5.2 (see Technical Report) the capillary pressures during drainage and imbibition as a function of water saturation were examined, In Task 5.3 (see 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. The behavior during cycles of increasing and decreasing methane pressure were 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, and 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 reveal 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 then be used in future tasks to model the growth of methane hydrate at these interfaces.

Project Start: October 1, 2006
Project End: September 30, 2010

Project Funding Information:
Phase 1: DOE Contribution: $590,875, Performer Contribution: $148,135
Phase 2: DOE Contribution: $682,111, Performer Contribution: $171,378
Planned Total Funding (if project continues through all project phases):
DOE Contribution: $1,272,986, 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)

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].

Technology Status Assessment [PDF-1MB] - November 2006

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

Quarterly Progress Report, October-December 2006 [PDF-84KB] - March 2007

Quarterly Progress Report, January-March 2007 [PDF-59KB] - April 2007

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

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

Quarterly Progress Report, April-June 2007 [PDF-213KB] - July 2007

Quarterly Progress Report, July-September 2007 [PDF-303KB] - October 2007

Quarterly Progress Report, October-December 2007 [PDF-1167KB] – January 2008

Quarterly Progress Report, January-March 2008 [PDF-862KB] – April 2008

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

Quarterly Progress Report, April - June 2008 [PDF-804KB] - August 2008

Quarterly Progress Report, July - September 2008 [PDF-7.05MB] - October 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

Quarterly Progress Report, October - December 2008 [PDF-692KB] - February 2009

Technical Report (Task 6) - Couple Gas/Water Interface Dynamics with Fracture Propagation [PDF-1.63MB] - February 2009 (movies mentioned in the report are shown above under "Current Status").