Selection of Rigs

Selecting the rigs in drilling operation is a crucial and important task. It depends upon the environment and the geology or the formations. Running drilling operation in this millennium requires advance technology and expertises. Technology in this case is Drilling Rig. Drilling rigs are a perfect blend of quality, modernity and versatility, highly competent and technology driven to drill for hydrocarbon as well as the minerals. Therefore, high expertises; engineers (Mechanical, Petroleum, Civil etc), geologists and geophysics are required. This section will cover the history of drilling rigs, main components of modern rigs and their basic operation, setting up the rigs and advance technologies using in drilling operation.





History of drilling rigs



Drilling or digging for hydrocarbon had been done in hundreds of years ago until now. The first oil wells were drilled in China in the 4th century (Rigs International, 2009). They had depths of up to 243 meters and were drilled using bits attached to bamboo poles.

In the late 19th century, the primary method for drilling rock involved muscle power is it human or animal. Rods were turned by hand, using clamps attached to the rod (Rigs International, 2009).

Corbett Portable drilling rig (Fig. 1) was invented in 1884 (Pees, S. T, nd).

Corbett1.jpg

Figure 1: The rig shown, having only a 600 foot depth capability, would be usable only in very shallow oil territory, like the flats bordering Oil Creek, and probably wouldn't reach the oil-bearing beds if the drill site was on the high flanks of the valley. Other Corbett models could drill deeper.

In 1878 Robert M. Downie of Butler County, Pennsylvania, designed a wheel-mounted cable tool rig called Ketstone Driller (Fig. 2) that held a vertical boiler, vertical steam engine and the operating or hoisting wheels (Pees. S. T, nd). The engine operated a spring pole, the ram end of which was anchored to the ground a short distance behind the drill wagon. The fulcrum of the spring pole arrangement was on the rear of the wagon. A tripod served as derrick or mast and the base of the poles were stuck in the ground.

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Figure 2: It was used to drill 1000 foot wells and for clean-out or work over purposes. It was run by a Waukesha four cylinder gasoline engine, seen at the back of the frame. The corrugated metal building was erected over the rig floor.

The portable rigs of the Star Drilling Machine (Fig. 3), Company of Akron, Ohio, date to the 1890's was very popular model in the shallow oil fields in the early 1900’s and teens (Pees, S. T, nd). It had capacity of 800 feet but would drill a little deeper.

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Figure 3: Operators often chose a model without the mounted vertical steam engine and T-boiler (or stripped them off), preferring to use separate power plants and changing when needed, i.e. steam to gas or gasoline, even electricity. In 1910, 20, and into the 30's all types of power plants were introduced to the oilfields. A lot of operators hauled their rigs rather than use the traction of the No. 2 machine type which needed assistance in the rugged topography anyway.

Most of them are self-propelled portable rigs with wheels employed steam as the propelling force and for the drilling, at first. Some of these rigs had the boiler and steam engine mounted directly on the apparatus. Others had to be pulled and the prime mover (engine) or at least the boilers were hauled separately to the location.






Modern Rigs

First rotary rig to use diesel engine was invented in1925. Rotary drilling rigs (Conventional Drilling) are used for almost all drilling done today. It consists of the main components such as power system, hoisting system, rotating equipment, circulation system, casing, and BOP.

Rig Power system

Drilling rigs are usually powered by steam engines or internal combustion engines which burn diesel-fuel oil to provide the main source of power. Steam engines are rarely used in present drilling operation. Internal combustion engines are most commonly used on modern rigs which classified as diesel-electric type (Electrical generators are powered by the diesel engines to provide electrical power engines) and direct-drive type (Mechanical system the rig uses torque converters, clutches, and transmissions powered by its own engines, often diesel) which attaches to the engine. The power system is using to run the operations such as rotating, hoisting, drilling fluid circulation, mud cleaning equipment, hydraulically operated blowout preventer and lighting.

