Ultradeep-Field Study: Extreme-Underbalanced HP/HT Coiled-Tubing-Conveyed Perforating
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At an initial stage in a project to carry out the first high-pressure/high-temperature (HP/HT) field development offshore Malaysia, a locally conventional initial design considered the use of coiled-tubing-conveyed perforating (CTCP) to perforate a 7-in. liner and then to complete the well in open hole with overbalanced completion fluid. This solution posed several challenges that would have affected the safety of the completion process and the future integrity of the well architecture. A different approach, in which a fully cased hole well was completed in underbalance and then perforated once the completion was installed and tested, was used by the operator for the first time.
The KN field is located west/northwest of Labuan offshore Sabah. The operator has been tasked to develop the KN ultradeep HP/HT reservoir (pressures exceeding 12,000 psi, temperature exceeding 300°F). The well production-casing envelope consisted of a single 10¾-×10⅛-×9⅞-in. production casing, while the reservoir section was drilled and then covered with a 7-in. production liner. This liner was cemented, secured at the top with a liner hanger, and secured at the bottom with a bridge plug.
Once these barriers were installed, the next operation in the well was to run a wellbore-cleanup assembly with a scraper, brushes, magnets, and an inflow-test packer. After wellbore cleanup, the drillpipe volume was displaced with inhibited fresh water, an initial inflow test was conducted, and, then, after good indication of well integrity, the well was displaced completely to inhibited fresh water. Immediately, an extended inflow test was conducted. At the time of the inflow test, the well was in an underbalanced condition of 6,000 psi at total depth. Fig. 1 shows the results.
The completion philosophy for the ultradeep well was a single completion string to be run in the closed well immediately after the wellbore-cleanup tools were out of the hole. After running the completion, the following operations were performed:
- Setting the tubing hanger
- Setting the production packer
- Pressure testing the production packer from above and below
- Nippling down the blowout preventer (BOP)
- Nippling up the tree
- Function and pressure testing the wellhead system
In the case of this well, these operations were conducted safely and within the allocated time and cost as per the approval for expenditure.
CTCP was implemented in conjunction with a dedicated gun-deployment system to complement the underbalanced-completion approach. Conventional retrievable-tubing-conveyed perforation was not an ideal solution for the HP/HT gas well because of the potential risk of gas migration during the running of the completion. A well-control situation during the deployment of the completion would have been difficult to manage.
The through-tubing approach was selected because the production tubing and accessories could be installed and landed safely in a controlled environment. Furthermore, additional barriers would be present, because the completion and tree would be already installed and pressure tested.
The combination of coiled tubing and the gun-deployment blowout preventer enabled the deployment of a long perforation interval (more than 400 m) in a single run without the requirement of additional completion equipment (i.e., a lubricator valve) that might have reduced the reliability of the HP/HT completion system.
Another major advantage of the CTCP system was its ability to perforate the complete interval in underbalance.
Design and Work Preparation for CTCP
CTCP is a widely used technique for HP/HT wells in the North Sea, the Gulf of Mexico, and the Middle East. However, in the case of Malaysia, the technique was applied for the first time in this well. This created a series of challenges ranging from quality control to sourcing a specific type of coiled tubing able to endure great depth and considerable explosive impact.
To overcome these challenges, a strategy was implemented during the early days of the field-development project. The most important initiative was to replicate the HP/HT completion design in less-complicated wells of the same field. This allowed the service provider and the rig contractor to gain experience in the system with the local crews and to register a series of lessons learned that were then implemented for the HP/HT operation.
A gun size of 2⅞-in. was selected for the 400 m of total interval to be perforated. To confirm that the reservoir depth could be reached with the selected gun size and the coiled tubing in use, a series of simulation runs was performed, simulating a range of environments. The results from these simulations are presented in the complete paper.
At the beginning of the project, the use of 2⅞-in. hexanitrostilbene charges was considered, given the reservoir conditions. However, these charges presented some disadvantages from the design data and simulation, which, combined with the maximum allowable gun size of 2⅞‑in., provided a limited depth of penetration, thus raising the possibility of potential underdelivery of required production.
After further evaluation of the temperature conditions and the potential time of exposure of the charges at the reservoir depth, and after analyzing all the potential options, the contractor proposed a new type of cyclotetramethylene trinitramine charge that could resist the reservoir conditions for up to 10 hours (enough time from reaching the depth to actual detonation as per design), which provided a maximum potential depth of penetration of 38.6 in. These charges proved to be very effective during deployment, with all charges firing and with well-testing and production confirmation of no skin or near-wellbore damage.
To achieve the 400 m of perforated interval in a single run, a modular deployment mechanism was used. The system uses the combination of a deployment BOP and a gun-detonation transfer connector to deploy the gun sections. The gun bodies are regular 2⅞-in. bodies that transmit the detonating sequence to the charges with a primacord. However, the deployment-system mechanism, which is initially sealed, operates by transferring the explosive sequence through a dedicated shaped charge that detonates into the next section, effectively transmitting the detonation. In this well, given the benefit of a great lubricator length, 12 latching sequences were carried out successfully.
It has been established from the experience of the KN ultradeep well that at least 4 days are required for the complete rig up of the coiled tubing and the gun-deployment system. During the back deployment of the guns, every time a gun body was disconnected, the lubricator was under high pressure (up to 10,000 psi) from the accelerated well percolation of gas from the reservoir. In order to bleed out the lubricator, a dedicated bleedoff line was installed and connected to the well-test choke manifold. Every time the lubricator was bled off, the fluids under pressure (a mix of inhibited fresh water and reservoir gas) were sent to the well-test separator and the hydrocarbons were flared through the burner booms. As part of the well design, the bleedoff line was connected directly to the well-test choke manifold. This created an additional high-pressure volume, given the length of the line. One major recommendation from this experience was to install a manual choke valve close to the lubricator in order to keep the bleedoff line depressurized at all times, and to keep the well-test choke open. In this way, the lubricator is depressurized from the rig floor, increasing the safety of the operation and significantly reducing the bleedoff time for the line.
Another major challenge for the perforating system was achievement of a good depth correlation. For this, a casing-collar-locating logging system in real time (by use of a solenoid system activated by pumping pressure) was used. This system was backed up during a dummy run with a memory logging tool attached to the bottom of the dummy-gun assembly.
Ultradeep-Field Study: Extreme-Underbalanced HP/HT Coiled-Tubing-Conveyed Perforating
01 April 2018
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16 April 2018