Solid, icelike combinations of water and natural gas. Gas hydrates occur wherever appropriately-sized gases and water are brought together in sufficient quantities under specific conditions of low temperature and moderately high pressure. Prior to the mid-1970s, the existence of gas hydrates in nature was unconfirmed. As the result of a series of scientific and engineering drilling programs, both in the Arctic and on the continental shelves, gas hydrates are now believed to host hydrocarbons, predominantly in the form of methane (CH4), in immense quantities (Fig. 1). The implications of this storehouse of organic carbon for previous concepts of long-term carbon cycling, near-term feedbacks to environmental change, various natural and industrial geohazards, and potential future energy supply, is the subject of accelerating global research. See also: Hydrate; Methane
Gas hydrates are clathrates, substances that consist of a rigid lattice of “host” molecules (in this case hydrogen-bonded water molecules) containing a regular array of open cavities that enclose, without direct chemical bonding, appropriately sized “guest” molecules (Fig. 1). Clathrates are unique materials in that they are crystalline but also nonstoichiometric, which means they do not have a fixed composition. In the case of gas hydrates, anywhere from 80% to 100% of the voids may include a guest gas molecule. While the abundance of CH4 in nature means that this gas most often fills voids in the clathrate structure, molecules of other low-molecular-weight gases can be taken up as well. Depending on the gases present, various geometries of lattice will form. The most common geometry, known as Structure I, forms in methane-rich environments. Structure II is less common in nature; it forms where larger and heavier hydrocarbons are present, such as in active sea-floor seeps in hydrocarbon-rich regions. A third structure, known as Structure H, is exceedingly rare in nature. E. D. Sloan and C. Koh provide a full review of the nature, physical properties, and thermodynamics of gas hydrates. See also: Clathrate compounds; Hydrogen bond
Gas hydrates were first synthesized in the laboratory in the late 1700s. For more than a century, the laboratory was thought to be the only place gas hydrates existed. In the 1930s, however, gas hydrates were found to be spontaneously forming hazardous blockages within natural-gas transport pipelines in cold climates. That finding spawned accelerated research to understand the nature of gas hydrates. Later, as oil and gas production moved into cold and high-pressure deep-water environments, spontaneous hydrate formation within production equipment was increasingly encountered. Mitigation of hydrate blockages, a key part of a discipline known as flow assurance, continues as a major ongoing research topic with extensive practical importance in the oil and gas industry. See also: Natural gas; Pipeline
In the mid-1960s, Dr. Yuri Makogon recognized that methane, water, and gas hydrate stability conditions all occur widely within the natural environment and predicted that gas hydrates should exist in nature in large volumes. By the mid-1970s, compelling evidence for naturally occurring gas hydrates had been observed in oil and gas wells from permafrost regions in Russia, Canada, and Alaska. Offshore seismic-reflection profiles from deep-water continental shelves were observed to commonly feature anomalous reflections (bottom-simulating reflectors), a phenomenon that coincided with the likely position of the base of gas-hydrate stability and attributed to the effect of gas hydrate on the physical properties of sediments. Beginning in the early 1980s, evidence for gas hydrates was regularly observed in shallow sediments in scientific drilling programs by the Deep Sea Drilling Program. In 1995, the government of Japan initiated a dedicated research program to evaluate gas hydrates as a future energy resource. Gas-hydrate research continues to be dominated by large national programs, with India, South Korea, Japan, the United States, and China having conducted multiple marine gas hydrate drilling programs in recent years. Private-sector research on naturally occurring gas hydrates is limited. The oil and gas industry has focused primarily on the hazards associated with drilling through gas hydrates in pursuit of deeper hydrocarbons. See also: Offshore oil and gas
