A well kill is the operation of placing a column of special fluids of the required density into a well bore in order to prevent the flow of reservoir fluids without the need for pressure control equipment at the surface.
Well kills may be planned in the case of advanced interventions such as workovers, or be contingency operations.
On occasion, the unintended accumulation of fluids, either from injection of chemicals like methanol from the surface, or from liquids produced from the reservoir, can be enough to kill the well, particularly gas wells, which are notoriously easy to kill.
Extensive training, testing, proof of competence, and experience are prerequisites for planning and performing a well kill, even a seemingly simple one.
The principle of a well kill revolves around the influence of the weight of a fluid column and hence the pressure exerted at the wellbore's bottom.
Where P is the pressure at a specific depth, h, within the column, g is the acceleration of gravity and ρ is the density of the fluid.
Simple conversion factors (0.433 for specific gravity and 0.052 for ppg) convert these values to a pressure gradient in psi per foot.
Since well-kill certification is normally (in the US/UK) done in "oil field units" (feet for length, inches for diameters, oilfield barrels for volume-pumped, psi for pressures), complex workarounds are often performed to keep the planned calculations in line with local regulations and industry "best practice".
During normal operation, it would remain closed sealing off the tubing and the annulus, but for events such as this, it would be opened to allow the free flow of fluids between the two regions.
If there is a sudden need to kill a well quickly, without the time for rigging up for circulation, the more blunt instrument of bullheading may be used.
However, it can be limited by the burst-pressure capabilities of the tubing or casing, and can risk damaging the reservoir by forcing undesired materials into it.
Though effective, it is not as desirable since it is preferred that the well bore fluids be displaced out to production, rather than the annulus.
It involves repeatedly pumping in small quantities of kill mud into the well bore and then bleeding off excess pressure.
It works on the principle that the heavier kill mud will sink below the lighter well bore fluids and so bleeding off the pressure will remove the latter, leaving an increasing quantity of kill mud in the well bore with successive steps.
This is done by pumping kill mud down the drill pipe, where it circulates out the bottom and into the well bore.
This involves injecting a gas at high pressure, usually nitrogen since it is inert and relatively cheap.
Low-density ("light") liquids such as diesel fuel, or the "base fluid" for a "(synthetic) oil-based mud" can also be used, depending on availability and pressure-management issues for a specific well.