Geomechanical Characterization of the Buntsandstein including a Fracture Model

Abstract

With an increased demand for renewable energy sources, geothermal developments have moved into the spotlight, including sedimentary structures. In the western part of the Netherlands, cretaceous sandstone bodies are already in use for the geothermal energy production. However, these reservoirs are generally producing water at temperatures less than 100°C from depths of 1500 to 3000 m. To reach higher temperatures, deeper reservoirs need to be targeted. Being more than 130 m thick in some parts and up to 4000 m deep, the Lower Triassic Buntsandstein would provide an ideal target reservoir for geothermal exploration in the West Netherlands Basin, given the high temperatures of over 140°C at this depth. However, the Buntsandstein is relatively heterogeneous in terms of porosity, as seen in numerous rock samples, and the overall permeability that was found in the Buntsandstein in various wells was generally low. Therefore a conventional development of a Buntsandstein reservoir, with a geothermal doublet in a natural hydrothermal reservoir, might not be applicable. To enhance the permeability and achieve economic flow rates, hydraulic stimulation can be performed. This technique has proven to be effective in many other reservoirs. To assess such effects on the Buntsandstein, this thesis focusses on the effect of fractures on the fluid flow in the Buntsandstein. Geomechanical characteristics and physical properties of the Buntsandstein at relevant pressure and temperature (PT) conditions are investigated in core experiments in uniaxial compression tests and triaxial tests. The laboratory experiments on cores provide reliable intrinsic information of permeability at relevant PT conditions and give a realistic estimation of fluid flow parameters. Triaxial deformation tests were conducted on different sandstones varying in porosity, clay content, degree of oxidization and initial permeability. Changes in permeability before and after deformation are compared. Moreover, the effect of increasing confining pressure on the permeability is measured on intact sandstones. Two principal modes of brittle fracture (extension fracture and shear fracture) were tested on samples under confining pressure. Experiments revealed that the permeability decreases by a factor of 17 in porous sandstones, from 85 mD to 5 mD, when inducing shear fractures at 30 MPa effective pressure. Buntsandstein samples with a low initial permeability (~1 mD) show a slight increase to 2 mD. For tensile fractures the permeability increased exponentially with the increase in apparent fracture aperture. Up to apertures of around 1000 µm, the permeability increased by factor of 2 for increments of 100 µm. Ultimately, effects of fractures and resulting changes in flow properties are analyzed for the impact on the production life of a geothermal injector-producer pair. Results from lab experiments are implemented in a field scale thermo-hydro-mechanical model, making it possible to predict the PT distribution in the reservoir in regard to a change in permeability. The laboratory experiments and the numerical modeling procedure indicate that the in-situ permeability is too low for a geothermal reservoir production and would require some sort of stimulation to enhance the permeability.