Gas Solubility in Formation Waters of the North German Basin - Implications for Geothermal Energy Recovery

Abstract

Geothermal heating power plants in the North German Basin produce 50 to 100?C fluids from Upper Triassic and Jurassic sandstones at depths of 1000 to 2500 m. The Na-Cl geothermal fluids are characterized by salinities as high as 250,000 ppm, pHís of 5-7, and noncondensable gas contents as high as 10% of the total mass flow.

Operation of the investigated heating plants requires that the geothermal fluid remains pressurized so that carbon dioxide gas can not exsolve and that oxygen can not enter the system and react with iron. Geochemical modeling with SOLMINEQ.88 confirms the operating experience that cooling and a drop in the pressure of the geothermal fluid will not result in oversaturation of the water as long as oxygen entry and a pressure drop below a plant specific water degassing level are avoided.

Oxygen measurements show that diffusion of oxygen into the system is minimal during routine operations. In contrast to normal production periods, oxygen can enter the system during plant shutdowns or when slop water (leakage water accumulated during maintenance work) is pumped back into the system.