Production casing The fourth but not necessarily the final string of pipe run in the hole is the production casing.  The production casing is used to control the hydrocarbon bearing zones that will be produced. This string of pipe adds structural integrity to the well-bore in the producing zones.  It is necessary to conduct the hydrocarbons to the surface Production casing should be set before completing the well for production. It should be cemented in a manner necessary to cover or isolate all zones which contain hydrocarbons. A calculated volume of cement sufficient to fill the annular space at least five hundred (500) feet above the top of the uppermost hydrocarbon zone should be used.  Casings must also be of such quality that they can withstand particularly corrosive media in the well (H2S, CO2 etc.), if expected to be exposed to such formations. This string would normally be the longest string run and may often be cemented in stages so as not to brake down the lower formation. It must also be of sufficient strength that should the production string (s) leak it will contain the formation pressure that will migrate to surface and should be design to cover the expected life span of the well. If there are indications of inadequate primary cementing (such as lost returns, cement channelling, or mechanical failure of equipment) in  the surface, intermediate, or production casing strings, the casings should be evaluate, by pressure testing the casing shoe, running a cement bond log or a cement evaluation tool log, running a temperature survey, or a combination  before continuing operations. If the evaluation indicates inadequate cementing, the casing should re-cement and if necessary perforated and squeezed with cement. When a liner is used as production casing, the testing of the seal between the liner top and next larger string must be conducted as in the case of intermediate liners.