Hydrate in Subsea Pipeline
05.51
Gas hydrates are of
great importance for a variety of reasons (Figure-1). In offshore hydrocarbon
drilling and production operations, gas hydrates cause major, and potentially
hazardous flow assurance problems.
Naturally occuring
methane clathrates are of great significance in their potential for as
strategic energy reserve, the possibilities for CO2 disposal by sequestration,
increasing awareness of the relationship between hydrates and subsea slope
stability, the potential dangers posed to deepwater drilling installations,
pipelines and subsea cables, and long-term considerations with respect to
hydrate stability, methane (a potent greenhouse gas) release, and global
climate change.
Drilling
In
drilling, record water depths are continuously being set by oil companies in
the search of hydrocarbon reserves in deep waters. Due to environmental
concerns and restrictions, water based drilling fluids are often more desirable
than oil based fluids, especially in offshore exploration. However, a
well-recognised hazard in deep water offshore drilling, using water based
fluids, is the formation of gas hydrates in the event of a gas kick.
In
deep-water drilling, the hydrostatic pressure of the column of drilling fluid
and the relatively low seabed temperature, could provide suitable thermodynamic
conditions for the formation of hydrates in the event of a gas kick. This can
cause serious well safety and control problems during the containment of the
kick. Hydrate formation incidents during deep-water drilling are rarely
reported in the literature, partly because they are not recognised, Two cases
have been reported in the literature where the losses in rig time were 70 and
50 days.
The
formation of gas hydrates in water based drilling fluids could cause problems
in at least two ways:
·
Gas hydrates could form in the
drill string, blow-out preventer (BOP) stack, choke and kill line. This could
result in potentially hazardous conditions, i.e., flow blockage, hindrance to
drill string movement, loss of circulation, and even abandonment of the well.
·
As gas hydrates consist of more
than 85 % water, their formation could remove significant amounts of water from
the drilling fluids, changing the properties of the fluid. This could result in
salt precipitation, an increase in fluid weight, or the formation of a solid
plug.
·
The hydrate formation condition of
a kick depends on the composition of the kick gas as well as the pressure and
temperature of the system. As a rule of thumb, the inhibition effect of a
saturated saline solution would not be adequate for avoiding hydrate formation
in water depth greater than 1000 m. Therefore, a combination of salts and
chemical inhibitors, which could provide the required inhibition, could be used
to avoid hydrate formation.
Production
The ongoing
development of offshore marginal oil and gas fields increases the risks of
facing operational difficulties caused by the presence of gas hydrates. A
typical area of concern is multiphase transfer lines from well-head to the
production platform where low seabed temperatures and high operation pressures
increase the risk of blockage due to gas hydrate formation (Figure-2). Other
facilities, such as wells and process equipment, can also be prone to hydrate
formation.
Different
methods are currently in use for reducing hydrate problems in hydrocarbon
transferlines and process facilities. The most practical methods are:
·
At fixed pressure, operating at
temperatures above the hydrate formation temperature. This can be achieved by
insulation or heating of the equipment.
·
At fixed temperature, operating at
pressures below hydrate formation pressure.
·
Dehydration, i.e., reducing water
concentration to an extent of avoiding hydrate formation.
·
Inhibition of the hydrate formation
conditions by using chemicals such as methanol and salts.
·
Changing the feed composition by
reducing the hydrate forming compounds or adding non hydrate forming compounds.
·
Preventing, or delaying hydrate
formation by adding kinetic inhibitors.
·
Preventing hydrate clustering by
using hydrate growth modifiers or coating of working surfaces with hydrophobic
substances.
·
Preventing, or delaying hydrate
formation by adding kinetic inhibitors.
Hydrates in The Natural Environment
Hydrates as a Potential Energy Resource
Two factors
make gas hydrates attractive as a potential energy resource: (1) the huge
volumes of methane that is apparently trapped as clathrate within the upper
2000 m of the Earth's surface, and (2) the wide geographical distribution of
gas hydrates.
Natural gas
is widely expected to be the fastest growing primary energy source in the world
over the next 20 years. In the U.S. Energy Information Administration's
International Energy Outlook 2002 (IEO2002) reference case, worldwide gas
consumption is projected to almost double to 162 trillion cubic feet in 2020
from 84 trillion cubic feet (standard conditions) in 1999. Given the attractive
features of gas hydrates, and the growing demand for natural gas, it seems
reasonable to conclude that gas hydrates could serve as a future energy
resource.
A number of
schemes for methane hydrate exploitation have been proposed, although at
present, technical and economic considerations restrict production to
experimental tests only. The Japan National Oil Company (JNOC) has been a
pioneer in this field, having already drilled experimental wells in the
Mackenzie Delta of Northern Canad with ambitious plans for further test wells
in sediments of offshore Japan.
One
interesting branch of research in this area is the possibility of CO2
sequestration. CO2 hydrate is thermodynamically more stable than methane
hydrate, so the possibility exists for sequestration of CO2 into existing
seafloor clathrates, whereby yielding methane. This process is particularly
attractive, as it would act as both a source and a sink with respect to
greenhouse gas emissions.
