Deep U-tube heat exchanger breakthrough: combining laser and cryogenics gas for geothermal energy exploitation &#8211; a perspective of laser-rock interactions
Authors: Slupski Pawel , Zampieri Enrico , Di Sipio Eloisa , Manzella Adele , Pasquali Riccardo , Pockele Luc , Romanowski Arno , Sassi Raffaele , Steinmeier Olaf , Galgaro Antonio 
Published: March 2024

The technology envisioned in the DeepU project (Deep U-tube heat exchanger) is expected to revolutionize the geothermal energy sector, increasing the accessibility of deep geothermal resources for low-carbon heating and power generation. The ultimate project goal is to create a deep (>4 km) closed-loop connection in the shape of a U-tube exchanger by developing a fast and effective laser drilling technology. The project comprises the development of a novel drilling technique and its application via geothermal modeling at selected sites. A prototype of a drill-head has been realized, combining the laser system with drill strings, sustaining the coupled action of laser and cryogenic gas. The fine particles of drilled rocks are ejected to the surface in the gas stream via the borehole annulus. This contribution focuses on the project&#8217;s activities related to the laser-rock interactions studied in the experimental laser drilling tests based on previous works (Seo et al., 2022; Li et al., 2022a, 2022b). Three types of lithologies were selected for initial laboratory tests: granite, sandstone, and limestone (50 x 35 x 15 cm). Constant rates of penetration (ROP) upwards of 20 m/h have been achieved in all lithologies with borehole diameter reaching 18 cm. The petro-thermo-mechanical phenomena occurring during laser drilling, such as spallation, melting, and evaporation, were recognized and described. The drilling process was investigated by thermocamera imaging providing information about the most effective process induced by heating the rocks, up to 700&#176;C. The laser working parameters and experimental setup were optimized regarding observed phenomena. In the next step, sections of boreholes were cut out and examined. The microscopic observations on the thermal unaffected and affected rocks&#8217; thin sections have been performed with the use of polarized optical microscopy and scanning electron microscopy revealing micro-fracturing patterns of the rock induced on rock samples by the heating processes. The change of physic-mechanical properties of rocks was investigated and acknowledged in geothermal models. This innovative and comprehensive study revealed macro- and micro-scale phenomena occurring during laser drilling, contributing to the successful development of this new drilling method and subsequently its application for exploitation of geothermal energy from depths below 4 km.This research is funded by the European Union (G.A. 101046937). However, the views and opinions expressed are those of the author(s) only and do not necessarily reflect those of the European Union or EISMEA. Neither the European Union nor the granting authority can be held responsible for them.ReferencesLi, G., Shi, D., Hu, S., Ma, C., He, D., and Yao, K., 2022a, Research on the mechanism of laser drilling alumina ceramics in shallow water: The International Journal of Advanced Manufacturing Technology, v. 118, p. 3631&#8211;3639, doi:10.1007/s00170-021-08190-0.Li, Q., Zhai, Y., Huang, Z., Chen, K., Zhang, W., and Liang, Y., 2022b, Research on crack cracking mechanism and damage evaluation method of granite under laser action: Optics Communications, v. 506, p. 127556, doi:10.1016/j.optcom.2021.127556.Seo, Y., Lee, D., and Pyo, S., 2022, The interaction of high-power fiber laser irradiation with intrusive rocks: Scientific Reports, v. 12, p. 680, doi:10.1038/s41598-021-04575-z.