Drilling

Now that we have found a promising geological

structure and acquired the petroleum rights, it is
time to go ahead and drill.
Regardless of all the sophisticated geological and
geophysical mapping that has been done, the only
way to find out if there is any oil or gas present
under your land is to drill a well
• Anticlines and other potential hydrocarbon
traps can be located using seismic and other
techniques, but
• The only way to find out if petroleum is
present is to drill a well.
• Here we have two identical geologic structures
side by side and only one of them contains oil
• Only about one in ten wildcat wells is a
discovery
 A Wildcat Well
• The well that we are about to drill is called a
wildcat well.
• This means that we will be drilling into a geologic
structure in which no oil or gas has yet been
discovered.
• We will be attempting to discover a new oilfield.
• If a discovery is made on this geologic structure,
we will need to drill more wells (delineation
wells) to determine the size of our oilfield.
• Drilling wells is an expensive and risky business.
• The costs can run into millions of dollars and the
chance of success for wildcats is only about one
in ten
 Drilling the Well

• Drilling a well involves a whole new cast of

characters.
• The drilling engineer, the driller, the tool
pusher, the roustabouts, the roughnecks, the
logging crew and many others.
• We will not get into detail regarding Drilling
 Drilling The Well (cont…)
• The responsibilities of each of these individuals, as this
reaches beyond the scope of this manual, but we will
provide a general description of the drilling process
and its challenges.
• The overall objective of drilling is to bore a hole (the
well bore) into the ground until you penetrate a target
• Rock formation, that has been identified by the
geologists and geophysicists as having the potential to
contain commercial hydrocarbons.
• We know that the formations through which we will
be drilling may be porous and permeable, and may
contain fluids (oil, water or natural gas) at very high
pressures.
• If we were to drill into such a formation without taking
the appropriate precautions, the fluids would spew
violently out of the hole and we'd have what's called a
blowout
• Blowouts can be extremely dangerous as well
as damaging to the environment.
• In the early days of the oil industry, before
drilling technology had evolved significantly,
when a well struck oil it often blew out.
• These blowouts frequently caught fire,
destroyed the rig, and sometimes cost rig
workers their lives.
 Blowout in Texas in 1901
spewed 50,000 barrels per day for 9 days
• In today's drilling industry, blowouts are very
rare.
• The entire technology of today’s drilling
industry is focused on the ideal of drilling
safely and economically into the pressurized
underground formations, without allowing
any uncontrolled flow of fluids out of the well
• The well is allowed to flow through a series of
valves only under controlled conditions
• But how do we accomplish this?
• What are the basic techniques of oil well
drilling?
 Drilling Technology
• Drilling a well is, in many ways, similar to drilling a hole into a
• piece of wood using a rotating drill bit.
• The bit, in this case is much larger and is attached to the
bottom of a hollow steel pipe called a drill string (or drill pipe)
that can be thousands of metres long
• The drill string is rotated by an engine at the surface
• The rock drill bit is not the screw-shaped type, but is a roller-
type
• bit made up of three cone-shaped, toothed cutters
• As this bit rotates, under several tons of downward pressure, it
crushes the rock.
• The drill string is composed of a number of 9 metre lengths of
high strength steel pipe that are screwed together end to end
• As the well is deepened, the drill string is lengthened by
screwing additional lengths of pipe to the top
• But now a couple of questions should come to mind.
Drill String
Drill Bit
 Drilling mud
• (1) How do we get these pieces of crushed
rock (rock cuttings) up out of that hole, which
may be thousands of metres deep?
• (2) How do we keep the high pressured liquids
and gases, that may be present in the rock
formations, from blowing out?
• Both of these concerns are addressed by the.
stop
 Drilling mud
• Drilling mud, which is a mixture of clay, high density solids
(weighting material), water and chemical additives
• It is pumped down through the inside of the hollow drill string
• Because the drill bit, and hence the borehole, is larger in diameter
than the drill string, a hollow space (the annulus) exists between
the drill string and the walls of the borehole.
• The mud that is being pumped down squeezes through holes in the
drill bit and circulates back to the surface through the annulus,
carrying the rock cuttings along with it
• Back on the surface, the rock cuttings are mechanically removed
from the drilling mud in the “shale shaker” and can be examined by
the wellsite geologist.
• The same mud can then be recirculated through the drill string and
the process continues while drilling is in progress.
• Drilling mud is also the key to preventing blowouts.
