Oil and gas wells can be serviced by the use of tools and devices run into the wells on a length of circular cross‑section steel line mounted on a powerful reel at the surface. ‑This is called a wireline unit.
The wireline is subjected to pressures ranging from a few pounds to several thousands of pounds per square inch. This pressure pushes against the cross sectional area of the wireline. If large diameter wireline were used, the weight needed to pull the wireline down the hole against this force would be too much. The small diameter wireline is elastic and this elasticity ‘s needed to assist in operating downhole devices. The large diameter wireline would also require large diameter reels and sheaves as the minimum bending radius of that wire would have to be large as well.
Operations that can be done by 'running' or 'pulling' the tools and equipment into and out of the well
bore by using a wireline are listed below.
· Measure the depth of the deepest well.
· Accurately measure bottom hole pressures and temperatures.
· Carry out gradient pressure and temperature surveys.
· Obtain fluid samples at any point in the well for laboratory analysis.
· Check tubing internal dimensions, used to aid in the detection of corrosion or scale
· Remove wax and sand deposits.
Besides the jobs listed above, a large number of special tools and equipment can be set, retrieved, or
moved to change the well status.
ADVANTAGES OF USING A WIRELINE
· It is more economical to use a wireline for workovers than a drilling rig because:
· A wireline unit can move onto the well site, rig up, do its job, rig down, and move off to
another well site in less time than a drilling rig.
· A wireline unit requires‑only 2 or 3 operators.
· A wireline unit normally requires no outside services or assistance to do its job as it is
self‑sufficient. In fact, wireline units are used as a support service to drilling and workover
WIRELINE RISK OF FAILURE
Wireline operations involve a large risk of failure if not done correctly.
This risk is due to the fact that the wireline specialist is working 'blind'. He has sensors and indicators
at the surface but he still has to operate tools and equipment that are on the end of a length of wire
several thousand metres away at the bottom of a hole in the ground.
The operator has only two movements available to him:
· He can run more wire into the hole.
· He can pull wire out of the hole.
Simply put, he can only move his tools up or down.
This risk is minimised by keeping good well records and maintaining all tools and equipment to the
This is the piece of equipment that operates the reel that contains the wireline.
Prime Mover (driver)
This unit supplies the hydraulic power. It consists of a diesel engine that drives a hydraulic pump.
The reel unit receives the flow of hydraulic oil from the pump via hoses and converts that
flow into rotational movement through a hydraulic motor.‑ The hydraulic motor rotates the reel that contains the wireline. Rotational movement direction is controlled through a 4‑ valve. Hydraulic horsepower (tension on the wireline) is controlled by a manually set, combination pressure regulator/bypass valve.
The line tension (pull on the line) and line speed can be varied through a very large range. The line direction, running into the well or pulling out of the well, can be changed instantly by movement of the 4‑way valve control handle. This can be compared to a car travelling forward at 100 kph and instantly changing direction to reverse at 100 kph. Such movements of the wireline are required when unexpected problems occur down the hole. This is done to prevent the wireline from breaking.
Many problems can occur while using the wireline. Some problems are caused by OPERATOR ERROR but others are UNPREDICTABLE PROBLEMS.
Operator Error Problems
Placing too much tension on the wire may cause it to break if its strength is exceeded.
Running an incorrect tool.
This may cause the tool to become stuck down the hole or fail to do its job.
Using the incorrect wire grade for the well conditions.
This can cause corrosion or embrittlement (becoming easy to break). If the wire then breaks it can fill the hole with small bits of wire.
Not equalising the pressure across the downhole devices.
This can cause the tool string to be blown up, or blown down the hole, resulting in broken wire and wire stuck in the hole.
Running a tool that is damaged or not correctly prepared.
This can cause the tool to be stuck down the hole or fail to do its job.
Running wireline that is too old.
This can cause the wire to break.
Note : All these problems can be put down to 'operator error', a man‑made mistake,
The success of the operation depends on the skill and experience of the wireline specialist
· A downhole device can malfunction.
· A downhole device can break due to metal fatigue or because it has become brittle.
· The downhole tubing can collapse or partially collapse trapping the tools.
· The tool or device that is thought to be in the well is not where it should be or it is of a different type than expected. (Well records are incorrect.)
· There is a tool or device in the well that should not be there. (Well records are not up to date.)
The diameter of the wireline depends on several factors.