Downhole Tractors and associated powered Intervention Technologies
The early 1990s saw the introduction and growth of highly deviated and horizontal wells in many oil & gas fields across the globe, required by offshore (particularly subsea) completion design and/or the desire to provide maximum reservoir contact for optimised reservoir drainage.
This meant that well intervention using gravity feed electric line or slickline deployment was no longer an option. As a result, coiled tubing became the main deployment method, however this was not an ideal solution considering its large and complex spread of equipment, crew and logistics. It was also a time consuming, less precise and excessive option if there were no pumping requirements associated with the intervention being undertaken.
This led to the development of electric line (e-line) deployed and powered tractor technology, designed to convey a “passenger tool” to task depth over the highly deviated section of the well, examples being logging tools, perforation guns or plugs and packers. The tractor was designed to fit in as an integral part of an e-line toolstring using the same surface deployment equipment, hence requiring minimal resources over and above that of a standard e-line operation with regards to equipment, personnel, logistics and time – providing an efficient, light, agile and cost-effective solution.
Principle of Operation
Downhole tractors utilise electrically powered drive mechanisms, usually with modular drive sections which provide traction force. The tortuosity and lateral extent of the wellbore, coupled with e-line cable weight and friction, determines the tractor force requirement to convey the toolstring and cable to depth. This in turn determines the number of drive sections required. Once the tractor has conveyed the toolstring to task depth, power is simply switched back to the passenger tool and it is operated as normal, with the e-line winch puling the tools out of hole.
There are also hydraulically driven coiled tubing tractors designed to “pull” coil beyond the point of helical buckling, experienced in extended reach laterals.
Following its first deployment in 1996, e-line tractor conveyance saw exponential growth in the North Sea, and soon became an accepted conveyance solution globally. Additional tractor tool sizes emerged, conveying tools capable of handling a wider spread of wellbore dimensions, both smaller and larger. Technology developments also evolved beyond conveyance only purposes, delivering a range of powered electrohydraulic mechanical tools providing mechanical dexterity with sufficient force to carry out rotational and high strength push/pull capabilities. These grew over the years to cover a range of intervention services, be that for wellbore cleanout, completion manipulation or wellbore access and recovery applications. These services were deployed not only in high deviation scenarios but also in vertical or low deviation wells where the technology efficiencies and precision was more suitable and hence preferable.
Examples of such powered intervention tools are
- debris removal tools, with and without collection e.g. brushes and honing devices, debris mills, suction tools
- bi-directional strokers for downhole valve manipulation
- milling tools for completion component removal
These technologies are now being used in intervention work scope across all phases of a wells life, engaged in greenfield drilling support and new well completion to brownfield production enhancement, repair and maintenance eventual P&A operations. Ingenious developments and the application of new technologies continue to bring enhancements as well as new capabilities to these conveyance and intervention services.