Presentation
Drilling a well
How wells are drilled
• The most common method of drilling wells uses rotary drilling (Figure 1). A
drilling bit is attached to the end of a long string of jointed, hollow drill
pipe, and the whole assembly is rotated by a motorized turntable at the
surface, the rotary table. Modern rigs use a top drive system for rotating the
drill pipe, an assembly that is guided up and down rails on the derrick. The
rotating bit cuts or crushes the rock. Drilling mud, consisting of water or an
oil-water mixture, solids, and various additives, is circulated down through
the drill pipe and out through nozzles in the drilling bit. The mud returns to
the surface up the annulus, the space outside of the drill pipe. The mud
lubricates the bit, prevents it from getting too hot because of friction, and
lifts the drilled rock cuttings up the hole. It should be dense enough to
overbalance any high-pressure formations encountered while drilling. If it
fails in this last action, the fluid in the formation will displace the mud up
the hole. This is called a kick. Should this hazardous situation not be dealt
with quickly, hydrocarbons will exit at the surface and a blowout results.
Scenes of oilmen dancing with glee as oil gushes over the drilling rig are for
the cinema only. In reality, oil field professionals are acutely aware of the
danger involved in the combustion and explosive blast that can result from
a hydrocarbon blowout.
Figure 1 The drilling
bit, attached to the end
of the drill pipe,
rotates, cutting and
crushing the rock
underneath it. Mud
passes out through the
nozzles in the drill bit,
cools the bit, and acts
to lift the rock cuttings
up to the surface.
• The scale and cost of a drilling operation differs between wells
onshore and those offshore. An onshore well is drilled with a
relatively cheap land rig (Figure 2); offshore, the operation is
several times more expensive.
• In shallow water, typically about 6–45 m (20–150 ft) deep,
drilling is conducted by a jackup rig. A jackup is a rig that has
three or more legs that sit on the sea floor. In moderately deep
water (more than 45 m [150 ft] deep), a floating or
semisubmersible rig is used. The semisubmersible rig is kept
in place by several anchors.[2]
• In deep water, a drill ship is the preferred option. Deep water
is defined as water depths between 500 and 2000 m (1640
and 6562 ft).[3] The drill ship is maintained in place by
dynamic positioning. Computers constantly calculate the
position of the drill ship using global positioning system
technology or in response to signals from transducers on the
sea bed. Signals are sent to propellers and lateral thrusters on
the sides of the vessel. These readjust the location of the ship
to keep it stable against the forces of wind and water currents.
Figure 2 Various types
of drilling operations,
offshore and onshore.
Drill ship Jack Ryan
courtesy
of BP (www.bp.com).
Jackup rig courtesy of
Maersk Oil and Gas
(www.media.maersk.co
m). Semisubmersible
rig and land rig in the
Sahara Desert, Libya,
courtesy of Woodside
Drilling problems
• Occasionally, something will go wrong and a piece of equipment is lost
down the well. For example, the drill pipe may twist off somewhere along its
length and fall to the bottom of the hole. The drilling operation will come to
a halt unless the foreign object or fish, as it is known, is "fished" out; that is,
physically removed from the hole.[5] Specialist tools are available for fishing
operations. Sometimes the fishing operation can last many days.
• Every now and again, the hole will collapse in on itself. This will happen
where the earth stresses exceed the rock strength. Salt sections or shale
sections at shallow depths containing water-sensitive clays are prone to
this. Water-sensitive clays can expand by reacting with drilling fluids,
particularly low salinity muds. This can cause the borehole wall to founder
and bury the drill bit irretrievably. A decision may then be made to branch
off from what hole is left, and this is called sidetracking.
• Another problem that can occur is lost circulation, whereby the drilling mud
is lost in large quantities into a fracture or a highly permeable interval.
Adding fibrous material to the mud will solve the problem. This clogs up the
lost circulation zone and prevents any further losses.
Drilling operation
• A well starts by being spudded as the drill bit encounters the first bit
of soil or subsea sediment. A well will not be drilled all the way
through in one go; instead, there will be several stages of drilling.
Each section will involve drilling the hole to a certain depth and
then running in and cementing metal casing onto the rock surface of
the borehole wall before going any farther (Figure 3). The simplest
reason for doing this is to prevent poorly consolidated sediment
from collapsing once the well has been drilled over a long interval,
although there may also be good reasons for isolating certain
problem formations.
• A typical well will have a similar geometry to an inverted telescope,
with the hole size and casing diameter decreasing incrementally
down the hole. Typical hole sizes and casing diameters are 36-in.
(91.44-cm) hole, 24-in. (60.96-cm) hole, cased with 18 5/8-in.
(47.29-cm) or 20-in. (50.8-cm) casing; 17 1/2-in. (44.45-cm) hole,
cased with 13 3/8-in. (33.95-cm) casing; 12 1/4-in. (31.11-cm) hole,
cased with 9 5/8-in. (24.43-cm) casing; and 8 1/2-in. (21.59-cm)
hole, cased with a 7-in. (17.78-cm) liner. A liner is a type of casing
that is not run all the way up the hole; instead, it is hung off inside
the lower part of a casing string (Figure 3).
Figure 3 A well is not drilled all
the way through. Metal casing
strings are run to isolate specific
sections of the hole before drilling
further. Sometimes the subsurface
team will require the reservoir
interval to be cored. This is carried
out with a special coring barrel
attached to the end of the drilling
assembly once the drill bit has
been removed. A doughnut shaped
coring head will cut a cylinder of
reservoir rock, and the cut core will
slide into the coring barrel,
typically about 18–27 m (60–90 ft)
long. Once full, the core barrel is
pulled back up to the surface for
retrieval. Several coring trips may
be required to core a reservoir
interval of interest. Given the trip
time for coring and the expensive
rig day rates, a coring operation is
costly.
• Figure 4 When a
perforated liner
completion is
run, the reservoir
interval is first
isolated from the
wellbore by the
cemented liner.
The liner is then
selectively
perforated with
holes so as to
allow fluid to flow
into the wellbore
from the specific
zones required
for production.
• It is necessary to change out the bit frequently because it will
become worn and inefficient after several days of drilling.
When this happens, the entire drill pipe needs to be pulled out
of the wellbore and then run in with a new bit attached. This
operation is known as tripping. A two-way trip, or round trip,
can take 12 hr or more in the deeper sections of the well.
• Specialist service personnel called mud loggers monitor the
drilling parameters and collect the drill cuttings for analysis.
There may also be a well site geologist present at the rig site
who will draw up a lithology log from examination of the
cuttings. The objective is to analyze the lithostratigraphy of
the interval being drilled in order to help make operational
decisions, such as when to run casing. The well site geologist
will also examine the cuttings for indications of hydrocarbon
shows. An ultraviolet light source will be used to check for
hydrocarbon fluorescence in the samples, a sign that oil is
present.