7 WHAT DOES A BASIC DRILL STRING FOR VERTICAL DRILLING LOOK LIKE?
7
WHAT DOES A BASIC DRILL STRING FOR
VERTICAL DRILLING LOOK LIKE?
A
drill string for a deep borehole is a hollow shaft comprised of individual drill pipes and
can be many kilometers long. Its purpose is to connect the drill bit in the ground both
mechanically and hydraulically to the drilling rig at the surface (illustration 24). From the
point of view of a mechanical engineer, the drill string is quite an awkward structure. The
length easily exceeds the diameter by a coefficient of 30,000 or more. Therefore, we are
dealing with a very extreme component. Using this shaky hollow shaft, we now want to
transmit several hundred horsepower of driving power to the drill bit at the bottom. At
the same time, we want to pump hundreds of gallons of viscous mud per minute down
the inner core while also drilling curves in any direction with this string of steel pipe.
A drill string, which simply consists of similar drill pipes screwed together, is not capable of accomplishing this task. Numerous other components
and special tools are required in the string in order
to do this. Each component serves a different purpose. In the course of this chapter, we will take a closer
look at the drill string and get to its components
better.
For the purpose of simplification, let’s first
Illustration 24: Pipe
assume that we would like to drill a vertical hole,
rack viewed from
thus taking the shortest path from the drilling rig
work floor
straight down.
7 WHAT DOES A BASIC DRILL STRING FOR VERTICAL DRILLING LOOK LIKE?
7.1 DRILL BIT
ROLLER CONE BIT OR ROCK BIT
Roller Cone Bit oder Rock Bit 59
Diamond Bit 60
Which is the better bit? 61
R
T
he drill bit is located at the bottom end of the drill string. It cuts the rock, creating
cuttings which are flushed to the surface by the drilling mud. It is crucial to match the
best bit for the rock and also to optimize the drilling parameters. If the cuttings are too
coarse, they cannot be removed steadily or completely from the borehole. If, on the other
hand, the cuttings are too fine, we have obviously wasted way too much of our valuable
operating power at the bit to pulverize the rock rather than using it to “make hole” (illustration 14).
The drill bit has an unbelievably difficult life.
From above, the drill string presses with multiple
tons of weight and below is the rock, often extremely hard, that is supposed to be drilled. Depending
on the type of bit and how it is driven, it rotates at
a rate of 60 to 1,000 revolutions per minute (RPM),
often performing several millions of revolutions during its lifespan and drilling many kilometers, sometimes in one run. Of course, it is supposed to remain
Illustration 25:
sharp for a long time, to drill fast and, as desired by the customer, to be
Roller cone bit for
reasonably affordable.
hard (left) and
Obviously not all of these wishes can be simultaneously fulfilled
softer formations
and, as in real life, one has to make compromises.
(middle), as well as
Drill bits can be divided into two major groups: roller cone and
a PDC bit (right)
diamond bits (illustration 25).
oller cone bits were invented more than a hundred years ago. Mostly they are equipped with three cones fitted with teeth. If the bit is pressed on the bottom and the drill
string is set into rotation, the cones will roll over the formation and break the rock.
Due to the fact that there are many different types of formations, there are also
many types of roller cone bits to match the specific properties of those formations. For
obvious reasons, we cannot go into every detail of the construction of those roller cone
bits, but we would at least like to point out the differences between tooth bits and insert
bits.
Tooth bits are used in softer formations. As the name suggests, their cones have
long teeth. When manufactured, the cones, as well as their teeth, are milled out of solid
pieces of raw material. Basically, a tooth on the bit works like the spade we use for gardening at home. The blade, or the tooth, is pressed firmly into the ground and then as much
material as possible is removed from the ground by the tilting action of the blade.
Insert bits, on the other hand, are used in hard to very hard formations. Their
cones are equipped with short spiky inserts made of hard metal. When they are rotated
on the bottom, only some of the numerous spikes come into contact with the formation
at the same time. This results in more force being applied to the formation, which then
causes the brittle rock to break under the high pressure.
