Linking Chemical Phase and Mechanical Properties to Evaluate the Use of Millimeter-Wave Induced Vitrified Basalt in Enhanced Geothermal Systems

Abstract Extraction of geothermal energy from Earth’s heat could significantly contribute to long-term energy needs, yet the current geothermal drilling process faces significant technical limitations. A promising advancement in enhanced geothermal systems is the use of a millimeter-wave (MMW) gyrotron, which enables faster and more efficient drilling. The MMW drilling process offers two key advantages over traditional methods: (1) rock is melted rather than mechanically drilled, leading to faster well hole advancement, and (2) the molten rock solidifies into a vitrified wall, eliminating the need for additional casing materials. This integrated drilling and casing method has the potential to save costs, time, and materials. This paper examines the strength, structural integrity, and microscale mechanical and chemical properties of the vitrified material formed during the mm-wave process, focusing on basalt as the test material. By employing a suite of experimental and analytical characterization techniques, this study aims to provide a comprehensive comparison of the structural, mechanical, and chemical changes in the rock before and after melting, offering insights into the effectiveness and implications of mm-wave drilling for enhanced geothermal systems. Highlights There is a clear change of phase between the basalt, the transition zone, and the melt, due to mm-wave exposure. The region exposed to mm-waves is completely vitrified, while there is partial melting of minerals within the zone right outside of the mm-wave beam. The transition zone created from mm-waves poses high risk to wellbore stability due to its variable mechanical strength and chemical composition. A better understanding of this new material can be achieved by overlaying a series of chemical and mechanical characterization data.