The creation of deep reservoirs for geothermal energy or oil and gas extraction is impeded by insu cient stimulation. Direction and extension of the created fractures are complex to control and, therefore, large stimulated and interconnected fracture networks are di cult to create. This poses an inherent risk of un- economic reservoirs, due to insu cient heat-sweep surfaces or hydraulic shortcuts. Therefore, we present a new technique, which locally increases the cross section of a borehole by utilizing a thermal spallation process on the sidewalls of the borehole. By controlled and local enlargement of the well bore diameter, initial fracture sources are created, potentially reducing the injection pressure during stimulation, initiating fracture growth, optimizing fracture propagation and increasing the number of accessible preexisting frac- tures. Consequently, local thermal borehole enlargement reduces project failure risks by providing better control on subsequent stimulation processes. In order to show the applicability of the suggested technique, we conducted a shallow field test in an underground rock laboratory. Two types of borehole enlargements were created in a 14.5 m deep borehole, confirming that the technology is applicable, with implications for improving the productivity of geothermal, oil and gas reservoirs.