7. DRILLING METHODS
1. Rotary drilling.
2. DTH Drilling.
3. Percussion Drilling.
Dr. Sharma gave the details mechanism and application on drilling methods. Percussion drilling is the oldest methods followed
in drilling methods which is generally done in hard rock areas. Rotary drilling methods are generally applied in the
areas containing alluvium formations whereas DTH (down to hole) methods are applied in the all type of formations .there are
also combination of drilling techniques done in GWB of Rajasthan which is discussed later.
The following is the demonstration of rotary drilling method. A bit is
a cutting tool, bit materials are generally alloy steel.
Rotary Drilling:
Rotary drilling uses a sharp, rotating drill bit to dig down through the Earth's crust. Much like a common hand held drill,
the spinning of the drill bit allows for penetration of even the hardest rock. The idea of using a rotary drill bit is not
new. In fact, archeological records show that as early as 3000 B.C., the Egyptians may have been using a similar technique.
Leonardo Da Vinci, as early as 1500, developed a design for a rotary drilling mechanism that bears much resemblance to technology
used today. Despite these precursors, rotary drilling did not rise in use or popularity until the early 1900's. Although rotary
drilling techniques had been patented as early as 1833, most of these early attempts at rotary drilling consisted of little
more than a mule, attached to a drilling device, walking in a circle! It was the success of the efforts of Captain Anthony
Lucas and Patillo Higgins in drilling their 1901 'Spindletop' well in Texas that catapulted rotary drilling to the forefront
of petroleum drilling technology.
While the concept for rotary drilling - using a sharp, spinning drill bit to delve into rock - is quite
simple, the actual mechanics of modern rigs are quite complicated. In addition, technology advances so rapidly that new innovations
are being introduced constantly. The basic rotary drilling system consists of four groups of components. The prime movers,
hoisting equipment, rotating equipment, and circulating equipment all combine to make rotary drilling possible
Most
drill rigs are of the rotary rig type. Today's rotary drill rig consists of multiple engines (gas, diesel) that supply power,
hoisting equipment that raises and lowers the drill string (drill pipe), and rotating equipment that turns the drill string
and the drill bit at the bottom of the hole. These engines also drive the circulating equipment that pumps liquids (mud) down
the hole to lubricate the drill string and drill bit (rotating in the hole), removes cuttings (loose bits of rock), and controls
downhole pressure to prevent blowouts (pressure in reservoir traps may blow up the open hole and ignite causing an explosion).
The conventional drill bit has three movable cones containing teeth made of tungsten carbide steel and
sometimes industrial diamonds. The rotating cones) are the cutting heads. The downward force on the drill bit is the result
of the weight of the overhead drill stem (steel pipe, pipe joints called collars) and drilling equipment on the derrick all
of which can amount to thousands of pounds. While the bits cuts into the bottom of the hole, pumps located at the surface
are forcing drilling fluids down the hole (lubricates, removes cuttings) through the inside of the drill pipe. This space
between the drill pipe and the walls of the hole is called the annulus space. The fluid then flows out the center of the drill
bit and is forced back up the outside of the drill pipe onto the surface of the ground where it is cleaned of debris and pumped
back down the hole. This is an endless cycle that is maintained as long as the drill bit is turning in the bottom of the hole.
The drilling crew is under the supervision of the Driller (operates the drilling and hoisting equipment).
The person who works on a platform high in the derrick is called a Derrickman; this very dangerous job is to handle the upper
part of the drill stem as it is raised and lowered out/into the hole. Roughnecks are the workers on the drill floor;
their job is to add new pipe joints as the well is drilled. All of these personnel and the entire operation of the drill rig
is under the supervision of the Tool Pusher. A typical drill rig will operate 24 hours per day, 7 days per week. It never
shuts down for holidays.
When drilling first began, drilling a straight vertical hole was the most desirable. Now days, however,
new drilling technology has developed methodologies for directional drilling (deviating from the vertical at some angle)
and horizontal drilling (starting with a vertical hole and then deviating at depth to a horizontal hole). A vertical well
has the advantage of penetrating more than one reservoir trap, but the disadvantage is that it can pull hydrocarbons from
only a small area within the vicinity of the well. Horizontal wells on the other hand, are much more limited in penetrating
multiple reservoir traps, but can remove hydrocarbons from greater areas within a given trap. Almost all wells drilled from
an offshore platform are of the directional type. Completing the bottom of the well so that it is ready to withdraw hydrocarbons
from the subsurface is done in the same manner regardless of whether it is a convention vertical hole or a horizontal hole.
