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Methane Recovery and Geothermal Energy Valorization in Oltenia, Romania

Valorization of dissolved gas (Oltenia, Romania)

Context:

Located in the lower Olt Valley, Romania, three hydrocarbon exploration wells revealed a geothermal reservoir within carbonate rocks resting on a crystalline substratum. This faulted and fissured geothermal system, featuring dual porosity, enables convection through hydraulically conductive fractures and a low-permeability porous medium.

The geothermal fluid in this system originates from a mix of:

  1. Meteoric inputs from a karst massif acting as a vast recharge zone.
  2. Local supply from the Olt River.
  3. Lateral supply of highly mineralized water enriched with dissolved methane and hydrogen sulfide (H2S) of microbiological origin.

Geothermal Fluid Characteristics:

  • Hydrocarbons: 85%, including 80% methane (CH4)
  • Nitrogen: 14%
  • CO2: 1%
  • H2S: 21%

Physical Properties:

  • Density (relative to air): 0.87%
  • Humidity Rate: 0.4 Nm³/Nm³ (40%)
  • Calorific Value: 28.8 kJ/Nm³ (6,900 kcal/kg)
  • Temperature: 95°C
  • Static Head Pressure: +6 barg

Geothermal Reservoir Characteristics:

  • Transmissivity: 3,500 millidarcys m (mdm)
  • Skin: +20 (perforated cemented casing)
  • Diagnosis: Low-performing reservoir, with undeveloped well, potentially improved by stimulation.
  • Modeling: 3D horizontal multilayer calibrated on well tests (300 hours).
  • Simulation Predictions:
    • Without stimulation: Depletion of 11 barg at 20 years under 40 m³/hr production.
    • Post-acid stimulation: Potential production of 120–150 m³/hr with less than 20–25 barg of drawdown.

Projected Developments and Equipment:

  • Current Capacity: 1,500 kWth / 5,500 MWhth
  • Future Capacity: 2,500 kWth / 10,000 MWhth

The recovery of methane significantly benefits domestic and collective use, such as heating for hotels and spa guests.

System Improvements:
The existing degassing system has been enhanced with the following innovations:

  1. Degassing Dome: Limits gas losses, eliminates water carryover, and lowers the outlet temperatures of separated gas.
  2. Droplet Separator (Demister): Reduces the humidity rate from 40% to below 5%, making the gas suitable for downstream applications.
  3. Silica Gel Cartridge System: Further drying capacity, reducing the humidity rate to 1%.
  4. Cooling Module: Meets summer and winter temperature and humidity standards:
    • Summer: 32.5°C / 40% Humidity
    • Winter: 25.5°C / 40% Humidity

A significant upgrade would involve implementing the treatment chain from the Paris Basin to recover biogas instead of burning it.

Figures:

  • Figure 1: Old degassing system.
  • Figure 2: Modified system with improvements.
  • Figure 3: Separation, treatment, and combustion chain in the Paris Basin.

Conclusion:

The valorization of dissolved methane from the Oltenia geothermal reservoir offers a sustainable energy solution for district and domestic heating. By improving the degassing system and enhancing methane recovery, the project provides a cleaner alternative to traditional fossil fuels and contributes to the region’s energy independence.