Gas Hydrate
Resource Potential
Gas hydrates are ice-like crystalline
substances occurring in nature where a solid water lattice accommodates
gas molecules (primarily methane, the major component of natural gas) in
a cage-like structure, also known as clathrate. These form under
conditions of relatively high pressure and low temperatures, such as
those found in the shallow subsurface under many of the world's oceans.
One cubic foot of hydrate at reservoir temperature and pressure yields
approximately 160 cubic feet of gas at atmospheric temperature and
pressure. The amount of natural gas in methane hydrate worldwide is
estimated to be far greater than the entire world's conventional natural
gas resources.
The MMS, in conjunction with the United
States Geological Survey (USGS) industry, and numerous universities, has
invested significant resources to date in an effort to better understand
methane hydrates. With the demand for natural gas expected to increase
significantly over the next 10 to 20 years, methane gas hydrates, which
are likely present on the OCS in significant quantities, may be a
potential source to meet both industrial and domestic needs for natural
gas.
Resource Assessment
The MMS is in the process of completing the
first comprehensive assessment of gas hydrate on the OCS since a 1995
assessment published by USGS. The results of the 1995 USGS assessment
are
available online
(271.55 KB PDF). Ultimately, the final results of the current
multi-year effort will provide estimates of the undiscovered in-place,
technically recoverable, and economically recoverable gas hydrate
resources for each OCS region (Gulf of Mexico, Atlantic, Pacific and
Alaska). As of February 2008, only the preliminary in-place results for
the Gulf of Mexico have been generated.
Background
Since very little is known about the
capability and reliability of conventional direct-detection technologies
as they apply to marine hydrate accumulations, the presence of gas
hydrate must be inferred through geologic modeling. Development of the
MMS assessment methodology, probabilistic model structure, and
underlying framework of input data has been underway for the past
several years. The physical model and methodology are updated
frequently, and at any time the current model represents our latest
understanding of the various physical conditions that affect the volume
and distribution of gas hydrate accumulations.
Model Summary
The modeling methodology and the physical
model developed for the OCS gas hydrate assessment are the product of a
collaborative body of experts from the MMS, USGS, private industry,
academia, and various research institutions. Due to the inherent
uncertainties associated with an assessment of undiscovered resources, a
stochastic model was developed to sample from distributions of selected
input parameters and to provide the results as a probabilistic range.
The physical model code is combined in
FORTRAN and contains three primary modules:
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Charge |
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Container |
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Concentration |
Each of the primary modules contains a
number of calculations and sub-models where parameters have been derived
from published literature, analogs, and in some cases, expert judgment.
The run order of model version 3.38 is described in the
Flow Diagram
(15.51 KB
PDF). The model is currently set
to run 1,000 trials per cell.
Regional Input Data
Input data for each of the four OCS margins
will vary according to the amount of local empirical data available. For
the Gulf of Mexico, the four primary input grids included bathymetry,
top of salt depth, distribution of sandy sediment in the shallow section
and location of surficial seismic anomalies. These inputs were generated
from seismic data from the GOM that covered 200,000 square miles. The
GOM was divided into 202,000 cells of equal size (5,000 ft x 5,000 ft.)
and a value generated from each input grid (i.e. water depth, salt
depth, and percent) was assigned to every cell.
Results
The results at this time comprise an
analysis of the in-place gas hydrate resources in the Gulf of Mexico
(GOM). The estimates presented here only represent in-place resources,
and do not include either technically recoverable or economically
recoverable resources. The mean in-place estimate for the GOM is 21,444
trillion cubic feet, which includes gas hydrate resources in a variety
of sediment hosts and trap types. The fraction of this in-place estimate
that can be technically extracted and brought to market will be
identified in the next phase of the project.
The GOM was chosen as the initial study area
due to the relative abundance of empirical data and MMS-controlled
proprietary data, both of which are attributable to the mature status of
the GOM as a conventional oil and gas province. The model will be
modified as necessary to accommodate the parameters unique to the
remaining three OCS margins (Atlantic, Pacific, and Alaska).
The
complete methodology report
(3770 KB PDF),
including preliminary in-place results, is now available.
Future Work
The MMS is currently working to modify the
preliminary in-place model to incorporate a number of changes, including
the addition of a thermogenic gas component and implementation of a
technically recoverable module. The modified model will be run on the
Atlantic, Alaska, and Pacific region input data as well as on the GOM
data. As more data becomes available, an economically recoverable
hydrate volume will be generated for all four regions of the OCS.
Additional information on the status of
MMS gas hydrate research, links of interest, and maps and
graphics are available.
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