Casing string is designed for further cementing of wells and isolation of pay zones during operations. The string is arranged of case pipes by way of their consecutive screwing together (sometimes welding). Case pipes used for drilling oil and gas wells are made mainly of steel with two threaded ends and one box end (sometimes inserted joint casing with flared end are used). Pipe thread may be of conical, triangular-shaped or special trapezoidal profile. To assure air-tightness under high oil/gas pressure (above 30 MPa) sealing elements are used in the joints.

Casing strings may be of three types:

  • hole directors;
  • intermediate string;
  • production string.

Intermediate casing strings are for casing the walls of the lower intervals of the wells. Normally hole directors and intermediate casing are cemented, but they also may be interchangeable (e.g., when drilling some exploration wells or deep wells – to prevent wear and tear of the previously run-in-hole intermediate casing strings). Production casing bridges pay zones. Oil and gas flow into the well through casing perforation and then are moved to the wellhead through tubing.

Casing strings are exposed to external pressure of liquids and gas in the reservoirs and mass rock, which are especially prominent in argillaceous and saline deposits; to internal pressure of oil, gas and also mud; to their own mass and pulling force of the string caused by temperature and pressure effects. Length, diameter and number of casing strings are determined by geological conditions when drilling (pressure differential of hydraulic fracture, reservoir pressures, stability of the drilled rock, etc.), by the level of technology and engineering applied when constructing a well, by the potential for prevention and liquidation of incidents, etc. Production strings diameter and well depth are the key factors for defining the intermediate string diameter. String design should be selected with account for economic feasibility and the forecasted operations period. Equipment used for RIH operations: derrick, hoist, pulley system, as well as mechanisms for suspension of string in the wellhead.