Current perspectives on gas hydrate resources

Abstract

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.

Formation of Methane Clathrate Hydrates in Coal Moisture: Implications for Coalbed Methane Resources and Reservoir Pressures

“…Some reflections in the plots are from hexagonal ice that may have formed from adsorbed water in the coal sample. The ratio of CH 4 hydrate was estimated to be 87 wt % and that of ice was 13 wt %. The lattice constant of the hydrate was 1.1920(1) nm at 173 K. This value is slightly larger than reported values for CH 4 hydrate that was synthesized from water and CH 4 gas [1.1903(3) nm at 173 K].…”

“…The ratio of CH 4 hydrate was estimated to be 87 wt % and that of ice was 13 wt %. The lattice constant of the hydrate was 1.1920(1) nm at 173 K. This value is slightly larger than reported values for CH 4 hydrate that was synthesized from water and CH 4 gas [1.1903(3) nm at 173 K]. 28 However, the results of the pattern fitting using the Rietveldt method (R wp ∼ 1.8% assuming full cage occupancy of CH 4 ) suggest that the hydrate crystal formed in the coal sample is the same as in the bulk hydrate.…”

“…Figure 6 shows the X-ray diffraction profiles from CH 4 hydrate as the hydrate transformed into hexagonal ice during an increase in temperature from 163 to 273 K. Reflections from ice are evident at 163 K, and these peaks increase with temperature while the CH 4 hydrate diffraction peak heights decrease with temperature. This indicates that the amount of hydrate decreased and the amount of ice increased; the dissociated CH 4 hydrate likely transforms into ice as it releases CH 4 gas. The integrated intensities of the (321) reflection of CH 4 hydrate and the (100) reflection of the hexagonal ice are plotted as a function of temperature in Figure 7.…”

“…The structure, referred to as a small S cage, is comprised of 12 pentagonal 5-member rings of water (5 12 ) with a cage dimension of 7.9 A, and the large cage has 12 pentagonal 5-member rings of water and 2 hexagonal 6-member rings (5 12 6 2 ) with a cage dimension of 0.86 nm. The stoichiometry is CH 4 ·5.75H 2 O, which, if the cages are fully occupied, yields about 180 volumes of CH 4 at standard temperature and pressure per volume of hydrate. 13 At extreme pressures (i.e., >0.8 GPa) the cubic methane hydrate structure gives way to a hexagonal one and subsequently to an orthorhombic phase.…”

“…Gas clathrate hydrates are crystalline solids formed by the entrapment of guest molecules, such as CH 4 and CO 2 , by water cages at low temperature and pressure. 1,2 The cage is formed by water molecules that are hydrogen bonded, and the structure is stabilized by the repulsive interactions between the water and the guest molecule.…”