Roller cone bits are versatile and comparably low in price. First of all, this is very nice for the
user. Much less favorable, however, is the fact that
the delicate ball bearings have a limited lifespan.
These bearings are necessary for cones to rotate
on the bit. Obviously, if a bit is composed of many
moving parts, there is a chance that something can
break. Therefore, roller cone bits do not last as long
as we might desire. After one or two days of drilling
in a challenging formation with a high load, the life
of a roller cone bit will come to an end, despite its careful and robust Illustration 26:
construction. You have to be careful that the cones do not come loose, Roller cone bit
or in extreme cases, fall off (illustration 26). Drilling cannot continue without and with
when there is junk in the borehole since every piece of hard metal will roller cones
damage the next bit and quickly render it useless.
Therefore, any lost cones have to be fished out of the borehole before the next
bit is run in the hole. These fishing attempts will cost the drilling company a lot of time
and money, which is highly undesirable. One could, of course, exchange the bit after only a
few hours of drilling in order to avoid anything falling off the bit. However, this also would
not be such a good idea because the constant tripping out and in of the drill string would
cost a lot of time, making the whole effort quite expensive again.
7 WHAT DOES A BASIC DRILL STRING FOR VERTICAL DRILLING LOOK LIKE?
DIAMOND BIT
D
iamonds are harder than any other material. Thus, it is suitable to use diamonds to cut
rock. In the 1950s bits were already being built that were covered with natural diamonds and could cut the rock like metal on a lath. The great advantage to roller cone bits
was that they did not have any movable parts, resulting in a much longer service life. On
the other hand, manufacturing those natural diamond bits was very expensive because
they required a large amount of diamonds in addition to a high amount of manual labor.
Since every natural diamond has a different shape, they had to be fit to the bit individually.
In the 1980’s the mass production of industrial diamonds started. These were
much cheaper than natural diamonds and it was possible to produce them in standardized sizes that were much easier to work with. Most suitable for the construction of drill
bits are Polycrystalline Diamond Cutters, or PDC’s. These consist of mostly triangular or
round shaped synthetic diamond plates that are soldered onto metal carriers. PDC bits
are equipped with a number of ribs, known as “blades” by experts. The cutters are fixed
to the front edge of the blades (illustration 25). PDC bits, along with roller cone bits, are
the most frequently used drilling bits in the field today. Often they can be used for a week
or even longer without losing their cutting edge.
Some formations are so abrasive and hard that they cannot be successfully cut
using either PDC bits or roller cone bits. The only way to effectively work on these formations is to sort of “sand them down.” To do this, special bits were developed that consist
of a matrix interspersed with minute splinters of diamonds. As the softer matrix is worn
off by the formation, new splinters of diamond appear and work on the rock. This type of
bit is called an “impregnated bit.”
However, they represent only a very small group and have a very small market
share percentage.
WHICH BIT IS BETTER?
O
f course, diamond bits are much more expensive than roller cone bits. The money needed to purchase a standard diamond bit would be enough to buy a good middle class
car, or five to ten roller cone bits. But since a diamond bit normally lasts so much longer
than a roller cone bit, one or more expensive roundtrips can be saved. The determination
of whether a roller cone bit or a diamond bit is the more economical way to drill a well can
only be made on a case by case basis. This decision depends primarily on the rock, as well
as other conditions.
On extremely expensive offshore rigs with rental costs easily reaching one million
dollars per day, the choice is obvious. If using a diamond bit can save even one roundtrip
that would cost one million dollars, I would gladly pay several ten thousand more for the
bit!
On simple and cost effective land rigs, time is not quite as important. At a daily
rate of 10,000 dollars, I’d rather accept having to take a roundtrip in order to swap out a
cheap bit, instead of purchasing a relatively outrageously expensive diamond bit.
