COIL TUBING
LECTURE NOTES BY PROF.SUSHANTA SARKAR
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1. Principle and and area of application The coil tubing unit consists of continuous metal pipe with a diameter of 3/4" to 1-1/2" or even more, coiled up on a spool ( or reel ).
It can be run in and pulled out of a pressurized well.
The pipe, with its end equipped with a check valve at least (usually), is maneuvered via an injector head through a safety system.
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1. Principle and and area of application The coil tubing unit consists of continuous metal pipe with a diameter of 3/4" to 1-1/2" or even more, coiled up on a spool ( or reel ).
It can be run in and pulled out of a pressurized well.
The pipe, with its end equipped with a check valve at least (usually), is maneuvered via an injector head through a safety system.
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The operation requires a special team of at least three people.
Coil tubing units can carry out a number of operations on pressurized wells rapidly (lightweight rig, no pipe length to be made up, etc) and are first and foremost a means of circulating in the well.
They are therefore used in particular to: •
make the hydrostatic column lighter prior to perforating (underbalanced perforating after equipment installation )
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start up or kick off a flowing well ( after a stimulation job for example ) by circulating a "light" fluid (i.e. exerting a hydrostatic pressure that is lower than the reservoir pressure ) or by injecting nitrogen. implement temporary gas lift ( during testing while drilling,when waiting for workover, etc ).
reduce, hence optimize, the flowpath through the tubing (in a well with heavy phase segregation problems after the flow rate has dropped )
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clean out the tubing (sand, salt, paraffins, hydrates,etc ) by circulating an appropriate fluid ( water,brine,hot oil,alcohol,etc)
clean out the bottom of the well by circulating ( sand fill,etc )
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spot acid, solvents, etc opposite the zone ( S ) that need to be treated.
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inject a killing fluid by circulating ( prior to workover,etc )
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Theoretically other operation are also feasible, but due consideration must be given beforehand to the difficulties and risk they entail.
Additionally, it is not sure that coil tubing is the most appropriate method and at the end of the day the lower costs may dictate the way the coiled tubing operations are deployed for such as:
spotting cement plugs
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small scale turbine plug,sediments,etc )
redrilling
(cement
some fishing job ( using an overhot,etc)
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Let us mention the special case of horizontal wells where coiled tubing can be used:
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to get tools into a horizontal drain hole, in particular for well logging ( in which case the electric cable will have been placed in the coiled tubing before it is spooled up on the reel ).
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2. Coiled tubing equipment Beside the tubing itself the equipment in a coiled tubing unit consists principally of the following;
a reel an injector head a safety assembly ancillary surface equipment: a cab, a power pack, a crane, etc downhole accessories.
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2.1. The tubing The tubing consists of a strip of mild steel rolled cylindrically and welded longitudinally.
Unit lengths of several hundred of meters (about a thousand feet) are butted together by radial welding to make up coils upto 6000 mts (20.000 ft) long.
The outside diameter is 3/4",1",1-1/4" or 1-1/2" and even more.
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2.2. The Reel The reel, with a diameter of about 2.5m (8 ft), can contain approximately 6000m (20,000 ft ) of 1" tubing.
It is driven by a hydraulic motor.
The tubing coiled up on the reel is connected by a reel swivel to the rotation axis.
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This allows the connection with a pumping system and enables pumping to continue while the reel is coiling or uncoiling the tubing in the well.
There is a system to sprinkle corrosion inhibitor onto the tubing when it is pulled out of the well.
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2.3. The Injector head The injector head uses a friction drive system actuated hydraulically (a series of half slips contoured according to the tubing diameter and mounted on two chains).
Depending on the wellhead pressure, it must be able to:
push the tubing into the well as long as the tubing weight is not greater than the force
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exerted by the pressure in the well on the total tubing cross-section. •
then to support it.
It is also equipped with a guide for the tubing (or gooseneck), a straightener, a depthometer and a weight sensor.
