MMS Report 2008-004, Preliminary Evaluation of In-Place Gas Hydrate Resources: Gulf of Mexico Outer Continental Shelf (3770.59 KB PDF File)

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:

	Charge
	Container
	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.