Wireline is a method of lowering specialised equipment into an oil or gas well, or raising specialised equipment from an oil or gas well. The principle of wireline is to attach a workstring or toolstring to the end of a reel of wire and by reeling out the wire the toolstring is lowered into the well. By either reeling in or reeling out the wire, the toolstring can be made to perform simple tasks downhole.
The toolstring consists of a variable combination of individual tools screwed together to form a working unit. A toolstring typically comprises a rope socket, a stem or sinker bar, an upstroke jar, a spang jar and a pulling and running tool. The rope socket attaches the toolstring to the wire. The stem or sinker bar comprises lengths of bars screwed together to give the toolstring weight and also provides a mass for jarring operations. The upstroke jar is a mechanical or hydraulically operated device which allows a delayed partial release of the upper portion of the toolstring to enable an upwardly directed high impact force to be generated by the toolstring. The spang jar is a very simple device and similar to an elongated chain link and allows immediate movement of the upper portion of the toolstring. Spang jars are used to allow upward or downward impact forces to be generated by the toolstring. At the end of the toolstring there are pulling tools and running tools. These various tools are available to be screwed directly to the end of the toolstring. These are used for either pulling equipment out of the well or setting and leaving equipment in the well.
Conventionally, there are two distinct types of upstroke jar available on the market. The first is a hydraulic jar and the second is a mechanical or spring jar. Both types of jar have different attributes and disadvantages.
The hydraulic jar is activated only when the bottom end of the jar is anchored and the top end is subjected to a constant pulling force. For simplicity the jar can be regarded as being a piston located in a cylinder which is filled with hydraulic oil. The piston, commonly known as the jar rod, is normally at the bottom end of its stroke within the cylinder, where the two are close fitting. Very limited fluid by-pass around the piston means that it takes considerable force and time to move the piston up the cylinder. The time factor allows a desired pull force to be reached before the piston reaches the point where the internal diameter of the cylinder opens out. When the piston reaches the opened out portion of the cylinder, the pulling force accelerates the piston to the top of its stroke where it will deliver an impact force upwardly when it is stopped by the jar housing itself. The piston usually contains a small check valve to enable a fast return stroke into the small internal diameter portion of the cylinder by allowing greater fluid by-pass in that direction only.
The advantages of hydraulic jars are that they are very versatile in use because a small pulling force will result in a small jar force and similarly a large pulling force will result in a large jar force. In addition, there is no need to remove these jars from the toolstring to adjust the release setting, as is necessary with mechanical jars. Hydraulic jars will also fire whatever the value of the pulling force that is used or is available.
However, hydraulic jars still have a number of disadvantages. As there is a seal around the jar rod itself, the ability of the jar to function depends on the life time of this seal. This seal is subjected to considerable wear and tear due to the violent motion of the jar rod. To ensure relocation of the piston back into the lower reduced internal diameter the jar rod is usually fairly short and this compromises the resulting jarring force available. Also, the whole tool is full of hydraulic oil which makes maintenance of the tool difficult.
Mechanical jars contain no hydraulic oil. The jar therefore has no seals. Again the jar can be regarded as a piston within a cylinder however this time the piston is held at the bottom end of its stroke by various mechanical mechanisms which are usually dependent on the manufacturers. Usually the mechanism comprises a coil spring or spring washer stack arrangement as part of the mechanism. The spring is used to pull against to allow the piston to be released and travel up its full stroke within the main housing of the jar when a certain known pull force is reached. This value is usually dependent on the spring rate.
The advantages of the mechanical jar are that there is no seal around the jar rod and there is an unhindered travel of the jar rod up to its full stroke, i.e. there is no hydraulic oil to be by-passed. It is also possible to obtain a larger jar rod stroke than can be achieved with a hydraulic jar.
However, there are also a number of disadvantages associated with mechanical jars. Mechanical jars must be removed from the toolstring in order to be adjusted to the desired pull force for activation downhole and the pull force at which the jar is set to fire must be applied to the jar before the jar will work. This value is often difficult to predict especially when it is used deep downhole. There is also a difficulty in maintenance due to the large number of parts which comprise the jar.
Coil tubing operations are similar to wireline operation and also use jar mechanisms to enable high impact forces to be generated by the toolstring during the coil tubing operation. However, with coil tubing operations there is the additional complexity that it is desirable to pump fluid through the toolstring during the operations, and this feature has been difficult to combine with conventional jar mechanisms.
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