The extraordinary promise and challenge of gas hydrates
EXPLORATION / EXPLOITATION REPORTThe extraordinary promise and challenge of gas hydratesEvidence is growing that vast reserves of natural gas, in the form of hydrate, exist in permafrost regions and continental margins. Current research efforts are intensifyingAllen Lowrie, Exploration Consultant, Picayune, Mississippi; and Michael D. Max, Consultant, Washington, DC
Introduction Secure supplies of petroleum oil are a depleting resource that is naturally limited and subject to short-term disruption by political events. However, attention need not be riveted on existing sources for conventional gas and petroleum. Other fuels are potentially available that could ensure supply of low-cost fuel well into the 21st century. In spring 1999, crude oil was selling for historically low prices. Through artificial supply constriction by OPEC, prices rebounded in early summer. Nonetheless, an upturn in price caused by a real supply shortfall will almost certainly occur during the next 20 years. Transition from an oil-based economy to some other type of energy-based economy is near enough to demand attention from both government and industry. The impact on industrial societies of irrevocably rising energy costs will activate market forces to develop other fuels. An array of alternate fuel sources such as ethanol, methanol, heavy oils, tar sands, oil shale and hydrogen systems, as well as conventional, deeper-ocean hydrocarbon resources, will all be brought into play. New engines and electricity-generating techniques, e.g., fuel cells, optimized gas-turbine electric generators and compound-engine vehicles, may facilitate using newer fuels. Environmental requirements also influence fuel selection. Suitability of alternative fuels depends on technical / economic breakthroughs, such as low-cost production of high-energy, liquid fuels from methane, methanol or ethanol, engineered for low environmental impact. An important future fuel is probably methane. Vast, newly recognized (since 1979) deposits of methane are held in the form of gas hydrate in the world’s oceans. These deposits are important for what they are — potentially the greatest single storehouse of combustible energy on earth — and for where they are located. The bulk of petroleum oil supplies are in the Middle East, including the Caspian basin and former Soviet Union. Sufficient hydrates may exist to offer the prospect of widespread energy independence for countries that are not self-sufficient in petroleum reserves. Methane Hydrate Gas hydrates are ice-like, crystalline accumulations formed from natural gas (mainly methane) and water, Fig. 1. Hydrates are thermodynamically stable, both at very low temperatures in permafrost regions and in low-temperature / high-pressure regimes present in deep oceans. The temperature / pressure (T-P) regime controlling the position of the gas-hydrate phase can change, causing hydrate disassociation. When such disassociation occurs, an endothermic reaction consumes heat locally, and will retard the rate, yet not the amount of hydrate that disassociates. In appearance, hydrates are intergrown, transparent-to-translucent, white-to-grey and yellow crystals, with poorly defined crystal form; yet, little is known about their natural habit. Hydrates may "cement" sediments in which they occur, imparting considerable mechanical strength to them, but they may also occur in pore spaces and nodules, uncemented to sediment grains. Hydrate formation forces methane molecules into closely packed, guest-lattice sites in hydrate crystals. This condensing has the effect of concentrating methane. One cubic meter of natural methane hydrate (90% of guest sites normally occupied) contains about 164 m3 of methane gas (at sea level) and about 0.87 m3 of water. Methane hydrate exists where conditions are appropriate and when both sufficient gas and water are available to form hydrates in pore spaces. Methane in hydrates may be derived both from within the Hydrate Stability Zone (HSZ) and from sediments below, Fig. 2. Both shallow, biogenic gas derived from bacteriological decomposition of organic matter, and deeper-sourced gas, produced by thermal "cracking" of hydrocarbons, have been found within hydrate deposits. In non-Polar, open-ocean conditions, the HSZ extends down from the seafloor, at about 450-m water depth, to sub-seafloor depths determined by rising sediment temperature, i.e., the geothermal gradient. In permafrost areas, the HSZ extends downward from about 200-m depth to over 1 km, where temperatures are about 12°C. Seafloor T-P and heat flow are primary controls of HSZ thickness. The HSZ is of fairly uniform thickness, commonly hundreds of meters at a given seafloor depth, and is thicker at greater pressure. Methane occurs both in solid hydrate and as free gas trapped beneath the HSZ. Methane hydrate has a high energy density — 84,000 Btu/ft3, compared with 1,150 Btu/ft3 for methane gas — but below LNG, at 470,000 Btu/ft3, and below standard fuel oil, at over 900,000 Btu/ft3. Hydrate occupies significant percentages of pore space in high-porosity sediments; and because it occurs in large, contiguous deposits, it constitutes a potential economic resource. Hydrate Reservoirs The role of reservoir is taken first by methane within hydrates, and second, by associated methane