Geothermal Drilling Overview

Objectives
Primary Objectives:
Obtain geothermal production from targets:
- Permeable formations, fractured formations and contact zones
- Shallow, deep and basement faults
- Assess likely future well productivity

Secondary Objectives:
Provide geological and reservoir data to assess geothermal potential of the area, future well plan, and reservoir management. Key information includes:
- Geological stratigraphy
- Pressure and temperature distribution
- Identification of faults
- Identification of any stratigraphic permeability suitable for production or injection
- Evidence of any change in reservoir temperature or chemistry conditions
- Determination of depth to basement

Characteristics
Geothermal Drilling – is unique and challenging
Uniqueness:

• Geothermal area is generally located in mountainous terrain
• Prolific lost circulation environment
• Geothermal energy sources are derived from the earth's crust. Often, bottom hole temperatures can exceed 450^oF (232°C)
• Hard and abrasive formations (volcanic rocks)

Challenges:

• Locations and drilling pad preparation
• Road access (steep, narrow, load limit, agricultural regions, etc)
• Community issues
• Hole cleaning challenges and stuck pipe risks
• Low penetration rate, vibrations, bottom hole assembly and bit selection

Design Consideration

• Formation Pressures:
  • Subnormal pore pressures
  • Fracture gradients decrease with depth
  • Objective of geothermal well is to encounter permeable fractures, resulting in total lost circulation limiting the usefulness of fracture gradients
  • Leak off test performed for shoe integrity, not to predict next shoe depth
• Formation Temperatures:
  • Reservoir temperatures in the range of 450^oF – 650^oF.
  • Flowline temperature monitoring
• Temperature Design Reduction Considerations:
  • Yield strength reduction at high temperature
  • Wellhead pressure rating
• Production Operating Conditions:
  • Maximum anticipated shut in wellhead pressure
  • Flowing wellhead pressure
  • Flowing wellhead temperature
  • Thermal stress during well kill operation (pumping cold water down heated well)