Autonomous tractoring

March 11, 2014

Wireless tractor on catwalk prior to picking up to the rig floor at onshore well test site. (BP)

The first version of a new autonomous tractor unit for well work, developed by Welltec and BP, undertook its first commercial deployment in January. Elaine Maslin found out more.

Well intervention operations continue to pose significant risk, cost, access, and complexity challenges to operators, regionally and globally.

However, the rewards from well intervention activities through incremental oil or gas rates can be significant, especially when compared to rig based drilling and completion activities.

The spread of well intervention equipment and application methods will largely dictate an operation’s complexity and technical challenge, when deploying coiled tubing, hydraulic workover or wireline/slickline methods. Increasing use of light well intervention vessels (LWI), for subsea wells further increases costs and deployment times. For the well intervention industry, arguably the last true innovation came with the introduction of the well intervention tractor in the mid-1990s, says Callum Munro, senior well engineer, BP.

“Tractors transformed horizontal well interventions from large coiled tubing operations to more nimble wireline operations, using fewer personnel, less equipment, and crucially resulted in less risk,” he says. “Tractor operations became ‘business as usual’ and with it, further innovations helped to increase the applications, from logging operations to a full suite of mechanical services, which includes milling, plug-setting, sleeve manipulation, perforating, and well clean out activities.”

BP was aware of several projects, which had started investigating the idea of down-hole robots, or autonomous intervention units (AIU). Development work in the US resulted in a caterpillartracked prototype tool running in and out of a US land well, proving the theory, Munro says. The project was developed by IIC, and the tool system was called micro rig. Further development of this system however failed to materialize. In 2009, BP and Welltec collaborated in a project to further develop Welltec’s patented Well Tractor system into an autonomous unit. Additionally, BP trade-marked the term “Wireless Wellwork,” for use in all downhole operations, where a physical link to surface was not used. Welltec also moved to adopt the service “independent well tractoring” or IWT.

The project aligned both company’s goals to establish a robotized means of intervention—i.e. carrying out activities without any physical links to surface, while providing data from the tools to surface, and ultimately being able to optimize the tool from an onshore desktop. Further development could result in robots “living” within the well space, to be activated as and when they are required to undertake either mechanical or surveillance functions.

The tractor during testing at the onshore live well site, UK. (BP)

Key advantages of this technology are, according to the project partners:

“The complexity and technical challenges of the project were not underestimated and as such three development phases/versions were identified, namely V1, V2 & V3,” Munro says.

V1 comprised standard tractor assembly, tethered to slickline, with forward drive only, and additional battery and mission controller sections, to control start & stop functions.

Its design was based on Welltec’s 2 1/8in. standard tractor system, including the wheels, body, and connections.

The aim was to deploy the tractor in a horizontal well, using traditional slickline methods until “lock up depth” was reached, i.e. the horizontal point where gravity would no longer pull the tools into the well bore. The mission controller would then signal tractor start up and the tool would be tractored into, and along, the horizontal well bore. The tool would then be recovered on slickline.

V1 was yard-trialed at Welltec’s R&D facility in Allerød, Denmark, in a 590ftlong, 5 ½in. test loop, then taken to an onshore well-site, where it was deployed and recovered successfully. V1 was then tested in a live well, with 300psi closedin tubing-head pressure, on slickline. A two-stage bottom hole assembly (BHA) rig up was completed, including permanent circulating equipment.

“The idea was to rig it up, put it in the lubricator, and set it so that, when it sees the pressure test in the lubricator, it goes through a test run–the tractor powers up, the wheels come out for about 30 seconds and then retract,” Munro says. At this point the tool is ready to be deployed and run into the well.

The starting system using a pressure switch, which is activated by a set value of hydrostatic pressure, attained or calculated once the tool is in the well. The tractor only stops once battery life is depleted. While operating, the tractor speed can be managed by maintaining back tension on the slickline.

Forward/reverse wheeled sections of wireless tractor. (BP)

The unit performed two runs, tractoring for about 1600ft, at 35-40ft/min on 40 minutes battery life. Debris in the well meant it couldn’t be run to the planned depth, limiting function-testing on the motor controller start stop, and a multifunctional timer.

“Unfortunately due to well debris the tractor didn’t reach target depth however important lessons were learned on rig up procedures, battery re-charging time along with speed and battery life of the unit ” Munro says.

The goal on V2 was to release the tool, at lock up depth after it had been conveyed there on slickline.

“It would then tractor in to a desired depth, while recording pressure and temperature data, before tractoring back out to the initiation depth, at which point it would be recovered using slickline or electric line,” Munro says.

“V2 was a significant technical challenge for the Welltec engineers, since it required a release mechanism, a reverse tractoring module, additional battery power, and a casing collar locator for depth correlation, in this case Welltec’s well hardware scanner (WHS).”

V2 was built and then tested in Denmark and Aberdeen. Despite several glitches with the release mechanism, and the loss of data during the yard trial, V2 was deployed, operated in forward and reverse modes, and recovered using electric line.

V2 was then taken to field trial onshore, for deployment with a memory production logging tool (MPLT), to simulate a full toolstring rig up, and to identify possible handling issues, due to the length of the assembly (>45ft). The rig up was successfully achieved (in two stages) and preparations made to run the tool, after a slickline run prior to the rig up identified a 950m lock-up depth.

“However, while in the lubricator the tool prematurely released during pressure testing, later traced to a firmware / software interface issue,” Munro says. “The back-up tools were then deployed, but failed to release at ‘lock up depth.’ This was traced to failed insulation on the release device. Time constraints at the well site meant that further operations were curtailed, and the equipment was returned to Denmark and subsequently repaired and retested in Welltec’s temperature/ pressure vessel successfully.”

V3 is now under development, and will include downhole power generation (DPG), so that the unit can recharge itself. The goal for V3, measuring 60ft or less, is for it to be automatically-deployed and retrieved from surface, without slickline, and with shallow wireless communications.

Trials on the DPG were successfully carried out in December, in Welltec’s test loop at Allerød. Using a production water flow of 1bbl/minute, 65-70-watt output was generated. Shallow acoustic communications were also trialed at a 180m range. The next step will be to integrate the DPG into the tractor, and for it to recharge the tool’s battery packs after depletion of power in the test loop. The future is V4. This will also be surface-launched, with some limited mechanical services, potentially a stoker, with real time data transmission to surface via wireless communications.

Munro says the key challenges going forward are: battery power life expectancy; wireless communications; and developing a surface deployment system.

“Battery technology for down hole is complicated by temperature issues, but we are looking at other batteries, which would give us a longer charge at higher temperatures,” he says.

Reducing the length and weight of the tractor will also be an aim. “We are accepting it is 20m, but once we have proved we are able to charge and operate it, the next step is to reduce the length,” he says. “We are looking at composite materials to make the assembly lighter, increasing the battery life, while reducing the battery pack length, and also optimizing the wheeled sections, to make them independently bi-directional.

“Longer term the partners see increasing sub-sea applications for this technology, improving functionality, reliability and cost effectiveness. Ideas also include ‘disposable AIU’s’ and of course permanent AIU’s living within the well bores.” V1 has since been used on its first commercial deployment was with Petronas, in January, on a field in Malaysia.