Drilling

DRILLING

What will be covered in this chapter

• Overview of how a well is drilled
• The basic equipment for drilling a well
• Casing and cementing

1. Well Characteristics

To produce hydrocarbons it is necessary to drill a well, a conduit from the sub-surface reservoir to
the surface. Edwin Drake drilled the first well in Pennsylvania, USA in 1859, to a depth of 69½ ft.
Most wells now days are a lot deeper, since most reservoirs are found in the range 1000 – 5000 m,
with a few being shallower or deeper. At its bottom, the well usually has a diameter of about 8″ (20
cm), so a well is very long and thin. All operations linking the reservoir to the surface have to take
place within this long, thin, one-dimensional space.

There are roughly three million producing wells in the world, about a million in the USA, a million in
the former Soviet Union and a million in the rest of the world, producing about 70 million barrels of
oil per day. So world-wide the average well produces about 25 barrels of oil per day.

2. Well Path

The path of the well is generally not straight. In a desert environment, it may be possible to drill
straight vertically from the surface to the desired location in the reservoir. However, in most cases
wells are not straight, but “deviated” as shown on Figs. 1. A lot of wells may be drilled from a single
drilling location or platform. During drilling, the direction of drilling can be controlled, and wells can
be steered in any chosen direction. It is now common for a large part of the well to be horizontal
within the reservoir, thus increasing the contact between the reservoir and the well.

Fig.1 Deviated Drilling

With deviated drilling it is currently possible to

reach targets at a horizontal distance of 10 km. A
typical well has a diameter of about 20 cm and a
length of 1000 – 12000 m.

If the well is drilled horizontally in the reservoir,

the length of the horizontal section may be very
long (e.g. 1000 m).

Recently, wells have been constructed with branches in the wellbore (see Fig.2). Such wells are
called “multilaterals”.Note: When drawing a schematic diagram of a well, we always draw it as
vertical, for simplicity.

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Fig. 2 Multilateral Well

3. Sequence for drilling a well

For safety reasons, a well is always drilled in stages; as will be discussed below. It is not possible to
drill the well in one stage. After each stage of drilling, the hole is lined with a steel tube, as shown in
Fig.2. These steel tubes are called “casing”. Some of the earlier casings are often given special names
(stove pipe, conductor, surface pipe). After each casing has been put in place, cement is pumped
behind it (see Fig.3), to seal off the gap between the rock and the casing. The cementing process is
discussed later.

Fig.3 Staged drilling of a well, with

placement of casing and cementing
after each stage

A well with casing all the way to the bottom

of the well is called a “cased hole”. In some
wells, and particularly in long horizontal
wells and multilaterals, casing may not be
placed in the bottom section of the well, in
the reservoir itself. Such wells, without
casing in the reservoir, are called “open
holes”. An open hole may be chosen either
because it is difficult to place the last casing,
or because it may give improved flow of oil
into the well.

Sometimes the last casing does not extend

all the way from the bottom to the top, but
just hangs in the bottom part of the well.
This is called a conductor.

4. Drilling the well – downhole

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The well is drilled using a drillbit. These are of two general

types: roller-cone bits and
diamond PDC bits (see Fig. 4)

Roller-cone bits have 3

(sometimes 2) roller cutters
(cones) which rotate and crush
and scrape the rock. Diamond
(PDC) bits have no moving
parts, but industrial diamond
inserts which scrape the rock.

Fig.4 Roller-cone bits (left) and PDC bits (right)

The bits are attached at the end of the drillstring, which is a made up of 10 m lengths of hollow steel
drillpipe, diameter about 10 cm, screwed together. The drillstring goes all the way to surface. The bit
is rotated to cut the rock, either by rotating the drillstring or by using a hydraulic mud-motor (see
below).

5. Drilling the well – surface

At surface, the well is drilled using a drilling rig (see

Fig. 5) A rig consists of a derrick, hoisting
equipment, rotating equipment and pumps.
The rig rotates the top of the drillstring, and this
power is translated downhole to the bit.

As the
well is

drilled and gets deeper, the drillstring must be

lengthened. This is done by screwing a new piece
of drillpipe to the drillstring, as shown in Fig 6.

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Fig.5 Schematic of a Drilling Rig Fig. 6

Screwing on a new piece of drillpipe

The drillstring has to be pulled out of the hole if it is necessary to

change the drillbit, or when it is time to set casing. This process of
pulling the drillstring out of the hole is called “tripping out”. To
save time, the pieces of drillpipe are unscrewed only at every
third connection, and the lengths of 3 pieces of drillpipe (length 30
m) are stored in the derrick, as shown in Figs. 7 and 8. These are
then screwed together again (“tripping in”) when drilling starts
again.
Fig.7 Tripping Out

Fig. 8 Drillpipe stored in the

derrick after tripping out

6. Drilling Fluids

During drilling, fluid is pumped down the hollow drillstring, out

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through holes in the drillbit and back up the annulus between the rock and the drillstring (see Fig. 9).
This fluid is known as “drilling fluid”, “drilling mud” or “mud”.

