Geothermal energy is a relatively sustainable energy source of which the essence is to extract heat from hot subsurface rocks. Circulating fluids serve as the transport agent of heat. The contact area between the fluids and the rocks is where the relevant heat transfer occurs, i.e., where the water is heated up. In some geothermal reservoirs, this circulation occurs naturally through porous matrix (mainly sediments) or through heavily fractured formations. Enhanced geothermal systems (EGS) are potentially favorable reservoirs where subsurface permeability is increased by means of artificial stimulation techniques. These stimulation techniques often involve hydraulic fracturing where a limited existing fracture network is expanded or “enhanced” by injecting fluids under high pressure conditions. While the geometry of the generated fractures influences permeability, the effect on heat exchange has received less attention. This thesis discusses the effects of fracture geometry on the heat transfer between solid and fluid. Along with laboratory experiments, numerical simulations were conducted. All investigations were performed on igneous granite rocks. Tensile fractures were generated to allow a fluid flow along the otherwise impermeable rock samples. Several parameters were varied throughout the experiments and simulations including volumetric flow rate, fracture aperture, rock temperature and fracture geometry and surface area in order to investigate their impact on heat transfer processes. Flow rate variations in the experiments have shown that higher flow rates cause the fluid to absorb less heat per unit volume and cause the rock to cool down more extensively, therefore thermal depletion of the reservoir is likely to occur within a shorter time frame. The dependency of exchanged heat on fracture aperture variations (in the range of 0.05 to 0.5 mm) did not yield a clear trend within the experiments, but does so in numerical simulations. Aperture variations in the numerical simulations did not cause notable differences in transferred heat as long as the volumetric flow rate is kept constant. However, as the fluid velocity is kept constant the amount of fluid flushed along the fracture per unit time is affected by varying apertures. This causes a difference in heat transfer as well. Increased fracture surface areas alone (more extensive topology/roughness) have shown a minimal impact on the heat production while a more extensive fracture network (additional branches) has shown notable enhancement in the amount of heat produced. Cooling behavior of the rock has shown correlations with Newton’s law of cooling and suggests a limitation of heat production by the heat conduction occurring within the rock.
Experimental findings cannot directly be compared with natural reservoir conditions. The reason for this is a thermal equilibrium that is achieved at each flow experiment, i.e., the heat withdrawn equals the heat resupplied by a heater. In natural reservoirs this is often not the case where a cold front propagates towards the production well and determines the lifetime of how long heat can efficiently be produced from a certain rock mass. This results in an unsteady heat conduction where the heat withdrawn does not equal the heat resupplied.

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.

This thesis aims at investigating the effect of a thermal shock of a couple of hundred degrees on the creation of the thermal microfractures and in what way the mechanical properties of volcanic rocks change. In the geothermal field, thermal microfractures can contribute to increase the permeability and produce more steam to the surface. We exposed basaltic andesite and andesite rocks from Tangkuban Perahu (Indonesia), a granite from Benin (West Africa), a basalt from the Eifel (Germany) and a basalt rock from Reynifsjara (Iceland) to temperatures of 200, 300, 400 and 500°C before cooling them rapidly by placing them in cold 20°C water. Initially, most of the sample rocks show less than 1% of porosity, excluding andesite Tangkuban Perahu (~6%) and basalt Reynisfjara (~14%), and permeability for all rocks is below detectable value (2.6 mD by Ruska gas permeameter for specified core geometry). In the geothermal field scenario, it indicates the pores were not connected and the steam may not easily flow through the rock. After the heating stage and thermal cooling, significantly increased values of porosity are observed in most of the rocks. However, only the two high porosity rocks gained permeability; the andesite Tangkuban Perahu (7-11 mD) and basalt Reynisfjara (4.45 mD). The changes in Young’s modulus, Poisson’s ratio and ultimate compressive strength of the various samples were also determined by using an unconfined Uniaxial Compressive Strength (UCS) apparatus in which both heat treated and non-heat treated samples were placed. The results show the rock strength decreases with increasing thermal shock. Similarly, we see a decrease in both Young’s modulus and Poisson’s ratio with increasing thermal shock. To recognize the thermal shock effect visually, the samples were imaged using a micro-CT scan before and after heating treatment. The resolution of the CT scanner was 30 μm at best when the whole sample is scanned. It is noticed that the porosity increases after the heating experiment. Additional mini-cores (~10 mm x 8 mm) from the whole core are also scanned at 14 μm resolution. The scans showed the porosity on the outside part of the cores is up to 10 times higher than the inner part. In Indonesia, the geothermal reservoir temperature varies from 200-300oC. By injecting cold water from the surface at a high rate, the water temperature remains low and can thus create the thermal shock that opens up existing fractures and forms new ones. In that way, it can increase fluid path ways around the well bore and along existing natural fractures.