The Downhole Safety Valve (DHSV) integrity tests of two water injection wells on the Nova subsea oil field on the Norwegian Continental Shelf failed after one month in operation. One of the two wells, W-1, also showed issues with the Injection Master Valve (IMV). The objective was to re-instate the functionality of all compromised valves as soon as possible.

First, the root cause for the malfunction was to be identified. Several hypotheses were developed and assessed, including mechanical and chemical issues. Both injectors (W-1 and W-4) are completed in the oil leg of the reservoir and have been cleaned up to rig before an injection test was conducted. The wells were then suspended for several months prior to initial start-up and commencement of water injection. Although wax inhibition was used during the clean-up, wax deposition at DHSV depth could not be fully discarded. Monoethylene glycol (MEG) has been deployed for hydrate mitigation after the injection tests and during initial well start-up. Pressure data indicated that at least partially, a column inversion within the tubing, from water to hydrocarbons, occurred during the suspension period. This observation gave support to that wax or hydrate deposition might restrict the DHSVs' flappers' movement.

Based on this hypothesis, an operation with an Inspection Maintenance and Repair (IMR) vessel was planned, organized and conducted within five weeks after the failed tests. The treatment concept included not only a wax dissolver, but also MEG and heated fluids to combine the benefits of temperature as well as chemical dissolution towards either potential type of deposit. Both wells were treated from the vessel as per plan. The operation successfully re-instated the functionality of all three compromised valves, allowing to safely commence water injection into the reservoir.

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