This structure is stable within the temperatures that prevail in the ocean floors at water pressures of tens of bars water depth of a few hundred meters. Thus when methane is available, the pore space of shallow marine sediments will contain not only brine but also solid hvdrate. Gas to solid volumetric ratio in covered hydrate sarnple may be as large as Fig. Because temperature in the sediments increases with depth, these hydrate become unstablea few hundred meters below the ocean floor Kvenvolden, This lower boundary gives rise to the so-called bottom simulating seismic reflectors BSRS which parallel the seafloor Fig.
These BSRS are most likely caused by a sharp elastic contrast between the sediments containing methane hydrate and the underlying sediments without them. Often free methane is trapped below the BSRS, enhancing or even dominating the seismic reflections ,and implying that the hydrated zone must be impermeable. The amount of concentrated carbon tied up in earth's subsea methane hydrates significantly exceeds that in all other hydrocarbon deposits on earth. This is why gas hydrates are beginning to be recognized as a potential future energy source. Willoughby, M. Chen, T. He, I. Novosel, K. Hyndman, G.
Spence, N. Chapman, R. Spence, R. Chapman, M. Riedel and R. Schwalenberg, E. Mir and R. Edwards, 'Marine gas hydrate signatures in Cascadia and their correlation with seismic blank zones', First Break , 23, , Edwards and K. Schwalenberg and R. Edwards, 'The effect of sea floor topography: An analytic formulation for the magnetotelluric fields in the presence of a harmonic interface', Geophysical Journal International , , , Latychev and R. Edwards, 'On the compliance method and the assessment of three dimensional gas hydrate deposits', Geophysical Journal International , , , Reidel, and R.
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Jianwen Yang and R. Yuan and R. Hyndman, N. Reidel, R. Edwards and J. Yuan, 'Cascadia margin, northeast Pacific Ocean: hydrate distribution from geophysical investigations', in Natural gas hydrate in oceanic and permafrost environments, Max, M D. Edwards and G. Matsubayashi and R. Walia, J. Gettrust and R. Jian Yuan and R. Edwards, 'The assessment of marine gas hydrate through electrical remote sounding: Hydrate without a BSR? Willoughby and R. N Edwards, Jian Yuan and G. Jianwen Yang, K. Jegen and R. Letters , 19, , Liming Yu and R.
Liming Yu, R. The goal with each is to manipulate the in situ stability conditions of the methane hydrate and induce in-place dissociation to release free gas and associated hydrate-bound pore water. Each of these methods is discussed in more detail below. Worldwide experience in production testing of methane hydrate is very limited. Makogon has proposed that the Messoyakha natural gas field in northern Siberia may have been capped by methane hydrate and that the production response of this field can be explained in part by dissociation of methane hydrate as the pressure of the free-gas reservoir declined with time.
However, this interpretation has been questioned Collett and Ginsburg, , and the scarcity of field data to confirm the initial in situ conditions or the detailed production response greatly limits any modern engineering evaluation. The only other full-scale methane hydrate production study to be undertaken has been at the Mallik field in the Mackenzie Delta Box 2. At Mallik, a thermal stimulation test was undertaken in by a five-country consortium, including participation by DOE Dallimore and Collett, Full-scale depressurization testing at the site was also undertaken by a Canadian-Japanese research program in A key challenge in synthesizing repeatable samples that closely represent natural methane hydrate—bearing sediment formations is that natural cores that form within the hydrate stability zone most likely exhibit a methane hydrate pore-filling morphology formed from an aqueous solution containing dissolved gas , whereas laboratory-synthesized hydrate samples which are typically formed from free gas generally result in methane hydrate cementing the sediment grains Sloan and Koh, ; Waite et al.
Therefore, the synthesis method strongly influences the pore-scale habit see figure below , thereby potentially affecting the structural and physical properties of the hydrated sample. Methane hydrate—bearing sediment samples synthesized with dissolved gas can exhibit a formation mechanism and morphology more closely replicating nature particularly hydrate formed within the hydrate stability zone and in coarse-grained sediment; Dallimore et al.
