Hydrates contain vast store of world gas resources

EMERGING U.S. GAS RESOURCES-4

Timothy S. Collett
U.S. Geological Survey
Denver

Vello A. Kuuskraa
Advanced Resources International Inc.
Arlington, Va.

The discovery of large gas hydrate accumulations on the North Slope of Alaska and off the U.S. southeast coast has heightened interest in gas hydrates as a possible energy resource of the future. However, significant geological uncertainties and possibly insurmountable technical issues need to be resolved before gas hydrates can be counted as a viable option for future, affordable supplies of natural gas.

Several countries, most recently Japan and India, are launching ambitious projects to further examine the viability of gas hydrates. These projects may help answer key questions on the properties of the host reservoir, the design of the production system, and, most importantly, the costs and economics of hydrate recovery. The U.S. Geological Survey has an active research program to document the geologic parameters that control the occurrence of gas hydrates and to assess the volume of gas stored in U.S. gas hydrate accumulations.

This article, fourth in a series, reviews the main geologic controls on the occurrence of gas hydrates and discusses the recently completed USGS national assessment for gas hydrates. It then examines the technological challenges of producing this vast, complex resource. It ends with an overview of the gas hydrate R&D programs in Japan and India.

Gas hydrates overview

Gas hydrates are naturally occurring crystalline substances composed of water and gas, in which a solid water-lattice accommodates gas molecules in a cage-like structure or clathrate.

Gas hydrates occur in numerous settings-in permafrost regions of Alaska and Siberia and beneath the sediment of the outer continental margins (Fig. 1 [45,682 bytes] and Fig. 2 [90,227 bytes]).

While methane, propane, and other gases can be included in the clathrate structure, methane hydrates are the most common. The total amount of methane sequestered in gas hydrates, adapted from Kvenvolden,1 ranges by almost three orders of magnitude, from about 100,000 to 270,000,000 tcf.

It is possible that the volume of gas in the world's hydrate reservoirs exceeds the volume of known conventional gas reserves. However, the resource estimates include numerous assumptions that need careful testing, such as the concentration and extent of the hydrate accumulations. Moreover, little work has been performed on the production potential or the economic feasibility of gas hydrates.

Hydrate structures

Under appropriate conditions of temperature and pressure (Fig. 3 [45,664 bytes]), gas hydrates form one of two basic crystal structures, called Structure I and Structure II. Each unit cell of Structure I gas hydrate consists of 46 water molecules that form two small dodecahedral voids and six large tetradecahedral voids.

Structure I gas hydrates can only hold small gas molecules such as methane and ethane, with molecular diameters not exceeding 5.2 angstroms. The unit cell of Structure II gas hydrate consists of 16 small dodecahedral and 8 large hexakaidecahedral voids formed by 136 water molecules.

Structure II gas hydrates may contain gases with molecular dimensions in the range of 5.9 to 6.9 angstroms, such as propane and isobutane. At standard temperature and pressure (STP), one volume of saturated methane hydrate (Structure I) may contain as much as 164 volumes of methane. Because of this large gas storage capacity, gas hydrates are an important storage site for gas.

On a macroscopic level, many of the gas hydrate mechanical properties resemble those of ice, because hydrates contain a minimum of 85% water on a molecular basis. Of interest are the phase-equilibrium properties of gas hydrates, which are mostly controlled by the fit of the guest gas molecules within the hydrate water cages. For example, the addition of propane to a pure methane hydrate changes the hydrate structure and broadens the conditions in which hydrates can occur.

For a more complete description of the properties and thermodynamics of hydrates, see the summaries by Sloan² and Kuuskraa.³

Permafrost hydrates

Onshore gas hydrates are reported in the West Siberian basin and are believed to occur in other permafrost areas of northern Russia, including the Timan-Pechora province, the eastern Siberian craton, and the northeastern Siberia and Kamchatka areas.

Direct evidence for gas hydrates on the Alaska North Slope comes from a core-test, and indirect evidence comes from well logs that suggest the presence of numerous gas hydrate layers in Prudhoe Bay and Kuparuk River oil fields³(Fig. 4 [21,519 bytes]). Well-log responses attributed to the presence of gas hydrates have been obtained in about one-fifth the wells drilled in the Mackenzie delta of Canada, and more than half the wells in the Arctic Islands are inferred to contain gas hydrates.

