Awell intervention, orwell work, is any operation carried out on anoil or gas well during, or at the end of, its productive life that alters the state of the well or well geometry, provides well diagnostics, or manages the production of the well.
Pumping is the simplest form of intervention as it does not involve putting hardware into the well itself. Frequently it simply involves rigging up to thekill wing valve on theChristmas tree and pumping in a fluid determined necessary for the particular well.
The complexity ofwellhead and Christmas tree maintenance can vary depending on the condition of the wellheads. Scheduled annual maintenance may simply involve greasing and pressure testing the valve on the hardware. Sometimes thedownhole safety valve is pressure tested as well.
Slickline operations may be used for fishing, gauge cutting, setting or removing plugs, deploying or removing wireline retrievable valves andmemory logging.
Braided line is more complex than slickline due to the need for a grease injection system in the rigup to ensure theblowout preventer (BOP) can seal around the braided contours of the wire. It also requires an additional shear-seal BOP as a tertiary barrier, as the upper master valve on theChristmas tree can only cut slickline. Braided line includes both the core-less variety used forheaving fishing and electric-line used forwell logging andperforating.
Coiled tubing is used when it is desired to pump chemicals directly to the bottom of the well, such as in a circulating operation or a chemical wash. It can also be used for tasks normally done by wireline if the deviation[clarification needed] in the well is too severe for gravity to lower thetoolstring and circumstances prevent the use of awireline tractor.
Snubbing, also known as hydraulic workover, involves forcing a string of pipe into the well against wellbore pressure to perform the required tasks.[clarification needed] The rigup is larger than forcoiled tubing and the pipe more rigid.
In some older wells, changing reservoir conditions or deteriorating condition of thecompletion may necessitate pulling it out to replace it with a fresh completion.
Subsea well intervention offers many challenges and requires much planning. The cost of subsea intervention has in the past inhibited the intervention but in the current economic climate it is much more viable. These interventions are commonly executed from light/medium intervention vessels, ormobile offshore drilling units (MODU) for the heavier interventions such as snubbing and workoverdrilling rigs. Light interventions are generally performed with the well live, and usually involve adjustments of things such as valves; while heavy interventions are generally performed with the well shut down, and may be used to replace parts such as tubing strings or pumps, or to plug and abandon the well.