Further studies performed on the pore-scale habit of hydrate-bearing sediment systems could add needed detail to these types of laboratory syntheses. The need to synthesize methane hydrate—bearing sediment samples that closely resemble natural samples has been recognized by most researchers and has also resulted in the ongoing laboratory synthesis efforts, and a shift away from using the model tetrahydrofuran THF hydrate system, which is stable at atmospheric pressure below 4. Conversely, similar mechanical properties have been suggested for hydrate-bearing sediment formed from dissolved gas and THF hydrate-bearing sediment at low hydrate saturations of less than 40 percent.
This similarity may result because both of these systems form hydrate in the pore space of the sediment Yun et al. Different pore-scale habits are obtained depending upon the hydrate formation mechanism. Grain cementing tends to occur when hydrate samples are formed from free gas plus liquid water sediment grains can be partially or fully water saturated Kneafsey, ; Waite et al. Pore filling tends to occur when hydrate is formed from gas dissolved in liquid water Kneafsey, ; Waite et al.
Core and well-log studies have confirmed high concentrations of methane hydrate within clastic sands, and the occurrence of methane hydrate as a matrix pore-filling material with an interconnected liquid-water interface with measurable permeability in the 0. The Japan Oil, Gas and Metals National Corporation, Natural Resources Canada, and Aurora College returned to the site in the winters of and to complete the first full-scale pressure drawdown production tests Dallimore et al. Field activities in the first year included drilling, borehole geophysics, and installation of production and monitoring infrastructure.
A meter test interval with high methane hydrate concentrations was selected for pressure drawdown testing. A short production test was undertaken by lowering the formation pressure below the methane hydrate phase equilibrium. The test results revealed the substantial mobility of methane hydrate—bearing sediments at Mallik when the methane hydrate, which bonds the sandy reservoir sediments, was dissociated.
Because of the loss of sediment strength, sand flowed into the well causing operational problems Dallimore et al. Operational problems encountered with sand inflow in were overcome in with the use of sand screens Yamamoto and Dallimore, , and a simpler operational sequence. A downhole heater was also used to prevent methane hydrate formation within the production tubing.
Although detailed production results remain confidential at this time, Yamamoto and Dallimore and Dallimore et al.
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The production testing at Mallik demonstrated sustained methane production from methane hydrate by depressurization from a clastic, sand-dominated methane hydrate reservoir. Continuous and significant gas how rates were observed and water production rates were judged to be manageable. The program successfully used conventional oil field technologies adapted for the unique physical properties of methane hydrate and can be considered in simple terms as demonstrating a proof of concept for this technique.
Cumulative production red line and derived bottom-hole pressure black line from pressure drawdown test at Mallik. Operational issues during which the pump did not operate are shown with dark gray vertical shading. Dallimore et al. Although more limited, additional data are also available from short-term drilling tests conducted by industry in the s Bily and Dick, and from small-scale, in situ tests of the methane hydrate formations conducted as part of the Mallik program Dallimore and Collett, , the Nankai drilling program Tsuji et al.
The depressurization technique is considered by many researchers to be the most cost-efficient and practical production method Max et al. The primary concept is to reduce the in situ pressure of the fluids in the porous rocks in contact with the methane hydrate reservoir. This technique can be applied by changing the pressure regime of the methane hydrate reservoir itself, or by reducing the pressure of the overlying or underlying sedimentary rocks in contact with the methane hydrate reservoir and transferring this pressure change to the reservoir.
The efficiency of this technique is significantly influenced by the manner in which the methane hydrate occurs i. The primary concept in the thermal stimulation technique is to increase the in situ temperature of the methane hydrate reservoir above the pressure-temperature stability threshold. The only full-scale thermal stimulation test was conducted at the Mallik site in During this test, hot brine was circulated across a meter perforated test interval, relying mainly on heat conduction into the formation to dissociate methane hydrate see Dallimore and Collett, Approximately m 3 of gas were recovered during the course of the hour thermal test.