The diameter of the hole that is to be drilled also has to be taken into consideration when selecting the bit. In the larger diameter sections in the upper part of the hole,
a roller cone bit will likely be used because such a large diamond bit would require a huge
amount of diamonds. This cannot be afforded even on an offshore rig! In the lower section
of the hole, where the diameter is smaller, it pays to use a diamond bit since the manufacturing of minute roller bearings would be too complicated.
As you can see, the question of which bit is the best cannot be easily answered
because it depends on a multitude of conditions. The selection of the correct drill bit is
therefore gladly left to the appropriate service companies.
7.2 DRILL PIPES
A
fter we have selected the correct bit for drilling the formation
down in the hole, we have to connect it somehow to the drilling
rig at the surface so that we can send the mud down and transmit
rotary power. What would be more natural than to use simple pipes
outfitted with thread connectors on either end so that they can be
screwed together? These pipes are called drill pipes. Like almost all
the components in the drill string, they have a length of about 30
feet each (illustration 27).
The actual drill pipe is relatively narrow with a diameter which
is standardized, as is almost everything in the drilling business. Both
ends are welded with connectors that are larger in diameter. One
connector has a female, or box thread, the other one has a matching
male, or pin thread. The threads of drilling equipment are almost exclusively tapered threads, meaning the diameter of the pin decreases
towards the free end. The box threads are correspondingly shaped.
Tapered threads are more robust than cylindrical threads and are
therefore better suited for the rough operating conditions on a drilling rig. Obviously the threaded ends of the pipe are hollow so that
the mud can flow freely through the drill pipe.
Illustration 27:
Drill pipes
(by R. Ritschel)
7 WHAT DOES A BASIC DRILL STRING FOR VERTICAL DRILLING LOOK LIKE?
Illustration 28:
Roughnecks using
“make-up” tongs.
Multiple drill pipe sections are screwed together to make up the drill string. In order to screw
them together, large “make-up” tongs hang from
the derrick above the work floor and are used by the
roughnecks (illustration 28).
A drill string is relatively thin and flexible,
therefore no downward pressure must be applied
from above. If pressure was applied, it would bend
and, in the worst case, could even lock up in the
borehole. Thus, the drill string is put together in a
way so that it is constantly kept under tension. In order to provide the
tension necessary at all times, heavy weights, known as drill collars,
are attached to the lower end.
Sculpture in a shopping mall in Edmonton, Canada
7.3 DRILL COLLARS
T
he bit down at the end of the drill string has to be pushed strongly into the rock so that
it drills quickly. But where does this weight on the bit (WOB) actually come from? No
weight can be transmitted to the bit via the thin drill string because it will buckle or give
way immediately when it is pushed from the surface. It will eventually
jam up the borehole, making it impossible to push forward.
Therefore, one has to provide the WOB using so-called drill collars which are located on the lower end of the drill string. Drill collars
are especially thick walled and, consequently, heavy drill pipes. While
their considerable weight presses the bit into the rock, it also keeps
the upper part of the drill string under constant tension, thus keeping
it from buckling. As a result, drill collars are an essential part of every
bottom hole assembly.
Most drill collars have grooves on their outer diameter along
their body that are spiral shaped (illustration 29). This is because the
drilling mud is prepared in a way so that the pressure inside the borehole is always slightly greater than in the pores of the surrounding rock.
As long as the drill string is rotating, everything is fine. But when a
Illustration 29:
thick drill pipe with a slick surface leans motionless onto the soft mud
Drill Collars
cake for some time, for example when a new drill pipe is being installed
(by R. Ritschel)
at the surface, it is possible that the drill pipe in the hole cannot be mo-
ved afterwards, even with a huge amount of power. This is because the mud has pressed it
firmly into the wall of the borehole. The high pressure inside the borehole presses the drill
collar onto the wall while the lower pressure in the pores of the rock cannot provide resistance and the soft mud cake acts like a seal between the two zones of different pressure.
Consequently, the drill string is stuck to the wall. Those grooves were invented to prevent
this from happening. Large slick surfaces enabling the drill pipe to stick to the wall do not
exist anymore, and the dreaded blocking, called “differential sticking”, can no longer occur.