Hoisting system

It is used to raise and lower the drill string during drilling (and tripping and POOH), to lower casing and specialist tools and equipment down the hole, to move equipment around the rig. It is consisted draw-works, crown block, travelling block, hook, drilling line, elevator and derrick and substructures.


a. Draw-works

Driving force generator which means applying force to the wire line to raise or lower drilling equipment in or out of the hole.

b. Crown block

It is the stationary end of the block and tackle.

c. Travelling block

It is moving end of the block and tackle. Together, they give a significant mechanical advantage for lifting.

d. Hook

It is used to hold the the entire drill strings and swivel.

e. Drilling line

It is a stranded metal cable threaded through the two blocks (traveling and crown) to raise and lower the drill string.

f. Elevator

It is hinged devices that is used to latch to the drill pipe or casing to facilitate the lowering or lifting (of pipe or casing) into or out of the wellbore.

g. Derrick
To supports structure for the equipment used to lower and raise the drill string into and out of the wellbore.



Rotating equipment

It is used for rotary drilling. It is consisted swivel, Kelly, rotary table, drill string (drill pipe, drill collars and heavy weight drill pipe) and drill bit.


a. Swivel

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

b. Kelly (four or six sided pipe)

It is used to transfers rotary motion to the turntable (rotary table) and drill string.

c. Turntable orrotary table

It is used to drives the rotating motion using power from electric motors.

d. Drill string

It is consisted of drill pipe (connected sections of about 30 ft / 10 m) and drill collars (larger diameter and heavier pipe) which fits around the drill pipe and places weight on the drill bit and heavy weight drill pipe which is placed on top of drill pipe and sometimes between drill pipe and drill collars to keeps the drill pipe in tension and prevents buckling or bending.

e. Drill bit

Drill bit is end of the drill that actually cuts up the rock by sliding, tearing, crushing, and gouging. It comes in many shapes and materials (tungsten carbide steel, diamond) that are specialized for various drilling tasks and rock formations.

Circulation system

Circulation system is using to circulate drilling mud (mixture of water, clay, weighting material and chemicals) under pressure through the hole. Mud is pumped down inside of strings, through bit (lubricates bit), then carries cuttings through the annulus to the surface and the cuttings then clean and recycle. The system is composed of drilling mud, mud mixing equipment, mud pit, pumps, cleaning equipment (shale shakers, de-silters, de-sanders, de-gassers, centrifuges) and hoses and standpipe.

a. Drilling mud

It is used to transporting bit cuttings as drilling progresses, clean the bottom hole, cool and lubricate the bit and drill stem, support the wall of the wellbore and prevent entry of the formation fluids into the wellbore.

b. Mud mixing equipment

The equipment is consisted of hopper mixing mud which is used to mix water, clay, weighting material and chemicals and then send to the mud pits.

c. Mud pit

Mud pit is used to store the drilling mud after mixing.

d. Pump

It is used to suck mud from the mud pits and pump it to the drilling apparatus.

e. Cleaning equipment

This equipment is consisted of shale shakers, de-silters, de-sanders, de-gassers and centrifuges. It is used to sieve that separates rock cuttings from the mud and store the cuttings in the reserve pit.

f. Standpipe and hoses

It connects pipe to drilling apparatus.


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Figure 4: Mud ciculation system in onshore drilling operation.

Casing

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


Blowout preventer (BOP)

It is used as high-pressure valves (located under the land rig) 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).






