Offshore gas hydrates : origins, development, and production

385 Pages·2015·21.495 MB·English

by  Rogers, Rudy E

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ffshOre
GAS HYDRATES
ORIGINS, DEVELOPMENT,
AND PRODUCTION
RUDY ROGERS
Professor Emeritus
Mississippi State University
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DEDICATION
Offshore Gas Hydrates is dedicated to the late Bob Woolsey, geologist ex-
traordinaire who was a leader in extending gas hydrate knowledge from a
laboratory curiosity to practical offshore applications. He was a pioneer in
forging this giant step for the advancement of gas hydrate knowledge – a
step toward alleviating worldwide energy deficiencies while introducing
intriguing insights into seafloor ecologies and continental slope instabilities.
Dr Woolsey envisioned an offshore Gas Hydrate Observatory for hydrate
studies and single-handedly procured funding, established site location at
Mississippi Canyon lease block 118, and assembled a Gulf Coast consortium
of government, industry, and academe to fulfill research goals. The book is a
salute to this outstanding individual.
PREFACE
A new, clean energy source is discovered in contemporary times, so abun-
dant that its resident carbon may exceed carbon in all known fossil energy
sources combined. Impossible you say? Enter, gas hydrates. Forming world-
wide in the frontier of deep ocean floors, the ice-like hydrate scavenges
and stores both thermogenic and biogenic methane. Perhaps its existence
in extreme environments devoid of free oxygen, or perhaps its decomposi-
tion during sediment retrieval with traditional unpressurized coring tools,
is responsible for the delayed discovery. In any case, there are fascinating
consequences of the offshore hydrate discoveries now presented as a unit
for the first time in this textbook.
Chapter 1 presents historic timelines of gas hydrate awareness and pres-
ents estimates of in-place hydrate gas regionally and universally. It assumes
readers approach the book already well versed in gas hydrate structural fun-
damentals.
The primary excitement of offshore hydrate discoveries derive from
prospects of producing hydrate’s methane as a fuel, for the ubiquitous hy-
drate accumulates near many nations whose fuel shortages hinder national
progress. Solutions are rapidly being made to technical, economic, and envi-
ronmental problems involved with producing hydrate gas. The text address-
es these production issues and, while doing so, discusses a suite of associated
but absorbing discoveries concerning science of extreme environments.
Chapter 2 initiates the production quest by discussing sediment–hydrate
relationships in seafloor hydrate zones critical to understanding search and
evaluation of future quality reservoir sites for hydrate-gas production. Be-
cause of its extensive infrastructure for conventional gas production, the
Gulf of Mexico is highlighted in Chapter 3 as a gas hydrate province having
prolific hydrate accumulations, occluding both biogenic and thermogenic
gases. It affords practical study for prospective hydrate-gas production. Dis-
cussed are Joint Industry Program (JIP) evaluations of seismic, logging, and
drilling at selected sites to establish protocol for hydrate reservoir develop-
ment. Then, Chapter 4 covers process and technique combinations now
being devised to produce hydrate gas: computer simulations, limited Arctic
production, conventional process modifications, and laboratory evaluations
of processes are made. A useful discussion ensues on a process to store and
transport gas produced from remote offshore locations. Chapter 5 reviews
xi
xii Preface
progressively increasing hydrate inhibitions required for hydrocarbon pro-
duction in ultra-deepwater operations – both conventional and hydrate-gas
productions. Besides state-of-the-art inhibitors, advanced inhibitors such as
antifreeze proteins and microbial cell wall materials are discussed.
Contemporary offshore gas hydrate discoveries, and their accompanying
focused seafloor studies, give insight into hydrate-associated seafloor insta-
bilities. Gas hydrates collect thermogenic and biogenic gases generated in
deep ocean sediments, and have been doing this over geologic time, to form
vast repositories primarily of methane and secondarily of carbon dioxide.
The natural storage process of seafloor hydrates beneficially modulates the
ejection of these gases into the atmosphere. But there is a caveat. The stored
gases, as a solid clathrate solution, generate extraordinarily high pressures if
the hydrates decompose in confined spaces from inadvertent heat injection,
for example, when hydrates surrounded by impermeable clayey sediments
are heated by a penetrating drill bit. Or on a larger scale, the instability phe-
nomenon is evidenced by prehistoric submarine landslides having occurred
on continental slopes when warmer or shallower waters triggered hydrate
dissociations that weakened shear strengths of inclined sediments. In these
and other diverse cases, then, hydrates initiate seafloor instabilities. In Chap-
ter 6 variations in instability manifestations are discussed.
