After a well has been drilled and the company has determined that the reservoir will be economical to produce, the work of setting the final string of casing, preparing the well for production, and bringing in the oil or gas begins.
A well completion is a logical facility that enables production from a reservoir or reservoirs into a production string. It acts as a conduit from/to one or more sets of perforations or open-hole intervals to the production string.
Completion equipment and the methods employed are quite varied, and operators make the decisions for an individual well based on the type of oil or gas accumulations involved, the requirements that may develop during the life of the well, and the economic circumstances at the time the work is done.
A Production string is normally represented by a Tubing String. It can be represented by a Casing String when the fluid flows directly through the casing. This usually happens in two cases: when there is no tubing string and where there is dual completion with one completion flowing through the Tubing String and another one flowing through the Casing String (or Annulus). An important reason for creating these logical groupings is that the petroleum engineer is sometimes not interested in, for example, the physical perforations created when the well was completed. This user needs to account for production from reservoirs regardless of the number of physical perforations created.
The production string either runs to the bottom of the hole and blocks off the production zone completely or, rarely, stops just above the production zone. If the casing stops above the production zone and leaves the hole open, it is an open-hole completion. The reservoir fluids simply flow from the formation into the uncased hole. Usually, however, the casing runs to the bottom of the hole. The hydrocarbons cannot get out of the reservoir and into the casing because the cement and casing walls seal it off. In this case, the crew perforates the casing, or shoots holes in it, so that the fluid can flow into the well. This is a perforated completion.