• If the drill bit penetrated a porous and permeable formation
containing oil and gas under high pressure, the oil and gas
would naturally tend to rush out of the formation and up the
borehole.
• But in order to do this, it would have to overcome the
downward pressure exerted by the thousands of kilograms of
slowly circulating mud.
• What usually happens in such a case is that only small traces
of oil or gas will flow out of the formation and circulate back
to the surface with the mud.
• This result is actually useful to the wellsite geologist as oil
returning with the mud is a good indication that there are
hydrocarbons down below.
• Drilling mud also helps to cool and lubricate the drill bit and
provides a protective and stabilizing coating on the walls of
the borehole, which helps to seal off porous formations and
keep the wellbore from caving in
 Casing the Well
• Although the drilling mud does help prevent the wellbore from caving in
and does help to seal off porous formations that have been drilled, a more
permanent and reliable method to accomplish this is to case the well.
• Casing the well involves lining the borehole with steel pipe and cementing it
in place.
• To case a well, the drill string and bit are removed from the hole.
• The drill string, of course, cannot be removed in one piece.
• Every third joint in the drill string will be unscrewed and the stands of pipe
will be stacked against the derrick.
• When all of the drill string and the bit have been removed, the casing is
lowered into the mud- filled hole by screwing together the 12 metre lengths
of casing pipe end to end.
• The casing is slightly smaller in diameter than the borehole and is secured in
place by pumping cement into the annular space between the outside of
the casing and the borehole.
• When the cement sets, drilling will continue using a slightly smaller bit than
before.
• As the well is deepened, additional casings of concentrically smaller
diameters are added as needed to seal off the deeper formations.
 Logging the Well
• Although the wellsite geologist can get many hints of the
types of rock formations in the well from the rock cuttings,
and sometimes gets hints of the presence of oil or gas by
the small amounts that come up with the drilling mud,
more information is needed to properly analyse the rock
formations that have been drilled.
• To help obtain this information, the geologist has some
associates who go by the name of the logging crew.
• This logging crew does not come equipped with chainsaws
or axes, but with geophysical logging devices.
• These logging tools are lowered into the mud- filled hole
(usually prior to casing being set in that part of the hole)
and measure the electrical, acoustic, and radioactive
properties of the different layers of rock.
• The results of these logging measurements are analyzed to
determine which of these layers are porous and permeable,
and likely to contain hydrocarbons
 The Drill Stem Test
• In the old days before the use of drilling mud, the drilling crew immediately knew
when they had hit a good oil reservoir because the well would blow out.
• This would be an event of great excitement as well as great danger.
• Fortunately, things are done differently today. The excitement and tension of
finally determining if the well is a discovery are still present, but much of the
danger has been removed.
• After the geologist has analyzed the logs and identified a formation that appears
(from the log analysis) to be porous, permeable and contain hydrocarbons, the
excitement begins to build.
• This is the time of reckoning for the well. It is time to do a drill stem test (DST). The
DST involves the lowering of specialized equipment into the well bore that isolates
the formation of interest from the pressure exerted by the drilling mud so as to
allow the fluids within the formation to flow to surface. This determines the flow
capability and the type of fluids (oil, natural gas or water) present in the formation.
• The DST also records valuable information on the formation pressure, which is
critical to determining whether the well can produce at a high enough rate and for
a long enough time to be profitable. For example, if a well flowed oil at a rate of
100 barrels an hour for 24 hours and showed very little decrease in reservoir
pressure, one could conclude that this was a pretty good well and should produce
for a long time.
• If, however, at the end of the test the pressure had dropped off by 50%, it would
mean that this oil reservoir was small and could not produce for a long enough
time to be profitable. A well that cannot produce hydrocarbons in commercial
quantities is called a dry hole.
 The Blowout Preventor
• It should be evident, from our discussion thus far, that a drilling rig is a very
complex piece of machinery containing a broad range of specialized equipment.
• Although we won’t go into a detailed discussion on the workings of every part of a
drilling rig here, there is one special piece of equipment that is critical to the safe
operation of the rig and warrants special mention.
• Have you considered what would happen if the bit drilled into a porous and
permeable formation, in which the pressure was so great that the weight of the
mud column could not hold the formation fluids back?
• What would happen is that the fluids in the formation would enter the annulus
and start heading for the surface
• The first indication at the surface would be that the mud level would start to rise
in the mud tanks because the mud would be coming out of the hole faster than it
is being pumped back down the drill pipe.