This low volume of gas. However, the combination of depressurization with modest thermal stimulation may offer the opportunity to both enhance reservoir production and overcome flow assurance issues within the production tubing. A critical challenge in this regard is to understand the endothermic change of the methane hydrate dissociation and the impact this change has on reformation temperatures and the produced water and gas.
One category of technique often used to characterize conventional and even unconventional hydrocarbon reservoirs is based on pressure testing or pressure transient testing and analysis. These kinds of techniques are complementary to other characterization techniques because 1 they fill a gap between the small-scale characterization based on cores and logs and large-scale characterization based on geophysical measurement and 2 they provide a measure of flow capacity e. Refinement of such techniques for methane hydrate reservoirs could prove advantageous. The original production concept for the chemical stimulation of methane hydrate was to modify the in situ methane hydrate equilibrium conditions by injecting hydrate inhibitors such as salts and alcohols; these inhibitors act to decrease methane hydrate stability and induce dissociation.
This technique has been used for decades to deal with methane hydrate blockages in pipelines, but it has not been seriously considered as an option for long-term production. Prohibitive issues include potential operational challenges to the introduction of the inhibitor into the formation, the significant expense of the method, and environmental issues related to disposing of the used chemicals after production. Some novel concepts to extract methane from methane hydrate have also been suggested with numerous technical patents being issued around the.
Perhaps the most promising of these is a variation of a chemical stimulation technique which involves injecting another gas species such as carbon dioxide into a methane hydrate reservoir, essentially sequestering carbon dioxide and liberating methane at the same time. This concept is based on laboratory observations and thermodynamic considerations Graue et al.
Although the laboratory and modeling studies are encouraging, the challenge of scaling this technique from the laboratory to field testing has yet to be undertaken. Other production concepts put forward or patented include techniques to induce in situ combustion of the methane hydrate; combustion would heat the formation and stimulate methane hydrate dissociation 10 Collett, ; Max et al.
In situ combustion has been pursued to stimulate production from tar sands; however, this concept has not been seriously considered for methane hydrate production. The possibility of seafloor strip mining has also been discussed as a potential approach to recover methane from near-seafloor methane hydrate deposits. Reservoir simulation models are computer models routinely used by engineers to simulate production from a hydrocarbon field over long timescales. They are valuable tools in the petroleum industry to evaluate the effectiveness of various production techniques and methods to stimulate or enhance production, and to consider the environmental consequences of production.
Although considerable experience exists worldwide in the use. Acceptance of a verified methane hydrate simulation model would enable prediction of methane production rates and formation responses from different production strategies e. The integration of modeling and field studies is essential to effectively evaluate different production strategies and responses.
Reservoir models can aid in predictions of both the production rates and responses, as well as in interpreting the experimental observations from the field tests. Reservoir simulators under development in the world are listed in Table 2. These numerical models incorporate coupled equations accounting for heat transfer, fluid flow, and kinetic mechanisms that govern methane production from hydrate reservoirs. Despite the progress made through history matching with the currently available short-term field production datasets Moridis et al.
However, attempts have been made to compare each model by. As described by Wilder et al. They predicted different hydrate front locations when ice formation was expected in some parts of the reservoir. All simulators showed that methane and water production rates increase when free pore water is present.
Reservoir simulation models need to be carefully validated and tested with long-term production field data. The geomechanical modeling is still in the early stages of development, and experimental and field data will also be critical to validate the geomechanical predictions.
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A recent, additional application of these simulations has been to develop economic models to estimate the commercial viability of methane production from methane hydrate on simulated methane hydrate reservoirs. These models, although very preliminary, are the first economic studies to be performed that estimate the price of natural gas that could lead to economically viable gas production from methane hydrate Hancock, ; Walsh et al. These economic models result in a range of gas prices for economic production of methane hydrate that is in the range of prices seen historically in North America.
