Rig Sizing and Selection

Drilling rigs are used to drill the hole, lower and cement casing in the well, and provide a means to perform various auxiliary functions such as logging and well testing. Today's rigs are complicated and require highly experienced, trained personnel for efficient operations. If improperly selected, the rig can be the cause for low penetration rates, formation damage from poor solids control, and high ultimate well costs.

In past years, rig selection was often based on footage rating. A 20,000ft rig would be selected for a 19,000-ft prospect. This meant the rig could drill safely to 20,000 ft. It also implied the rig could perform all associated activities required to drill and complete a well at 20,000 ft. Some 20,000-ft-rated rigs cannot meet the rigors of a 20,000-ft hole for any of the following reasons:

  • large-diameter, full-length production casing strings cannot he used since the derrick or substructure will not support the casing load
  • mud pumps, pump horsepower, rotary systems, or drawworks cannot operate in the deep environment
  • ineffective solids control system for high-density muds
  • poor BOP systems

Many companies are applying more effort in rig selection since it is important for the safety, efficiency, and cost of the well.

The proper procedure for rig selection is to size or design the various loads that will be placed on the equipment and to select the most cost-effective rig that will satisfy these requirements. Drilling contractors provide detailed rig specifications for this purpose. When these specifications are compared with the well prognosis, the proper rig can be selected. Fig, 1-2 illustrated that rig sizing is an integral part of the well planning process; however, it must be completed after the actual well has been designed.

Drilling rigs may be subdivided into several component systems for design and sizing. Although the following systems groupings are arbitrary, they serve as a basis for the selection process:

  • power
  • hoisting
  • derrick and substructure
  • rotary
  • circulating
  • pressure control

These groups, although broad in nature, cover most important aspects of rig design requirements.

Rig selection, which occurs after rig sizing, is not completely quantitative. Although the objective is to select the most cost-effective rig that will drill the well, some factors must be considered:

  • technical design requirements
  • qualifications of the rig's manpower, i.e., experience and training t track record
  • logistics handling
  • rig-site requirements

Rig selection is even more complicated if availability is poor or long-term contractual commitments on several rigs require use of rigs ill-suited for a particular well.

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