Class Ofimary B - Week 12 Lecture Notes

Introduction

• The lecture continues from last week, focusing on detecting and predicting overpressure during drilling.
• Last week: Discussed using normalized or exponential drilling speed.
• This week: Discussing using information from drill cuttings.

Drill Cuttings and Cavings

• Cuttings: Small rock fragments from the drill bit impact.
• Cavings: Associated with borehole collapse, not drill bit impact.

Analyzing Drill Cuttings

1. Shale Density

   • Measure density of shale cuttings to detect overpressure.
   • Overpressure can be generated by:
     • Loading mechanism: Rock fails to compact, high porosity.
     • Fluid expansion mechanism: Overpressure without density anomaly.

2. Shale Factor (Cation Exchange Capacity - CEC)

   • Different clay minerals have distinct CECs (e.g., smectite vs. illite).
   • Transformation from smectite to illite can indicate overpressure.

3. Size and Shape of Cuttings

   • Infer subsurface pressure.
   • Blocky caving: Indicates shear failure.
   • Splintery caving: Indicates tensile failure.

4. Abundance of Cuttings/Cavings

   • Used to detect and predict overpressure.

5. Cutting Gas

   • To be discussed next week.

Detecting Overpressure

• Shale Density as an Indicator

  • Overpressure from loading mechanism: High porosity, low density.
  • No density anomaly in fluid expansion mechanism.

• Shale Factor Analysis

  • Measure CEC to determine dominant mineralogy.
  • Transformation from smectite to illite indicates potential overpressure.

• Size and Shape Analysis

  • Blocky caving: From shear failure, stronger rock, or underbalanced drilling.
  • Splintery caving: From tensile failure, indicates underbalanced drilling.

Mechanical Instability and Stress

• Near-field stress: Concentration around borehole due to drilling.
• Blocky caving: Can be due to mechanical instability or underbalanced drilling.
• Splintery caving: Indicative of underbalanced drilling, not mechanical instability.

Conclusion

• Importance of understanding geological conditions and mechanisms in predicting overpressure.
• Multiple parameters should be used in combination for accurate detection.

Further Discussion

• Suggestions, questions, or comments are welcome in the video comments section.

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This lecture provides insights into understanding and predicting overpressure using drill cuttings, emphasizing the combination of different analysis techniques.