As described previously, methane hydrate in certain marine and permafrost environments is thought to constitute a significant storehouse of natural gas. In addition to the energy potential of methane hydrate, considerable. This section focuses on geologic processes that may be related to methane hydrate degassing, including methane seepage in marine and terrestrial environments, biological processes, submarine landslides, inferred gas venting structures, and methane hydrate as an atmospheric greenhouse gas source. Detailed field studies have demonstrated that methane seepage is ubiquitous in various marine settings where pressure and temperature conditions are appropriate for methane hydrate to be stable at or close to the seafloor.
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Active methane seepage has been observed to occur in deep waters along essentially all the continental margins of the world including off the U. Gulf Coast e. Although these observations seem to confirm that methane can migrate as free gas within the methane hydrate stability field, the detailed processes involved with this migration remain uncertain and the explicit link to methane hydrate is tenuous. A number of authors have also suggested that methane seepage may occur where natural processes have either warmed formation temperatures or reduced pressure, causing methane hydrate dissociation.
Some of the most perturbed methane hydrate deposits in the world occur in the Arctic in terrestrial permafrost environments with very cold mean annual surface temperatures. Paull et al. Methane seepage that may be related to degassing of transgressed permafrost methane hydrate accumulations has also been observed in the shallow waters of the East Siberian. Shelf of the Laptev Sea Semiletov and Gustafsson, Recent oceanbottom warming and inferred down-slope retreat of the landward limit of methane hydrate stability conditions is also implicated in the formation of numerous gas vents observed offshore Svalbard Westbrook et al.
In terrestrial Arctic settings, Holocene warming both from atmospheric temperature changes and the formation of lakes or river channels has also significantly perturbed the geothermal regime in the Arctic. Dissociating methane hydrate has been implicated as a possible source of methane release observed in lakes on the North Slope of Alaska and Siberia Walter et al. Gas venting and methane hydrate occurrences are in some places linked with distinctive seafloor features or processes such as extrusion of sediment onto the seafloor e.
However, detailed physical explanations as to how gas venting and methane hydrate dynamics actually form these features have yet to be developed. The presence of methane within and at the seafloor in these environments also generates biogeochemical impacts. In subseafloor environments where upward-migrating methane meets sulfate diffusing downward from the overlying seawater, populations of microorganisms anaerobically oxidize methane Boetius et al. This process converts the methane carbon into bicarbonate, and the sulfate into hydrogen sulfide which then is used in iron sulfide mineral formation, and thus alters the local environment.
The addition of bicarbonate to the pore waters can stimulate the precipitation of carbonate, which can cement the near-seafloor sediment Ritger et al. This process can result in a. In some areas carbonate-cored mounds have been inferred to grow up from the seafloor, creating considerable local topography Teichert et al.
The availability of either methane or sulfide on the seafloor will stimulate the development of chemosynthetic biological communities. Some seep source estimates have been compiled to indicate the relative importance of various seeps and vents. However, the vast majority of the seeping methane dissolves into the surrounding waters and is consumed by bacteria. Thus, very little of the methane from seafloor seeps in deep water reaches the atmosphere Reeburgh, Many authors have tentatively associated major submarine landslides on continental margins with methane hydrate occurrences e.
The potential causal link is the changes in mechanical properties and the geopressure regime when methane hydrate decomposes. When methane hydrate decomposes, the solid hydrate transforms into water and dissolved or gaseous methane, causing a consequent decrease in long-term sediment strength, thus making failure more likely. The evidence linking methane hydrate to slope failures is consistently indirect; for example, seismic evidence in headwall sediments suggests that the landslide failure plane is coincident with or at least near a BSR seen in seismic profiles.
The potential role of methane hydrate in natural slope stability and sediment dynamics remains largely an academic topic except in cases in which conventional oil and gas seafloor developments are being considered within potential slides and with corresponding tsunami potential Solheim et al. From a climate change perspective the natural dissociation of even a small part of the extremely large global methane hydrate occurrence that exists on Earth see section Methane Hydrate Resource Assessment could potentially release significant amounts of methane and water into the surrounding environment.
