Geothermal well drilling differs from oil well drilling on some points, although both use the same rotay drilling methods. The primary point in drilling geothermal wells may be the formation temperature which will be higher than typical oil wells even at the same depths. In addition to the extremely high temerature environment, frequent occurrence of severe lost circulations in naturally fractured or fault zones and very low rate of penetration due to the volcanic or granitic hard and abrasive formations are also the critical issues. Increase in drilling cost caused by these problems peculiar to geothermal well drilling will act as a barrier particularly for EGS (enhanced geothermal system) development as a future promising renewable energy. To reduce the drilling cost in EGS well drilling, applicabilities of emerging technologies being developed in oil well drilling industry were surveyed in this study. For example, a normal managed pressure drilling (MPD) system combined with continuous mud circulation system or non-stop driller is very effective for keeping the bottomhole pressure constant to avoid lost circulation. Furthermore, the continuous circulation system is also effective for keeping the bottomhole temperature sufficiently low during tripping operations. Also, development of new types of drill bits and non-mechanical novel drilling technologies such as laser drilling or other high energy drilling systems are now progressing. Effectiveness and feasibility of these emerging drilling technologies for EGS development were evaluated and the future tasks are discussed.

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