Casing Tensile Loads and Design Considerations Quiz

Overview of Casing Design

• Casing design parameters are affected by location's geological conditions, formation pore pressure profile, and fracture pressure profile.

Types of Casings

• Conductor casing: provides structural support, prevents washouts of soft formations, and serves as the first flow conduit.
• Surface casing: isolates fresh/groundwater formations, allows BOP installation for pressure control.
• Intermediate casing: addresses borehole stability issues, isolates differing pressure regimes, and isolates problematic formations.
• Production casing: isolates the production zone, provides tubing leak protection, and serves as an injection/gas lift conduit.
• Liners: provide isolation or protection for an interval of concern and can be converted to full strings with tiebacks.

Principal Loads on Casing

• Operational limits (drag, margin of overpull) and doglegs & buckling affect casing design.

The Casing Design Process

Strength Parameters

• Casing dimensional specifications (size, OD, ID)
• Yield strength
• Wall thickness
• Material properties (Young's Modulus, Poisson's Ratio)
• Composition, hardness, toughness, etc.

Design Uncertainties

• Changes in load cases (worst-case scenario)
• Load parameters (changes based on operational requirements)
• Failure mode

Data Requirements for Casing Design Process

• Rig type
• Pressures (pore and fracture)
• Temperature profile
• Contaminants level (reservoir and other formations)
• Directional survey – planned trajectory
• Casing OD and minimum ID
• Tentative casing setting depth based on project requirements
• Casing type (subject to procurement plan)
• Mud density
• Cement density

The Components of a Casing String

• A casing string is composed of individual joints (40 ft in length) connected by threaded connections.
• A guide shoe or casing shoe is attached to the bottom of the casing string.
• A casing hanger allows the casing to be suspended from the wellhead.
• Other equipment may be included in the casing string or attached to the outside of the casing:
  • Float collar
  • Centralizers
  • Scratchers

Casing Terminology

• Casing strings are named based on their functions.
• Liner: a short casing string suspended from the inside of the previous casing string by a device known as a liner hanger.

Introduction to Casing

• Wells are drilled in sections, with each section sealed off by lining the inside of the borehole with steel pipe (casing).
• The annular space between the casing string and the borehole is filled with cement before drilling the subsequent hole section.
• Casing costs account for 20-30% of the total well cost.
• Great care must be taken when designing a casing program that meets the requirements of the well.

Reasons for Casing

• To prevent unstable formations from caving in.
• To protect weak formations from high mudweights that may be required in subsequent hole sections.
• To isolate zones with abnormally high pore pressure from deeper zones which may be normally pressured.

Casing Terminology – Intermediate Casing

• Intermediate (or protection) casing strings isolate troublesome formations between the surface casing setting depth and the production casing setting depth.

Casing Terminology – Production Casing

• The production casing is run through the pay zone or set just above the pay zone (for an open hole completion or prior to running a liner).
• The main purpose of this casing is to isolate the production interval from other formations (e.g., water-bearing sands) and act as a conduit for the production tubing.