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2.4. The safety assembly The assembly consists of a stripper (components with sealing elements) that provides a seal in the dynamic phases (tubing being run in or pulled out) and a stack of ram-type BOPs that fulfill the safety function in the static phases.
The stripper is located above the BOPs.
The sealing element is actuated hydraulically at a pressure that depends on the wellhead pressure.
The element is in two parts and can be changed during tripping after having closed the relevent BOP.
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The BOP stack is made up of four preventers, from bottom to top:
pipe rams (that close and seal on the tubing)
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slip rams ( that hold the tubing )
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cutters ( that cut the tubing )
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blind rams ( that give a total closure either over a cut tubing or by crushing the pipe )
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Furthermore, there are also valves to equalize the pressure on either side of the rams and to allow pumping
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through the tubing if it has been cut off, thereby placing the well in safety condition.
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2.5. Ancillary surface equipment Other equipment is required such as:
A hydraulic crane with a collapsible boom or a hydraulic actuated mast to install and retrieve the safety assembly and the injector head.
A
power pack consisting of a diesel engine driving a hydraulic pump that provides the energy required by all the equipment.
An
adjustable height control cab placed behind the reel, the injector head and the safety assembly.
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All
the controls, guages and warning lights need to operate, monitor and keep the unit safe as a whole are located there.
If need be, a nitrogen unit, i.e. in particular low temperature storage tank for liquid nitrogen, a pumping unit with a cryogenic booster pump and heat phase converter.
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2.6. Downhole accessories. In all operations where the coiled tubing is exposed to the well's pressure and fluids it is recommended to use check valves mounted on the lower end of the tubing ( at least two for safety ).
This helps limit the risks in the event of a leak at the reel swivel or in the part of the tubing that is on the surface.
Other accessories can also be used: •
jet tools with radial and/or end nozzles to get a better cleaning action. ������� ����� �� ���� ������ �� �������������
hydraulic motors used with a drilling bit (however, in the case of a small coiled tubing diameter, a large part of the hydraulic power is consumed in tubing itself before it gets to motor and little weight can be put on the bit ).
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Overshots
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These accessories are connected to the tubing either by crimping or by a screwed assembly.
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3. Operating consideration The tubing works in the plastic range at the reel and at the gooseneck on the top of the injector head (to remain in the plastic elastic range with a 1" pipe a curvature radius of approximately 5 m, or 17 ft would be necessary and thus is not feasible in practice).
As such, pipe fatigue must be taken into account.
It is related more to the number of times the pipe is coiled and uncoiled rather than the depth it has worked at.
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Experience seems to show that the pipe can be coiled and uncoiled at least 29 to 40 times without any excessive risks in routine operations.
Acid jobs and other specific conditions would curtail this number.
Beyond this point the pipe should be scrapped.
The following operational limits can be mentioned: Even if the capacity rating of the unit allows higher speeds, it is advisable to limit speeds to 20 m/min (63 ft/min ) when running in, 40 to 50 m/min (130 to 165 ft ) when pulling out and 0.3 m/min (1ft/min)
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penetration rate in sediments (in 5 to 6 m, or 16 to 20 ft stretches at the most ). As for internal yield pressure, the limit is generally 5000 psi, which allows circulation flow rates of around 80 liters/min (1.5 bpm ),160 liters/min (1 bpm ) and 240 liters/min (1.5 bpm ) of fresh water for coil tubings with a pipe diameter of 1",1-1/4" and 1-1/2" respectively and a 5000 m (around 16000 ft ) reel.
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These are relatively small flows that may be too low to raise sediments properly (the required speed is around 30 to 40 m/min or 100 to 139 ft/min with water ).
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Viscosified water, foam or nitrogen slugs,etc, must then be used.
As for collapse pressure, the limit is generally 1000 or 2000 psi to take account of the influence of tension on collapse strength.
As for tension, depending on the diameter and thickness of the pipe the limit corresponds in fact to the weight of about 4000 to 5000 m (13000 to 16000 ft ) of tubing in air (unless the tubing is composite pipe, i,e. thinner at the bottom and thicker at the top .
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