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

Abstract

The occurrence of enhanced concentration of the radium triplet ²²⁶Ra, ²²⁸Ra and ²²⁴Ra is a frequently observed property of highly saline anoxic deep water as used e.g. in geothermal plants. In the present study we develop a model to explain the observed activity levels in the brines. The model considers processes at the rock-fluid interface of the aquifer like alpha recoil, sorption and surface precipitation and is implemented by means of a Monte Carlo simulation. The outcomes of the simulations indicate the dominating role of fine-grained constituents of the reservoir rock, e.g. claystone with enhanced specific activities of the natural decay chains. Mass fractions of such material in the order of a few percent are sufficient to result in radium fluid concentrations >1 Bq l^-1. Also a generally valid relation between the Th/U ratio in the aquifer rock and the ²²⁸Ra/²²⁶Ra activity ratio in the fluid was found. This link improves the agreement between radium fluid data and the mean Th/U ratio of the Earth's crust. The ²²⁴Ra/²²⁸Ra fluid ratios reflect the transport time from the location of last radium release to the sampling point. The model findings were applied to a well investigated aquifer used in a geothermal plant in the North German Basin. An eight component system of the aquifer rock was established as the basis for the simulation of the radium concentrations in the deep fluid. The comparison between simulation and fluid analyses revealed a degree of radium sorption of about 50 %, which is necessary to match the model's results with the measurements. On the other hand, the ²²⁸Ra/²²⁶Ra fluid ratio of the brine was well reproduced by the simulation, showing the suitability of the model even in complex heterogeneous reservoirs. From the ²²⁴Ra/²²⁸Ra fluid ratios a transition from pore-to fracture-guided transport < 10 m distance from the production well is suggested. Precipitates from such deep fluids occurring after changes of the thermodynamic conditions are able to accumulate radium isotopes in Ba/Sr-sulphate phases. The time dependence of the radioactive disequilibrium between ²²⁶Ra, ²²⁸Ra and its child ²²⁸Th in such scales is described by a mathematical model and is applied to two different uptake models. Based on this approach, age determinations on precipitates found in different components of a geothermal plant are conducted. They reveal the triggering of scale formation due to modifications in the plant. The results are suitable for drawing conclusions about the operation of the system, which result in a reduction in the amount of scale and a reduction in downtimes.

Keywords: Age determination; Alpha recoil; Anoxic brines; Deep water; Radium isotopes; Scale formation; Sorption.