--> Luttelgeest Case Study Illustrating The Design Workflow Of A Geothermal System

AAPG European Region, Geothermal Cross Over Technology Workshop, Part II

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Luttelgeest Case Study Illustrating The Design Workflow Of A Geothermal System

Abstract

In the coming years a set of geothermal systems will be realised in Luttelgeest in the Dutch province of Flevoland to provide heat for greenhouses. Geothermal experts from IF Technology have completed extensive research into the geological and reservoir engineering parameters in order to design multiple geothermal systems around Luttelgeest. Seismic data was interpreted to derive the depth of the Slochteren Formation which acts as the geothermal reservoir. Petrophysical analysis of nine wells was performed to determine the reservoir properties. Special attention was paid to the probability of reservoir compartmentalization and seismic hazards. Based on the study results a geothermal system with two production and four injection wells was designed. As part of the geological study the top and base of the Slochteren Formation were interpreted on a selection of 46 2D seismic lines. Information from five hydrocarbon exploration wells were used to tie the seismic. A 2D depth conversion method was applied. The seismic interpretation results illustrate that the Slochteren Formation reaches a depth of approximately 1750 m within the Luttelgeest geothermal exploration license area. The thickness of the Slochteren Formation was derived by interpolating well data. The Slochteren Formation has an average gross thickness of 70 m within the Luttelgeest geothermal exploration license area. Petrophysical analysis using data from nine wells in the vicinity of the project location was performed to determine the reservoir properties (net-to-gross, porosity, permeability and salinity) of the Slochteren Formation in the study area. Based on a Monte-Carlo analysis it was derived that the average porosity of the Slochteren Formation is 23% and the average permeability is 600 mD. IF Technology reservoir engineers ran a 1D reservoir model using the DoubletCalc 1D application (Mijnlief et al., 2014) to calculate the indicative power of the Luttelgeest geothermal system. A geothermal system with two production and four injection wells was designed, because this configuration leads to the most optimal results in terms of geothermal capacity. The designed geothermal system has an expected total capacity of 58 MWth. A 2D reservoir model using the DoubletCalc 2D application (Veldkamp et al., 2015) was also ran to analyse the pressure and temperature development around the proposed production and injection wells. To de-risk the occurrence of reservoir compartmentalization a fracture analysis was performed. Three methods were used: seismic attribute (similarity) analysis, shale gauge ratio analysis and stress analysis. The seismic similarity displays illustrated that no reservoir compartments are to be expected. The shale gauge ratio was estimated to be 11% in the study area. The vertical stress, and maximum and minimum horizontal stress are analysed based on information available from the world stress map (www.world-stress-map.org). The results of the seismic attribute, shale gauge ratio and stress analysis implicate that faults present in the study area are not forming hydrological barriers at reservoir level. As part of the geothermal system design process the potential for induced seismicity was analysed. The study area was assessed on a selection of geological parameters (i.e. distance to faults, connection to basement) and production parameters (i.e. re-injection pressure, injected volume). Based on this assessment it was concluded that the study area has a low potential for induced seismicity. The Luttelgeest case study illustrates the extensive research that is required to design an optimally and safely performing geothermal system with multiple production and injection wells.