Gas-hydrate formation requires pressure and temperature conditions that occur only in deep marine settings (below 300–500 m of water, depending largely on latitude and bottom-water temperature) or in areas with thick permafrost (Fig. 2). Gas-hydrate stability is also affected by gas chemistry (Structure II is stable to higher temperatures, for example) and water chemistry (high salinity inhibits hydrate formation), with additional subtle controls such as sediment grain size and mineralogy. Most gas hydrate is found in the marine environment, and there, most is concentrated along marine continental margins. Hydrates are thought to be relatively rare in the abyssal (deep) oceans because of lack of available methane. See also: Marine sediments; Permafrost
As is the case with all hydrocarbons, the source of the methane in gas hydrates derives from the microbial (“rotting”) or thermogenic (“cooking”) alteration of buried organic matter. Because gas hydrate is found in relatively shallow sediments as compared to other hydrocarbons, and because it is often dominated by isotopically light methane, its origin is generally assigned to microbial processes acting on relatively recently deposited organic matter. However, in those areas where gas hydrates are anomalously concentrated, it is likely that some (or most) of the methane migrated long distances from deep thermogenic sources. In this sense, gas hydrates can be viewed as the shallowest component of larger regional petroleum systems. T. Collett and colleagues have reviewed gas-hydrate systems in nature. See also: Marine microbiology
Gas hydrates exhibit a variety of forms that reflect both the abundance of methane and the physical properties of the sediment that enclose them (Fig. 3). Where gas is venting directly to the ocean, gas hydrates can form massive seafloor “mounds,” up to tens of meters in size, which may host unique chemosynthetic communities. Although they are the most visible manifestation of gas hydrates, seafloor mounds likely represent a very small share of total gas hydrates. In fine-grained, clay-rich sediments (which represent the bulk of shallow marine sediments worldwide), gas hydrates are typically finely disseminated at very low (<5% of pore space) concentrations. Where the gas supply has been more robust and focused, gas-hydrate formation can displace the fine-grained sediments and take the form of macroscopic nodules and veins at modest or low concentrations (up to 30% of pore space). In areas of greater gas supply, accumulations can form where gas hydrates can range up to 80% of the total sediment volume over thick sections. In sand- and silt-rich sediments, which are intrinsically more porous and permeable, gas hydrates will accumulate in disseminated form within the existing pore space without grain displacement and typically can achieve concentrations up to 80% of pore space. In virtually all cases, the remaining pore space is filled with liquid water. In sand-rich and coarse-silt-rich sediments, some share of that water remains free to flow through the sediment, which has profound implications for potential energy recovery, as will be discussed in a later section. W. Waite and colleagues have reviewed the physical properties of gas hydrate bearing sediments. Ongoing efforts focus primarily on the composite physical properties of gas-hydrate-bearing sediments.
The total volume of gas enclosed in gas hydrates is poorly constrained and potentially extremely large. Current estimates of the total (or in-place) resources range from 100,000 trillion ft3 (2832 trillion m3) to more than 1,000,000 trillion ft3 (28,320 trillion m3) [the volume of the gas at standard temperature and pressure]. This large uncertainty relates to the difficulties in upscaling to global occurrences based on the insights gained at the small number of natural accumulations that have been drilled thus far. Regardless of the actual global inventory of gas hydrate, not all hydrate deposits have the same relevance to potential energy supply or to the environment.
