Typically, the temperature in those vast regions of the accessible crystalline basement rock increases with depth. This geothermal gradient, which is the principal HDR resource variable, ranges from less than 20 °C/km to over 60 °C/km, depending upon location. The concomitant HDR economic variable is the cost of drilling to depths at which rock temperatures are sufficiently high to permit the development of a suitable reservoir.Tester, J. W., Herzog, H. J., Chen, Z., Potter, R. M., and Frank, M. G., 1994. “Prospects for universal geothermal energy from heat mining,” Science and Global Security Vol. 5, pp. 99–121 The advent of new technologies for drilling hard crystalline basement rocks, such as new PDC (polycrystalline diamond compact) drill bits, drilling turbines or fluid-driven percussive technologies (such as Mudhammer Souchal, R., 2017, High-Power Mudhammer Drilling: A promising solution for deep geothermal reservoirs, Geotherm Deep Geothermal Energy Congress proceedings) may significantly improve HDR economics in the near future.
