| Abstract |
Fluvial hot sedimentary aquifers (Hsa) Are composed of permeable sandstone bodies surrounded by impermeable floodplain claystones. In this type of aquifers, the sandstone bodies form the fluid pathways. The aquifer architecture describes the distribution of the sand- and claystone bodies. One of its main parameters is the net-to-gross. This is the ratio of total sandstone body volume and the total aquifer volume including both the permeable and impermeable bodies. If the net-to-gross is high, the sandstone bodies are more likely to form larger and fewer clusters. If in contrast, the net-to-gross is low, more and smaller, isolated clusters are formed. This is reflected by the connectivity of the aquifer which is the ratio of the volume of the largest sandstone body cluster and the total sandstone body volume (Larue & hovadik, 2006). For example, if the connectivity is 100% all sandstone bodies form one big network. If the connectivity is 20%, the largest sandstone body volume is 20% of the total sandstone volume in the aquifer. Low connectivity is a risk in the development of hsa because isolated clusters do not contribute to the heat production. The connectivity is furthermore influenced by the geometries and paleo flow direction range of the sandstone bodies (Cuevas gazolo et al., 1997; Larue & hovadik, 2006). Both the net-to-gross and the connectivity influence the life time of a geothermal doublet. During the production, a cold-water plume develops around the injection well. The moment at which the cold water plume reached the production well is defined as the thermal breakthrough moment. After this moment the production temperature decreases. In heterogeneous fluvial sedimentary aquifers, the cold water 'fingers' through high permeability sandstone bodies. Bypassed low permeability aquifer bodies supply heat to the more permeable bodies through heat diffusion (SalimI et al., 2012, poulsen et al., 2015). The aquifer architecture therefore influences the thermal breakthrough moment and the following speed of the production temperature reduction. In homogeneous or layered reservoir models (Mijnlieff et al., 2007; Mottaghy et al., 2011; Ekneligoda et al., 2014), the cold-water plume development will have a cylindrical shape. To improve the life time estimations of geothermal doublets in fluvial aquifers, detailed reservoir architecture models have to be taken into account. The effect of reservoir heterogeneities on reservoir performance are extensively studied for oil and gas production (Larue & hovadik, 2006; Larue & hovadik, 2008) And to a more limited extend for geothermal energy production (Mijnlief et al., 2007; Mottaghy et al., 2011; Ekneligoda et al., 2014) And (Combined) Co2 sequestration (SalimI et al., 2012; Issautier et al., 2013; Issautier et al., 2014). This study aims to improve life time estimations of geothermal doublets in fluvial hot sedimentary aquifers. For this purpose, hundreds of facies models are generated with process-based facies modelling software based on a geological dataset of a lower cretaceous fluvial interval in the west netherlands basin. In this way facies models are created in which the sandstone bodies have a sedimentologically based spatial relation (Karssenberg et al., 2001). This is crucial for the connectivity analysis. The net-to-gross in the facies models ranges from 10 to 100%. In addition, a group of facies models is generated with random facies distribution. The net-to-gross in this group of models also ranges from 10-100%. First the connectivity is determined in every facies model and related to the net-to-gross of the model. Second, a finite-element approach is utilized to study the geothermal energy production in all models. In every model, a vertical production- and injection well are placed at a 1000m distance. The initial reservoir temperature is 75 °c; The re-injected water has a temperature of 35°c. Geothermal energy production is simulated in the doublet models with a 100 m3/H production flow rate. The life time of a doublet model is defined as the time at which the production temperature degreased to 74 °c. The life time in all flow rate and minimum production temperature scenarios is related to the net-to-gross and connectivity of the facies models. The facies modelling process, the connectivity analysis and production simulation results are described in the following paragraphs. |