Drilling for Natural Gas
People, for one reason
or another, have been digging into the earth's surface since the beginning of
recorded history. The first wells were mainly used for drinking water or
irrigation. These hand dug wells were usually shallow (relative to today's oil
and gas wells), only scratching the surface of the Earth. At first, tools of
stone and wood were used to dig these primitive wells, eventually, these tools
were replaced by metal tools that were more efficient. Debris from the well was
often hauled out of the hole in buckets. As early as 600 B. C. the Chinese were
using percussion tools to dig wells through brine formations. These wells used
bamboo shafts with a metal bit to pound through the layers of brine.
Once an exploration team
has determined a site for drilling, the location of the trap will determine, to
a large extent, the equipment used on the surface for drilling the well. If the
target is a relatively shallow formation, then a cable drilling rig might be
used to dig the well. However, deeper formations require the use of rotary
drilling rigs. The nature of the rock formations that must be drilled through
will also be a factor in determining the kind of drilling equipment that is
chosen for a certain well.
cable-tool drilling, is characterized by repeatedly raising and dropping a heavy
metal bit into the Earth's surface- eventually pounding a hole downwards into
the ground. This process is still widely used today for drilling water wells.
Periodically, bailers, or containers that remove debris, must be lowered into
the shaft to clear out loose soil and rock chips so that the bit will have a
clear shot at the bottom of the well.
David and Joseph Ruffner
are credited with an important development in well drilling: the first well that
used cashing for the sides to prevent collapse. They were drilling through brine
near Charleston, West Virginia where their holes kept collapsing in and ruining
their well. To remedy the situation, they used hollow tree trunks to reenforce
the sidewalls of the well. Today, steel pipe serves the same purpose.
Egyptians are credited with
another first in drilling technology: they used rotary drilling mechanisms to
drill into the Earth as early as 3000 B.C.. Much later, in 1500 Leonardo DaVinci
developed a design for a drilling rig that is similar to many of today's rigs.
Today, about 85% of the wells drilled use conventional rotary drilling rigs to
dig their deep wells.
The rotary drilling
method differs from the percussion method in that it relies on a sharp bit to
literally drill through earth and rock layers. It is also used to lift waste
materials out of the wellbore. On the surface, a complex system of cables,
engines, support mechanisms, lubricating devices and pulleys control the
rotation of the bit below the surface, as well as keeping the bit lubricated,
and bringing debris out of the wellbore.
Underneath the surface,
the bit is attached to a long drill pipe. Its job is to break up the bottom
layer of rock or Earth to allow the well to progress deeper into the crust.
There are many different varieties of drilling bits, each with a
different specialty. Some major varieties are Steel Tooth Rotary Bits, Insert
Bits, Polycrystalline Diamond Compact Bits, and Diamond Bits. In addition there
are hybrid bits that combine features from different variations. When drilling
deep wells through different layers of rock, several different bits might be
used on a single well. The decision to change bits is not made easily, however,
because it takes time and extra equipment to remove the entire drill pipe and
bit, and then to reassemble it using a different bit. Sometimes, changing a bit
could involve removing close to a mile of drill piping.
Drilling fluids also play an
important role in rotary drilling. These fluids cool the bit, remove cuttings
and debris, and coat the wellbore with a cake. Most fluids have a clay base, and
are customized for the specific formations that are encountered at a given site.
The cake that forms from the fluids serves to coat the walls of the wellbore
until a steel casting can be put in place to prevent collapse.
Advances in the latter
part of the century have allowed drilling companies to cut costs, gather more
information about well holes, and increase the value of wells through a variety
of technologies. In the past, all information about the depth and conditions at
any point below the surface in a well had to be obtained from the surface
analysis of materials that were brought up by the drilling rig. This was
unsatisfactory because in deeper wells there was often a considerable delay
between the time when a bit contacted a new layer of rock, or new conditions,
and when evidence of this contact came to the surface.
A new class of technology, known
as measurement while drilling (MWD), includes all devices that help drilling
crews by providing information about down-hole conditions. They often constantly
provide information to crews at the surface, thus eliminating the need to cease
drilling operations in order to take measurements, and they also eliminate the
amount of lag time between contact with new conditions and the surface crew's
awareness of the change in drilling environment. These technologies have
increased drilling safety, as well as efficiency.
One of the most exciting
and productive new technologies is horizontal drilling. It is heralded today as,
"...causing the greatest change in the industry since the invention of the
rotary bit." The first patent for horizontal drilling tools was issued in
1891 to Robert E. Lee (the Civil War general). Lee drilled a horizontal drain
for a vertical oil well.
The notion of drilling
wells that are not aligned vertically with the surface is not new. For years,
companies have been employing slant drilling to drill wells at an angle in order
to reach areas where rigs could not feasibly be placed. Slant or deviation
drilling has also been traditionally utilized in offshore sites where the
expense of platforms prohibits the construction of multiple platforms. In order
to reduce costs, several slanted wells are drilled from a single platform,
reaching several different traps or oil fields. In some cases 20 or more slanted
wells can be drilled from a single platform. The difference between slant wells
and today's horizontal wells is that slant wells would take as much as 2000ft
or more to bend from vertical to horizontal. Today's technology allows a 90°
shift within a few feet. The benefits of horizontal drilling include:
A horizontal well can penetrate more than one reservoir, and can
produce up to six or seven times as much gas or oil from as an equivalent
Salt water production can be minimalized
The 'traditional' primary recovery life of a well can be increased
from 25% of the oil in place to 50 to 75%.
Oil and gas can be taken from a well, even while drilling
Lastly, automation of
drilling rigs has reduced the cost of manpower for a given drilling operation.
Automated rigs are safer and less expensive to operate than traditional drilling
rigs because people no longer have to do many of the hazardous jobs involved in
drilling a well. Automated rigs also reduce drilling time by 12 to 15%, saving
time and money for the gas or oil producer.