The combined information from Arctic gas hydrate studies shows that, in permafrost regions, gas hydrates may exist at subsurface depths ranging from about 130-2,000 m.

Marine gas hydrates

The presence of gas hydrates in offshore continental margins has been inferred mainly from anomalous seismic reflectors that coincide with the predicted phase boundary at the base of the gas hydrate stability zone. This reflector is commonly called a bottom-simulating reflector. BSRs have been mapped at depths below the sea floor ranging from about 100-1,100 m. ¹

Gas hydrates have been recovered in gravity cores within 10 m of the sea floor in sediment of the Gulf of Mexico, the offshore portion of the Eel River basin of California, the Black Sea, the Caspian Sea, and the Sea of Ok- hotsk. Gas hydrates have been recovered at greater sub-bottom depths during research coring along the southeastern coast of the U.S. on the Blake Ridge, in the Gulf of Mexico, in the Cascadia basin near Oregon, the Middle America trench, off Peru, and on Japan's eastern and western margins.

Even with this extensive investigation, the areal and vertical extents of gas hydrate accumulations are based primarily on the theoretical conditions of hydrate stability rather than direct measurements of the presence and concentrations of hydrate deposits.

Resource assessment

World hydrates

World estimates for the amount of gas in gas hydrate deposits range from 5.0 x 10 ² to 1.2 x 10 ⁶ tcf for permafrost areas and from 1.1 x 10 ⁵ to 2.7 x 10 ⁸ tcf for oceanic sediments, according to data adapted from Kvenvolden. ¹

The oceanic sediments seem to hold the largest volumes of hydrates as well as the most uncertainty in the resource estimate. Median estimates of the amount of methane in the worldwide gas hydrate accumulations are in rough accord, at about 7 x 10⁵ tcf.

USGS hydrate estimate

Recently, the U.S. Geological Survey estimated U.S. onshore and offshore gas hydrate resources. The appraisal was based on play-analysis applied on a province by province basis, similar to the methodology used for conventional resources in the 1995 National Oil and Gas Assessment. ⁵

The USGS assessment provided an estimate of the in-place gas hydrate resources-that is, the amount of gas that may exist within the gas hydrates without reference to its recoverability. Prospects (potential hydrocarbon accumulations) were grouped according to their geologic characteristics into plays. The geologic settings of the hydrocarbon occurrences in the play were then modeled. Probabilities were assigned to the geologic attributes within the model necessary for generation and accumulation of hydrocarbons.

In the assessment, 13 plays were identified within four offshore and one onshore petroleum provinces (Fig. 5 [62,341 bytes]). The in-place gas hydrate resource was calculated for each play and then aggregated to produce the estimate of total U.S. gas hydrate resources.⁶

The offshore petroleum provinces assessed consist of the U.S. Exclusive Economic Zone (EEZ) adjacent to the lower 48 states and Alaska. The only onshore province assessed was the North Slope of Alaska, which included state waters and some federal waters. The five provinces (Fig. 5) group the individual petroleum provinces along broad geographic and geologic lines.

Maps depicting the geologic data used in this assessment have been included in the USGS 1995 National Oil and Gas Assessment.⁵ Maps of bathymetry, sedimentary thickness, total organic carbon content of the sediments, seabed temperature, geothermal gradient, and hydrate stability zone thickness have been published on the Assessment CD-ROM for all four offshore provinces, along with a map of hydrate stability zone thickness for northern Alaska. Fig. 6 [65,881 bytes]provides an example of a map of a known gas hydrate accumulation within the southeastern Atlantic Ocean play.

Recent marine research drilling by the Ocean Drilling Program (ODP Leg 164) within the EEZ along the eastern margin of the U.S. has provided information on one of the offshore plays. This drilling and assessment identified substantial quantities of methane stored as solid gas hydrate and as free gas trapped below the gas hydrate on the Blake ridge⁷(Fig. 7 [140,242 bytes]).

U.S. hydrate resources

Gas in place in U.S. hydrates is estimated to range from 113,000 to 676,000 tcf at the 0.95 and 0.05 probability levels, respectively ( Table 1 [113,855 bytes], Fig. 8 [32,766 bytes]). The mean in-place value for the entire U.S. is estimated at 320,000 tcf of gas.

