This article by the Wall Street Journal discusses one of the requirements for economical accessing of the Earth’s deep heat: drilling.
The radically new technologies such as millimeter wave drilling, laser drilling, plasma drilling, and a few others that are less well known, have been around for a few years. Of course, technology continues to advance, but consideration of these new approaches must be assessed dispassionately, and repeatedly as developments ensue. I attach a few comments below, but there are also projects to increase the rate of penetration of more conventional methods.
Recently, in the FORGE Project context in Utah (https://utahforge.com/), improvements in conventional rotary drilling using polycrystalline diamond compact (PDC) bits have increased the rate of penetration in the local granites by a factor of four! This is an astounding improvement, and I refer you to the website and to this JPT article for further details:
Another approach mentioned in the article is the ORCHYD Project in Europe (www.orchyd.eu). They are pursuing a combination of percussive drilling (highly effective in brittle rock), high pressure water jets actuated downhole to help break the granite, new bit shapes, and a couple of other tweaks to the process. Achieving rate-of-penetration values in excess of 15 metres/hour for a 20 cm hole diameter during active drilling time-on-bottom seems possible. Excellent advances over the last decades in percussive drilling in the mining industry tend to support such an optimistic outlook.
I believe that more automation and real-time AI and penetration rate optimization will further increase the penetration rate by a factor of 1.5.
However, the latter two developments tend to be overshadowed by claims for the other processes (millimeter wave, plasma…).
Some issues that these technologies must solve are:
Borehole stability when heating the rock massively (high tangential stresses are generated, accelerating spalling).
Borehole stability in deep stress fields that can lead to such high stresses that even a borehole in granite is difficult to keep open. This is especially true if the two horizontal stresses are significantly different, which is often the case.
Returning the cuttings to surface from great depth without a drilling fluid, or somehow getting rid of the rock waste to generate a stable hole, is challenging.
Supporting the bottom-hole assembly for plasma or mm-wave drilling with steel drill pipe (or wave guides) for depths greater than 8-9 km is a great challenge because of the need to carry all that weight.
These are not trivial requirements. The “highly novel” drilling technology claims must be demonstrated “on the ground”.
In any case, great advances in drilling speed are hoped for in the next 10-20 years. These will go a long way to bring deeper geothermal projects into the realm of commercial interest because many projects find that 70% of the costs of a project are associated with drilling wells. Also, the rapid percussive drilling being pursued by ORCHYD will have many applications in the mining industry and in the shallower heat storage approaches involving arrays of 100-300 m deep borehole heat exchangers to create seasonal heat storage systems. These are already being emplaced (e.g.,https://www.geosourceenergy.com/) underneath high rises in Toronto and other places, and one may envision their installation in subdivisions equipped with direct heating capabilities. High-speed drilling will reduce their cost.
The nice thing about geothermal approaches is that they are carbon-free and environmentally benign. Something everyone wants, but are reluctant to pay more for.
