| Authors |
Jan-Diederik VAN WEES, Siavash KAHROBAEI, Sander OSINGA, Brecht WASSING, Loes BUIJZE, Thibault CANDELA, Peter FOKKER Jan TER HEEGE, Mark VRIJLANDT |
| Abstract |
The role of geothermal energy production as a source of a sustainable energy for district heating in the Netherlands is expected to grow, from 20 geothermal doublets currently in operation to around 175 doublets in 2030 . Current geothermal doublets and planned doublets produce from porous sandstone aquifers of Tertiary, Cretaceous/Jurassic, Permian (Rotliegend), and Triassic age and fractured carbonate rocks of Dinantian age. Production temperatures of the conventional doublets are generally between 65 – 100 °C, and fluids are re-injected at temperatures between 20 - 45°C. Cooling of reservoir rocks and surrounding rock mass can lead to significant thermal stresses, which, superimposed on the pressure induced stress changes, may affect fault stability and may lead to an increased seismicity hazard. In particular, doublets drilled in competent rock types and marked by large temperature contrasts are prone to a high likelihood for the build-up of significant thermal stresses over time ( more than more than 1MPa). For safe and effective operations, it is important to assess the long-term combined effect of pore pressure and temperature changes in geothermal operations on fault stability and associated seismicity, taking into account (1) operational parameters such as injection temperatures, pressures, flow rates volumes and (2) in-situ geological, geohydrological and geomechanical factors. We developed a 3D workflow, capable of assessing both pressure and thermal evolution and its effects on stress changes on faults, which can be used to evaluate seismic hazard. The workflow is designed for complex faulted reservoirs, taking into account the afore-mentioned operational and in-situ factors. The workflow easily quantifies the effect of long-term cooling during geothermal operations on fault stresses, which can be used for fault reactivation potential and seismic hazard, for typical conventional geothermal doublets. In this paper the workflow is demonstrated for relatively simple reservoir geometries in the most common geological settings in The Netherlands, i.e. homogeneous porous sandstone reservoir (representative of the Cretaceous/Jurassic, Rotliegend and Triassic reservoirs in the south-western and northern part of The Netherlands). Results for homogeneous and faulted porous sandstone reservoirs indicated that geothermal doublet operations, can cause thermal stressing causes a significant increase of Coulomb stresses and can have a destabilizing effect on fault stability within the vicinity of the geothermal doublets. Increased pore pressures can cause additional positive Coulomb stressing of the faults; however, the effect of pore pressure is limited in areal extent and relatively small compared to the effect of thermal stresses, except in the first years of operation. Acknowledments This work was part of a project that received funding by the European Union’s Horizon 2020 research and innovation programme Destress under grant agreement number 691728. |