However, the scientific evaluation of such releases is complex and involves considerations such as the response time of methane hydrate to change, and the geologic, biologic, and oceanographic processes that ultimately control connection between methane release associated with methane hydrate decomposition and methane release to the atmosphere from many other sources. Climate change researchers have generally approached these issues from the perspective of considering climate change in the geologic past, modeling studies, and, only very recently, field investigations.
Dickens suggests that large excursions in the carbon isotopic records of carbonates in oceanic sediments from the Paleocene-Eocene Thermal Maximum may have been attributed to massive dissociation of methane hydrate. Kennett et al. Although stimulating much discussion in the literature, these theories remain unproven. More recently, modeling studies have explored the possible past and future interactions between methane hydrate and the global carbon cycle Archer et al.
An example of imposed surface temperature change comes from the Arctic region, where the last major warming began at least 10, years ago. These studies indicate that most methane hydrate occurrences can take thousands of years to respond because of the attenuation of the temperature change versus depth, and the endothermic nature of the dissociation process itself. The challenge to distinguish between methane seepage occurring from natural processes and seepage from active disturbance of methane hydrate during drilling and production is substantial, and many aspects of this field of investigation remain uncertain.
In a traditional hydrocarbon context, all methane hydrate deposits occur at relatively shallow burial depths and therefore have the potential to induce either seafloor or surface displacements as long-term field development is undertaken. Exploratory wells drilled in permafrost environments in the s and s encountered some uncontrolled gas releases from relatively shallow depths in which methane hydrate was also identified Bily and Dick, ; Yakushev and Collett, ; Collett and Dallimore, The released gas was suggested as being generated either by 1 methane hydrate decomposition while drilling with warm drilling fluids or drilling fluids containing methane hydrate inhibitors such as glycol or 2 encountering preexisting overpressured gas pockets within the methane hydrate stability zone see Box 1.
Although drilling with chilled fluids and more carefully selected mud was shown to prevent decomposition of methane hydrate while drilling Bily and Dick, , the issue of whether overpressured gas pockets were encountered within the methane hydrate stability zone Weaver and Stewart, has never been resolved. In the s, the Arctic was the only area where commercial drilling was conducted in association with potential methane hydrate—bearing sediments, both on- and offshore.
Perceived, but unproven, safety issues related to methane hydrate—bearing sediments in commercial drilling projects in the Arctic resulted in a policy of categorically avoiding any drilling operations where methane hydrate was suspected to occur Paull and Ussler, In practice this approach meant avoiding areas where BSRs were. Black shading represents methane hydrate—bearing sedimentary rocks. The surface on which the drill rig rests could be either the sea or land surface. The well bore in A-C is drawn at an exaggerated scale in order to demonstrate possible relationships with the methane hydrate and released methane gas.
A Release of methane into the wellbore from methane hydrate-associated sediment. B Release of methane directly into the sediment column surrounding the wellbore. C Potential casing collapse associated with pressures generated through methane hydrate decomposition. Despite the concerns about hazards from methane hydrate, addressing the issue with confident scientific and technical approaches remains a challenge because very little data and research exist to support or refute existing theories for understanding of methane hydrate as a geohazard.
Current industry practices advocate simply avoiding methane hydrate—bearing occurrences when drilling for conventional oil and gas plays. Despite this practice, a number of unintentional encounters in marine environments. These projects have detected no adverse effects on the drilling operations. The committee is unaware of documented bore-hole problems that have been attributed to methane hydrate during drilling of exploratory or development holes in deepwater settings where the holes passed through the depths associated with BSRs observed in seismic reflection data over the well site.
The combined experiences of these drilling activities have also shown that the amount of interstitial gas needed to generate BSRs and the probable overpressures in marine methane hydrate are modest e. Considerable experience now exists for drilling operations in non-Arctic marine settings and in the terrestrial Arctic with few operational issues attributed to drilling through methane hydrate e.
Conversely, some evidence sugges ts that overpressures may occur at shallow depths on the submerged parts of the Arctic shelf where methane hydrate may be undergoing decomposition associated with long-term warming stimulated by the last deglacial transgression Paull et al. These overpressures may be significant enough to extrude sediments in the form of gas vents and structures on the Arctic shelf. The identification of these features has occurred in the same areas where Bily and Dick introduced the concept that methane hydrate may contribute to overpressured conditions in the subsurface.