Gas hydrate formation is a crystallization process, where nucleation
sites classically initiate crystal growth. But additional site criteria on a mi-
croscopic scale exist for hydrate nucleation. Sufficient gas molecules must
propitiously converge with oriented water molecules properly arranged
by hydrogen bonding – a stochastic occurrence often involving lengthy
times. Offshore hydrate studies reveal microbe–mineral–hydrate synergies
that create unique nucleation sites to accelerate hydrate initiation. This fas-
cinating phenomenon affects hydrate formation and stability in extreme
environments and is developed in the text. Chapter 7 introduces the bio-
genic hydrate provinces. Blake Ridge, Cascadia margin subduction zone,
Nankai Trough, and other regions where offshore hydrates primarily collect
biogenic gases are characterized. Chapter 8 discusses the intriguing role
of microbes in catalyzing gas hydrate formations in seafloors, followed by
mechanisms whereby the microorganisms enter and thrive in interstitial
spaces of hydrate masses.
Explorations for hydrate gas conducive to production have advanced
deep-sea knowledge. Amid gas hydrate accumulations, which often pro-
trude above the seafloor, unique ecologies develop with complex interde-
pendencies. Why? Carbon, essential to microbial metabolism but generally
Preface xiii
scarce in these extreme environments, is abundant in the hydrocarbons of
hydrate-zone cold seeps and methane collected in gas hydrates. Conse-
quently, extensive microbial species proliferate around these oases of carbon.
For example, Archaea and sulfate-reducing bacteria team in clusters to an-
aerobically oxidize methane (AOM), utilizing cold-seep gases and seawater
sulfates/nitrates. As a result, massive carbonates litter the seafloor, marking
current and previous seep locations. Although AOM produces hydrogen
sulfide, the toxic compound promotes chemosynthetic communities and on
occasion gas hydrates. Beggiatoa mats spread over hydrate outcrops and sea-
floor, extracting and converting hydrogen sulfide to help replenish sediment
sulfates in a sustaining cycle. Extensive root systems of tube worms scav-
enge hydrogen sulfide from sediments to support their internal symbiotic
microbes. The text describes gas hydrates fitting into a complex ecological
scenario. Chapter 9 is devoted to the ecology of the hydrate zone.
Bolstered by contemporary feedback from planetary scientific probes,
interest intensifies in the possibility of extensive gas hydrates within Mar-
tian subsurfaces. By analogies based on hydrate information acquired from
Earth’s extreme ocean and polar environments, the final Chapter 10 reviews
gas hydrate feasibility under Martian conditions. If hydrates form in the
Martian subsurface environment, as the data review indicates probable, our
analogy with seafloor hydrates introduces an intriguing circumstance where
any microbes on Mars would have the best protection and sustenance for
longevity provided within the subsurface hydrates.
ACKNOWLEDGMENTS
The author gratefully acknowledges support of individuals and groups that
made the work possible. The Gulf Coast Gas Hydrate Consortium, adminis-
tered by MMRI of the University of Mississippi, provided a valuable forum
over the years to present and discuss university, industry, and government
hydrate research. The interest, support, and funding of NETL, NOAA, and
Department of the Interior for hydrate research were irreplaceable. The
author is appreciative of exceptionally good graduate students who col-
laborated with him in gas hydrate research at Mississippi State University.
Finally, the work was made achievable by the kind support of the author’s
wife, Beth.
xv
CHAPTER ONE
Introduction
1.1 THE MYSTIQUE OF GAS HYDRATES
For more than a century after Sir Humphrey Davy reported the first
gas hydrate (chlorine) to the Royal Society (Davy, 1811), gas hydrates re-
mained curiosities in informal, disparate laboratory experiments. But during
October 1931, on activation of the first natural gas pipeline, an expensive
24 in. diameter line operating under 600 psi and traversing 1000 miles from
the newly discovered energy riches of the Texas Gulf Coast to Chicago
(Ziegenhain, 1931), inaugural gas failed to flow. A massive, solid plug was
found blocking the line. The plug resembled ice, yet at no time or place had
line temperatures dropped below 40°F. The final verdict: a massive Davy-
type gas hydrate was the culprit (Hammerschmidt, 1934).
During the next 50 years as oil and gas drilling expanded to offshore and
proceeded into deeper waters, hydrate research focused on economically
preventing clathrate blockages from disrupting offshore operations – an elu-
sive goal still being pursued. But the insight generated by this preoccupation
perhaps contributed to the next in a series of gas hydrate discoveries.
The Ocean Drilling Program (ODP), funded to advance scientific
knowledge of deep ocean environments, discovered ice-like gas hydrates
in ocean sediments – found them to exist superabundantly. Consider the
massive hydrate protuberances photographed from manned submersibles
offshore Vancouver as well as the Gulf of Mexico (GOM). For example,
a hydrate mound discovered and photographed by Professor Chapman
and associates is presented in Figure 1.1 (Chapman, R.N., 2015. Professor,
School of Earth and Ocean Sciences, University of Victoria, Victoria, BC,
personal communication).
