Gas hydrates are of great importance for a variety of reasons (Figure-1). In offshore hydrocarbon drilling and production operations, gas hydrates cause major, and potentially hazardous flow assurance problems.
Naturally occuring methane clathrates are of great significance in their potential for as strategic energy reserve, the possibilities for CO2 disposal by sequestration, increasing awareness of the relationship between hydrates and subsea slope stability, the potential dangers posed to deepwater drilling installations, pipelines and subsea cables, and long-term considerations with respect to hydrate stability, methane (a potent greenhouse gas) release, and global climate change.
Figure-1 Major issues of gas hydrates. Click Image to Enlarge.
In drilling, record water depths are continuously being set by oil companies in the search of hydrocarbon reserves in deep waters. Due to environmental concerns and restrictions, water based drilling fluids are often more desirable than oil based fluids, especially in offshore exploration. However, a well-recognised hazard in deep water offshore drilling, using water based fluids, is the formation of gas hydrates in the event of a gas kick.
In deep-water drilling, the hydrostatic pressure of the column of drilling fluid and the relatively low seabed temperature, could provide suitable thermodynamic conditions for the formation of hydrates in the event of a gas kick. This can cause serious well safety and control problems during the containment of the kick. Hydrate formation incidents during deep-water drilling are rarely reported in the literature, partly because they are not recognised, Two cases have been reported in the literature where the losses in rig time were 70 and 50 days.
The formation of gas hydrates in water based drilling fluids could cause problems in at least two ways:
The ongoing development of offshore marginal oil and gas fields increases the risks of facing operational difficulties caused by the presence of gas hydrates. A typical area of concern is multiphase transfer lines from well-head to the production platform where low seabed temperatures and high operation pressures increase the risk of blockage due to gas hydrate formation (Figure-2). Other facilities, such as wells and process equipment, can also be prone to hydrate formation.
Different methods are currently in use for reducing hydrate problems in hydrocarbon transferlines and process facilities. The most practical methods are:
Figure-2 A large gas hydrate plug formed in a subsea hydrocarbon pipeline. Picture from Petrobras (Brazil)
Two factors make gas hydrates attractive as a potential energy resource: (1) the huge volumes of methane that is apparently trapped as clathrate within the upper 2000 m of the Earth's surface, and (2) the wide geographical distribution of gas hydrates.
Natural gas is widely expected to be the fastest growing primary energy source in the world over the next 20 years. In the U.S. Energy Information Administration's International Energy Outlook 2002 (IEO2002) reference case, worldwide gas consumption is projected to almost double to 162 trillion cubic feet in 2020 from 84 trillion cubic feet (standard conditions) in 1999. Given the attractive features of gas hydrates, and the growing demand for natural gas, it seems reasonable to conclude that gas hydrates could serve as a future energy resource.
A number of schemes for methane hydrate exploitation have been proposed, although at present, technical and economic considerations restrict production to experimental tests only. The Japan National Oil Company (JNOC) has been a pioneer in this field, having already drilled experimental wells in the Mackenzie Delta of Northern Canad with ambitious plans for further test wells in sediments of offshore Japan.
One interesting branch of research in this area is the possibility of CO2 sequestration. CO2 hydrate is thermodynamically more stable than methane hydrate, so the possibility exists for sequestration of CO2 into existing seafloor clathrates, whereby yielding methane. This process is particularly attractive, as it would act as both a source and a sink with respect to greenhouse gas emissions.
The aspect of gas hydrates which has the biggest implications for human welfare at present, is their potential as a geohazard. Of particular concern is the danger posed to deepwater drilling and production operations, and the large body of evidence which now exists linking gas hydrates with seafloor stability.
With conventional oil and gas exploration extending into progressively deeper waters, the potential hazard gas hydrates pose to operations is gaining increasing recognition. Hazards can be considered as arising from two possible events: (1) the release of over-pressured gas (or fluids) trapped below the zone of hydrate stability, or (2) destabilization of in-situ hydrates.
The presence of BSRs has previously been a cause of concern, as they could be considered evidence for the existence of free gas (possibly at high-pressure) beneath the HSZ. More recent analysis suggests however, that as long as excess water is present, there should not be a build-up of gas pressure beneath the HSZ. This is because, at the base of hydrate stability, the system approximates to 3-phase equilibrium, where pressure is fixed (generally at hydrostatic), and temperature occupies the available degree of freedom. This means that any excess gas will be converted to hydrate, returning the system to its equilibrium pressure (assuming there is no major barrier to the mass transfer of salt). This case is likely to predominate in many hydrate-bearing sediments, although gas seeps and mud volcanoes, common to thermogenic hydrate areas (e.g. Gulf of Mexico, Caspian Sea), could be considered evidence for excess gas and pore-fluid pressures at shallow depths.