Set up the rig

Rig set up is 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 barges for work on inland water where there is no foundation to support a rig (as in marshes or lakes). It must be situated in right position and accurately assemblage. Once the equipment is at the site, the rig is set up. The procedures of setting up the rig such as:

1. Setting up plastic layer in reserve pit in order to prevent the contamination on soil

2. Dig a rat and mouse holes

3. It is required approximately 75 people and 35 semi trucks to move and assembly the equipment

4. When the equipment is at the site it needs to be inspected by the engineers and the technician to make sure all need specification and safety standard

5. Starting assembly (Screw up the equipment) in right place and position

6. Normally it takes 3 and a half days to set up.



Here are the major systems of a land oil rig (Fig. 5):

Oil_Rig_NT8.jpg
Figure 5: The major systems of a land oil rig with whole assembly.
Legend:
  1. Mud tank
  2. Shale shakers
  3. Suction line (mud pump)
  4. Mud pump
  5. Motor or power source
  6. Vibrating hose
  7. Draw-works (winch)
  8. Standpipe
  9. Kelly hose
  10. Goose-neck
  11. Traveling block
  12. Drill line
  13. Crown block
  14. Derrick
  15. Monkey board
  16. Stand(of drill pipe)
  17. Pipe rack (floor)
  18. Swivel(On newer rigs this may be replaced by a top drive)
  19. Kelly drive
  20. Rotary table
  21. Drill floor
  22. Bell nipple
  23. Blowout preventer (BOP) Annular
  24. Blowout preventers (BOPs) pipe ram & shear ram//
  25. Drill string
  26. Drill bit
  27. Casing head
  28. Flow line.




Advance drilling technologies

Advance technologies in drilling operation have been developed and used in world wide. They vary in operating system, their components and purposes according to the environment, geology (formation) and depth. They are such as lightning rig, truck mounted/portable rigs and coiled tubing drilling or CTD.

Lightning rig

It is considered as a modern rig. It was manufactured in Houston, Texas in the United States and cost AUS $32 million (Geodynamic, n.d). It is a top drive drilling rig.

Some of the key attributes of the LeTourneau Lightning rig are:

a. It uses modern AC technology that is becoming the preferred technology to power rigs, with both energy efficiency and control advantages

b. It has an effective 3,000 HP draw works (hoisting system); It has the capacity to drill geothermal wells to 6,000 metres

c. It has a mast and base structure capacity of one million pounds (approximately 450 tonnes)

d. It has a clearance under the drilling floor of over 8 metres, needed for the pressure control systems on top of the well during drilling

e. It has the capability to erect the base structure and mast without the need for cranes; It will need around 32 truck loads to move, and moves should be able to be effected in around three days (compared with around 60 truck loads and three weeks for older rigs)

f. It is designed to operate in the extreme desert conditions that exist in the Cooper Basin (Geodynamic, n.d).


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Figure 6: Typical Lightning Rig fully assembled.


Truck mounted drilling rig/ portable rig

It can be drilled in shallow fields at depth from 1,500-3,500 ft (Copeland, M n.d). In assembling this rig a 71/2-10 ton truck chasis, with 225-inch wheel base is used. The mud pump is a second motor and draw works are all mounted integrally on the truck. The unit transports itself from location to location. The motors are rated at 112 hp and will develop 140 brake horsepower at 2,100 rpm. There are two motor operating the systems. First motor is a power take-off from the truck motor which drives the mud pump and lighting plant which placed on top of the pump and second motor drives the draw works and rotary run table.

Draw works are mounted on the rear end of the truck.

In rigging up, the unit is backed up to the derrick, and the rear end of the truck frame is to set on the derrick floor. The truck frame is levelled up with the blocking placed on timbers so that all weight is taken off the truck springs to give the unit a solid foundation while drilling. A steel header is placed over the draw works and turn buckled to the derrick sills or foundation. The crew requires are such one driller, one derrick man, and two floor man on tour and a mechanic.

Disadvantage is when considering the mud pump due to weight and space limitations.


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Figure 7: Truck-Mounted Rotary-Drilling Rig.


