The Radium triplet ²²⁶Ra, ²²⁸Ra, ²²⁴Ra in saline deep water - a valuable information source

Area of Science:

• Geochemistry
• Hydrogeology
• Nuclear Science

Background:

• Elevated concentrations of radium isotopes (226Ra, 228Ra, 224Ra) are common in saline, anoxic deep waters, particularly those used in geothermal energy production.
• Understanding the processes governing radium behavior in these environments is crucial for managing geothermal operations and interpreting hydrogeochemical data.

Purpose of the Study:

• To develop and validate a model explaining the observed radium activity levels in deep saline brines.
• To investigate the influence of rock-fluid interactions, such as alpha recoil, sorption, and precipitation, on radium concentrations.
• To apply the model to geothermal brines and precipitates for operational insights.

Main Methods:

• Development of a Monte Carlo simulation model incorporating rock-fluid interface processes (alpha recoil, sorption, surface precipitation).
• Application of the model to analyze radium concentrations and activity ratios (228Ra/226Ra, 224Ra/228Ra) in geothermal brines.
• Mathematical modeling of radioactive disequilibrium in mineral precipitates for age determination.

Main Results:

• Fine-grained reservoir rock constituents (e.g., claystone) with high natural decay chain activities are the primary source of elevated radium in brines.
• A correlation between the Th/U ratio in aquifer rock and the 228Ra/226Ra activity ratio in fluid was established.
• Model simulations accurately reproduced radium fluid data from a North German Basin geothermal plant, indicating significant radium sorption (approx. 50%) and suggesting a transition from pore- to fracture-guided transport near production wells.
• Age determination of sulfate scale precipitates revealed correlations with plant operational modifications.

Conclusions:

• The developed Monte Carlo model effectively explains radium isotope behavior in geothermal brines.
• Radium sorption and transport mechanisms play significant roles in controlling fluid concentrations.
• Analysis of mineral precipitates provides valuable insights into geothermal plant operation, aiding in scale reduction and minimizing downtime.