Offshore Gas Hydrates (eBook)

Origins, Development, and Production

Rudy Rogers (Autor)

eBook Download: PDF | EPUB

2015 | 1. Auflage
398 Seiten
Elsevier Science (Verlag)
978-0-12-802556-7 (ISBN)

Gas hydrates collect and store both thermogenic and biogenic methane generated in deep ocean sediments that, over geologic time, forms vast methane repositories. Offshore Gas Hydrates: Origins, Development, and Production presents gas hydrates as an emerging, clean energy source possibly more abundant than all other fossil fuels and especially important for countries geographically and economically restricted from conventional fossil fuel resources. The book explores feasible methods to produce offshore hydrate gas, the means to store and transport the remotely produced gas, new hydrate inhibitors for conventional and hydrate production in ultra-deep waters, instability manifestations of seafloor hydrates, and hydrate roles in complex ecological scenarios. Complementing production and drilling method presentations are computer simulation studies, hydrate field tests, and seismic and logging developments. Offshore Gas Hydrates delivers a well-developed framework for both the oil and gas researcher and corporate engineer to better exploit this future unconventional resource, empowering the oil and gas professional with the latest data and information on sophisticated challenges that offshore hydrates present.

Addresses the technical, economic, and environmental problems of producing hydrate gas.
Introduces the overlooked and unchartered role of microbes in catalyzing offshore hydrate formations with attendant effects on stability/dissociation.
Reviews the latest world-wide field tests, research, and case studies involving seafloor hydrates, inclusive of most known hydrate provinces.
Displays two videos within the e-book only: (1) hydrates, carbonates, chemosynthetic communities, and natural hydrocarbon leakages on the seafloor at the Mississippi Canyon hydrate observatory site; (2) hydrate nucleation, migration and self-packing in a laboratory test cell under the influence of anionic surfactants.
Extends deep-water hydrate knowledge regarding the hydrate formation and protective cover for microbes within the extreme environment of Mars.

Rudy Rogers, Professor Emeritus in Chemical Engineering at Mississippi State University, holds BS, MS, and PhD degrees in Chemical Engineering from the University of Arizona and the University of Alabama. Beginning in 1977, Dr. Rogers spent thirty-three years teaching petroleum engineering and chemical engineering at MSU, including eight years as Petroleum Engineering Chairman. During twenty yearsof gas hydrate research, he garnered nearly two million dollars of grants, participated in four scientific cruises to the Gas Hydrate Observatory in the Gulf of Mexico, received three U.S. patents, authored fifteen hydrate papers in peer-reviewed journals, gave over thirty presentations on gas hydrates as author or coauthor at national and international conferences, had numerous hydrate publications in proceedings, and introduced a senior/graduate-level hydrate course.

Gas hydrates collect and store both thermogenic and biogenic methane generated in deep ocean sediments that, over geologic time, forms vast methane repositories. Offshore Gas Hydrates: Origins, Development, and Production presents gas hydrates as an emerging, clean energy source possibly more abundant than all other fossil fuels and especially important for countries geographically and economically restricted from conventional fossil fuel resources. The book explores feasible methods to produce offshore hydrate gas, the means to store and transport the remotely produced gas, new hydrate inhibitors for conventional and hydrate production in ultra-deep waters, instability manifestations of seafloor hydrates, and hydrate roles in complex ecological scenarios. Complementing production and drilling method presentations are computer simulation studies, hydrate field tests, and seismic and logging developments. Offshore Gas Hydrates delivers a well-developed framework for both the oil and gas researcher and corporate engineer to better exploit this future unconventional resource, empowering the oil and gas professional with the latest data and information on sophisticated challenges that offshore hydrates present. Addresses the technical, economic, and environmental problems of producing hydrate gas. Introduces the overlooked and unchartered role of microbes in catalyzing offshore hydrate formations with attendant effects on stability/dissociation. Reviews the latest world-wide field tests, research, and case studies involving seafloor hydrates, inclusive of most known hydrate provinces. Displays two videos within the e-book only: (1) hydrates, carbonates, chemosynthetic communities, and natural hydrocarbon leakages on the seafloor at the Mississippi Canyon hydrate observatory site; (2) hydrate nucleation, migration and self-packing in a laboratory test cell under the influence of anionic surfactants. Extends deep-water hydrate knowledge regarding the hydrate formation and protective cover for microbes within the extreme environment of Mars.