Progressively improving analyses aided by advanced coring, drilling,
seismic, and logging techniques gradually revealed seafloor hydrate de-
posits spanning all latitudes. Calculations accompanying discoveries in-
dicated hydrate-occluded methane perhaps on a scale of the combined
energy found in all fossil fuels. In seacoast countries long dependent on
imported hydrocarbons, the discoveries introduced national hopes of fuel
independence.
Offshore Gas Hydrates. http://dx.doi.org/10.1016/B978-0-12-802319-8.00001-2
Copyright © 2015 Elsevier Inc. All rights reserved. 1
2 Offshore Gas Hydrates
Figure 1.1 Massive gas hydrate mound discovered on seafloor, Cascadia margin. Beg-
giatoa mats cover white hydrates protruding into ocean. (Source: Chapman, R.N., 2015.
Professor, School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, personal
communication.)
How a new energy source, an elephant in the room, remained so
long unknown adds to an evolving gas hydrate enigma. One physical
property helped disguise the presence. Because hydrates decompose as
pressures are lowered or temperatures are raised beyond an instability
threshold, early nonpressurized coring tools allowed hydrate decom-
position during extraction–retrieval of cores from deepwater ocean
sediments.
Early discoveries initiated quests for seafloor hydrate fundamentals, es-
pecially mechanisms of formation, dissociation, and kinetics. But the dis-
coveries initiated additional interests that will be outlined in the remaining
section.
One way the hydrate mystique reemerges is a microbial association with
seafloor hydrate accumulations. In seafloor sediments, microbes proliferate
around near-surface gas hydrates. Why? Microbes require carbon for metab-
olism and an inordinate carbon supply is stored in hydrates. It is not surpris-
ing, therefore, that the microorganisms promote hydrate accumulations by
providing catalysis and hydrate crystal nucleation sites. In fact, the activity
creates an intriguing phenomenon of seafloor synergy between microbes,
minerals, and hydrates. Throughout the text, threads of microbe–hydrate–
mineral synergistic interactions are discussed.
Associated with hydrates on the ocean floor are communities of or-
ganisms sustaining life by chemosynthesis in the absence of light or
oxygen – tubeworms, ice worms, clams, and Beggiatoa mats. Aided by sym-
biotic bacteria, an extravagant and fascinating ecosystem exists around sea-
floor gas hydrates.
Introduction 3
Gas hydrates have caused seafloor instabilities of catastrophic proportions
over geologic time. Such episodes are even woven into the story fabric of
works of fiction. On continental slopes, gas hydrates cement unconsolidated
sediments to impart additional shear strength and stability. Warmer or shal-
lower ocean waters eventually breach the temperature–pressure threshold
of hydrate stability to lower shear strengths along the slope’s bottom plane.
Ultimately, mudslides propel downslope the hydrate-sediment overburden,
releasing from ensuing rubble large quantities of methane gas. Evidence
indicates this happened in the ancient Storegga Slide offshore Norway. Pos-
sible consequences of submarine mudslides include tsunamis and sudden
large imposition of greenhouse gases into the atmosphere.
Consider some dangers gas hydrates pose to deepwater drilling: (1)
1 m3 of solid hydrate may contain 150–180 m3 of hydrocarbon gases at
standard temperature and pressure (STP); (2) gas hydrates in sediments
decrease permeability to the flow of gas; (3) energy input decomposes hy-
drates, perhaps from drilling-bit friction or exothermic well-cementing
reactions. Consequently, hydrates decomposed within surrounding po-
rous media made impermeable by hydrates and shales build extraordinary
pressures.
Voluminous gas discharges into deep ocean waters from naturally oc-
curring submarine mud volcanoes or from well blowouts create plumes of
gas, bounded by the upper limit of hydrate stability, drifting long distances
in the water subsurface. Hydrate stability may extend upward in the water
column to within 400–500 m of the surface. On initial emission into the
high pressures and low temperatures of deep waters, each bubble of hydro-
carbon gas is rapidly enveloped by hydrate, delaying gas dissipation. Efforts
to control the Deepwater Horizon blowout in 1544 m deep waters of the
GOM experienced debilitating interference by extensive hydrate formation
at the point of seafloor eruption.
Finally, many scientists now believe gas hydrates exist in the Martian
subsurface, heightening the hydrate enigma while exploration proceeds on
the planet. Scientific knowledge gained from studying Earth’s seafloor hy-
drates points favorably toward hydrates forming under extreme Martian
conditions. For the first time, a hypothesis connects the significance of hy-
drates to an enhanced possibility of microbes on Mars. Sequestered within
Earth’s seafloor hydrate masses and accessible to occluded carbon as well as
a lifeline of diffusing nutrients, microbes thrive while sheltered from the
vagaries of changing and hostile exterior environments. Does an analogy
hold for Mars? The last chapter discusses the possibility.