gas. Since recognition of concentrated, naturally occurring methane hydrate in the late 1960s and early 1970s, hydrates have been found within marine sediments in most continental slopes, Fig. 2. Huge quantities of methane are bound within, and present below, hydrates. The amount of methane held as gas hydrates worldwide is estimated to be at least 10,000 gigatons of fixed carbon. Estimates of methane volumes in the deep oceans are converging on about twice the methane equivalent of all known fossil-fuel deposits ever found on continents and their margins, including coal, oil and natural gas. However, questions of hydrate concentration, commerciality and extractability are not completely resolved at present. The industry view is that when hydrate development is required, challenges will be met, just as industry has mastered other difficult environments in energy resource development, e.g., deepwater exploration. Mainly onshore, permafrost hydrates probably hold less than 2% of the methane bound within oceanic hydrate, but are more accessible than oceanic deposits. They occur as part of a compound water-ice and hydrate permafrost on land and adjacent continental shelves of Alaska, Canada and Russia. Methane has been recovered from hydrates at Messoyakha field in western Siberia, and recovery tests of methane from gas hydrates in Prudhoe Bay field of Alaska have yielded methane-recovery rates similar to those in Russia. Drilled in permafrost hydrate in the Mackenzie Delta of northern Canada in March 1998, the Mallik test well proved a significant resource of 150 Bcf methane/km2. A well-known oceanic gas hydrate system occurrence is in the Blake Outer Ridge along the SE U.S. Coast. Located in water depths of 2.5 to 3.5 km, the Blake Ridge area may contain more than a thousand Tcf of methane in a single, contiguous deposit (current U.S. methane consumption is about 20 Tcfg/yr). The total resource of methane associated with hydrates in U.S. waters may approach 200,000 Tcf. If a small percentage of this is recoverable, it constitutes a stupendous store of energy. Hydrate Variability Granting the huge volumes of fuel apparently stored within known HSZs, there is great uncertainty in just how hydrates accumulated. They may be found in massive amounts (filling all guest sites), disseminated (filling some available sites) or offset (due to faults and/or fault zones). They may also lie in patterns not yet recognized. Hydrates may disassociate, resulting in overpressured, buoyant fluids rising within subsiding, compacting and dewatering sediments (some 1/3 to 2/3 of all waters initially incorporated upon deposition are subsequently expelled prior to lithification). And natural gases may rise within the sediment wedge without ever becoming hydrates. These fluids are appropriate for creation of pockmarks, gas vents and mud volcanoes on seafloors. Faulting, Hydrates And Slumping Increased resolution of bathymetry and side-scan sonar in ever-more areas along active and passive margins reveals increasingly complex faults, usually extensional. Deep-tow, high-resolution, seismic-reflection data shows complex faulting patterns, even over areas apparently not known for tectonics, such as the Blake Outer Ridge. The participation of hydrates / free gas with numerous small, large and regional / massive slumps is increasingly well documented, for example, the Cape Fear landslide, whose apex is the shelf break at the geographic head of Blake Ridge. This area contains a main portion of the central East-Coast hydrate accumulation. A classic seismic section (Fig. 3 and Fig. 4) shows a hydrate indicator, called the Bottom Simulating Reflector (BSR), as well as "bright spots" beneath the BSR. Also note the sediment deformation, presumably caused by the Cape Fear landslide, in Fig. 3. The landslide may have had multiple initiators: decomposition of hydrates building high pore pressure, salt diapir intrusion oversteepening the lower slope, or earthquakes. Continental Margin Instability The following considerations point toward the fragility of hydrates:
The apparent fragility may also be the result of growing awareness that continental margins, especially passive margins as now seen, are in a state of near collapse, replete with "shattered sediments." Hydrate fragility may have serious implications for well siting, drilling and casing programs. Hydrate Releases and BSRs Hydrate leakage appears to range from semi-continuous to explosive. Appreciable amounts of hydrates have been deposited along the Louisiana shelf-slope seafloor associated with salt domes and seafloor-intersecting faults. There seems to be semi-continuous hydrocarbon (including methane) releases into the ocean, as noted on satellite photos. Historical reports of persistent oil slicks, dating from the 1840s, have been published, and they correlate with known producing oil fields along the shelf. On occasion, buoyant gases collect beneath salt overhangs, then migrate to the ocean floor, creating pockmarks. 