Log Quality Control

Log Quality Control

Log Quality Control (LQC)

Log Quality Control (LQC) is essential for ensuring that well logs are accurate and reliable. Incorrect logs can lead to erroneous interpretations, making it critical to identify and correct errors early.

Objectives of LQC

• Understand how to read field logs/prints.
• Learn how to perform LQC.
• Prevent wrong interpretations caused by errors in raw logs.

LQC is conducted in two steps:

1. At the Wellsite (Field Engineer)
2. At the Office (Log Analyst)

Understanding tool physics and the environment allows errors in logs to be corrected before final analysis.

Common Log Errors

Errors in well logs can occur due to:

• Measurement physics and tool responses.
• Environmental conditions affecting tool operation.
• Calibration issues, depth matching, or improper tool configuration (e.g., centralization).
• Borehole conditions or tool failure.

Main Components of LQC

1. Presentation
   • Curves, pips, scales, and inserts should be correctly labeled and neat.
   • Units must be noted, and scales should match customer requirements.
2. Calibration
   • Calibration ensures measurement accuracy and is performed at three stages:
     • Master Calibration: Conducted monthly to ensure tools are functioning correctly.
     • Before Survey Calibration: Performed at the wellsite to check for any drift since the last calibration.
     • After Survey Calibration: Done after logging to verify sensor accuracy and ensure data reliability.
3. Operating Techniques
   • Response in Known Conditions:
     • Compare tool responses to known values in similar formations, using geological information, mud reports, or logs from nearby wells.
     • Check for expected values in formations such as salt, anhydrite, or coal.
   • Logging Speed:
     • The maximum logging speed for a tool depends on factors such as:
       • Tool design (most important).
       • Desired repeatability and accuracy.
       • Formation characteristics and borehole conditions.
     • Slower speeds may be required in unusual borehole conditions or difficult formations to ensure accuracy.
   • Tool Positioning:
     • Tools must be positioned correctly within the borehole:
       • Centralized tools (e.g., laterolog, sonic devices) require centralizers to ensure accurate readings.
       • Eccentered tools and others may require specific positioning or a standoff from the borehole wall.
   • Depth Matching:
     • The first log run in a well is typically the master log run, and it should serve as the depth reference for all subsequent logs to ensure accuracy.
4. Environmental Effects
   • Factors like mud type, salinity, borehole fluids, and external noises can affect tool accuracy.
   • Ensure tools operate within their specifications for temperature, hole size, and deviation.