110
The Last Resource
thick layers of common salt and layers of limestone. Where the
impermeable layers overlap the coal measures, they have trapped
the hydrocarbons in the sandstone layer sandwiched between them.
In some places the Zechstein rocks are themselves broken and
porous and where capped by rock salt have in their turn become
reservoirs for natural gas. Some gas has been found in even higher
layers of the North Sea rocks (see gure 10).
The main constituent of natural gas is methane (‘marsh gas’).
The other constituents vary considerably from one gas eld to
another. North Sea gas is on average 90 to 95 per
cent methane with
about 4 per cent nitrogen, small amounts of more complex liquid
hydrocarbons, and traces of benzene and sulphur compounds. There
is also a considerable amount of water mixed in with the gas. All of
the solids, and some of the water, are usually removed at the well—
head and, before piping the gas ashore, other substances may be
added to prevent hydrates from forming a scaly deposit inside the
pipeline. As the gas passes towards the shore installations, liquids
separate out and collect at low points in the pipe. Periodically these
collections of impurities are swept away by sending spheres or other
plugs, known in the business as ‘Go-devils’ or ‘Pigs’, along the pipe
to push them out. After the removal of any remaining undesirable
impurities, at the shore terminal, the gas is ready for conventional
processing, distribution or storage.
The search for oil in the Gulf of Mexico has led to development of
_
another mineral resource. Drilling down to a promising salt dome,
the Humble Oil Company of the United States came across con—
siderable quantities of sulphur in the limestone capping the dome.
The deposit, which stretches over several hundred acres and varies
in thickness from 220 to 425 feet, is the third largest in the United
States. lt lies under about 2,000 feet of sediments about seven miles
seaward of Grand Isle, Louisiana. The sulphur is now being mined
using a modied Frasch process. Dr Herman Frasch, a petroleum
engineer, came to the aid of sulphur miners in the 18905. The miners
had found extensive terrestrial deposits of sulphur but the lethal
hydrogen sulphide gas found with the sulphur prevented it from
being excavated. Frasch hit upon the idea of making use of the low
melting point of
not, he argued, send in hot water
instead of men to melt and ush out the sulphur? By the early
1900s his process was a commercial success.
The secret of its success is a special ‘threc-in—one’ pipe which is
The Last Resource
thick layers of common salt and layers of limestone. Where the
impermeable layers overlap the coal measures, they have trapped
the hydrocarbons in the sandstone layer sandwiched between them.
In some places the Zechstein rocks are themselves broken and
porous and where capped by rock salt have in their turn become
reservoirs for natural gas. Some gas has been found in even higher
layers of the North Sea rocks (see gure 10).
The main constituent of natural gas is methane (‘marsh gas’).
The other constituents vary considerably from one gas eld to
another. North Sea gas is on average 90 to 95 per
cent methane with
about 4 per cent nitrogen, small amounts of more complex liquid
hydrocarbons, and traces of benzene and sulphur compounds. There
is also a considerable amount of water mixed in with the gas. All of
the solids, and some of the water, are usually removed at the well—
head and, before piping the gas ashore, other substances may be
added to prevent hydrates from forming a scaly deposit inside the
pipeline. As the gas passes towards the shore installations, liquids
separate out and collect at low points in the pipe. Periodically these
collections of impurities are swept away by sending spheres or other
plugs, known in the business as ‘Go-devils’ or ‘Pigs’, along the pipe
to push them out. After the removal of any remaining undesirable
impurities, at the shore terminal, the gas is ready for conventional
processing, distribution or storage.
The search for oil in the Gulf of Mexico has led to development of
_
another mineral resource. Drilling down to a promising salt dome,
the Humble Oil Company of the United States came across con—
siderable quantities of sulphur in the limestone capping the dome.
The deposit, which stretches over several hundred acres and varies
in thickness from 220 to 425 feet, is the third largest in the United
States. lt lies under about 2,000 feet of sediments about seven miles
seaward of Grand Isle, Louisiana. The sulphur is now being mined
using a modied Frasch process. Dr Herman Frasch, a petroleum
engineer, came to the aid of sulphur miners in the 18905. The miners
had found extensive terrestrial deposits of sulphur but the lethal
hydrogen sulphide gas found with the sulphur prevented it from
being excavated. Frasch hit upon the idea of making use of the low
melting point of
not, he argued, send in hot water
instead of men to melt and ush out the sulphur? By the early
1900s his process was a commercial success.
The secret of its success is a special ‘threc-in—one’ pipe which is