"Scalings im Thermalwasserkreislauf"
Abstract
In the Bavarian Molasse Baisin, the water-bearing sediments of the Upper Jurassic with reservoir temperatures up to 150 °C are exploited for geothermal energy production. However, particularly the most ambitious geothermal facilities which produce water with high temperatures > 100 °C) and high production rates (> 100 L/s) are facing chemical precipitations (scalings) which have a negative effect on the operation and on the economic efficiency of the plant. These mainly carbonatic precipitations are caused by a shift of the lime-carbonic-acid-equilibrium. The crystallisation of calcite is triggered by locally reduced pressures leading to degassing and stripping of CO2 . For a quantitive prediction and control of these scalings, a comprehensive investigation of the scalings and of the scaling-relevant parameters is of essential importance. The operational parameters of 14 facilities which are affected by scalings or are prone to scalings due to the present knowledge, is collected. Scalings appear at temperatures ≥ 98 °C. Facilities which are unaffected by scalings despite temperatures ≥ 98 °C have production rates < 50 L/s with one exception. This exceptional site has no scaling inspite of production temperature of 128 °C and a flow rate of 125 L/s. Six of the invesitigated geothermal sites have massive scaling problems. At these facilities, scaling samples of the whole geothermal cycle (casing, production pipe, particle filters, heat exchanger, groundlevel- and injection pipe) are collected, partially repeatedly at different dates. Multiple analysis methods confirmed a primarily calcitic composition of the scalings. The scaling thickness in the casing below the submersible pump reaches values of up to 10 mm with a maximum just below the pump. The scaling thickness in the production pipes shows a strong increase from the pump outlet (0.0 - 1.3 mm) to the well head (0.3 - 10 mm). The resulting calcite scaling rates at the well head range from 1 · 10−6 to 4 · 10−6 mol/(m2 ·s). Only 0.5 - 3.5 % of dissolved calcium precipitates throughout the production pipe. The crystallisation rate is largely controlled by the surface reaction kinetics and not by the available mass. In terms of produced water volume, the scaling growth is 0.5 - 1.5 mm/Mio m3 , at the well head. The number of pump starts and shut-offs doesn’t correlate with the scaling rate. At least, up to a scaling thickness of 10 mm, the scaling rate is constant. In this region the increasing shear forces due to the reduced flow surface area doesn’t limit the scaling thickness. Smooth pipe surfaces yield no long term technical advantage, because they only can retard calcite precipitation at the initial phase, while they promote scaling remobilisation, after that. At 13 of the investigated facilities, the hydrochemical composition of the geothermal waters including the contained gases was dermined in a consistent sampling procedure. Neither calcium ion concentration nor hydrogen carbonate ion concentration nor CO2 content are decisive for the occurence of scalings. The facilities with the most severe scaling problems show the smallest salinities with a low concentration of calcium- and hydrogen carbonate ions as well as carbon dioxide. If mass limitation occurs locally, mass concentration of calcium ions exists.