Drilling

INTRODUCTION

Drilling is major part in many explorations such as Oil, Coal, Water, Civil Constructions etc.Below mentioned is just an short example of oil exploration technique, to get general idea of how the drilling industry works.

Preparing to Drill
Once the site has been selected, it must be surveyed to determine its boundaries, and environmental impact studies may be done. Lease agreements, titles and right-of way accesses for the land must be obtained and evaluated legally. For off-shore sites, legal jurisdiction must be determined.

Once the legal issues have been settled, the crew goes about preparing the land:

1]The land is cleared and leveled, and access roads may be built.
2]Because water is used in drilling, there must be a source of water nearby. If there is no natural source, they drill water well.
3]They dig a reserve pit, which is used to dispose of rock cuttings and drilling mud during the drilling process, and line it with plastic to protect the environment. If the site is an ecologically sensitive area, such as a marsh or wilderness, then the cuttings and mud must be disposed offsite -- trucked away instead of placed in a pit.
Once the land has been prepared, several holes must be dug to make way for the rig and the main hole. A rectangular pit, called a cellar, is dug around the location of the actual drilling hole. The cellar provides a work space around the hole, for the workers and drilling accessories. The crew then begins drilling the main hole, often with a small drill truck rather than the main rig. The first part of the hole is larger and shallower than the main portion, and is lined with a large-diameter conductor pipe. Additional holes are dug off to the side to temporarily store equipment -- when these holes are finished, the rig equipment can be brought in and set up.

Setting Up the Rig:

Depending upon the remoteness of the drill site and its access, equipment may be transported to the site by truck, helicopter or barge. Some rigs are built on ships or barges for work on inland water where there is no foundation to support a rig (as in marshes or lakes). Once the equipment is at the site, the rig is set up. Here are the major systems of a land oil rig:

Anatomy of an oil rig

I] Power system: -

• large diesel engines - burn diesel-fuel oil to provide the main source of power
  


• electrical generators - powered by the diesel engines to provide electrical power

II] Mechanical system: - driven by electric motors

• Hoisting system - used for lifting heavy loads; consists of a mechanical winch (drawworks) with a large steel cable spool, a block-and-tackle pulley and a receiving storage reel for the cable
  


• Turntable - part of the drilling apparatus
  

III] Rotating equipment: - used for rotary drilling

• Swivel - large handle that holds the weight of the drill string; allows the string to rotate and makes a pressure-tight seal on the hole
  


• Kelly - four- or six-sided pipe that transfers rotary motion to the turntable and drill string
  


• Turntable or Rotary table - drives the rotating motion using power from electric motors
  


• Drill string - consists of drill pipe (connected sections of about 30 ft / 10 m) and drill collars (larger diameter, heavier pipe that fits around the drill pipe and places weight on the drill bit)
  


• Drill bit(s) - end of the drill that actually cuts up the rock; comes in many shapes and materials (tungsten carbide steel, diamond) that are specialized for various drilling tasks and rock formations
  


• Casing - large-diameter concrete pipe that lines the drill hole, prevents the hole from collapsing, and allows drilling mud to circulate

IV] Circulation system: - pumps drilling mud (mixture of water, clay, weighting material and chemicals, used to lift rock cuttings from the drill bit to the surface) under pressure through the kelly, rotary table, drill pipes and drill collars

• Pump - sucks mud from the mud pits and pumps it to the drilling apparatus
  pipes and hoses - connects pump to drilling apparatus
  


• Mud-return line - returns mud from hole
  shale shaker - shaker/sieve that separates rock cuttings from the mud
  


• Shale slide - conveys cuttings to the reserve pit
  


• Reserve pit - collects rock cuttings separated from the mud
  Photo courtesy Institute of PetroleumMud circulation in the hole
  


• Mud pits - where drilling mud is mixed and recycled
  


• Mud-mixing hopper - where new mud is mixed and then sent to the mud pits

Drill-mud circulation system

V] Derrick - support structure that holds the drilling apparatus; tall enough to allow new sections of drill pipe to be added to the drilling apparatus as drilling progresses

VI] Blowout preventer - high-pressure valves (located under the land rig or on the sea floor) that seal the high-pressure drill lines and relieve pressure when necessary to prevent a blowout (uncontrolled gush of gas or oil to the surface, often associated with fire)

Drilling

The crew sets up the rig and starts the drilling operations. First, from the starter hole, they drill a surface hole down to a pre-set depth, which is somewhere above where they think the oil trap is located. There are five basic steps to drilling the surface hole:

• Place the drill bit, collar and drill pipe in the hole.
  
• Attach the kelly and turntable and begin drilling.
  
• As drilling progresses, circulate mud through the pipe and out of the bit to float the rock cuttings out of the hole.
  
• Add new sections (joints) of drill pipes as the hole gets deeper.
  
• Remove (trip out) the drill pipe, collar and bit when the pre-set depth (anywhere from a few hundred to a couple-thousand feet) is reached.

