Heat Exchange in Tensile Fractures

Abstract

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.