The assessment is made without reference to hydrate recoverability. Future resource assessments are needed to further address issues dealing with gas hydrate concentration and recoverability.

Hydrates production

A major motivation for seeking to produce gas from hydrates, aside from the vast size of the hydrate resource, is its high concentration of energy.

As shown on Table 2 [18,937 bytes], a cubic foot of hydrate in a reservoir rock (with 30% porosity) may hold 50 cf of gas, many times greater than can be stored in other gas sources at moderate reservoir depths.³ This high energy concentration is comparable in energy content with heavy oil and tar sands and gives hope that thermal injection, pressure reduction, or other recovery methods may be applicable for hydrate production.

Favorable setting need

The geologic setting of hydrate entrapment and accumulation is essential, but little is known about the nature of gas hydrate occurrences.

Fig. 9 [54,838 bytes] provides three alternative situations where a hydrate layer exists adjacent to a free gas trap. These geologic settings would be attractive for using pressure reduction as the hydrate production scheme. Other settings, where the hydrate is in a distinct but continuous reservoir zone, may lend themselves to using thermal injection as the recovery technology.

Production alternatives

The proposed methods of gas recovery from hydrates usually involve releasing the methane molecule from the in-situ gas hydrate by:

1. Heating the reservoir beyond hydrate formation temperatures;
2. Decreasing the reservoir pressure below hydrate equilibrium, allowing the gas hydrate to dissociate; or
3. Injecting an inhibitor, such as methanol or glycol, into the reservoir to decrease hydrate stability.

Fig. 10 [105,533 bytes] provides a schematic of gas hydrate production using thermal injection, depressurization, and hydraulic mining. Table 3 [86,494 bytes] reviews and critiques the alternative gas hydrates production methods under current consideration.

Gas recovery from hydrates is hindered because the gas is in a solid form and because hydrates are usually widely dispersed in hostile Arctic and deep marine environments.

First order thermal stimulation models (incorporating heat and mass balance) have been developed to evaluate hydrate gas production using hot water and steam injection. These models have shown that gas can be produced from hydrates at sufficient rates to make hydrates a technically recoverable resource but that major difficulties exist in establishing and maintaining reservoir flow paths.⁸

Similarly, the use of gas hydrate inhibitors for the dissociation of gas from hydrates has been shown to be technically feasible. However, the use of large volumes of solvents such as methanol may be economically prohibitive. Currently, depressurization is considered to be the most promising method for producing gas from hydrates.

Field example

Messoyakha gas field in the northern West Siberian Basin is often used as an example of gas production from in-situ gas hydrates. Production data and other pertinent geologic information have documented the presence of gas hydrates within the upper part of Messoyakha field. ⁹

It has been suggested that the field's production history demonstrates that gas hydrate production can be started and maintained by the production and depressurization of the reservoir.

As production began from the lower free-gas part of the field in 1969, the measured reservoir pressures followed a predicted decline. However, by 1971, the reservoir pressures began to deviate from expected values. This deviation has been attributed to the liberation of free gas from the adjacent dissociating gas hydrates seal.

Based on a review of Messoyakha field's production history, it is estimated that about 36% (about 183 bcf) of the gas produced is from the dissociation of the gas hydrate seal.⁹

Recently, however, several studies suggest that gas hydrates may not be significantly contributing to gas production at Messoyakha field (reviewed by Collett and Ginsburg¹⁰).

Production economics

Because of uncertainties about the geologic settings and feasible production technology, few economic studies have been published on gas hydrates.

The National Petroleum Council, in its major 1992 gas study of gas,¹¹ published one of the few available economic assessments of hydrate production (Table 4 [51,909 bytes]) .

This information, extracted from MacDonald,¹² assesses the relative economics of hydrate recovery using thermal injection and depressurization. It also benchmarks the costs of hydrate production with the costs of conventional gas production on Alaska's North Slope.

Since the date of this NPC publication, the costs of conventional gas production on the North Slope and elsewhere in the U.S. have declined.

International research

In the last two years, government agencies in Japan and India have initiated research programs to recover gas from oceanic hydrates.

Japan National Oil Corp., with funding from the Ministry of International Trade and Industry (MITI), launched a five year study to assess the domestic resource potential of gas hydrates. In numerous statements, MITI has indicated that "methane hydrates could be the next generation source of producible domestic energy."