Once they reach the pre-set depth, they must run and cement the casing -- place casing-pipe sections into the hole to prevent it from collapsing in on itself. The casing pipe has spacers around the outside to keep it centered in the hole.

The casing crew puts the casing pipe in the hole. The cement crew pumps cement down the casing pipe using a bottom plug, a cement slurry, a top plug and drill mud. The pressure from the drill mud causes the cement slurry to move through the casing and fill the space between the outside of the casing and the hole. Finally, the cement is allowed to harden and then tested for such properties as hardness, alignment and a proper seal.

Drilling continues in stages: They drill, then run and cement new casings, then drill again. When the rock cuttings from the mud reveal the oil sand from the reservoir rock, they may have reached the final depth. At this point, they remove the drilling apparatus from the hole and perform several tests to confirm this finding:

• Well logging - lowering electrical and gas sensors into the hole to take measurements of the rock formations there
  
• Drill-stem testing - lowering a device into the hole to measure the pressures, which will reveal whether reservoir rock has been reached
  
• Core samples - taking samples of rock to look for characteristics of reservoir rock

Once they have reached the final depth, the crew completes the well to allow oil to flow into the casing in a controlled manner. First, they lower a perforating gun into the well to the production depth. The gun has explosive charges to create holes in the casing through which oil can flow. After the casing has been perforated, they run a small-diameter pipe (tubing) into the hole as a conduit for oil and gas to flow up the well. A device called a packer is run down the outside of the tubing. When the packer is set at the production level, it is expanded to form a seal around the outside of the tubing. Finally, they connect a multi-valved structure called a Christmas tree to the top of the tubing and cement it to the top of the casing. The Christmas tree allows them to control the flow of oil from the well.

Once the well is completed, they must start the flow of oil into the well. For limestone reservoir rock, acid is pumped down the well and out the perforations. The acid dissolves channels in the limestone that lead oil into the well. For sandstone reservoir rock, a specially blended fluid containing proppants (sand, walnut shells, aluminum pellets) is pumped down the well and out the perforations. The pressure from this fluid makes small fractures in the sandstone that allow oil to flow into the well, while the proppants hold these fractures open. Once the oil is flowing, the oil rig is removed from the site and production equipment is set up to extract the oil from the well.

Photo courtesy Phillips Petroleum Co.Rotary workers trip drill pipe

Types of Drilling


The common classification for types of drilling is by the most distinctive feature of the type. For example: -

• ROTARY DRILLS – rotary cutting action
  
• DIAMOND DRILLS – diamond bits
  
• CABLE TOOL DRILLS – cable percussion operation
  

Further classification of drilling can be done on the basis of: -

Ø The hole making action:

· Whether drilling action is a hammering or a turning action.
· Whether the hole making action is driven by air, hydraulic, electrical or mechanical means.

This classification is very important and should be seriously considered before actual application. Whether the hole making and hole clearing method selected for the particular site is suitable for large or small holes, for shallow or deep holes etc.


Ø CLASSIFICATION BY HOLE MAKING METHODS: -

· Breaking rock
· Stirring unconsolidated sediments

Hole making also includes the clearing of chips and loose material from in front of the bit so that breaking or stirring action can continue. It can be mechanical or hydraulic.

· Mechanical breaking or stirring methods:
o Cable tool drilling
o Rotary auger drilling
o Rotary drilling
o Slim hole rotary drilling

I] CABLE TOOL DRILLING

Principle: -
Cable tool drilling is carried out by breaking or stirring the formation, then make slurry of the cuttings into the depth itself and lastly bailing them out.

Also if the sampling is required, then a sample tube is driven into the formation for making a hole. This achieved by either of the following methods: -

· Repeated blows are made to the sample tube by “Spudding” action.
· The sample tube is repeatedly raised and dropped.
· Or just impact the sample tube into the bottom using one long drop.

The last two mentioned techniques are often used, as it does not need any specially equipped machines.


Techniques: -
The drilling machine used for this type is capable of handling many types of drilling applications. The techniques thus used are also many, and are selected according to the need and the conditions that are faced on the drilling site. Here are some of the commonly used techniques.

Normal cable tool operation: -
The cable tool is freely dropped, and is caught before it reaches to the bottom, so that cable is stretched when the bit reaches the bottom. As soon as this happens the tool is again pulled upwards instantly and the bit is rotated with help of the swivel by taking off the weight from the cable.

Casing plug drilling: -
In this technique the heavy casing is driven into the formation, and the core is retained with the help of powerful casing pulling assembly on the rig.

Shell drilling: -
The normal drill stem assembly id fitted with tube tool rather than a chopping/mixing bit.

Mud drilling: -
To prevent the hole caving, normally the casing is driven into the hole. But, in the large diameter holes it is proffered to stabilize the formation by filling the hole with mud.


Advantages: -

• Low capital investment is needed. It is less than third that is required for the rotary drilling. Maintenance does not require precision fitting and most of the parts are cheap as compared to the complex machines.
  
• High quality of sampling is attained for the geological purposes such as alluvial mineral investigations.
  
