Resource Potential of Gas Hydrates
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.
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 onlin
(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.
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.
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.
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.
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 (3770.59
KB PDF), including
preliminary in-place results, is now available.
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. |
