| Abstract |
Hydro-mechanical processes involved in geothermal reservoir engineering are highly complex. Apart from additional thermal and chemical effects, their dynamic interaction has been the focus of attention of many geoscientists addressing hydraulic fracturing in natural and engineered geosystems. In particular, hydraulic fracturing represents a key component in enhanced geothermal systems (EGS). Especially the fracture path and geometry in a layered reservoir is influenced by a variety of factors such as different mechanical and hydraulic material properties, different stress regimes in the respective layers, material heterogeneities, interaction with pre-existing fractures, and others. The North German Basin with its typical lowpermeable sedimentary veneer has been the subject of recent intensive geothermal studies to evaluate the potentiality of cost-effective extraction of geothermal energy. Among the prominent projects currently in progress, the project “Hydro-mechanical response of geothermal reservoirs in the stress field generated by complex geological structures is a subproject of the interdisciplinary research association “Geothermal energy and high-performance drilling techniques” (Geothermie und Hochleistungsbohrtechnik “gebo”) in Lower Saxony, Germany. The goal of this subproject is to advance and refine the understanding of the hydro-mechanical behavior of geothermal reservoirs typical of the North German Basin. Using FRACOD as two-dimensional boundary element code, a series of numerical models involving relevant scenarios were tested. Different numerical simulations with dissimilar layer sequences characteristic of the North German Basin at targeted depths were performed. By means of this a broad range of possible scenarios was examined. Loading conditions provided by previous modelling and data on the stress field in the region under investigation as well as material properties from laboratory data were varied over a wide parameter space. Specifically, the drilling demonstration project GeneSys-Borehole GT1 in Hanover Groß-Buchholz together with vast data obtained from laboratory measurements on specimens typical of the study area have provided valuable constraints on the hydraulic and mechanical properties of the modeled geothermal reservoirs. A multiple fracture scenario is also included to study fracture interaction, an applicable scenario in deeper targets such as vulcanite. First, preliminary modelling results show that the difference in elastic properties such as Young’s modulus and Poisson’s ratio between the sedimentary layers has little influence on the fracture trajectory. The difference in these elastic properties does not lead to fracture containment or arrest at material interfaces, but rather has an influence on fracture aperture. This corroborates previous field observations but disagrees with recent numerical modelling. Difference in mechanical properties like fracture toughness in mode I and II and their ratio around sediment interfaces proofed to have a significant impact on the fracture path and mode of deformation. Model results demonstrate that with specific but in laboratory measured values of this parameter in both modes of deformation, fracture paths through interfaces may be clinked or splayed and switch to a mixed mode of deformation. Model results of multiple fracture scenarios reveal the complex interaction of pre-existing fractures with a hydraulically induced fracture. Pre-existing cracks experience displacement and hydraulic changes before they are hit by the hydraulically induced fracture. Moreover, when this latter hits the natural cracks it does not continue its previous path but rather the preexisting cracks propagate in the direction of maximum shear stress. |