• It is particularly suited for the water well works as it provides the samples of each water horizon.
  
• Best suited for the drilling operations carried out in the remote areas as, it requires very less amount of fuel, water and other materials.
  
• Very easy to setup, drill bits can be replaced onsite thus very helpful in remote areas.

Disadvantages: -

• Productive output measured in hole produced per day is relatively low in most cases.
  
• Hard rock penetration rate is very low.
  
• When casing is required, deep drilling presents problems in keeping the casing free.
  
• The heavy hampering causes disturbances and damages the formation thus, the mixed samples are received.
  
  
  
  
  
  

  II] ROTARY AUGER DRILLING
  

Principle: -
Auger drilling is done with a helical screw which is driven into the ground with rotation; the earth is lifted up the borehole by the blade of the screw. Auger drilling is used for well drilling, fence construction, soil engineering and geochemistry reconnaissance work in exploration for mineral deposits. In some cases, mine shafts are dug with auger drills. Small augers can be mounted on the back of a utility truck, with large augers used for sinking piles for bridge foundations.
Auger drilling is restricted to generally soft unconsolidated material or weak weathered rock. It is cheap and fast.

Techniques: -
Auger drills are mainly used in soil and unconsolidated groundwater investigations and fr drilling for contruction in soils and very soft rocks. These techniques are employed when there is need of robust machines capable of high torque at low rotation speeds. The following are the commonly used techniques.

Continues flight augers: -
Driven by any top-drive ritary machine with adequate torque rating. In deep, small diameter holes, contineous flight augers are supported by the hole. Cuttings are carried to the surface on the helical flights.

Advantages:

• Low equipment & operating costs.
  
• Fast drilling in suitable formations.
  
• No fluids required, thus no contamination of samples.
  

Disadvantages:

• Poor penetration in coarse formations.
  
• Will not drill through rock or boulders.
  
  
  
  
  

Hollow augers: -
Hollow augers are contineous flight augers which has a hollow centre tube. They are normally used with a plug bit held in place by a secondary internal rod string, or simply by friction.
Advantage of this type is that, if hard rock is encountered while drilling, the diamond core drill can be employed by plcing the assemply through the hollow casing of this type.

Plate augers: -Plate augers are used for large diameter holes. The augers are pulled out of the formation with the cuttings loaded on it, then these cuttings are spun off on the surface.

Advantages:

• Produces clean & dry holes.
  
• Drills large diameter holes.

Disadvantages:

• Water used in the process washes cutting from auger flights.
  
  
  
  
  
  
  
  
  

Bucket augers: -
The drill bit of this technique is kind of a big bucket depending on the bore size. The cuttings are then carried out in this bucket and unloaded on the surface.

Advantages:

• Drills larger holes.
  
• Drills below water & operates in mud filled holes.
  
• Large rotatry drive prides sufficient torque to drill through harder materials.
  

Disadvantages:

• Depth capacity is limited.
  
  
  
  
  
  

III] ROTARY DRILLING

Principle: -
This type of drilling is done by rotating a bit on the bottom of hole. It is rotational drilling as opposed to the up & down action of percussion drilling. Also these types of bits are mostly larger than the drill pipes and rotary drilling uses fluid circulation to clear the cuttings.

Techniques: -
The rotary drills cuts the formations by rotating action as well as maintaining the pressure on the formation while the process in going on. Larger rotary holes are used in near-vertical holes. Since the holes drilled downwards, the required pressure on the bit can be supplied by the weight of the drill pipe. Especially heavy pipes are run immediately after the drill bit to provide the necessary pressure.

Apart from the above stated principle of rotary drilling the following technique is also used frequently as per the need.

Rotary down-hole hammer drilling: -
When it is required to drill the hard formations, this type of drilling is used, as some of the formations can not be drilled simply by rotating action. Thus, additional hammering action is also provided to the bit to efficiently break the formation. Hence, this type of drilling uses both, the Percussion as well as the rotary action simultaneously.

Rotary fluid circulation: -
Different techniques for the removal of cuttings can be used in rotary drilling. Some of the frequently used techniques are given below: -

• Conventional air circulation: -
  Hammer bits are operated on rigs using air circulation. The air drives the hammer. The exhaust from the hammer clears the cuttings from the bit and carries them up the hole.
  
• Rotary mud/water circulation: -
  Water or mud, which should be an engineered water or oil-based polymer or clay suspension used for circulation. Liquids can move cuttings from the bit as well as maintain the temperature of the bit there by acting as a cooling system in heavy drilling operations. Mud also makes the wall of the hole more stable.
  
• Reverse circulation: -
  Reverse circulation is same as the above stated mud/water circulation method, the only difference being that the liquid used for the circulation purpose flows in reverse direction. The liquid is forced from outside the drill pipe, and it removes the cuttings by exiting from inside the drill bit.

References: -

1] www.howstuffworks .com

2] Google Image Search

3] DRILLING (A book by "Australian Drilling Industry Training Committee Limited")