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Well control stack for CT operations
The well-control stack system is a critical part of the coiled tubing (CT) unit pressure containment package and is composed of a stripper assembly and hydraulically operated rams, which perform the functions described next.
For typical well-intervention service, the four ram compartments are equipped (from top down) with:
- Blind rams
- Tubing shear rams
- Slip rams
- Pipe rams (Fig. 1).
The blind rams are used to seal the wellbore off at the surface when well control is lost. Sealing of the blind rams occurs when the elastomer elements in the rams are compressed against each other. For the blind rams to work properly, the tubing or other obstructions across the ram bonnets must be removed.
Tubing shear rams
The tubing shear rams are used to mechanically break the CT in the event the pipe gets stuck within the well-control stack or whenever it is required to cut the tube and remove the surface equipment from the well. As the shearing blades are closed onto the CT, the forces imparted will mechanically yield the body of the tube to failure. The cut is deformed and typically must be dressed to return to the proper geometry.
The slip rams should be equipped with bidirectional teeth, which, when activated, secure against the tubing and support the weight of the CT and bottomhole assembly (BHA) below. An additional utility of the slip rams is the ability to close onto the tube and secure movement in the event that well-pressure risks blowing the tubing out of the borehole. The slip rams are outfitted with guide sleeves that properly center the CT into the grooved recesses of the ram body as the slips are being closed.
The pipe rams are equipped with elastomer seals preformed to the specified outside diameter (OD) size of CT in service. When closed against the CT, the pipe rams are used to isolate the wellbore annulus pressure below the rams. These rams are also outfitted with guide sleeves that properly center the CT into the preformed recess as the rams are being closed.
Assembly of well-control stack
Typically, a kill-line flange inlet is positioned directly below the tubing shear ram set and above the slip ram set in the well-control stack. Two valves rated to the maximum allowable working pressure (MAWP) of the well-control stack are mounted onto the kill-line flange, which typically includes a high-pressure check valve installed in the high-pressure chicksan line run to the high-pressure pump. The practice of taking returns through the kill line is not recommended because it exposes the lower sets of rams and bonnets to accumulation of solids, debris, and other return fluids that may adversely affect the performance of the rams.
On all well-intervention services that require the circulation of wellbore returns to surface (solids, debris, spent acid, etc.), the use of a separate “flow-tee” or “flow-cross” mounted directly below the primary well-control stack rams is recommended. This flow-tee or flow-cross connection should be equipped with a minimum of two high-pressure isolation valves and rated to the same working pressure and National Association of Corrosion Engineers (NACE) classification as the well-control stack rams.
On most well-control stack assemblies, the blind ram and pipe ram compartments are equipped with ports that, when activated, allow pressure to equalize within the ram body. This allows for differential pressure to be equalized across the ram compartments before opening the rams.
The union positioned at the top of the well-control ram stack typically connects to the stripper assembly located on the bottom of the injector. The recommended connection at the bottom of the well-control ram stack is an integral high-pressure flange assembly or another suitable metal-to-metal seal connection. The pressure rating and arrangement of the well-control stack components for a given CT operation will typically depend upon the type of application employed and the maximum anticipated surface pressure in the well. When preparing for a CT well-intervention or drilling operation, the well-control equipment must be in compliance with the local regulatory authority and should reference applicable industry best practices, e.g., American Petroleum Inst. (API), Intl. Organization for Standardization (ISO), etc.
Noteworthy papers in OnePetro
Alexander Sas-Jaworsky and Ali Ghalambor 2002. Considerations For Conducting Coiled-Tubing Well-Control Operations To Minimize Formation Impairment, SPE Production & Facilities Volume 17, Number 4, 80290-PA, http://dx.doi.org/10.2118/80290-PA.
D. Egbert and H. Kendall 2008. Development of a Coiled Tubing Specific Well-Control Training Program, SPE Annual Technical Conference and Exhibition, 21-24 September 2008, 116343-MS, http://dx.doi.org/10.2118/116343-MS