The completion phase starts when it is determined from testing that the well should be commercial. The first step is to set the casing while the big rig is still on location. The casing size varies based on a number of different things, but primarily it is based on the diameter of the wellbore. You need a space between the casing and the wellbore to have cement. The casing is set into the well with the drilling mud still present. The drilling mud is necessary to hold back the oil and gas in the formations. Once the casing is set, cement is forced down the center of the casing to the bottom and then it will move up the outside of the casing. As it does this, it displaces the drilling mud. When completed, the casing will be empty with nothing in it and cement will be between the casing and the walls of the wellbore. The cement will be allowed to dry for a day or two and a cement bond log will be run to make sure there is a good seal everywhere. A poor bond could allow water from one formation to damage a good oil or gas formation. This is very important so you do not have different formations in communication.
Schematic of a Well Bore
Perforating requires specialized crews to transport, assemble, and operate the equipment. The well is perforated in the zone of interest.
The tool is lowered into the well and an explosive shaped charge is set off to shoot out small metal balls that make holes through the casing and cement into the producing formation. Production tubing is set, which is where the actual oil and gas will flow out of well and a packer is set to seal off that zone. On the surface there will be a pump jack or a Christmas tree.
Perf. Shot Penetrating Casing, Cement and Formation
1) Perf gun inside casing 2) Shooting charges 3) oil flowing into casing