3-D seismic and bathymetry show such pockmarks, often near salt domes, in the seafloor and sub-surface. The Louisiana continental margin is certainly known as petroliferous, and hydrates do exist. Yet, there are no reported BSRs in this area. For a BSR to show on seismic records, large volumes of free gas must have accumulated beneath an extensive hydrate layer. A suggestion for the absence of BSRs is that the tectonically active margin has not allowed the opportunity for enough gas to collect. The extensive Louann Salt deposits have had a dynamic history, moving laterally and vertically up to hundreds of miles and thousands of feet, respectively, often at rates of centimeters/yr. Review of increasingly available 3-D seismic data indicates that, along much of the Louisiana margin, sediments appear shattered, discontinuous, and separated by discontinuities and faults of all dimensions. Along these fractures / faults, available buoyant fluids will rise. With an abundance of escape, there may not be enough methane available to create all the steps necessary to generate a BSR. Present State Of Play A number of developments have taken place since the first U.S. National Gas Hydrate Workshop was co-hosted by the U.S. Department of Energy (DOE), the U.S. Geological Survey (USGS) and the Naval Research Laboratory in Washington, DC, during spring 1991. Interest in developing hydrate as a source of methane gas has intensified. In addition, it has been recognized that hydrates may be important in the global carbon cycle, with an impact on climate, seafloor stability and safety. Use of methane or its liquid derivative, methyl alcohol, in place of oil-based fuels, would provide for dramatic reduction in unwelcome combustion byproducts. Use of methane or methanol would undoubtedly be environmentally positive. Non-U.S. governments have indicated an interest in paying premium prices to obtain stable, domestic sources of methane-based energy supplies. The first, large-scale recovery of methane from hydrate was initiated at the Messoyakha field in Siberia in the mid 1970s. The Japanese government initiated a program of hydrate research in 1995 to recover methane from nearby oceanic hydrates. As part of the Japanese hydrate-research program, a drill-test of the hydrate-permafrost zone in Canada’s Mackenzie Delta, February 1998, involved extensive geological, geochemical and geophysical surveying, including vertical seismic profiling and cross-well tomography (JNOC, 1998). Participants in this program, headed by Japan Petroleum Exploration Co. Ltd., include Japanese industrial companies, USGS, Geological Survey of Canada, U.S. DOE and various universities. The Indian government, acting through the Gas Authority of India Ltd., began a parallel program of resource evaluation in 1996. In addition, the European Union and other countries have now begun to allocate hydrate-research funding. In the U.S., the President’s Committee of Advisors for Science and Technology recommended — Congress has legislated — S.330, the Methane Hydrate Research and Development Act of 1999. Introduced in the Senate, Jan. 28, 1999, it establishes a gas-hydrate research and development program. In 1997, the U.S. DOE reactivated its pilot gas-hydrate-research program at its Federal Energy Technology Center in Morgantown, West Virginia. The present program involves the USGS and the U.S. Navy. Geopolitical Impact / Conclusion The impact of methane-hydrate development has the potential to transform the existing geopolitical paradigm. Countries such as the U.S., Japan and India could potentially become energy independent, an event that would strongly affect international affairs and foreign policy. Fuel use would become more environmentally friendly, with methane increasingly replacing petroleum. This could also promote new technologies and practices in power generation using fuel cells. The existing world trade in energy would be completely altered, as would the foreign currency balance of existing major importers. The exploration / exploitation of hydrates will require much ingenuity from all concerned. Rather than rushing toward an energy-starved future, it may well be that exploitation of new, unconventional energy resources will ensure available and affordable energy for centuries. Acknowledgment Special thanks to P. Meeks, Specialty Office Services, Bay Saint Louis, Mississippi, for help and guidance with this research. Literature Cited
 The authorsAllen Lowrie earned a BA from Columbia University in 1962. His 37-year career in geologic exploration includes research at Lamont-Doherty Earth Observatory, Naval Research Labs, Naval Oceanographic Office (Navy acoustics) and hydrocarbon exploration for Mobil Oil. He is the author of some 75 publications and two books. Lowrie has taught graduate and undergraduate courses at Tulane University, University of Southern Mississippi and continuing education courses for SEG and AAPG. Michael Max earned a BSc from the University of Wisconsin, Madison, an MSc from the University of Wyoming, Laramie, and a PhD from Trinity College, Dublin, Ireland. He has worked for the Geological Survey of Ireland and the Naval Research Labs. Max has been involved in metamorphic and structural studies, nearshore marine studies and sonar applications, and has been carrying out research in marine gas hydrates since 1988.
Copyright © 1999 World
Oil |
his article presents
the rationale behind the growing interest in methane hydrates as a potential resource. It
discusses what hydrates are, how and where they form and are found, as well as how their
fragile nature creates a geohazard. The authors further discuss the current state of
international interest, exploration and geopolitical impact.