If the link between overpressures and these features is correct, methane hydrate decomposition may, in fact, contribute to the overpressure of sediments,. Again, however, the available data are i nadequate to confirm or refute these assertions. A number of issues may be associated with the presence of methane hydrate in the host sediments outside the well casing or the supporting well infrastructure Figure 2. However, most of the scenarios that may suggest methane hydrate is a geohazard to traditional hydrocarbon infrastructure do not manifest themselves at the time the well is being drilled, but rather result as a consequence of the long-term warming of the sediment associated with hydrocarbon production Figures 2.
These concerns relate to the substantial changes in sediment strength and permeability experienced when methane hydrate deposits are dissociated during production when the production well passes through a hydrate-bearing zone and when the production is from the hydrate-bearing zone. During the life of a potential producing methane hydrate field the. Strength and consolidation changes in the near well-bore area and production-induced regional subsidence could induce significant forces on the well casing with the possibility both of building pressure and developing significant casing strain, potentially resulting in casing failure Figure 2.
Because most methane hydrate—bearing sediments are unconsolidated, potential also exists for sediment migration into the producing well, resulting in operational problems Dallimore et al. These issues, when combined with permeability changes induced by dissociation, could cause poor sediment contact with the production casing, potentially resulting in failure of the casing cement bond and the creation of vertical migration pathways for gas migration. Although the petroleum industry has considerable experience worldwide in dealing with these types of problems, specific challenges may exist related to methane hydrate, especially if production schemes such as the use of horizontal wells are considered.
Permafrost also experiences significant changes in physical properties strength, porosity, permeability when the permafrost thaws in the near well-bore area in conventional oil and gas fields. Typical approaches to the predictions of permafrost response have been based on laboratory measurements of the consolidation effects on permafrost core samples and the development of a geomechanical model to predict both the near well-bore and field response to oil and gas extraction at depth.
In the situation of a producing methane hydrate field, the case is more complex because the producing and responding interval are the same, and at present, no published laboratory measurements exist of the consolidation response of methane hydrate samples. Another largely understudied topic is the amount and chemistry of the produced water that may be released when methane hydrate deposits dissociate.
Some reservoir simulation models suggest that the pore water liberated when methane hydrate dissociates will be highly mobile and will. The volume of produced water associated with methane hydrate production will directly impact the design of the well completion i. An important environmental consideration in any gas field is the risk of gas migration away from the production well infrastructure interacting with other geologic strata at depth or reaching the surface. In both cases a critical consideration is the seal integrity or the overlying permeability barrier above the production interval in the near well bore and also away from the well bore.
For most methane hydrate deposits, the nature of the seals may differ significantly from traditional hydrocarbon reservoirs. In some settings, methane hydrate itself or a permafrost layer may act as a seal and trap free gas below Grauls, The relatively shallow depths of methane hydrate occurrences also may mean that secondary sealing by the overlying sediment may only be weakly developed.
At present the mobility of the gas and water released from methane hydrate decomposition is unknown, including their potential to migrate to the surface e. Migrating methane can also reform into methane hydrate within the cold ocean bottom waters and form on top of the bottom-hole well assembly, potentially compromising the blow-out prevention systems. Erosion of the seafloor around wellheads could compromise these structures Figure 2. The level of progress and sophistication in methane hydrate research has been advancing at an exponential rate Figure 2.
As outlined in this chapter, observations, data, and analysis acquired from multidisciplinary. Dendy Sloan. Although these advances in knowledge testify to the great interest in the potential of methane hydrate to serve as a future energy source, they belie the need for considerably more information on methane hydrate including its behavior in nature, during drilling, and in production settings, and the approaches needed to identify and reliably produce methane from this type of occurrence. Chapter 3 reviews the research projects that the Program has supported during the past 5 years in pursuit of some of these outstanding issues.
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