The drilling fluid has five functions:

• Remove the pieces of rock cut by the drillbit (the cuttings)

• Cool the bit
• Lubricate the drillstring
• Provide pressure on the borehole wall to stop it collapsing
• Provide pressure on the fluids in the pores of the reservoir
rock, to stop the pore fluids flowing into the well

Fig. 9 Flow of drilling

fluid

The drilling fluid may also be used to supply hydraulic power to a mud motor (see later).

To remove the rock cuttings, the drilling fluid must be pumped fast enough and have sufficient
viscosity to carry the cuttings up the well.

The pressure P in the drilling mud at depth D is given by P = ρgD where ρ is the density of the fluid
and g the acceleration due to gravity. By changing the mud density, we can change the pressure. If
the mud pressure is too low, fluid will flow into the well from the surrounding rock. This is very
dangerous if this fluid is oil and extremely dangerous if it is gas. So it is necessary that
P > Pore-fluid pressure

But if P is greater than the pore pressure, drilling fluid will flow into the pores of the rock and be lost.
To prevent this, solid material is put into the drilling mud, and this “plasters” the rock surface, forming
a layer (called “filter cake”) which stops the flow of fluid into the pores (see chapter on Pressure
Control).

The properties of a drilling fluid are therefore:

• good cutting-carrying properties
• adjustable density to control the downhole pressure.
• good plastering properties to stop leak-off of drilling fluid

Drilling fluids and their properties are discussed in more detail in the Chapter on Oil Field Fluids

7. Drilling Fluid Treatment System

During drilling, the drilling fluid becomes contaminated with

the rock cuttings, oil and gas. The drilling rig therefore has a
treatment system, for keeping the drilling fluid in the required
condition.

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The path followed by the drilling fluid is

• From the mud storage tank to the mud pump
• down the drillstring and out through the bit
• up the annular space between the drillstring and the borehole, carrying the cuttings
• through the blowout preventer stack (see chapter on Pressure Control)
• to the “shale shaker”, a vibrating metal grid which filters out the larger rock cuttings
• to the settling tank, where small sand particles and cuttings settle out
• back to the mud storage tank
Fig. 10 Shale shaker for removing cuttings

The properties of the mud (viscosity, density) are continually checked, and fresh mud is added as
required to maintain the mud in good condition.

8. Cementing

The procedure for cementing a casing as follows, see Fig. 11

• The drillpipe is pulled

out of the well
• The well is still full of
drilling mud
• The casing is inserted
(just as for the drillpipe,
it is screwed together,
length by length).
• A float collar and a float
shoe have been installed
The are valves to
prevent flowback.
• Later the bottom and
top plug are pumped.
When they reach the
float collar, they act
together to seal off the
casing and prevent
further flow.
• Water (or a special fluid
called a spacer fluid) is
pumped into the casing,
followed by the bottom

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plug.

Fig.11 Cementing

• The cement slurry is then pumped into the well. The cement slurry weight is monitored
continuously during mix and samples are taken. The volume of cement mixed is usually controlled
by noting the amount of mix-water consumed.
• When all cement has been pumped, the top plug is released.
• The top plug is followed by the small amount of cement slurry which is in the flowlines, and a
small water spacer behind the cement.
• Drilling mud is pumped in after the water spacer at a pre-determined speed. (Normally greater
than 90m/min annular velocity is required to achieve turbulent flow in the annulus.) The pump
speed is reduced as the top plug approaches the float collar, so that it engages gently. Pumping
stops when the plug reaches the collar.
• After the cement has set, the float collar and plugs, the cement inside the casing and the float
shoe are drilled out.
• Drilling of the next section starts.

9. Directional Drilling

With a drillstring and bit, as described above, it is only possible to drill in a straight line. To achieve a
change in direction it is necessary to insert a steel wedge in the well (called a whipstock) which
forces the drillstring to change direction. After this change
of direction, the drillstring will carry on drilling in a straight
line. Note that the change of direction is small (a few
degrees), so the metal drillpipe can bend sufficiently.

Fig. 12 Using a Whipstock to change direction

Fig.13 Downhole mud motor

The downhole mud motor (Fig.13) was introduced in the

1980’s, and has been used extensively for the last 10
years. It allows continuous changes of direction, thus the
well can be steered to follow any desired trajectory.
Briefly, the mud motor works as follows. The drillbit can
be rotated independently of the drillstring, using the mud
motor, powered by the hydraulic pressure of the drilling
mud. A flexible joint (the bent housing in Fig.13) allows the direction of the drillbit to be set at a small
angle from the direction of the drillstring. If the drillstring is not rotated, then rotation of the drillbit will
result in a circular trajectory., with the drillstring “sliding” along the hole.

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Sliding motion can result in problems (the drillstring can get stuck, because it is not being rotated).
Recently, systems have been developed in which the drillstring is always rotated, but full steering is
possible. These are called “Rotary steerable systems” and are likely to be the way directional drilling
is done in the future.

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