A well intervention, or 'well work', is any operation carried out on an oil or gas well

Oil well

An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...

 during, or at the end of its productive life, that alters the state of the well and or well geometry, provides well diagnostics or manages the production of the well.
This is the simplest form of intervention as it does not involve putting hardware into the well itself. Frequently it simply involves rigging up to the kill wing valve on the Christmas tree

Christmas tree (oil well)

In petroleum and natural gas extraction, a Christmas tree, or "tree", is an assembly of valves, spools, and fittings used for an oil well, gas well, water injection well, water disposal well, gas injection well, condensate well and other types of wells...

 and pumping the chemicals into the well.
The complexity of this operation can vary depending on the condition of the wellhead

Wellhead

A wellhead is a general term used to describe the component at the surface of an oil or gas well that provides the structural and pressure-containing interface for the drilling and production equipment....

s.