In the absence of gas traps, hydrates still pose a hazard due to their potential for destabilization. This danger is particularly apparent in the case of conventional oil and gas exploration, for which drilling methods contrast quite markedly to the shallow piston-coring approach used by ODP in hydrate areas.
Conventional rotary drilling operations could cause rapid pressure, temperature or chemical changes in the surrounding sediment. An increase in temperature could be caused by a hot drill bit, warm drilling fluids, or later as high-temperature reservoir fluids rise through the well, while the addition of hydrate inhibitors to drilling muds (to prevent hydrate formation in the well-bore or drill string in the event of a gas-kick) could change sediment pore-fluid chemistry. Some, or all of these changes, could result in localized dissociation of gas hydrates in sediments surrounding wells. A similar case would apply to seafloor pipelines, where the transportation of hot fluids could cause dissociation of hydrates in proximal sediments. In a worst-case scenario, clathrate dissociation could lead to catastrophic gas release, and/or destabilization of the seafloor.
The hazards associated with drilling in gas hydrate areas are exemplified by cases from the Alaskan Arctic, where subsurface permafrost hydrate destabilization has resulted in gas kicks, blowouts, and even fires.
A significant part of the gas hydrate geohazard problem is related to how they alter the physical properties of a sediment. If no hydrate is present, fluids and gas are generally free to migrate within the pore space of sediments. However, the growth of hydrates converts what was a previously a liquid phase into a solid, reducing permeability, and restricting the normal processes of sediment consolidation, fluid expulsion and cementation. These processes can be largely stalled until the BHSZ is reached, where hydrate dissociation will occur. Dissociation of hydrates at the BHSZ can arise through an increase in temperature due to increasing burial depth (assuming continued sedimentation) or an increase in sea bottom-water temperatures, and/or a decrease in pressure (e.g., lowering of sea level). Upon dissociation, what was once solid hydrate will become liquid water and gas. This could lead to increased pore-fluid pressures in under-consolidated sediments, with a reduced cohesive strength compared to overlying hydrate-bearing sediments, forming a zone of weakness. This zone of weakness could act as a site of failure in the event of increased gravitational loading or seismic activity (Figure-3).
The link between seafloor failure and gas hydrate destabilization is a well established phenomenon, particularly in relation to previous glacial-interglacial eustatic sea-level changes. Slope failure can be considered to pose a significant hazard to underwater installations, pipelines and cables, and, in extreme cases, to coastal populations through the generation of tsunamis.
Figure-3 Potential scenario whereby dissociation of gas hydrates may give rise to subsea slope failure and massive methane gas release
Methane is a particularly strong greenhouse gas, being ten times more potent than carbon dioxide. Increasing evidence points to the periodic massive release of methane into the atmosphere over geological timescales. However, whether such enormous releases of methane are a cause or an effect with respect to global climate chnages remains the subject of much debate.
Global warming may cause hydrate destabilsation and gas release through a rise in ocean bottom water temperatures. Methane release in turn would be expected to accelerate warming, causing further dissociation, potentially resulting in run away global warming. However, coversely, sea level rise during warm periods may act to stabilise hydrates by increasing hydrostatic pressure, acting as a check on warming.
A further possiblity is that hydrate dissociaton may act as a check on glaciation, whereby reduced sea levels (due to the growth of ice sheets) may cause seafloor hydrate dissociation, releasing methane and warming the climate.
The strong link between naturally occurring gas hydrates and the Earth's climate is an increasingly recognised phenomenon. However, there is still little understanding concerning the exact role gas hydrates play in global climate change.
There are many Journal and Conference publications covering all aspects of gas hydrates available in the literature. Centre for Gas Hydrate Research publication reprints are available as Adobe .pdf files on request. See our publications page for details. We recommend the following books for a good summary of the main issues:
Henriet, J.-P. and Mienert, J. (eds.), Gas Hydrates: Relevance to World Margin Stability and Climatic Change, Geological Society of London Special Publication 137 (1998).
Max, D.: Natural Gas Hydrate in Oceanic and Permafrost Environments, Kluwer Academic Publishers, Dordrecht, Netherlands (2000).
Sloan, E.D.: Clathrate Hydrates of Natural Gases, Marcel Dekker Inc., New York (1998).