Coiled Tubing Drilling (CTD)

This type of drilling does not use individual sections of drill pipe that are screwed together (replace traditional drill pipe). Coiled tubing eliminates the time and cost associated with the continuous joining, reinstallation, and removal of drilling pipes. It has a smaller diameter than traditional drill pipe so a smaller volume of cuttings is generated. CTD systems are much more compact, require smaller drilling crews, can be mobilized and demobilized much faster, and offer a higher degree of safety and control (Clow, G Koci, B 2002).These drills are now commonly used to produce slim hole wells (diameters < 13 cm), drill horizontal sidetracks out to 900 m from the main wellbore, install completion tubing, log high-angle boreholes, and deliver sophisticated treatment fluids to specific ones down hole (Clow, G Koci, B 2002).



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Figure 8: New advance drilling technology introducing in drilling operation, Coiled Tubing Drilling (CTD).

Coiled tubing drilling is used in horizontal drilling in some extend when the reservoir/ well is beneath the population area such as houses, farming areas etc. It can be used as

directional drilling which allows many wells to be drilled from a single well pad, and for wells to be spaced much closer together. Coiled tubing drilling is capable to drill through several hundred meters of ice by utilizing a high-pressure pump to deliver fluid to a steerable down hole hydraulic motor that drives a cutting bit and using an injector on the surface to guide the tubing into and out of the hole and to maintain the correct pressure on the bit. CTD should be able to achieve effective drilling rates of 40 m/hr in polar ice (Clow, G Koci, B 2002). CTD has been successfully used by ARCO, BP Amoco PLC and Petroleum Development Oman (PDO) to access hard-to-reach, overlooked, and depleted reservoirs from existing wellbores. CTD is used to perform horizontal sidetracks (Gaddy, D 2000).

Several advantages and benefits of using coiled tubing for open-hole slim-hole drilling application are driving the technology forward:

a. Costs are reduced with coiled-tubing operations
Many of the cost saving attainable with coiled-tubing drilling arise from the small size of the rig. The inherent automation of coiled-tubing rigs, and other savings enjoyed in slim-hole operation. Costs other than drilling time, such as mobilization, site size and preparation, and expendables, often account for more than 50% of conventional costs.
b. Coiled-tubing drilling operations have smaller surface lease requirements than most conventional rigs due to a smaller footprint (usually less than 50% conventional) for the coiled-tubing system. Therefore, costs in several categories can be significantly reduced with coiled-tubing slim hole system.
c. Drill-string top time is reduced
Continuous tubing eliminates the need for drill-string connection, thus reducing trip times and increasing safety. Many rig-floor accidents and stuck-pipe incidents occur when drilling is stopped to make a connection.
d. Under balanced drilling is practical with coiled tubing
The design of coiled-tubing pressure-control equipment and systems allows the tubing to be run safely in and out of live wells. Drilling can be performed in under balanced conditions, which minimizes formation damage, increase rate of penetration, and eliminates differential sticking. Reducing formation damage, increases rate of penetration, and eliminates differential sticking. Reducing formation damage can lead to increased well productivity and eliminates the need for stimulation or damage removal treatments during completion operations.
e. Coiled tubing allows continuous circulation
A fluid swivel joint installed on the axle of the tubing reel allows circulation through coiled tubing while tripping. This design simplifies well control techniques and helps maintain good hole conditions. Continuous circulation also allows continuous drilling, facilitating the use of foam as a low-weight drilling fluid when appropriate.
f. Coiled tubing has no joints to make and break
There are several benefits to eliminating tool joints in the drill string. Among them are no mud spillage while making joints, elimination of noise from pipe-handling equipment, and increased safety on the rig floor.
g. Coiled tubing is readily adapted for wire line telemetry.
Wire line is routinely installed inside coiled tubing. high-speed continuous telemetry is practical with coiled tubing for MWD (measurement while drilling) and FEMWD (formation evaluation). The same wire line can also be used steering-tool data orientation-tool control.

Overall, selecting the appropriate rig in drilling operation is an important task requiring expertises such as engineers and technicians. The rig also needs to place in right position and assemblage accurately. Developing advance technologies such as Lightning Rig, Truck-Mounted Drilling Rig and Coiled Tubing Drilling (CTD) have helped oil companies to drill more effectively and safely.