This is because moving the platform to a new site is too costly.
A drilling operation produces waste material that includes drilling mud, rock cuttings, and salt water
brine (highly concentrated salt water generally about 80,000 PPM Sodium) which flows out
of a reservoir trap and up the well to the surface. These materials must be disposed of properly. The reserve pond is often
dug to temporarily hold the brine and drilling mud, but environmental regulations require such ponds be lined with materials
that will prevent the contaminants from entering into the subsurface. Often the brines are pumped back into the hole being
drilled. In any event, neither the drill mud nor the salt water brine is allowed to remain at the drilling site. All waste
materials must be removed off site and sent to a properly licensed landfill for disposal.
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The size of the bits is set by the IADC which is International Association of
Drilling Contractors. IADC uses different codes to specify the different bit sizes and shapes viz. 3-1-1 is the codes for
rock formation, similarly there are other codes used for different geological conditions.
The dimension of the borehole may be 2 7/5”,3 ½”, for water the
diameter may be 4 ½”
To break the mud cakes air is pumped which breaks.
DTH Drilling:
DTH(down to hole methods are the most popular method used in GWB, in layman’s language it is the combination of both
the rotary and percussion drilling equipments. There is rotary bits along with the hammer just like the percussion methods.
In DTH air in high pressure is passed (compressed about 350psi),the cuttings comes out from the bore hole due to the air pressure. 
Drilling fluids:
Typically, liquid drilling fluids are used.
The most common liquid drilling fluid, known as 'mud', may contain clay, chemicals, weighting materials, water, oil, or gases.
'Air drilling' is the practice of using gasses as the drilling fluid, rather than a liquid. Gases used include natural gas,
air, or engine exhaust. Air drilling can significantly cut down on drilling time, as well as drilling fluid costs. The drilling
fluid, much like the bit, is custom designed and chosen depending on what type of subsurface conditions are expected or experienced.
For example, if drilling is occurring through underground salt formations, freshwater would not be used, as this would risk
dissolving the subsurface salt. Similarly, if drilling near sources of fresh water, salt water would not be used for fear
of contaminating the fresh water.
The drilling fluid chosen must have a number of properties to allow it to accomplish its tasks. It must be light and thin
enough to circulate through the drill bit, cooling the bit as it drills as well as lubricating the moving parts. The fluid
must be heavy enough to carry drill cuttings away from the bit and back to the surface, as well as control upward pressure
that may be experienced in the well to prevent blowouts. The drilling fluid engineer ensures that the weight of the drilling
fluid is greater than the upward pressure of escaping gas that may be encountered while drilling. In addition, the drilling
fluid must be thick enough to coat the well bore with a cake, which serves to temporarily seal the walls of the well until
casing can be installed.
The circulating system consists of a starting point, the mud pit, where the drilling fluid ingredients
are stored. Mixing takes place at the mud mixing hopper, from which the fluid is forced through pumps up to the swivel and
down all the way through the drill pipe, emerging through the drill bit itself. From there, the drilling fluid circulates
through the bit, picking up debris and drill cuttings, to be circulated back up the well, traveling between the drill string
and the walls of the well (also called the 'annular space'). Once reaching the surface, the drilling fluid is filtered to
recover the reusable fluid.
Percussion Drilling:
It is the simplest, cheapest,
and most reliable technology available of earth boring for water well and environmental drilling. It will drill any material
and is completely self contained - requires no mud, mud pits, pumps, chemicals, yet will drill through rock.
The basic concept for cable
tool drilling consists of repeatedly dropping a heavy metal bit into the ground, eventually breaking through rock and punching
a hole through to the desired depth. The bit, usually a blunt, chisel shaped instrument, can vary with the type of rock that
is being drilled. Water is used in the well hole to combine with all of the drill cuttings, and is periodically bailed out
of the well when this 'mud' interferes with the effectiveness of the drill bit.
