Structural and thermo-hydraulic-chemical effects on geothermal energy extraction from complexly deformed carbonate reservoirs: Insights from Kohlenkalk in Weisweiler, North Rhine-Westfalia (NRW)

Kohlenkalk, a lower-Carboniferous carbonate, is targeted for geothermal exploration in Weisweiler, NRW. In the area, both the NNW-directed Variscan shortening and the Tertiary WSW-ENE Lower Rhine Graben extension significantly affected Kohlenkalk, creating structures on the m- to km-scale that likely control geothermal fluid pathways. To understand these controls, we have classified and characterised structural elements of Kohlenkalk outcrops into fluid conduits (bedding characteristics, joints, fractures, karsts, fold hinge collapse zones) and barriers (clay smears on flexural slip folds and normal/thrust faults). Our data allow the definition of Hydraulic Flow Units (HFU) within the deformed Kohlenkalk sequence in Weisweiler and thus the identification of the most likely geothermal flow pathways.

To assess the sensitivity of these flow pathways to changes in the chemistry of the geothermal fluid, we performed a series of flowthrough experiments on Kohlenkalk sub-members. 4D microCT and post-mortem SEM data, as well as ion chromatography analyses on pore fluids yielded a time-resolved sequence of evolving transport properties, host rock mineralogy, and pore morphology under geothermal conditions.

Our two-pronged investigation combines field- and laboratory data to contribute additional insights on the multi-scale thermo-hydraulic-chemical behaviour of Kohlenkalk in Weisweiler. The dataset is ready to be utilised by digital modelling, which will improve exploration accuracy, reduce risks, and ensure the long-term sustainability of geothermal energy production in carbonates.