What is a Downhole Tractor?
A downhole tractor is used for propelling well logging tools and other well intervention downhole tools. Well logging tools and well intervention downhole tools are deployed by the tractor generally in deviated or horizontal wellbores in order to perform a number of essential downhole operations in the well bore that can range from removal of debris and retrieval of downhole tools from the wellbore, to well logging which is usually performed to measure various properties of a hydrocarbon bearing rock formation in order to establish well flow potential. In addition downhole tools may be used to establish other downhole conditions including local temperature and pressure conditions under flowing and non-flowing conditions.
There are three means of powering and deploying downhole tractors. The first type is a mechanically driven device sometimes referred to as a ‘skipper’ which is run into the well on a conventional slickline and is driven forward into the well by manipulation of the slickline at the surface. This type of downhole tractor is recovered to the surface by overpull at the wireline (slickline unit at surface).
The second type is an electrically driven tractor powered by electric motors with an electrical supply provided by a conducting power cable installed to the surface. Drive force is applied to the wellbore or casing through hydraulically or mechanically driven wheels attached to extending arms which apply traction to the wall of the production tubing, casing or well bore driving the tractor forwards. Again this type of tractor is normally recovered to the surface by overpull at the wireline (electric line) unit at surface.
The third type of tractor is used with small bore tubing injected into the well at surface tubing know as coiled tubing. Conventional coiled tubing injection is restricted in the distance it can enter a deviated or horizontal oil or gas well due to a combination of friction (against production tubing, casing or wellbore), and the tendency for coil tubing to coil in the same manner as a spring under compression. This coiling of the tubing can limit the intervention distance to which injected coiled tubing can travel in a well bore. This is known as coiling, coiled tubing bunching or ‘bunching’. To overcome the problem of coiling or ‘bunching’, a coiled tubing tractor can be attached to one end of the coiled tubing in order to apply a force, so enabling the coiled tubing to remain straight and achieve much greater intervention distances into the wellbore.
There are two main types of existing downhole tractors suited to coiled tubing deployment. The first type is an electrically driven tractor powered by electric motors with an electrical supply provided by a conducting power cable installed in the bore of the coiled tubing. A drive force is applied to the wellbore or casing through hydraulically or mechanically driven wheels attached to extending arms. The wheels apply traction to the wall of the production tubing, casing or well bore.
The second type of downhole tractor is a hydraulically driven tractor which uses a series of sequentially phased hydraulic rams, with extending armatures that sequentially set against the tubing or well bore wall so drawing the tractor forwards, in a ‘caterpillar’ fashion. Once a first set of extending armatures is set (thereby providing an anchor to the wellbore or wall of the well tubing) hydraulic pressure is transferred to a hydraulic ram forward of the extending armatures. This urges a second set of armatures in a forward direction, until the limit of their forward extension is reached. Hydraulic pressure causes flow to pass to a second set of extending armatures which expand (under the hydraulic pressure), anchoring the tractor against the wellbore or tubing wall. When the extending armatures are fully set, the hydraulic ram is retracted pulling the coiled tubing forward in a ‘caterpillar’ type motion. If required, multiple tractor sections can be fitted to a single well intervention tractor tool string.
Both systems are effective but each suffers from inherent problems.
Electrically powered systems suffer from problems associated with electrical motor burn out and/or time delays due to a need to stop the intervention process to allow the electric motor cool down. Wheel driven well intervention tractors are also restricted by the limited traction they can apply to the tubing wall or well bore, due to the small foot print of the drive wheels. Such relatively small wheels can result in a loss of traction and ‘wheel spin’, frequently causing the electric motor to overheat or burn out. As a consequence the mass of the payload that can be conveyed by electrically powered wheel driven coiled tubing tractors and the amount of traction force they can exert, is limited.
Hydraulic tractor systems tend to be more powerful than electrically driven tractors, however the safe intervention distance will always be limited by the overpull available at the surface and the integral strength of both the coil tubing and tool connections. Excessive surface overpull can result in coiled tubing and electric line failure/parting, and the tractor, and payload becoming detached from the coiled tubing.
Accordingly unless a surface controlled ‘reverse gear’ is available to the operator which has a means of proportional power control, (i.e. a control system where the power output of the tractor does not outpace the overpull rate from surface) powered egress out of the wellbore will not be reliable. Consequently well logging interventions that use these types of tractor systems are usually only performed in a single direction (that is coming upwards out of the wellbore) under surface overpull, so as to provide ‘smooth’ readings, when the tools are being withdrawn from a well bore. Under these circumstances such downhole tools require retrieval during a sub-assemblies event ‘fishing’ well intervention costing both rig time (therefore lost production) and money, as well as possibly jeopardising the tool and indeed the well.
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