| dc.description.abstract | Thermal fracturing is aim to increase the permeability of the reservoir formation through thermal stress cracking. It occurs in enhanced geothermal system (EGS) wells and other types of geothermal wells where the temperature difference between injection fluid and reservoir temperature is high. Although the hydraulic fracturing is the effective technology to improve the production wells with low permeability in EGS, thermal fracturing is observed to further enhance the permeability of reservoir. However, the mechanism of thermal fracturing in downhole conditions is still not well understood. In the thesis, we aim to provide a deeper understanding of the thermal fracturing and thermal shock mechanisms by an experimental study in a laboratory environment mimicking EGS wellbores.
The laboratory study investigates the behavior of thermal fracturing stimulation by using room temperature water injected into hot concrete and granite blocks without confining pressure. An experimental test system and test procedures were developed and used to apply thermal shock to wellbores in block specimens with elevated temperatures. Water usage, borehole pressure, and temperature were monitored continuously during the fracturing process. The direct and indirect assessments of fractures were made by visual inspection, bubble leakages, pressure decay, and acoustic signatures before and after the experiments. In addition, the mechanical and thermal properties are measured and analyzed before and after treatment.
Experimental data showed that the permeability of the treated specimens was enhanced by the macro/micro cracks induced by thermal loading during thermal stimulations. The profiles of borehole pressure decay obtained before and after each stage of stimulation show that the thermal shock increased the permeability of treated specimens. The thermally driven fractures were usually initiated from the borehole surfaces and propagated adjacent to the boreholes to some extent. Those fractures was confirmed by the acoustic measurement and visually demonstrated by bubble leakage tests. These “seed” fractures created during the thermal stimulations may help reduce the breakdown pressure levels of pressure-based fracturing methods, and improve fracturing efficiency by creating multiple thermal fracture surfaces around a wellbore. | en |
