Learning Experience 1.6
Drilling
The only sure way to find oil or gas under the ground is by actually drilling a hole
down through the earth, using special drilling equipment. We commonly call a hole
like this a “well”. Geoscientists and engineers are interested to find out as much
information as possible, to answer questions such as:
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Is there any oil or gas present?
In what kind of rock is the oil or gas trapped?
How deep is the oil or gas?
How thick is the oil or gas reservoir, and so, how much is there?
At what pressure is the oil or gas?
Drilling involves a lot of special equipment, some of it designed to make drilling
easier, faster and safer, and some of it designed to take scientific measurements. The
scientific measurements involve collecting data from in and around the hole, deep
under the ground, so that the geoscientists and engineers can answer the questions
listed above.
Drilling engineers (drillers) raise and lower the “drill pipe” as required using powerful
engines, within the “derrick” structure, shown above. At the bottom end of the drill
pipe is a “drill bit”, which is a hard metal cutting tool. The rotary drive makes the
drill pipe and drill bit rotate and this action causes the drill bit to cut down and make a
hole into the earth. As the well is drilled deeper and deeper it encounters higher and
higher pressures.
Drillers have to prevent high pressure water, oil and gas unexpectedly moving
violently up the hole to the surface, causing what is called a “blow-out”. They pump
heavy mud down the hole from the “mud pits”, using the “mud pumps” to force the
water, oil and gas to stay underground. Special equipment called “blow-out
prevention equipment” is also installed, which is used to close off the well if water, oil
and gas starts to shoot up the drill pipe. To stop the sides of the hole collapsing
inwards, drillers insert a metal tube called the “casing” into the hole and held in place
using cement pumped down from the surface.
In order to collect physical measurements of the rock that has been drilled through,
and of the oil and gas that has been found, special electronic measuring devices are
lowered down the well, either attached to the drill pipe, or attached to metal cables.
This activity is called “logging” and the measuring devices are called “logging tools”.
Small broken chips of rock, which come up the hole to the surface and are known as
“cuttings”, are collected at the surface. Special down-hole cutting equipment can also
be used to produce solid rock “cores”. Geologists use the logging, cuttings and core
results together to describe the properties of the rock, such as its type, age, density,
porosity and which kinds of fluids it contains- whether water, oil or gas.
When drilling at sea, equipment similar to that shown and described above is mounted
on a floating vessel, together with accommodation for the geoscientists, engineers,
and crew. In shallow water, “jack-up” rigs are used, in which the platform holding
the equipment and accommodation rests on three or four strong steel legs. The legs
can be raised and lowered, depending on the water depth.
In deeper water, “semi-submersible” rigs are used, in which the legs have floating
pontoons attached, so that the rig actually floats. There is also an engine, so that the
rig can be propelled around like a boat from one drilling location to the next. Whilst a
well is being drilled, anchors are used to make sure the rig does not move too much in
the wind and sea currents.
In very deep water, typically greater than 1500 m, drilling ships are used. In this case,
the drilling equipment is mounted on a ship. Rather than using anchors, the ship uses
special water thrusters and GPS technology to make sure the ship stays relatively still
whilst drilling.
Wells used to explore for new accumulations of gas and oil are called “exploration
wells”. Once an accumulation has been discovered, further wells are drilled to collect
more data on the properties of the field, such as those listed above. These are called
“appraisal wells”.
Finally, if the company decides to produce hydrocarbons from the field, then
“production wells” are drilled. At the bottom of such wells, perforations or meshes
are constructed using special tools, which allow the oil and gas out of the rocks and up
the well bore to the surface, where they may be produced and transported to the
refinery or gas plant for processing.
Often, the oil or gas will come out of its own accord, due to the high pressure found at
these great depths below the surface. If the pressure too low, the oil and gas may need
a little help. With oil fields, this is done by injecting water, gas or even steam into the
reservoir down additional wells called “injection wells” to push the oil out. Gas at
low pressure is often extracted by ‘sucking’ it out, using huge turbine engines at the
surface. The following experimental activity illustrates how some of these processes
work.
A Model Oil Well
Source: http://earthnet-geonet.ca/activities/activity9_e.php
Objectives:
1. To demonstrate how a fluid such as oil or water can flow out of an
underground reservoir and up to the surface via a well.
2. To discover the characteristics of a reservoir (porosity/permeability) and the
important characteristics of a seal.
3. To investigate removal of fluids using a well.
Background Information:
Some sedimentary rocks are filled with fluid. At the time of deposition, water
associated with the sediment grains is trapped within the pores. Burial and subsequent
hardening will seal much of the water within the sediment. The migration of
hydrocarbons will often displace the water where porosity and permeability permits.
Movement of fluids within a sediment layer which possesses permeability is primarily
controlled by pressure. Fluids will flow upward (toward lower pressure) until they
reach an impermeable layer (seal).
Materials:
 a clear container preferably 12-15 cm (minimum) across the bottom (large
beaker, juice bottle or small aquarium);
 2 glass tubes or transparent drinking straws
 a funnel
 a piece of plastic (rubber) tubing which will fit both the tubing (straws) and
the funnel
 aquarium or garden gravel (approx. 5 mm sized grains)
 sand
 Plasticine or "Play Dough" to simulate clay
 water (food colouring should be added)
 cooking oil
Procedure:
1. Fit one tube (straw) with the rubber tubing and funnel.
2. Place the aquarium gravel (coarse sand) in the bottom of the container and fill
to a height of 8-10 cm.
3. Make a layer of "clay" at least 2 cm in thickness that approximately fits the
shape of the container. Shape holes in the clay on opposite sides of the layer,
that fit around the tubing.
4. Place the “clay” layer on top of the gravel. Push each piece of tubing through
the holes to a depth of 2 cm below the base of the clay. The tubes should be at
opposite sides of the container, next to the outside so the fluid may be
observed.
5. Seal carefully around each tube and against the side of the container, ensuring
that there are no visible gaps or cracks.
6. Add a layer of sand (3-4 cm) on top of the "clay" layer.
7. Pour coloured water into the funnel and observe the flow of water. Add more
water until it fills one third of the gravel layer. Observe the results.
8. Gradually add cooking oil to the funnel until the fluid level reaches the base of
the “clay” layer. Observe the initial results and again after 5-10 minutes.
9. Using your mouth gently blow air into the funnel and observe what happens.
Blow harder and observe.
Source: http://earthnet-geonet.ca/activities/activity9_e.php
Questions:
1. Make a sketch of the model and show the path of the water. What happened to the
air between the pores of the gravel as you poured in the water?
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2. What role did the “clay” layer play? What would have happened if it was not there?
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3. What happened to the oil initially, then after a few minutes? Explain your
observations.
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4. What effect did you have when you blew into the funnel? Explain your observations.
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5. What happened when you blew harder into the funnel? Explain your observations.
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6. Assuming the liquid was already in the gravel layer, what other means could be used
to remove it?
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