Chapter Two

Deep Ocean Sediment–Hydrate Relationships

Abstract

Detailing offshore gas hydrate accumulations involves many study disciplines. This chapter surveys selected fundamentals and definitions helpful to begin the offshore hydrate narrative, providing practical details for later referral. Discussion begins with origins of gases occluded in seafloor hydrates, analytically distinguishing biogenic and thermogenic sources. Continuing discourse covers acoustic wipeout zones identified by seismic data, as these seafloor instabilities often associate with concentrated hydrate accumulations. Further seismic techniques help identify hydrate boundaries, especially bottom limits to gas hydrate stability. Basic hydrate morphologies are discussed, addressing why specific forms develop in ocean sediments. Throughout the chapter, sediment–hydrate relationships are explored: influences of physical properties of sediments such as porosity, permeability, thermal conductivity, particle sizes, faulting impact hydrate extent, morphology, and pore saturation.

Keywords

acoustic wipeout zone

hydrate morphology

BSR

methane carbon-13 isotopes

geothermal gradient

seafloor instability

seismic techniques

CT-scan hydrate cores

A seismic wipeout zone is a fairway above bottom of gas hydrate stability (BGHS) where natural gases, having come from forceful disruption of a low-permeability interface at BGHS, enrich but disrupt sediment continuity while permeating to the seafloor via faults, fractures, and chimney-like structures. Hydrates, accessible carbon, and microbes abound in these fairways.

Early efforts to systematically locate seafloor gas hydrates focused on finding bottom-simulating reflectors (BSRs), which manifest seismic reflections outlining floors of hydrate occurrence. Envisioned was a means to economically explore ocean floors for hydrate reservoirs. Although useful, BSRs proved to have limitations. Accomplishments and shortcomings of seismic BSRs in hydrate exploration are discussed in this chapter, as well as well logging advancements that help profile gas hydrates qualitatively and quantitatively.

Gas hydrate morphologies in fine-particle ocean sediments and hydrate morphology in coarse-sand reservoirs are discussed in this chapter. Studies of the hydrate morphologies are aided by noninvasive core analyses utilizing computed tomography (CT) scans.

Especially important for near-surface hydrate studies are fracture propagations in fine sediments, polygonal faulting, and the filling of prevailing fissures with gas hydrates. Overall, the sediment–hydrate matrix relationships involving particle size, permeability, porosity, hydrate saturation, thermal conductivity, geothermal gradient, and heat flux are complex, but empirical bases are presented in this chapter for later determinations of their consequential effect on hydrate gas production.

2.1. Determining origin of hydrate-occluded gases

2.1.1. Carbon Isotope Analysis

Carbon isotope analyses of fairway methane and methane/ethane molecular ratios help determine origins of the hydrate-occluded gas. Carbon has two stable isotopes, carbon-12 and carbon-13, which find use in determining origins of carbon-containing gases associated with hydrates. An unstable isotope, carbon-14, has traditionally been used for archaeological dating. These three isotopes occur throughout nature in the characteristic amounts and are used in primary applications, as presented in Table 2.1.

Table 2.1

Carbon isotope concentrations in nature

Carbon isotope

Terrestrial content (%)

Comments

References

Carbon-12

98.89

IUPAC specifies its molecular weight as basis of all elements. Stable

Rounick and Winterbourn (1986)

Carbon-13

1.11

Allows distinguishing microbial source. Stable

Rounick and Winterbourn (1986)

Carbon-14

Trace

Radioactive, half-life 5730 years; dates wood. Unstable

Burdige (2006)

IUPAC, International Union of Pure and Applied Chemistry.

Because of weaker bond energies, compounds with the lighter carbon-12 isotope are preferentially processed by bacteria. An enriched substrate of carbon-13 is left behind, depleting that isotope in the gaseous bioproduct. To distinguish microbial sources of hydrate carbon compounds from thermogenic sources, delta values of the stable isotope δ13C comprising methane are calculated and compared with a standard. The standard is Pee Dee Belemnite (PDB), a calcium carbonate marine fossil from Cretaceous times found in the Pee Dee formation of South Carolina (Rounick and Winterbourn, 1986; Whiticar, 1999). Extremely depleted in the carbon-12 isotope, the delta value of the fossil was arbitrarily set as zero by the National Bureau of Standards.