JNOC in 1996 conducted seismic, gravity, and magnetic surveys off Japan's northern and southeastern continental margins. JNOC plans to drill in 1999 a gas hydrate test well in the Nankai trough (near Tokyo). This area has been speculated to hold as much as 1,800 tcf of methane within gas hydrates.

JNOC recently entered into a cooperative agreement with the Geological Survey of Canada and the USGS to drill an onshore permafrost associated gas hydrate test well in the Mackenzie delta of northern Canada.

India also initiated an ambitious National Gas Hydrate Project. The government of India announced new exploration licensing policies in March 1997 that included the release of several deepwater ( 400 m) lease blocks along the east coast of India between Madras and Calcutta.

Preliminary interpretations of recently acquired seismic data have revealed evidence of gas hydrates on the proposed lease blocks. Also announced was a large gas hydrate prospect in the Andaman Sea between India and Myanmar, which is estimated to contain over 200 tcf.

The government indicated that gas hydrates are of utmost importance to meet growing domestic energy needs. India's National Gas Hydrate Project calls for drilling five gas hydrate test wells by the end of this century.

Acknowledgments

The authors appreciate the Gas Research Institute, particularly Tom H. Fate and Charles F. Brandenburg, for supporting this study of emerging gas resources.

References

1. Kvenvolden, K.A., Gas hydrates as a potential energy resource-A review of their methane content, in Howell, D.G. (ed.), The future of energy gases, USGS Professional Paper 1570, 1993, pp. 555-561.
2. Sloan, E.D., Clathrate hydrates of natural gases, Marcel Dekker, Inc., New York, 1990, 641 p.
3. Kuuskraa, V.A., and Hammershaimb, E.C., Handbook of gas hydrate properties and occurrence: U.S. DOE/METC, 1983, p. 234.
4. Collett, T.S., Natural gas hydrates of the Prudhoe Bay and Kuparuk River area, North Slope, Alaska: AAPG Bull., Vol. 77, No. 5, 1993, pp. 793-812.
5. Gautier, D.L., Dolton, G.L., Takahashi, K.I., and Varnes, K.L., National assessment of U.S. oil and gas resources on CD-ROM, USGS Digital Data Series 30, 1995.
6. Collett, T.S., Gas hydrate resources of the U.S., in Gautier, D.L., Dolton, G.L., Takahashi, K.I., and Varnes, K.L. (eds.), National assessment of U.S. oil and gas resources-results, methodology, and supporting Data, USGS DDS-30, 1995.
7. Shipboard Scientific Party, Sites 994, 995, and 997, in Paull, C.K., and others, Proceedings, Ocean Drilling Program, Initial Reports, College Station, Tex., Vol. 164, 1996, pp. 99-334.
8. Kuuskraa, V.A., Hammershaimb, E.C., and Sawyer, W., Conceptual models for gas hydrates, DOE/ME/19239-1422, National Technical Information Center, DE83015129, 1995.
9. Collett, T.S., Potential of gas hydrates outlined, OGJ, June 22, 1992, pp. 84-87.
10. Collett, T.S., and Ginsburg, G.D., Gas hydrates in the Messoyakha gas field of the West Siberian basin-a re-examination of the geologic evidence, Proceedings, Vol. 1, of 7th International Offshore and Polar Engineering Conference, May 25-30, 1997, Honolulu, 1997, pp. 96-103.
11. National Petroleum Council, The potential for natural gas in the U.S., volumes I and II, NPC, 1992, p. 520 (combined).
12. MacDonald, G.J., The future of methane as an energy resource, Annual Rev. Energy, Vol. 15, 1990, pp. 53-83.

This series

Part 1-Kuuskraa, Vello A., Outlook bright for U.S. natural gas resources, OGJ, Apr. 13, 1998, p. 92. Part 2-Dyman, Thaddeus S., Schmoker, James W., and Root, David H., USGS assesses deep undiscovered gas resource, OGJ, Apr. 20, 1998, p. 99. Part 3-Reeves, S.R., Kuuskraa, J.A., and Kuuskraa, V.A., Deep gas poses opportunities, challenges to U.S. operators, OGJ, May 4, 1998, p. 133. End Part 4 of 6

The Authors