Assessing the potential impact of fractures on heat extraction from tight Triassic reservoirs in the West Netherlands Basin

Delft University of Technology, The Netherlands (1); PanTerra Geoconsultans (2); CGG The Hague (3) Heat production from low permeability reservoirs is believed to be significantly aided by the presence of open natural fractures (e.g. Upper Rhine Graben). However, in hot sedimentary aquifer studies these structural features are rarely included when estimating their economic viability. Therefore, we present a case study which assesses the impact of natural fractures on heat extraction from the tight Main Buntsandstein Subgroup targeted by the recently drilled NLW-GT-01 well (West Netherlands Basin (WNB)). We integrate: 1) reservoir property characterisation using petrophysical-analysis and geostatistical-inversion, 2) image log- and core interpretation, 3) large-scale seismic fault extraction and characterisation, 4) Discrete Fracture Network (DFN) modelling and permeability upscaling, and 5) fluid-flow and temperature modelling. First, the results of the petrophysical analysis and geostatistical inversion indicate that the Triassic reservoir layers have almost no intrinsic porosity or permeability. Second, the image-log interpretation shows predominately NW-SE orientated fractures having log-normal length - and negative-power-law aperture behaviour, respectively. Third, the seismic fault analysis depicts pattern which is to be expected from the structural setting of WNB. Fourth, inspection of the reservoir scale 2D DFN’s and upscaled permeability models suggest that if open, natural fractures significantly enhance the effective permeability (up to 500 mD). Finally, the performed 2D fluid-flow/temperature simulations indicate that open natural fractures significantly increase in the geothermal heat extracted from the normally tight reservoir rock. However, while the presented results are positive, it should be noted that the prediction and characterisation of natural fractures in the sub-surface is a relatively uncertain process.