Tomorrow's
Fuel Is Ready Today
Not so
long ago, natural gas was little more than a waste product of the oil
industry. Today it is the fuel of choice for the 21st century and the
enabler of the emerging hydrogen fuel cell economy. Gas production differs
from oil production in that upstream and downstream activities are far
more integral with - and mutually reliant upon - one another. As a result,
delivering complex projects requires knowledge of the entire gas chain
and the ability to contribute at every stage.
The exact
composition of natural gas varies from reservoir to reservoir. In particular,
the proportion of hydrocarbons such as ethane, propane and butane, called
natural gas liquids (NGLs), contained in the natural gas may differ.
If a reserve of gas is especially rich in these hydrocarbons, it may
be commercially viable to extract them for re-sale independently. Equally,
it may be necessary to remove these hydrocarbons in order to make the
gas meet a customer's requirements.
Liquefied
natural gas (LNG) is natural gas that has been cooled to approximately
-162 degrees C, to its liquid state. LNG is composed primarily of methane
and may also contain smaller quantities of ethane, propane and other
heavier hydrocarbons. Other components that may be found in pipeline
gas are water, carbon dioxide, nitrogen, oxygen and sulfur compounds.
The oxygen, carbon dioxide, sulfur compounds and water are removed during
the liquefaction process, which can purify the LNG to nearly 100-percent
methane.
LNG takes
up about one-six-hundredth of the volume of natural gas found at the
stove burner tip and weighs about 45 percent as much as water. It is
odorless, colorless, non-corrosive and nontoxic, and when mixed with
air, or vaporised, it burns only in concentrations of 5% to 15%. Neither
LNG nor its vapours can explode in unconfined environments.
Safety
First
While
many misconceptions exist regarding LNG, it is a very safe fuel. In
fact, years of controlled testing by independent laboratories, using
hundreds of thousands of gallons of intentionally spilled LNG, has never
resulted in a single explosion of an ignited vapour cloud. Testing has
even been done on initiating combustion of the gas cloud with high explosives,
and the strength of the resulting detonation was no stronger than that
delivered by the explosives themselves. However from the safety standpoint,
another factor to note is that the size of a LNG vapour cloud and the
speed that it travels correlate directly to the size and rate of a LNG
spill. To a smaller degree, the size and surface of the area where the
LNG has spilled, atmospheric conditions and LNG pressure also play a
part. Small LNG spills will quickly vaporise upon contact with the ground.
LNG vapour becomes buoyant in the air at about -162 degree C, and therefore
it dissipates rapidly into the atmosphere. Any spill large enough to
form a denser vapour cloud will similarly dissipate as a result of heat
from the ground over which the cloud migrates. Wind will also affect
the vapour cloud. Little or no wind will reduce movement of the cloud
away from the source, while higher winds will cause rapid dispersion
of the cloud due to mixing of vapours with the ambient air.
LNG
Overview
Pipelines,
Strengthening the Backbone Natural gas pipelines are the 'backbone'
of the natural gas industry, providing the primary physical means of
delivering gas from sources of supply to areas of demand. Over the past
few decades, these vital transportation arteries have grown from local
supply chains to international and pan-continental networks.
Offshore:
The gas required to manufacture LNG is provided generally from offshore,
due to the large amount of gas requirements. A complex maze of offshore
platforms, pipelines and facilities handles the gas, which is produced
from two different kinds of reservoirs, each requiring a different processing
technique. The majority of gas production is from non-associated gas
reservoirs. This is generally produced at high pressure and can be directly
piped to shore. This rest could be produced from oil reservoirs, most
of which also have substantial gas contents. This gas is either in solution
with oil, or overlying the oil reservoir in gas caps.
Gas produced
in solution with oil is separated out on offshore platforms. It is at
low pressure and has to be compressed before it is piped Gas overlying
oil reservoirs can be produced in a similar way to non-associated gas,
but the underlying oil has to be recovered first using the natural pressure
of the gas to aid production.
Some of
the gas produced could be compressed and injected back into oil wells
as 'gas lift gas' to help lighten the oil and increase the rate of production.
This gas is continuously recovered and fed back into the gas system.
If the
gas is relatively high in carbon dioxide (CO2), and impurities, they
are removed before liquefaction.
Geology:
Oil and gas-bearing rock is typically sandstone, with shale acting
as an impermeable barrier against which oil and gas collects. Stacked
in complex layers fragmented by myriad vertical faults, the reservoirs
range in size from 16 billion cubic metres. A typical gas well might
penetrate 50 reservoirs vertically. This degree of complexity increases
the technical challenge of assessing reserves and extracting the gas.
Though
the LNG manufacturers need to have enough access to gas reserves to
meet long term LNG sales contracts. Nevertheless, exploration continues
to find both new fields and to build a better picture of current prospects.
One of the key tools for this work is three dimensional (3D) seismic
surveying, the application. The techniques, which have revolutionised
exploration and development work, enables explorers to build a much
more detailed picture of underlying rock formations and identify prospects
that remained hidden on conventional two-dimensional seismic surveys.
To process the mass of data that 3D seismic generates, one has to runs
the most powerful computer.
Varied
Approach In recent years, LNG plant design and reliability have improved
considerably. Current real costs of liquefaction are some 30% lower
than in the 1960s, through developments such as the use of very large
gas turbines.
Each liquefaction
in a plant is unique, reflecting technological developments over time
as well as different feed gas compositions, local site conditions and
the design and process preferences of the plant owners. Many components
or units, each with a specific treatment purpose, are joined together
to make a 'train'. Almost all LNG plants in the world now incorporate
at least two trains, which are operated in parallel and can be shut
down sequentially for periodic maintenance and repair. Provided the
plant is well designed and constructed, properly maintained and refurbished
as necessary, it should have a useful operating life of 40 or 50 years,
possibly longer.
