OTCG Introduction

Tubing Casing Introduction

This chapter discusses types of casing and tubing types

There are two basic types of pipes used in oil and gas exploration and production and standardized by the American Petroleum Institute (API) and the International Standards Organization (ISO).

For in-well services (i.e., below the wellhead oil country tubular goods [OCTG]):

  • Casing: API 5CT/ISO 11960 with API 5B/ISO 10422 for threads;
  • Tubing: API 5CT/ISO 11960 with API 5B/ISO 10422 for threads.

Per API, the specification differences between casing and tubing are:

  • Length of the drift mandrel: 6 in. or 12 in. for casing and 42 in. for tubing;
  • Joint strength calculation method: Minimum tensile strength for casing, and minimum yield strength for tubing.

Drive, structural and conductor casing

The main purpose of this first string of pipe is to protect unconsolidated shallow formations from erosion by drilling fluids. Additional functions of the first casing string include:

  • Allows for installation of a full mud circulation system, when formations are sufficiently  stable;
  • Guides the drill string and subsequent casing into the hole;
  • Can form a part of the piling system offshore for a wellhead jacket or piled platform. In subsea wells the conductor may form an integral part of the structural support for the wellhead system;
  • Provide centralization for the inner casing strings, which limits potential buckling of subsequent casing strings;
  • Minimize shallow lost returns;
  • Provides a mount in onshore applications for a diverter system that would be used in the event of an unexpected shallow influx.

Conductor casings can be driven or jetted to depth or, alter- natively, run into a predrilled or jetted hole and cemented.

Surface casing

Surface casing is installed to:

  • Prevent poorly consolidated shallow formations from sloughing into the hole;
  • Enable full mud circulation;
  • Protect fresh water sands from contamination by drilling mud;
  • Provide protection against hydrocarbons found at shallow depths;
  • Provide initial support for the blowout preventers;
  • Provide kick resistance for deeper  drilling;
  • Support the wellhead system and all subsequent casing strings.

The surface casing string is typically cemented to the surface or seabed.

It is usually the first casing on which blowout preventers are installed.

The amount of protection provided against internal pressure will only be as effective as the formation strength at the casing shoe.

Intermediate casing

Intermediate casing is used to ensure there is adequate blowout protection for deeper drilling and to isolate formations that could cause drilling problems.

The first intermediate string is typically the first casing providing full blowout protection.

An intermediate casing string is nearly always set in the transition zone associated with the onset of significant overpressures.

If the well could encounter severe lost circulation zone(s), intermediate casing would normally be set in a competent formation below the loss zone.