Hydrates as a Geohazard
The aspect
of gas hydrates which has the biggest implications for human welfare at
present, is their potential as a geohazard. Of particular concern is the danger
posed to deepwater drilling and production operations, and the large body of
evidence which now exists linking gas hydrates with seafloor stability.
With
conventional oil and gas exploration extending into progressively deeper
waters, the potential hazard gas hydrates pose to operations is gaining
increasing recognition. Hazards can be considered as arising from two possible
events: (1) the release of over-pressured gas (or fluids) trapped below the
zone of hydrate stability, or (2) destabilization of in-situ hydrates.
The
presence of BSRs has previously been a cause of concern, as they could be
considered evidence for the existence of free gas (possibly at high-pressure)
beneath the HSZ. More recent analysis suggests however, that as long as excess
water is present, there should not be a build-up of gas pressure beneath the
HSZ. This is because, at the base of hydrate stability, the system approximates
to 3-phase equilibrium, where pressure is fixed (generally at hydrostatic), and
temperature occupies the available degree of freedom. This means that any
excess gas will be converted to hydrate, returning the system to its
equilibrium pressure (assuming there is no major barrier to the mass transfer
of salt). This case is likely to predominate in many hydrate-bearing sediments,
although gas seeps and mud volcanoes, common to thermogenic hydrate areas (e.g.
Gulf of Mexico, Caspian Sea), could be considered evidence for excess gas and
pore-fluid pressures at shallow depths.
In the
absence of gas traps, hydrates still pose a hazard due to their potential for
destabilization. This danger is particularly apparent in the case of
conventional oil and gas exploration, for which drilling methods contrast quite
markedly to the shallow piston-coring approach used by ODP in hydrate areas.
Conventional
rotary drilling operations could cause rapid pressure, temperature or chemical
changes in the surrounding sediment. An increase in temperature could be caused
by a hot drill bit, warm drilling fluids, or later as high-temperature
reservoir fluids rise through the well, while the addition of hydrate
inhibitors to drilling muds (to prevent hydrate formation in the well-bore or
drill string in the event of a gas-kick) could change sediment pore-fluid
chemistry. Some, or all of these changes, could result in localized dissociation
of gas hydrates in sediments surrounding wells. A similar case would apply to
seafloor pipelines, where the transportation of hot fluids could cause
dissociation of hydrates in proximal sediments. In a worst-case scenario,
clathrate dissociation could lead to catastrophic gas release, and/or
destabilization of the seafloor.
The hazards
associated with drilling in gas hydrate areas are exemplified by cases from the
Alaskan Arctic, where subsurface permafrost hydrate destabilization has
resulted in gas kicks, blowouts, and even fires.
Hydrates and Seafloor Stability
A
significant part of the gas hydrate geohazard problem is related to how they
alter the physical properties of a sediment. If no hydrate is present, fluids
and gas are generally free to migrate within the pore space of sediments.
However, the growth of hydrates converts what was a previously a liquid phase
into a solid, reducing permeability, and restricting the normal processes of
sediment consolidation, fluid expulsion and cementation. These processes can be
largely stalled until the BHSZ is reached, where hydrate dissociation will
occur. Dissociation of hydrates at the BHSZ can arise through an increase in
temperature due to increasing burial depth (assuming continued sedimentation)
or an increase in sea bottom-water temperatures, and/or a decrease in pressure
(e.g., lowering of sea level). Upon dissociation, what was once solid hydrate
will become liquid water and gas. This could lead to increased pore-fluid
pressures in under-consolidated sediments, with a reduced cohesive strength
compared to overlying hydrate-bearing sediments, forming a zone of weakness.
This zone of weakness could act as a site of failure in the event of increased
gravitational loading or seismic activity (Figure-3).
The link
between seafloor failure and gas hydrate destabilization is a well established
phenomenon, particularly in relation to previous glacial-interglacial eustatic
sea-level changes. Slope failure can be considered to pose a significant hazard
to underwater installations, pipelines and cables, and, in extreme cases, to
coastal populations through the generation of tsunamis.
Hydrates and Global Climate Change
Methane is
a particularly strong greenhouse gas, being ten times more potent than carbon
dioxide. Increasing evidence points to the periodic massive release of methane
into the atmosphere over geological timescales. However, whether such enormous
releases of methane are a cause or an effect with respect to global climate
chnages remains the subject of much debate.
Global
warming may cause hydrate destabilsation and gas release through a rise in
ocean bottom water temperatures. Methane release in turn would be expected to
accelerate warming, causing further dissociation, potentially resulting in run
away global warming. However, coversely, sea level rise during warm periods may
act to stabilise hydrates by increasing hydrostatic pressure, acting as a check
on warming.
A further
possiblity is that hydrate dissociaton may act as a check on glaciation,
whereby reduced sea levels (due to the growth of ice sheets) may cause seafloor
hydrate dissociation, releasing methane and warming the climate.
The strong
link between naturally occurring gas hydrates and the Earth's climate is an
increasingly recognised phenomenon. However, there is still little
understanding concerning the exact role gas hydrates play in global climate
change.
Sumber:
http://www.pet.hw.ac.uk/research/hydrate/hydrates_why.cfm
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