Free Point

on . Posted in Drilling Engineering

Free point is the location in the wellbore where the drill string is not stuck and can move freely.  Determining the free point is crucial when a portion of the drill string becomes stuck, as it helps in planning the recovery and freeing operations.  In other words, it is the highest point above which the pipe is not stuck and can move freely.  Determining the free point is crucial when a section of the pipe becomes stuck, as it allows operators to identify the precise location where the pipe can still be manipulated and where it is immobilized.

Importance of Determining the Free Point

  • Stuck Pipe Recovery  -  Identifying the free point allows operators to understand where the drill string is free and where it is stuck, which is essential for deploying tools and techniques to free the stuck section.
  • Minimize Damage  -  Accurate determination of the free point helps in avoiding unnecessary force on the free sections of the drill string, minimizing the risk of additional damage or breaking the string.
  • Operational Efficiency  -  Knowing the exact location of the stuck point can significantly reduce non-productive time (NPT) by streamlining the recovery operations.

 

Free Point formula

\( L \;=\;  E \; S \; A_c \;/\; P \; 12 \)    (Free Point)

\( E \;=\;  L \; P \; 12  \;/\; S \; A_c \)

\( S \;=\;  L \; P \; 12  \;/\; E \; A_c \)

\( A_c \;=\;  L \; P \; 12  \;/\; E \; S \)

\( P \;=\;  E \; S \; A_c  \;/\; L \; 12 \)

Symbol English Metric
\( L \) = length of free pipe \(ft\) -
\( E \) = modulus of elasticity of steel (psi) \(lbf\;/\;in^2\) -
\( S \) = stretch in drill pipe (elongation) \(in\) -
\( A_c \) = pipe wall area cross-section \(in^2\) -
\( P \) = overpull difference between max. and min. pull of pipe \(lbf\) -

    

Methods to Determine the Free Point

There are several methods used to determine the free point in a wellbore:

  • Free Point Indicator (FPI) Tool  -  This tool is run down the wellbore on a wireline to the suspected stuck point.  It measures the stretch and torque applied to the drill string.  The FPI tool detects where the pipe is free to stretch and rotate and where it is stuck, providing accurate depth measurements.
  • Stretch Measurement  -  Tension is applied to the drill string, and the stretch is measured.  The free section will stretch according to the applied force, while the stuck section will show little to no stretch.  This method requires careful calculation and interpretation to avoid misidentifying the stuck point.
  • Torque Measurement  -  Torque is applied to the drill string, and the rotation response is monitored.  The free section will rotate, whereas the stuck section will not.  The point where the rotation stops indicates the beginning of the stuck section.
  • Sonic or Ultrasonic Tools  -  These tools use sound waves to detect changes in the mechanical properties of the drill string.  They can identify the location where the drill string changes from being free to stuck.

Remedial Actions Based on Free Point Data

  • Jarring Operations
    • Hydraulic or Mechanical Jars  -  These tools apply repeated impacts to the stuck pipe to dislodge it.  The free point data helps position the jars optimally for effective impact.
  • Cutting Operations
    • Pipe Cutters  -  Based on the free point data, operators can use mechanical, chemical, or explosive cutters to sever the pipe at or above the stuck point, allowing for the retrieval of the free portion.
  • Fishing Tools
    • Overshots and Spears  -  These tools grip the stuck pipe or equipment, facilitating retrieval.  Accurate free point data ensures the tools are used effectively.
  • Chemical Treatments
    • Spotting Fluids  -  Chemicals can be pumped downhole to reduce friction or dissolve obstructions.  Free point data helps target the treatment area precisely.
  • Back-Off Operations
    • Pipe Back-Off  -  By applying torque to unscrew the drill string at a specific threaded connection above the stuck point, the free portion can be retrieved.  Free point data ensures the correct location for back-off.

Determining the free point is a critical step in managing stuck pipe incidents in drilling operations.  It enables operators to localize the problem and apply appropriate techniques to recover the drill string efficiently and safely.  By using tools like the free point indicator and measuring stretch and torque, operators can accurately identify the free point and minimize downtime and operational risks.    

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Tags: Drilling