Downhole Completion Tools: Tubing, Packers, SSSV and Nipples

Part 4 of 10 in the series From Reservoir to Surface: Oil and Gas Production for Beginners. Previous part: Perforation in Oil Wells.

Downhole completion tools are the pieces of equipment run inside a well, usually as part of the production tubing string, to carry fluids to surface, seal the annulus, control flow and keep the well safe. The most important ones are the production tubing, the packer, the subsurface safety valve, landing nipples, sliding sleeves and side pocket mandrels.

The Completion String

The completion string is the full assembly of tubing and tools installed in the well. Together with the production casing and the wellhead, it forms a pressure system that keeps reservoir fluids inside, except where they are meant to flow: up the tubing and out through the Christmas tree.

A basic completion string must be able to:

• Carry fluids to surface with low pressure loss and stable flow.
• Isolate the casing-tubing annulus.
• Shut the well in downhole during an emergency.
• Circulate fluid between tubing and annulus when needed, for example to kill the well.
• Accept a plug so the tubing can be isolated and pressure tested.

Production Tubing

Production tubing is the steel pipe that carries the produced fluids. It is hung from a tubing hanger in the wellhead. Tubing size matters: tubing that is too small causes high friction losses, while tubing that is too large can let gas and liquid separate, which makes flow unstable. Tubing is chosen for size, strength and resistance to corrosive fluids such as H₂S and CO₂.

Packers

A packer seals the annulus between the tubing and the production casing. It has a rubber sealing element that expands to fill the gap, and slips with teeth that grip the casing wall.

What a packer does in an oil well

• Forces all flow into the tubing, so the casing is not exposed to produced fluids.
• Prevents gas collecting in the annulus and causing unstable flow.
• Isolates zones from each other in multiple completions.
• Keeps injection pressure off the casing in injection wells.

Types of packers

• Permanent packers: held in place by opposing slips and usually set on wireline or tubing. They are removed by milling.
• Retrievable packers: can be released and pulled out. They are set by weight, mechanically (by rotation) or hydraulically (by tubing pressure).

The packer is normally set close to the top of the reservoir to keep the trapped volume below it small. Tubing length changes with temperature and pressure, so a seal assembly or expansion joint lets the tubing move without damaging the packer.

Subsurface Safety Valve (SSSV)

If the Christmas tree is damaged, for example by fire or impact, the well needs a second way to shut in. The subsurface safety valve is installed in the tubing below the surface and closes automatically. For wells that can flow naturally, it is usually mandatory.

• Surface-controlled SSSV: held open by hydraulic pressure through a small control line strapped to the tubing. If pressure is lost, a spring closes a flapper or ball. This fail-safe design is the most common.
• Direct-controlled (subsurface-controlled) valve: closes by itself when flow rate or pressure passes a preset limit. These are often called “storm chokes”.

SSSVs can be tubing-retrievable, part of the tubing itself, or wireline-retrievable, set inside a special nipple.

Landing Nipples

A landing nipple is a short tube with a machined internal profile. Wireline tools, such as plugs, gauges, chokes or safety valves, can be run down and locked into it. Nipples let engineers:

• Plug the tubing for pressure testing or before a workover.
• Hang pressure and temperature gauges for a bottom hole pressure survey.
• Install a downhole choke or a wireline safety valve.

Sliding Sleeve (Sliding Side Door)

A sliding sleeve is a tubing component with ports that can be opened or closed by a wireline shifting tool. When open, it connects the tubing and the annulus without perforating the tubing. It is used to circulate kill fluid, to switch between zones, or to bring a zone on line. It can be hard to operate after many years in the well because of scale or debris.

Side Pocket Mandrel

A side pocket mandrel has an offset pocket beside the main bore. A valve can be placed in the pocket with wireline while the main bore stays open. Side pocket mandrels hold:

• Gas lift valves (see Part 8 of this series).
• Chemical injection valves for corrosion or scale inhibitors.
• Circulation valves or dummy valves.

Other Common Components

• Flow couplings: thick-walled tubing joints above and below diameter changes, where turbulence causes erosion.
• Blast joints: heavy-walled joints placed opposite perforations to resist erosion from the jets of produced fluid, common in dual completions.
• Perforated joint: a section of pipe with holes, giving an alternative flow entry below the packer.
• Wireline entry guide: a bevelled end at the bottom of the tubing that guides wireline tools back into the tubing.
• Downhole gauges: permanent sensors that send pressure and temperature readings to surface.

Frequently Asked Questions

What is the purpose of a packer in an oil well?

It seals the space between the tubing and casing, so fluids flow only up the tubing. This protects the casing, stabilizes flow and isolates zones.

Is a subsurface safety valve always required?

In most areas, it is required by regulation for wells capable of flowing naturally, especially offshore. Rules vary by country and operator.

What is the difference between a nipple and a mandrel?

A nipple is a profile in the tubing that receives a locking tool. A mandrel is the device or body that holds a valve or plug, such as a side pocket mandrel.

Conclusion

Downhole completion tools turn a string of tubing into a safe, controllable production system. The packer seals the annulus, the SSSV gives emergency shut-in, nipples accept plugs and gauges, sliding sleeves allow circulation, and side pocket mandrels hold gas lift and chemical injection valves.

Next in this series: Part 5, Wellhead and Christmas Tree.