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In early 2019, Eni began a new development campaign on the Nene Marine XII Field offshore Congo. The development was to include multiple wells. However, after completing the second well of the campaign, a leak was observed on a 4.5-inch tubing retrievable, surface-controlled, subsurface safety valve (TRSCSSV). The valve could be operated to open, but a hydraulic leak was preventing it from remaining open.

A TRSCSSV is a rod-piston, curved flapper-type safety valve designed to shut in a well in the event of uncontrolled flow caused by equipment failure or damage. An integral part of the completion string, it is controlled by a single hydraulic control line. Application of control-line pressure keeps the valve in the open position; when pressure is bled off, the valve closes to protect property, personnel and the environment.

To fix this issue, the newly settled completion had to be removed. To change the completion, a mechanical cutter must first be run in the hole to cut the cut-to-release packer. However, due to the safety valve malfunction, it was first required to lock open the valve by deploying a specially designed deformation lock-out tool intended to deform the safety valve flow tube. Its purpose is to prevent the flapper from traveling back into the close position by deforming the flow tube, thus keeping the flapper in a lock-open position.

The well was equipped with a tubing hanger treated with a plasma nitriding process, which increases the tubing wall hardness, equivalent to 13 Cr. The tubing hanger had a minimum diameter of 3.81 inches (97mm). However, the deformation lock-open tool (which had an outer diameter of 3.813 inches) could not be run through it. Therefore, the tubing hanger had to be milled from 3.81 inches to 3.975 +/- 0.1 inches to allow the deployment of the deformation lock-out tool.

To overcome this, Eni issued a challenge to the service industry to enlarge the hardened tubing hanger while also preserving its mechanical integrity. Another challenge was the high cost of the drilling unit that was already on site, so any solution needed to be deployed rapidly.

The chosen solution utilized a wireline deployed milling tool string equipped with a diamond coated bit. The surface readout system allowed fine control of the milling operation. Then the combinability of the tools allowed for adequate planning of potential fish recovery while retaining well barriers on a live well. Additionally, the size of the equipment allowed for rapid overseas mobilization. The milling operation was completed in just a single run, with a total milling time of 1 hour and 47 minutes.

Case study

The operator accepted the value proposition of the Wireline Well Tractor with the Well Miller to enlarge the tubing hanger and allow the safe deployment of the deformation lock-out tool. The tubing hanger had three different internal diameters, with the top one slightly bigger than the target area. Therefore, the milling operation had to be started from the top 99.3-mm section to avoid getting wedged stuck due to the small clearance between the upper section and the milling bit to be used for the lower section.

Welltec Well Tractor
An e-line deployed, self-propelled, robotic device, Welltec Well Tractor overcomes the limitations of traditional e-line operations in highly deviated wells by driving the tools to the toe of the wellbore. (Source: Welltec)

Because of the speed required, the service company engineering team had to identify a bit capable of milling the target hardware. Within a couple of hours of technical discussions, the team confirmed the parameters of the milling bits suitable to mill such material, and corresponding bits were sourced for the job.

As the operation was to be carried out at the surface with a live well, a customized rigup setup was required. To comply with the operator well integrity policy, two gate valves were placed above the xmas tree and one above the rig floor.

The gate valves worked as barriers in case the milling string became stuck at any stage of the milling operation. Additionally, it was required that the pressure control equipment (PCE) connections where the milling tool string was operating were interconnected with flanges instead of threads, as threads could risk backing off due to the counter torque generated by the milling string.

Furthermore, paint marks across the joints were added and monitored during the milling operations to ensure this was not an issue.

Equipment deployment 

The operation required highly customized tools that were not immediately available in Congo, and due to the tight schedule, an extensive, detailed and precise job mobilization plan was prepared. Equipment movement in any of the African countries represents a challenge to all the services providers. For this operation, a specialized team was mobilized to accelerate the complicated custom exportation and importation process.

The tools came from multiples countries worldwide. Thanks to the compact nature of the wireline tools, which are modular and owing to the perfect alignment between the operations and the supply teams, the complete equipment list was made available in the country in time for the operations, with enough spare time to function check the complete equipment. During the entire process, the operator was constantly updated to coordinate with the offshore drilling and completion team.

Established technology

Wireline conveyed milling tool string technology was introduced in 2004 and is commonly used on complex milling operations where high precision and efficiency is required. The tool that is designed for milling this type of hardware consists of power and drive sections operated by electronics components that provide the rotation to the milling bit.

The tool is deployed using wireline, self-propelled, and it has two hydraulic systems. The first system oversees to extend/retract the tool wheel arms, which are designed to engage the tubing walls to provide traction and lateral force. The second system oversees providing the rotation of the wheels, making the wireline tractor (and the entire downhole tool assembly) move forward, providing the weight on bit.

Welltec Well Tractor
The Welltec Well Tractor can carry out operations throughout the entire length of the wellbore regardless of the deviation and extension of the well. In openhole wells, the Well Tractor can apply the force required to push large tool strings as well as navigate past washouts. (Source: Welltec)

Because of the tight clearance of the bit with the PCE and the Xmas tree, it was required to drift all the accessories with the bit to be used. After the drift was successfully conducted, the milling tool string was rigged up and run inside the PCE. Challenges were experienced while performing the correlation passes due to the tight clearance in the PCE/Xmas tree. The rig did not appear perfectly centralized over the wellhead, leading to a slight bend in the PCE.

The PCE was straightened to reduce its bending. This improved the clearance and enabled the milling string to tag the tubing hanger. The tool string was initiated at the top of the tubing hanger and immediately started milling. The surface readout allowed the team to monitor real-time progress and precisely know when the bit had passed the tubing hanger.

The total milling operation was carried out over two days, mainly driven by the complication with the PCE setup. Despite the operational challenges during the complex rigup setup, the milling operation was conducted within 4 hours and 20 minutes, and the actual milling of the tubing hanger was done in 1 hour and 47 minutes. The milling job was considered a significant success for the operator since it achieved the objectives through excellent technical and operational collaboration between the service vendor and client from job design to execution.