For the past three decades, discussion of naturally-occurring gas hydrates has been framed by a series of assessments that indicate enormous global volumes of methane present within gas hydrate accumulations. At present, these estimates continue to range over several orders of magnitude, creating great uncertainty in assessing those two gas hydrate issues that relate most directly to resource volumes -gas hydrate's potential as an energy resource and its possible role in ongoing climate change. However, a series of recent field expeditions have provided new insights into the nature of gas hydrate occurrence; perhaps most notably, the understanding that gas hydrates occur in a wide variety of geologic settings and modes of occurrence. These fundamental differences -which include gas hydrate concentration, host lithology, distribution within the sediment matrix, burial depth, water depth, and many others -can now be incorporated into evaluations of gas hydrate energy resource and environmental issues. With regard to energy supply potential, field data combined with advanced numerical simulation have identified gas-hydrate-bearing sands as the most feasible initial targets for energy recovery. The first assessments of potential technically-recoverable resources are now occurring, enabling a preliminary estimate of ultimate global recoverable volumes on the order of ~3 Â 10 13 m 3 (10 15 ft 3 ; $15 GtC). Other occurrences, such as gas hydrate-filled fractures in clay-dominated reservoirs, may also become potential energy production targets in the future; but as yet, no production concept has been demonstrated. With regard to the climate implications of gas hydrate, an analogous partitioning of global resources to determine that portion most prone to dissociation during specific future warming scenarios is needed. At present, it appears that these two portions of total gas hydrate resources (those that are the most likely targets for gas extraction and those that are the most likely to respond in a meaningful way to climate change) will be largely exclusive, as those deposits that are the most amenable to production (the more deeply buried and localized accumulations) are also those that are the most poorly coupled to oceanic and atmospheric conditions.

Abstract. We offer a first-principle-based effective medium model for elastic-wave velocity in unconsolidated, high porosity, ocean bottom sediments containing gas hydrate. The dry sediment frame elastic constants depend on porosity, elastic moduli of the solid phase, and effective pressure. Elastic moduli of saturated sediment are calculated from those of the dry frame using Gassmann's equation. To model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions: (a) hydrate modifies the pore fluid elastic properties without affecting the frame; (b) hydrate becomes a component of the solid phase, modifying the elasticity of the frame. The goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data. We apply the model to sonic and VSP data from ODP Hole 995 and obtain hydrate concentration estimates from assumption (b) consistent with estimates obtained from resistivity, chlorinity and evolved gas data.

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Spanning a latitudinal range typical for deserts, the Indian peninsula is fertile instead and sustains over a billion people through monsoonal rains. Despite the strong link between climate and society, our knowledge of the long‐term monsoon variability is incomplete over the Indian subcontinent. Here we reconstruct the Holocene paleoclimate in the core monsoon zone (CMZ) of the Indian peninsula using a sediment core recovered offshore from the mouth of Godavari River. Carbon isotopes of sedimentary leaf waxes provide an integrated and regionally extensive record of the flora in the CMZ and document a gradual increase in aridity‐adapted vegetation from ∼4,000 until 1,700 years ago followed by the persistence of aridity‐adapted plants after that. The oxygen isotopic composition of planktonic foraminiferGlobigerinoides ruberdetects unprecedented high salinity events in the Bay of Bengal over the last 3,000 years, and especially after 1,700 years ago, which suggest that the CMZ aridification intensified in the late Holocene through a series of sub‐millennial dry episodes. Cultural changes occurred across the Indian subcontinent as the climate became more arid after ∼4,000 years. Sedentary agriculture took hold in the drying central and south India, while the urban Harappan civilization collapsed in the already arid Indus basin. The establishment of a more variable hydroclimate over the last ca. 1,700 years may have led to the rapid proliferation of water‐conservation technology in south India.

the program were to (1) determine the feasibility of gas injection into hydrate-bearing sand reservoirs and (2) observe reservoir response upon subsequent flowback in order to assess the potential for CO 2 exchange for CH 4 in naturally occurring gas hydrate reservoirs. Initial modeling determined that no feasible means of injection of pure CO 2 was likely, given the presence of free water in the reservoir. Laboratory and numerical modeling studies indicated that the injection of a mixture of CO 2 and N 2 offered the best potential for gas injection and exchange. The test featured the following primary operational phases: (1) injection of a gaseous phase mixture of CO 2 , N 2 , and chemical tracers; (2) flowback conducted at downhole pressures above the stability threshold for native CH 4 hydrate; and (3) an extended (30-days) flowback at pressures near, and then below, the stability threshold of native CH 4 hydrate. The test findings indicate that the formation of a range of mixed-gas hydrates resulted in a net exchange of CO 2 for CH 4 in the reservoir, although the complexity of the subsurface environment renders the nature, extent, and efficiency of the exchange reaction uncertain. The next steps in the evaluation of exchange technology should feature multiple well applications; however, such field test programs will require extensive preparatory experimental and numerical modeling studies and will likely be a secondary priority to further field testing of production through depressurization. Additional insights gained from the field program include the following: (1) gas hydrate destabilization is self-limiting, dispelling any notion of the potential for uncontrolled destabilization; (2) gas hydrate test wells must be carefully designed to enable rapid remediation of wellbore blockages that will occur during any cessation in operations; (3) sand production during hydrate production likely can be managed through standard engineering controls; and (4) reservoir heat exchange during depressurization was more favorable than expectedmitigating concerns for near-wellbore freezing and enabling consideration of more aggressive pressure reduction.

