Evaluation des parametres influencant les temperatures d'operation des puits a colonne permanente

A Standing column well (SCW) is a type of geothermal well which uses groundwater as its heat carrier fluid. The groundwater is usually pumped from the base of the well through a dip tube and sent through a heat exchanger before being reintroduced at the top of the well, in-between the dip tube and the borehole wall. Although SCWs are generally built within deep boreholes, the thermal performance of each well is such that, in comparison to closed loop geothermal wells, it allows a significant reduction in total required drilling depths for a given load. Reductions in total drilling depths will generally have a significant impact on the construction costs of geothermal systems. Existing SCW models generally assume a homogeneous and isotropic aquifer, where the components of the geological matrix are practically in instantaneous thermal equilibrium. According to Banks (2008), these assumptions are less accurate in geological formations characterised by fractured, lithified bedrock, which are generally the most favourable to SCW application. The objective of this work is to demonstrate that the presence of a fracture zone can have a significant impact on the operation of the SCW.;This study demonstrates that the presence of a single fracture zone within the bedrock can have a significant beneficial effect on the performance of SCWs. Temperature differences between the porous medium model and the fractured model were particularly important when the SCW was operated at bleed ratios of 10 % to 20 %, corresponding to typical bleed percentages. Several parameters which characterise the geological medium have a significant impact on the performance of operating SCWs. The parameters having the most significant impact on operating temperatures are the thermal and hydraulic conductivities of the bedrock while porosity has little effect on these results. The analysis showed that hydraulic conductivity had a particularly significant impact while the operating system was in transient mode, prior to the settlement of the drawdown cone. However, over a long period of time, thermal-hydraulic conditions in the vicinity of the well tended to stabilize at the same temperature regardless of the hydraulic conductivity. It should be noted that the hydraulic conductivity is the only parameter affecting the drawdown at the well. However, this impact is significant and it is therefore recommended to conduct a hydrogeological study to assess this parameter before the construction of a SCW.;The proposed approach is to develop and validate a numerical model based on the assumption of flow in an equivalent porous media. The model will then be transformed to include a high hydraulic conductivity layer representing fractured bedrock. Since little information on the thermo-hydraulic response of SCWs is accessible to designers, the models will be subjected to a series of tests to illustrate their behaviour under various operating conditions.