Review of natural gas hydrates as an energy resource: Prospects and challenges ☆

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 × 1014 m3 (1016 ft3; ∼150 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.

Current perspectives on gas hydrate resources

The flow of a single-phase fluid through a rough-walled rock fracture is discussed within the context of fluid mechanics. The derivation of the ‘cubic law’ is given as the solution to the Navier-Stokes equations for flow between smooth, parallel plates - the only fracture geometry that is amenable to exact treatment. The various geometric and kinematic conditions that are necessary in order for the Navier-Stokes equations to be replaced by the more tractable lubrication or Hele-Shaw equations are studied and quantified. In general, this requires a sufficiently low flow rate, and some restrictions on the spatial rate of change of the aperture profile. Various analytical and numerical results are reviewed pertaining to the problem of relating the effective hydraulic aperture to the statistics of the aperture distribution. These studies all lead to the conclusion that the effective hydraulic aperture is less than the mean aperture, by a factor that depends on the ratio of the mean value of the aperture to its standard deviation. The tortuosity effect caused by regions where the rock walls are in contact with each other is studied using the Hele-Shaw equations, leading to a simple correction factor that depends on the area fraction occupied by the contact regions. Finally, the predicted hydraulic apertures are compared to measured values for eight data sets from the literature for which aperture and conductivity data were available on the same fracture. It is found that reasonably accurate predictions of hydraulic conductivity can be made based solely on the first two moments of the aperture distribution function, and the proportion of contact area.

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Hydraulic conductivity of rock fractures

Coal mine methane: A review of capture and utilization practices with benefits to mining safety and to greenhouse gas reduction

The fundamental challenge of the 21st century that mankind has to face is definitely energy supply, its storage and conversion in a way that necessarily protects the environment. For 250 years, the tremendous development of humanity has been founded on the harnessing of fossil fuels (coal, crude oil then natural gas) as primary energy due to their high energy density values and the easiness of access. However, this global pattern of energy supply and use is unsustainable. Global warming and finite fossil-fuel supplies call for a radical change in the energy mix to favour renewable energy sources. Without being exhaustive, we tackle in this article the tricky energy question and associated environmental issues as personally perceived. The eminent role of electric energy produced from decarbonized sources in a future sustainable economy is particularly highlighted as well as the issues of its needed storage. The possible and foreseen hindrances of electrochemical energy storage devices, focusing on the lithium-ion technology, are presented in parallel with the possible pathways to make such a technology greener in synergy with the rise of a biomass-based industry.

Clean energy new deal for a sustainable world: from non-CO2 generating energy sources to greener electrochemical storage devices

Natural gas-hydrates — A potential energy source for the 21st Century

Tight gas sands are defined as sandstone formations with less than 0.1 millidarcy permeability. The millidarcy is a unit of measurement related to the ability of a fluid to pass through a porous medium. The degree of permeability depends upon the size and shape of the pores, the size and shape of their interconnections, and the extent of the latter. It has been estimated that total-gas-in-place in the United States may exceed 15,000 trillion cubic feet. The size, location, and quality of the resource varies.

Tight Gas Sands

This study presents results which were generated and analyzed over a 2-yr period. Several hundred computer runs were made during this project and an extensive amount of rock property data was reviewed to insure that these data were representative of tight gas reservoirs. The following conclusions reflect the combined information which was generated during the study. The reservoir properties, such as capillary pressure, change of capillary pressure in damaged zones, and relative permeability (in low permeability gas reservoirs) are primary factors in determining the behavior of a fractured well during cleanup. If the reservoir rock permeability is not damaged by frac fluid invasion, no serious water blockage to gas flow will occur when (1) the pressure drawdown is much greater than the capillary pressure in the formation, or (2) the capillary pressure and water mobility are large enough to rapidly imbibe the frac water into the formation. If the reservoir rock permeability is not damaged by frac fluid invasion, a complete water block to gas flow cannot occur; however, gas production can be severely curtailed if the pressure drawdown does not exceed the formation capillary pressure and the water mobility is so low that the frac water remains immobile nextmore » to the fracture face.« less

Stephen A. Holditch

Papers

Natural gas-hydrates — A potential energy source for the 21st Century

Tight Gas Sands

Factors Affecting Water Blocking and Gas Flow From Hydraulically Fractured Gas Wells

The Effects of Non-Darcy Flow on the Behavior of Hydraulically Fractured Gas Wells (includes associated paper 6417 )

Unconventional oil and gas resource development – Let’s do it right