Gas hydrates are a vast energy resource with global distribution in the permafrost and in the oceans. Even if conservative estimates are considered and only a small fraction is recoverable, the sheer size of the resource is so large that it demands evaluation as a potential energy source. In this review paper, we discuss the distribution of natural gas hydrate accumulations, the status of the primary international R&D programs, and the remaining science and technological challenges facing commercialization of production. After a brief examination of gas hydrate accumulations that are well characterized and appear to be models for future development and gas production, we analyze the role of numerical simulation in the assessment of the hydrate production potential, identify the data needs for reliable predictions, evaluate the status of knowledge with regard to these needs, discuss knowledge gaps and their impact, and reach the conclusion that the numerical simulation capabilities are quite advanced and that the related gaps are either not significant or are being addressed. We review the current body of literature relevant to potential productivity from different types of gas hydrate deposits, and determine that there are consistent indications of a large production potential at high rates over long periods from a wide variety of hydrate deposits. Finally, we identify (a) features, conditions, geology and techniques that are desirable in potential production targets, (b) methods to maximize production, and (c) some of the conditions and characteristics that render certain gas hydrate deposits undesirable for production. Introduction Background. Gas hydrates are solid crystalline compounds in which gas molecules (referred to as guests) occupy the lattices of ice-like crystal structures called hosts. Under suitable conditions of low temperature T and high pressure P, the hydration reaction of a gas G is described by the general equation G + N H H 2 O = G•N H H 2 O,……………………………………………………………………………………………………(1) where N H is the hydration number. Hydrate deposits occur in two distinctly different geographic settings where the necessary conditions of low T and high P exist for their formation and stability: in the permafrost and in deep ocean sediments (Kvenvolden, 1988). The majority of naturally occurring hydrocarbon gas hydrates contain CH 4 in overwhelming abundance. Simple CH 4-hydrates concentrate methane volumetrically by a factor of 164 when compared to standard P and T conditions (STP). Some modeling suggests that the energy needed for dissociation could be less than 15% of the recovered energy (Sloan and Koh, 2008). Natural CH 4-hydrates crystallize mostly in the structure I form, which contains 46 H 2 O molecules per unit cell. Structure I hydrates have a N H ranging from 5.77 to 7.4, with N H = 6 being the average hydration number and N H = 5.75 corresponding to complete hydration (Sloan and Koh, 2008). Natural gas hydrates can also contain other hydrocarbons (alkanes C ν H 2ν+2 , ν = 2 to 4), but may also co...

The Mallik site represents an onshore permafrost-associated gas hydrate accumulation in the Mackenzie Delta, Northwest Territories, Canada. A gas hydrate research well was drilled at the site in 1998. The objective of this study is the analysis of various gas production scenarios from several gas-hydrate-bearing zones at the Mallik site. The TOUGH2 general-purpose simulator with the EOSHYDR2 module were used for the analysis. EOSHYDR2 is designed to model the non-isothermal CH 4 release, phase behavior and flow under conditions typical of methanehydrate deposits by solving the coupled equations of mass and heat balance, and can describe any combination of gas hydrate dissociation mechanisms. Numerical simulations indicated that significant gas hydrate production at the Mallik site was possible by drawing down the pressure on a thin free-gas zone at the base of the hydrate stability field. Gas hydrate zones with underlying aquifers yielded significant gas production entirely from dissociated gas hydrate, but large amounts of produced water. Lithologically isolated gas-hydrate-bearing reservoirs with no underlying free gas or water zones, and gas-hydrate saturations of at least 50% were also studied. In these cases, it was assumed that thermal stimulation by circulating hot water in the well was the method used to induce dissociation. Sensitivity studies indicated that the methane release from the hydrate accumulations increases with gas-hydrate saturation, the initial formation temperature, the temperature of the circulating water in the well, and the formation thermal conductivity. Methane production appears to be less sensitive to the rock and hydrate specific heat and permeability of the formation.

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