Gas hydrates were once considered an exotic potential resource that could only be exploited using concepts and technologies other than those used for conventional gas production. However, the discovery of highly saturated sand-hosted hydrates in both arctic and marine settings and their recognition as the primary targets for energy recovery have allowed gas-hydrate evaluation to use tailored approaches of existing exploration and production concepts. The total volume of gas in these favorable settings has been estimated at 40,000 trillion ft3 (1133 trillion m3). A global review of 197 basins revealed 14 basins with good potential for resource-grade gas-hydrate accumulations. Direct exploration for high-quality gas-hydrate prospects based on geophysical surveys (which can detect directly the strong mechanical contrast between highly gas-hydrate-saturated sediments and those softer and unconsolidated sediments that surround them) has shown good promise, and has replaced prior concepts built around features such as bottom-simulating reflectors (Fig. 4). See also: Energy sources
Production concepts for sand- and coarse silt-hosted gas hydrates focus on drilling into a reservoir and using the well to destabilize (“dissociate”) the hydrate into gas and water within the reservoir, and then producing the released gas to the surface via the well. Based on recent successful field programs in Canada and Japan, the primary means of dissociation will be reservoir “depressurization.” In this method, fluids pumped from the wellbore draw the free water from the hydrate reservoir, shifting the pressure out of hydrate stability conditions and releasing gas and water. Hydrate production is not prone to uncontrolled reaction. The primary challenge will be to sustain the dissociation reaction, which requires continuous pressure reduction as well as an active influx of sufficient heat to counter the endothermic (heat-consuming) nature of the dissociation reaction. Other methods of hydrate dissociation, including injection of hot fluids or chemical destabilizers, such as carbon dioxide (CO2), have been tested but show less potential to achieve the gas flow rates that appear possible with depressurization. As gas hydrate production systems mature, it is likely that depressurization will be supplemented by these and other reservoir stimulation and maintenance approaches as dictated by local geologic and operating conditions. To date, only a limited number of field production tests have occurred, all of relatively short duration (60 days or less). Attempts to extrapolate the findings of those field tests to potential multiyear well production profiles suggest that viable production rates could be achievable in certain circumstances. Economics will not be the sole driver of the pace of gas-hydrate development, however. Other incentives include the desire for increased energy independence for those nations that are highly reliant on imported energy. As experience with other hydrocarbon sources has shown, technology and ingenuity can be expected to allow more complex and challenging accumulations to be accessed with time (Fig. 5).
On 70% of the surface of the Earth, gas generated within sediments and migrating toward the surface must traverse a gas-hydrate stability zone prior to release to the ocean–atmosphere system. In most instances (exceptions occur where vigorous gas flow occurs along well-established pathways), this gas will be sequestered in hydrate form. As such, gas hydrate mediates the movement of methane within the carbon cycle. At small spatial and temporal scales, gas hydrate continually forms and dissociates around the margins of its occurrence in response to natural variations in environmental conditions. At larger scales, the gas-hydrate system expands, contracts, and hosts varying amounts of hydrate in response to changes in water temperature, sea-level rise, local and regional tectonism, rates of methane generation, and other processes. Globally, while the details of the system are complex, the driving process is water temperature (which readily overwhelms the effect of pressure changes due to sea-level fluctuation), with net uptake of methane in gas hydrate systems during cold climatic periods and net release of methane during warm periods. At present, much attention is being paid to the potential that ongoing global warming could accelerate methane release from gas hydrates, adding an unappreciated deleterious feedback to the accumulation of greenhouse gases in the atmosphere. See also: Biogeochemistry; Global warming; Greenhouse effect
Critical to the proper evaluation of gas hydrate–climate interactions is full evaluation of the actual occurrence of gas hydrates in the most climate-sensitive locations (Fig. 6; for example, most of the global gas-hydrate resource is deeply buried and well insulated from the ocean–atmosphere system), the time scales for various reactions (such as the transmission of bottom-water temperature changes into the subsurface), and recognition of the numerous sinks for released methane that exist within the sediments at the seafloor and within the water column. In areas where recently discovered venting has been linked to climate-induced gas-hydrate dissociation (for example, off the western margin of Svalbard, Norway), it now appears that the gas is not hydrate-derived and that the venting predates anthropogenic warming. C. Ruppel and J. Kessler have reviewed the issues associated with the interaction between gas-hydrates and global climate and have found that the near-term risks are minimal.
On geologic time scales, numerous large-scale climate events in the Earth's deep past exhibit features consistent with widespread gas-hydrate dissociation. Unraveling the causes from the effects in such events is complex and requires precise resolution in the dating of events. In many instances, particularly those in the most recent past, the role of gas hydrate appears unclear or minimal. In all cases (including the much-debated case of the Paleocene–Eocene Thermal Maximum), it appears that gas hydrate cannot trigger climate-warming events, but instead will only respond to, and temporarily exacerbate, ongoing changes that are driven by other global processes.