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Abstract (overall description) The downhole safety valve (DHSV) is a critical well safety component which prevents the uncontrolled release of reservoir fluids in the event of a surface disaster or other near-surface well integrity failure. The DHSV described in this case study is the common unidirectional flapper design. This fail-safe flapper design employs a flow tube or control sleeve which is moved down using pressure from surface control lines, pushing the flapper open when the well is producing. Spring force within the DHSV pushes the flow tube up when no pressure is applied from surface, allowing the flapper to close and isolating the top of the well from the reservoir fluids below. The sealing and opening function of the safety valve can be lost due to different factors such as scale build-up or control line pressure failures. DHSV failures can leave the flapper stuck open, prevent proper sealing when closed, or prevent any surface control of the flapper position. The case study presented in this paper describes an intervention with an electromechanical wireline shifting tool on a subsea well with a failed DHSV. The well had been shut in for several years due to the inability to function the safety valve open using control line pressure from surface. Due to it being a subsea well, the cost and complexity of intervention as well as the difficulty of diagnosis was significantly increased relative to dry tree wells on platforms and onshore. There were a number of hypotheses in place regarding the potential failure mechanism of the valve, but the primary theory was that the flow tube was stuck due to either corrosion or scale. The conventional form of intervention in this situation would be a slickline DHSV remediation package to treat, clean and physically shift the flow tube. However, slickline intervention has limitations due to the lack of real-time tool control or downhole measurement making diagnosis of the downhole condition difficult to impossible. As the failure cause was unknown, and efficiency was of especially high importance due to the subsea nature of the well and associated costs of intervention, a project was initiated to innovate on an existing electromechanical sleeve shifting tool to allow it to shift a safety valve flow tube. The electromechanical sleeve shifting tool has inbuilt sensors and control modules for shifting distance and force which would increase the efficiency of the intervention and provide critical downhole information on the failure mechanism of the valve. To enable smooth bore shifting of the DHSV flow tube, special friction grip pads had to be designed, as well as a custom clamp-on no-go for depth accuracy. This setup was then rigorously tested with high opening forces to assess whether there was any risk of flow tube deformation. In the case study, the smooth bore shifting configuration of the electro-mechanical sleeve shifting tool was successfully used to investigate the failed DHSV in the subsea well in the UK North Sea. This enabled surface tool control and feedback during the manipulation of the flow tube and direct measurement of the forces applied and distances that the flow tube was shifted. Realtime control and feedback were key to minimizing the risk of damage to the flow tube and allowed real-time decisions to be made. The wireline tool measurements clearly showed that the flow tube was free to move full stroke in both directions and indicated that the failure mechanism was likely in the control line. The unique design of the electro-mechanical sleeve shifting tool allowed the setting of the tool in "float" mode where it would measure displacement and force whilst being free to extend and retract. Surface pressure was exacted on the control lines, and the tool showed no movement or force downhole, indicating definitively that the failure mechanism lay with the surface control lines and allowing the operator to confidently plan the next intervention steps.
Soerensen et al. (Mon,) studied this question.