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)

What is Geothermal Energy

Geothermal
Geo = Earth; Thermal = Heat

Geothermal energy is the heat contained within the Earth which can be recovered and exploited for heating and other direct uses (residential, industrial), or generation of electricity. Geothermal system is a transfer of heat energy from the inner part of the Earth to the surface.

Geothermal power is sustainable, cost effective, and environmentally friendly. Recent technology advances have expanded the range and size of viable resources, especially for direct uses such as home heating. Geothermal power plants release greenhouse gases, but the emissions are much lower per energy unit than fossil fuels. As a result, geothermal power has potential to mitigate global warming, if it is deployed in place of fossil fuels.

Geothermal Reservoir
Requirements for a Geothermal Reservoir to Exist:

• Heat source (Magma)
• Permeability (flow capacity)
• Porosity (storage capacity)
• Recharge – natural or artificial (injection)
• Discharge – required to allow circulation to occur

Resource types:

1. single phase; water dominated; T = 100°C - 300°C (212°F - 570°F)
2. two-phase; steam and water; T = >230°C (>450°F)
3. single phase; steam dominated; T = >240°C (>465°F)
4. special conditions:

• critical fluid conditions T>374.15°C (>705.44°F)
• high salinity brines (up to 30 wt-% solids)
• high gas contents (up to 10 wt-%)

Geothermal Resources –
Located by:

1. surface geology and tectonic setting
2. surface manifestations – Geochemistry
3. geophysical techniques – to find anomalies: resistivity surveys, gravity, magnetic, etc.

Extracted by:

1. Drilling wells – wells may self flow or require pump
2. Pipes
3. Surface facilities (HE, separators, turbine, cooling towers, fluid treatment)

Energy utilization:

1. Direct Use (snow melting, heating, drying/dehydration, agriculture - greenhouse, etc.)
2. Power Generation (Steam turbine generator; Binary plant)
3. Hot Dry Rock (Enhanced Geothermal Systems)