• This is called a kick.
• If proper and decisive action is not taken by the driller, such a kick could lead to a
blowout.
• When a kick occurs the driller immediately activates the backup system - the
Blowout Preventor (BOP).
• The BOP is a system of powerful hydraulic rams anchored to the casing which can,
at the flick of a switch, be closed and sealed around the outside of the drill pipe
preventing any further flow of mud or formation fluid up the annulus.
• When the flow has stopped, weighting material is added to the mud making it
heavy enough to hold back the formation fluids.
• When this is accomplished, drilling resumes
 A Hostile Environment
• Today, offshore drilling takes place on the open ocean
and, in addition to dealing with the usual difficulties
associated with drilling onshore,
• It must also contend with waves, currents, fog and
deep water.
• In some areas, as on the Grand Banks, there are the
additional complications of cold temperatures,
icebergs, pack ice, storms that bring Winds of more
than 160 kilometres per hour, and waves that can be
more than 30 metres high.
• In this chapter, we will learn how the drilling industry
meets the challenges of drilling in such a hostile
environment.
 The Technology of Offshore Drilling
• Perhaps the most obvious difference between drilling offshore and
onshore is that an onshore rig sits directly on the surface through which it
is to drill.
• However an offshore rig can be floating anywhere from a few tens of
meters to a couple of thousand metres above the sea bed, and must
therefore be attached to the sea floor by a large steel pipe called a marine
riser
• The riser then acts as a conduit for the drill string and the drilling mud.
• In essence, it is an extension of the casing that allows the drilling mud and
rock cuttings to be isolated from the water column and circulated back to
the rig.
• Another complication in offshore drilling is that, since the sea is seldom
calm, the rig is constantly heaving and rolling in the swell.
• Tensioners and motion compensators allow the riser and drill string to
remain relatively stationary, and keep a constant weight on the drill bit
while the rig moves with the swell.
• But what of the winds and currents that tend to move the rig off the
drilling location? It is easy to imagine the problems that even a small
amount of drifting would cause when the drill pipe is extended thousands
of metres into the sea bed.
• The industry has adopted two solutions to this problem
• The first involves an elaborate system of 8 to 12 anchors,
each weighing up to 20 tons.
• Mooring lines comprised of heavy steel chains can extend a
mile or more, depending on water depth.
• A second system that is being increasingly used is the
dynamic positioning system or DPS.
• The DPS can either supplement or replace the anchors with
a computer controlled system of propellers or thrusters.
• Through an array of motion sensors, the computer can
immediately detect any movement of the rig and then
apply just enough power to the appropriate thrusters to
compensate and keep the rig exactly on location.
• This type of system makes it possible to drill in much
deeper water, where anchors would not be feasible.
• Wells can now be drilled in water depths exceeding 2000
metres.
• Another important advantage of DPS is that it allows the
drilling rig to quickly disconnect the riser and move off the
location in case of an emergency (such as the approach of
an iceberg).
• As you can imagine, the retrieval of several twenty-ton
anchors would be quite time consuming and would require
a lot more lead time when making the important decision
to leave the drilling location.
• The significance of such a decision becomes apparent when
one realizes that it costs in the neighbourhood of $250,000
per day to operate a rig on the Grand Banks.
• In this kind of operation, any time lost becomes very
expensive.
 Supply Vessels
• An offshore drilling rig located hundreds of
kilometres from land with a crew of about 100
people, needs a lot of supplies.
• These include food for the crew, fuel for the
engines and generators, sacks of cement and
sacks of bentonite and barite which are mixed
with water to make the drilling mud.
• It needs hundreds of tons of drill pipe, casing,
and, of course, the huge and massive sections of
pipe that are put together to form the riser.
• It is not always possible for a rig to store all of
these provisions on board and so each rig has a
supporting cast of two or three supply vessels.
• The supply vessels, in addition to delivering supplies
and equipment to the rig, perform other important
duties.
• They assist in the deployment and retrieval of anchors
and at least one vessel stands by the rig at all times in
case it must be abandoned.
• They sometimes participate in the transfer of
personnel to and from the drilling rig (the crew
changes every 3 weeks), although this is usually
handled by helicopters.
• They tow the rig, when it is not self-propelled, from
one location to the
• next and in some areas, as on the Grand Banks, they
are required to tow icebergs that may be drifting into
the vicinity of the rig.