Restoring well integrity Congo-style

October 1, 2012

An innovative cased-hole expandable liner system has been deployed to repair casing corrosion in an offshore Congo well. Weatherford’s Scott Durment and Doug Farley explain why and how.

Casing damage in older wells comes in many forms and from many sources, including deformation or complete separation due to formation movement; erosional wear by workovers or drilling activity; and corrosion due to the properties of the well effluent.

An operator working in a mature shallow-water (197ft) field off the coast of the Republic of Congo needed a cost-effective and long-term solution to repair a section of corroded casing at a depth of 385m to 423m on one of its wells. Over the course of its nearly 40-year operating history, the low-pressure reservoir, located 820-1312ft below the sea floor, has produced viscous oil and experienced significant water encroachment with water cut of 96% due to corrosion. In addition, the well in question was in a remote location and produced from an offshore platform with both deck-space and weight limitations, which prevented the operator from using a typical workover rig. This, combined with the presence of H2S gas, makes production operations challenging and calls for platform personnel to consistently remain vigilant of any potential health, safety and environmental risks.

The objective was to repair the corroded casing to regain well integrity, commence oil production and shut off water flow while maintaining a safe working environment.

In addition, given the economic balance between a mature-field remediation program and its cost, the desired solution also needed to be efficient and provide a long-term benefit that precluded the need for another intervention.

congo platformCongo platform showing the basic parameters of the rig, crane and blowout preventers.

Second skin for casing

Resolving these situations to get drilling or production back online requires an intervention. Several standard approaches exist, but have demonstrated varying degrees of success and may negatively impact the economics of the well. For example, additional liners, scab liners or straddle-pack assemblies all restrict the original hole size, thus reducing production.

In addition, if cement squeezes are not properly deployed, they can prove unpredictable requiring multiple squeezes to be run. Short term, the cement may break down. Longer term, they can become high risk, exposing the formation to high pressures and contamination of the reservoir from the cement during the squeeze.

Weatherford has developed an alternative casing-remediation solution with the MetalSkin cased-hole solid expandable liner system. Designed to bridge the casing-repair gap between cement squeezing and scab liners, the solid expandable system provides permanent isolation to the damaged section of the casing, regardless of the damage mechanism and provides a larger ID and better access to reserves than scab liners for future drilling, completion, production or injection operations.

The installation process involves positioning the liner system to the desired location downhole and applying surface pressure to expand and set an internal jacking system to anchor the liner. By using a downhole hydraulic jack there is no need for high-volume pumping services. The rig then simply pulls a cone through the liner, thus expanding it from the bottom up. The integral carbide anchoring mechanism positively locates the liner in position and prevents any further movement. Alternatively if the rig or work string is limited, the system can be reset and can be expanded with repeated stroking of the jack system.

The inner diameter of the expandable liner and the connections are never exposed to expansion pressure, thereby maintaining the integrity of the system. Because multiple joints of this cased-hole liner can be connected downhole, any length of casing can be isolated depending on well objectives.

The system is typically installed in one day and in a single trip. Because there are no darts, plugs or shoes required to generate pressure when the anchor is set, no drillout is required. The casing can be tested immediately and the subsequent operations can commence. Once installed, the cased-hole liner provides minimal ID reduction to the casing, which helps optimize completion designs for higher production or injection rates.

metal skinThe MetalSkin expandable cased-hole liner.

Planning and risk assessment

Because of the economic and long-term benefits of solid expandable liner, the company was selected to install an H2S-resistant, 51/2in x 7in, 29# expandable cased-hole liner in the Congo well between 1224ft and 1608ft. The remote location and safety concerns made the intervention more difficult and required significant pre-job planning.

Because of deck space and weight limitations and no present workover rig, special consideration and planning was conducted prior to the installation. The liner casing would be deployed using a mast with a maximum over-pull of 12te, and a maximum height under hook of 41ft . The equipment would be conveyed into the wellbore using a surface jack system that is unique to this field given platform’s limitations with a working travel of 63in, using 1.6m cylinders capable of 80 tons of force. Equipment handling and peripheral operations would be performed using a crane with a maximum load of 3.8te at 18m. Blowout preventers are located 6.6ft below the main deck.

In addition, prior to the installation, the company collaborated with the operator to evaluate the potential risks in working in the H2S environment. These risks were documented and a mitigation plan was implemented that was designed to address the potential release of H2S during the operation.

High priority was given to adhering to and implementing the safety policies developed jointly between Weatherford and the client. Safety meetings and job safety analysis (JSA) were conducted before each phase of the equipment and well preparation and prior to the expandable installation to ensure that all personnel were completely aware of the actions during each segment of the installation process. These meetings offered an opportunity for all personnel to ask questions, make comments or suggestions and be made aware that they were all empowered to shut down the operation if they questioned an activity or if they observed anything that could be unsafe. At times, work was stopped to ensure that safety remained the utmost priority.

doug farleyDoug Farley is global product line manager for Solid Expandable Systems at Weatherford. A graduate of Texas State Technical College, he has over 30 years’ experience in operations, manufacturing, and engineering of drilling and completion tools and has been involved in international manufacturing facilities start-up.
albert durmentAlbert Durment is a project coordinator for the Critical Wells Team Wellbore Construction group at Weatherford. Based in Houston, he has over 30 years’ oil & gas industry experience, with a diverse background ranging from positions as a floorhand, technical advisor, to his current role.

Liner installation

Pre-job operations included a clean-out trip with bit and mill assembly to ensure that all downhole obstructions or tight spots were cleared, as well as a caliper log to verify the inside diameter of casing down to and across the planned setting depth of the expandable liner.

Once satisfied with the condition of the wellbore and casing to accept the expandable liner, installation commenced. Thirteen connected liner joints were tripped in hole with a workstring to TD at a maximum run-in-hole speed of 60ft/min. The entire liner assembly was run 2-3ft past setting depth and picked back up to the setting depth mark on the pipe, which closed the slide valve in the string.

The liner was then expanded using a setting tool requiring 5000psi inside the workstring to achieve expansion and anchoring the liner. Once anchoring was confirmed, a jack system pulled the workstring out of hole so as to expand the liner in 5ft intervals. Expansion continued using an over-pull of 36-40 tons. During this time, the rig was pumping water in the hole to prevent the release of gas. The cone section was pulled out of the top of the liner on joint number 13, which was verified by a weight drop from nine tons to three tons on the indicator. The liner top was confirmed at a depth of 1224ft.


The expandable liner was successfully installed and expanded at the proper depth in a single trip, isolating the corroded zone. The installed liner provided a post-expansion drift of 5.265in to enable the well to be produced at a rate of 250b/d, with no water production. The minimal ID restriction in the wellbore post remediation allowed the client to continue production with the same completion strategy without having to downsize the electric submersible pump.

Finally, through continual communication with the client, all operations were performed without injury in a safe and professional manner, allowing the client to resume production with full HSE integrity. OE

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