Abstract
Barite scales in geothermal installations are a highly unwanted effect of circulating deep saline fluids. They build up in the reservoir if supersaturated fluids are re-injected, leading to irreversible loss of injectivity. A model is presented for calculating the total expected barite precipitation. To determine the related injectivity decline over time, the spatial precipitation distribution in the subsurface near the injection well is assessed by modelling barite growth kinetics in a radially diverging Darcy flow domain. Flow and reservoir properties as well as fluid chemistry are chosen to represent reservoirs subject to geothermal exploration located in the North German Basin (NGB) and the Upper Rhine Graben (URG) in Germany. Fluids encountered at similar depths are hotter in the URG, while they are more saline in the NGB. The associated scaling amount normalised to flow rate is similar for both regions. The predicted injectivity decline after 10 years, on the other hand, is far greater for the NGB (64%) compared to the URG (24%), due to the temperature- and salinity-dependent precipitation rate. The systems in the NGB are at higher risk. Finally, a lightweight score is developed for approximating the injectivity loss using the Damköhler number, flow rate and total barite scaling potential. This formula can be easily applied to geothermal installations without running complex reactive transport simulations.
Highlights
The North German Basin (NGB) and the Upper Rhine Graben (URG) are two of the main regions in Germany for geothermal exploration, exhibiting promising hydrothermal resources [1]
Barite is a low-soluble scaling mineral, typically found at geothermal installations located in the NGB and the URG area that handle fluids produced from reservoirs of depths greater 2000 m [9,11,12]
This paper provides a new approach for calculating potential barite scaling amounts near the injection well and assessing the resulting impact on injectivity loss for geothermal systems in the NGB
Summary
The North German Basin (NGB) and the Upper Rhine Graben (URG) are two of the main regions in Germany for geothermal exploration, exhibiting promising hydrothermal resources [1]. One of which the present study focused on is barite scale formation (BaSO4 ), originating from lowering the fluid’s state temperature and pressure during the production–injection cycle. Barite is a low-soluble scaling mineral, typically found at geothermal installations located in the NGB and the URG area that handle fluids produced from reservoirs of depths greater 2000 m [9,11,12]. Scalings are undesirable because they lower tubing diameters or reduce efficiency of heat exchangers, which leads to restoration costs [3,9,12,13]. In case of barite scaling, this leads to irreversible injectivity loss and reduced overall efficiency [13,14]
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