Delta values (δ13C) may be calculated from Equation 2.1:

13CPDB=( 13C/ 12C)Sample( 13C/ 12C)PDB standard−1×103

(2.1)

The isotopic ratio δ13C/δ12CPDB effectively determines whether the subject carbon has been fractionated by microbial activity. If δ13CPDB values of hydrate-occluded methane calculated by Equation 2.1 are more negative than −60‰ (in units of parts per thousand), the gases are considered to be of microbial origin, but δ13CPDB values of methane more positive than −50‰ are considered to be of thermogenic origin (Paull et al., 2005; Bernard et al., 1978).

Some representative δ13C values of methane associated with seafloor gas hydrates are presented in Table 2.2.

Table 2.2

Representative δ13C values of methane in seafloor gas hydrates

Location

δ13C (‰)

References

Comments

Indonesia

−70.6 to −52.6

Sassen and Curiale (2006)

0–6 m < seafloor

1396–1989 m water depth

MC-118, Gulf of Mexico

−45.7

Sassen et al. (2006)

Vent gas 0.5 m above seafloor

Below 840 m water column

Bush Hill, Gulf of Mexico

−44.1

Sassen et al. (1999)

Vent gas

Sea of Okhotsk

−49.5 to −65.8

−31.7 to −77.5

Cho et al. (2005)
Shakirov and Obzhirov (2011)

Seeps

Both thermogenic and biogenic

Nankai Trough

−96 to −63

Waseda and Uchida (2002)

Upper 300 m of sediments

Nankai Trough

−48 to −35

Waseda and Uchida (2002)

Gases deeper than 1500 mbsf

Cascadia margin

−71.5 to −62.4

Suess et al. (1999)

Methane released from gas hydrates

Eastern margin of the Sea of Japan

−36.2 to −40.33

Lu et al. (2011a)

Carbon isotope content of methane in retrieved gas hydrates

2.1.2. Molecular Structure Ratios

Further assistance in determining whether a hydrate gas source is biogenic or thermogenic is given by the criterion in Equation 2.2 of the molecular structure ratio [C1 (methane)]/[C2 (ethane) + C3 (propane)]. A ratio greater than 1000 defines a predominantly biogenic origin of the gas mixture because of the relatively large amounts of methane (Bernard et al., 1978):

1C2+C3>1000

(2.2)

For example, sediment gas samples from the Nankai Trough exhibiting molecular structure ratios greater than 4000 are decidedly of biogenic origin (Waseda and Uchida, 2002).

Combinations of isotopic ratio and molecular structure ratio become more informative, as well as reliable, in designating gas source. In the preceding Nankai sample, a δ13C analysis and use of Equation 2.1 yields delta values −71 to −66‰, well beyond the minimum −60‰ criterion for biogenic origin. Then, combination of results from Equations 2.1 and 2.2 firmly establishes hydrate gas origin as biogenic (Bernard et al., 1976; Whiticar, 1999).

Thermogenic origins are indicated when the molecular structure ratios are less than 100, as given by Equation 2.3:

1C2+C3<100

(2.3)

For example, a Green Canyon vent gas exemplifies thermogenic sources (Sassen et al., 1999). Composed of 90.4% methane, 4.5% ethane, 3.7% propane, and 1.4% of heavier hydrocarbons, the 11.0 molecular structure ratio is well below the 100 upper limit value specified by Equation 2.3.

Molecular ratios between 1000 and 100 represent mixed gases from both...

Erscheint lt. Verlag	24.8.2015
Sprache	englisch
Themenwelt	Naturwissenschaften ► Geowissenschaften ► Hydrologie / Ozeanografie
	Naturwissenschaften ► Physik / Astronomie
	Technik ► Elektrotechnik / Energietechnik
ISBN-10	0-12-802556-5 / 0128025565
ISBN-13	978-0-12-802556-7 / 9780128025567

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