Alteration products of reservoir rocks from the Upper Rhine Graben under geothermal conditions

Abstract

Deep seated fault systems play an important role in the utilization of deep geothermal energy, especially in non-volcanic enhanced geothermal systems with low natural permeability. In such settings, the fluid migration and heat convection take place through the connected fracture network that is often saturated with saline geothermal fluids, interacting with the primary and secondary mineral content of the host rock. The aim of this research is to comprehend the mineralogical alterations that occur, when the primary mineral assemblage is subjected to a synthetic geothermal fluid at temperatures above 200 °C.
Batch-type experiments with 2 molar Na-Cl solutions and cylindrical hard rock samples have been performed. Samples were taken from four different lithologies representing potential reservoirs for future geothermal exploration: Malsburg granite (crystalline basement), Tennenbach sandstone (Lower Buntsandstein) and Pfinztal sandstone (Upper Buntsandstein). Both, the solid samples as well as the fluid samples will be studied in order to draw a more general picture of the reaction process. The rock sample will be analyzed for dissolution processes, ion exchange reactions of primary minerals and precipitation of secondary minerals.
After extraction of the granite samples from the experiment, the surface mineral grains are disintegrated from the matrix. The main dissolution features that can be observed in SEM images are deep holes in quartz grains. The feldspars, however, do not show substantial dissolution, but are often covered with newly formed clay precipitates. Near-surface biotite grains are totally altered to greenish chlorite. The two sandstone samples show a different reaction behavior. Quartz dissolution is less pronounced than in the granite samples. While the feldspar remains mostly unaffected, muscovite grains show substantial alteration. The most prominent precipitate that can be found in the SEM images is analcime that occurs in
spherical shape. It is usually associated with few-?m small hexagonal kaolinite.