Drilling into the Earth in the
hopes of uncovering valuable resources is nothing new. In fact, the digging of water and irrigation wells dates back to the
beginning of recorded history. At first, these wells were primarily dug by hand, then by crude stone or wood tools. Metallurgy
brought about the use of iron and bronze tools to delve beneath the Earth's surface, and innovations led to more efficient
ways of removing debris from the newly dug hole.
How the Percussion Drill Works
Percussion drilling gets its name from the action of its drill which raises and falls to beat upon the
earth and chop up the soil and rock. It is more popularly known today as "Cable Tool Drilling" since the more modern drilling
rigs use steel cable. It has many variations, like the Chinese springboard, the American springpole, and the walking beam,
but all employ the same basic means of cutting the earth and clearing out the hole.
The drill involves a heavy steel bit attached to a rope which is lifted, either by hand or by machine,
and then dropped to cut the earth. As the bit chops the earth, water is added to the well hole so that the bit makes mud out
of the earth it has cut. After the hole is filled with several feet of mud, the heavy bit is withdrawn and a tool called a
bailer is attached to the rope and lowered into the hole. The bailer is a hollow tube with a door at the bottom. The door,
called a flap valve, opens when it hits the mud to allow the mud to fill the bailer, and then closes to trap the mud inside
the tube so that the mud can be lifted to the surface. The tube is emptied at the surface and the procedure is repeated until
the hole is clear.
The bit is then re-attached to the rope and the above process begins again.
If the earth being drilled will not cave in, then the drilling continues until water is reached. But if
the earth is made of loose material like sand, a large heavy steel pipe is driven into the hole to keep the walls from falling
in.
The large pipe is called casing and it holds the hole open until the drilling is done. It is removed after
the permanent casing (usually smaller) is installed. The larger steel casing can be used many times.
When the water level has been reached the drilling stops and the permanent casing, which is smaller than
the well hole, is installed. Then the rest of the hole around the casing is filled and sealed. A concrete slab is laid about
the new well and a hand pump is installed.
There are two
main types of drilling. Percussion, or 'cable tool' drilling, consists of raising and dropping a heavy metal bit into
the ground, effectively punching a hole down through the Earth. Cable tool drilling is usually used for shallow, low pressure
formations. The second drilling method is known as rotary drilling, and consists of a sharp, rotating metal bit used to drill
through the Earth's crust. This type of drilling is used primarily for deeper wells that may be under high downhole pressure.
Cable tool drilling uses tools suspended
on a wire cable attached to a winch, which lifts and drops them to break up and recover material in the bottom of a hole.
Other tools like clay cutters and stubbers allow rapid drilling in clays and in sands having some clay content, employing
the 'long drop' technique to cut and remove solid plugs of material, taking advantage the free-fall characteristics of the
special winch.
It is a non-invasive and sensitive drilling method which is suitable for smaller water sources, as
it allows the discovery, testing and development of a water source as drilling proceeds, rather than simply finding a potential
geology, which may be dry. The method, in addition to water drilling, is appropriate for environmental drilling, site investigation,
remediation, and prospecting; it has has a similar degree of sensitivity to trial pit excavation
Advantages:
Cable percussion (cable tool) drilling is the simplest, cheapest, and most reliable technology available
of earth boring for water well and environmental drilling. It will drill any material and is completely self contained - requires
no mud, mud pits, pumps, chemicals, yet will drill through rock.
Cable tool drilling uses tools suspended on a wire
cable attached to a winch, which lifts and drops them to break up and recover material in the bottom of a hole. Other tools
like clay cutters and stubbers allow rapid drilling in clays and in sands having some clay content, employing the 'long drop'
technique to cut and remove solid plugs of material, taking advantage the free-fall characteristics of the special winch.
It is a non-invasive and sensitive drilling method which is suitable for smaller water sources, as it allows the discovery,
testing and development of a water source as drilling proceeds, rather than simply finding a potential geology, which may
be dry. The method, in addition to water drilling, is appropriate for environmental drilling, site investigation, remediation,
and prospecting; it has has a similar degree of sensitivity to trial pit excavation
The main advantage of percussion method is that when there is a cavity in subsurface where the drilling
is conducted.
