COLLETT, TIMOTHY S., U.S. Geological Survey
Abstract: Energy Resource Potential of Natural Gas Hydrates
The discovery of large gas hydrate accumulations in permafrost regions of the Arctic and beneath the sea along the outer continental margins of the world's oceans has heightened interest in gas hydrates as a possible energy resource. Gas hydrates are crystalline substances composed of water and gas, in which a solid water-lattice accommodates gas molecules in a cage-like structure, or clathrate. The estimated amount of gas in the hydrate reservoirs of the world greatly exceeds the volume of known conventional gas reserves. However, the role that gas hydrates will play in contributing to the world's energy requirements will depend ultimately on the availability of sufficient gas hydrate resources and the "cost" to extract them. Yet considerable uncertainty and disagreement prevails concerning the world's gas hydrate resources.
Gas hydrate as an energy commodity is often grouped with other unconventional hydrocarbon resources that are either expensive to extract or require new technology for extraction. Unlike gas hydrates, most unconventional natural gas resources have some component of their total world volume being commercially produced. In most cases the evolution of a non-producible unconventional gas resource to a producible energy resource has relied on significant capital investment and technology development. To evaluate the energy resource potential of gas hydrates will also require the support of sustained research and development programs. It has been proposed that the evolution of gas hydrates as a viable source of natural gas, like any other unconventional energy resource, will follow a predictable path from research and discovery to implementation (Figure 1). However, insurmountable barriers may exist along this pathway.
Today, most of the gas hydrate research community is focused on three fundamental issues:WHERE do gas hydrates occur, HOW do gas hydrates occur in nature, and WHY do gas hydrates occur in a particular setting. However, relatively little has been done to integrate these distinct research topics or evaluate how collectively they affect the ultimate resource potential of gas hydrates. Only after understanding the fundamental aspects of WHERE-HOW-WHY gas hydrates occur in nature will we be able to make accurate estimates of how much gas is trapped within the gas hydrate accumulations of the world. Even with the confirmation that gas hydrates may exist in considerable volumes, significant technical, economic, and political issues need to be resolved before gas hydrates can be considered a viable energy resource.
Gas hydrates occur in sedimentary deposits under conditions of pressure and temperature present in permafrost regions and beneath the sea in outer continental margins (reviewed by Kvenvolden, 1993). Cold surface temperatures at high latitudes on earth are conducive to the development of onshore permafrost and gas hydrate in the subsurface. The combined information from Arctic gas-hydrate studies shows that, in permafrost regions, gas hydrates may exist at subsurface depths ranging from about 130 to 2,000 m (reviewed by Kvenvolden, 1993). The presence of gas hydrates in offshore continental margins (Figure 2) 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 or BSR. BSRs have been mapped at depths below the sea floor ranging from about 100 to 1,100 m. (reviewed by Kvenvolden, 1993). Gas hydrates have been recovered during research coring along the southeastern coast of the United States on the Blake Ridge, in the Gulf of Mexico, in the Cascadia Basin near Oregon and California, the Middle America Trench, offshore Peru, the Black Sea, the Caspian Sea, the Sea of Okhotsk and on both the eastern and western margins of Japan.
The amount of methane sequestered in gas hydrates is probably enormous, but estimates of the amounts are speculative and range over three orders-of-magnitude. World estimates for the amount of natural gas in gas hydrate deposits range from 14 to 34,000 trillion cubic meters for permafrost areas and from 3,100 to 7,600,000 trillion cubic meters for oceanic sediments (modified from Kvenvolden, 1993). Current estimates of the amount of methane in the world gas hydrate accumulations are in rough accord at about 20,000 trillion cubic meters (reviewed by Kvenvolden, 1993). If these estimates are valid, the amount of methane in gas hydrates is almost two orders of magnitude larger than the estimated total remaining recoverable conventional methane resources, estimated to be about 250 trillion cubic meters (Masters and others, 1991).
Even though gas hydrates are known to occur in numerous marine and Arctic settings, little is known about the technology necessary the produce gas hydrates. Most of the existing gas hydrate "resource" assessments do not address the problem of gas hydrate recoverability. Proposed methods of gas recovery from hydrates (Figure 3) usually deal with dissociating or "melting" in-situ gas hydrates by (1) heating the reservoir beyond hydrate formation temperatures, (2) decreasing the reservoir pressure below hydrate equilibrium, or (3) injecting an inhibitor, such as methanol or glycol, into the reservoir to decrease hydrate stability conditions. 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 computer models (incorporating heat and mass balance) have been developed to evaluate hydrate gas production from hot water and steam floods, which have shown that gas can be produced from hydrates at sufficient rates to make gas hydrates a technically recoverable resource (Sloan, 1990). However, the economic cost associated with these types of enhanced gas recovery techniques would be prohibitive. Similarly, the use of gas hydrate inhibitors in the production of gas from hydrates has been shown to be technically feasible (Sloan, 1990), however, the use of large volumes of chemicals such as methanol comes with a high economic and environmental cost. Among the various techniques for production of natural gas from in-situ gas hydrates, the most economically promising method is considered to be the depressurization technique. The Messoyakha gas field in the northern part of the West Siberian Basin is often used as an example of a hydrocarbon accumulation from which gas has been produced from in-situ natural gas hydrates (Sloan, 1990). Long-term production from the gas-hydrate part of the Messoyakha field is presumed to have been achieved by the simple depressurization scheme.
Economics and Outlook
As previously discussed in this paper, significant if not insurmountable technical issues need to be resolved before gas hydrates can be counted as a viable option for future supplies of natural gas. In most cases, the viability of an energy resource is based almost solely on economics. It is important to note, however, that in some cases the viability of a particular hydrocarbon resource can be controlled by unique local economic and non-technical factors. For example, countries with little domestic energy production usually pay considerably more for their energy needs since they rely more on imported hydrocarbons, which often come with additional tariffs and transportation expenses. Energy security is often a concern to resource poor countries, which in comparison to energy rich countries will often invest more money in relatively expensive unconventional domestic energy resources. In some cases the uniqueness of a particular location, such as distance to a conventional energy resource, may lead to the development of otherwise non-economic unconventional resource.
World estimates of the amount of methane sequestered in gas hydrates are enormous, but published estimates are highly speculative. Despite the fact that relatively little is known about the ultimate resource potential of natural gas hydrates, it is certain that gas hydrates are a vast storehouse of natural gas and significant technical challenges need to be addressed before this enormous resource can be considered a economically producible reserve.
Kvenvolden, K.A. 1993. Gas
hydrates as a potential energy resource -- a review of their methane content.
in Howell, D.G., ed.,The Future of Energy Gases. U.S. Geological Survey
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Masters, C.D., Root, D.H., and Attanasi, E.D. 1991. Resource constraints in petroleum production potential. Science, v. 253, p. 146-152.
Sloan, E.D. 1990. Clathrate hydrates of natural gases. Marcel Dekker Inc., Publishers, New York, New York, 641 p.
AAPG Search and Discovery Article #[email protected] International Conference and Exhibition, Birmingham, England