My half-bakery project is another drill-to-the-mantle geothermal project.
I would like to drill a dry shaft all the way to where the mantle meets the crust, and then have a dense gas like xenon carry the heat to a generator, or something similar.
For this project, I would develop a drilling method
for dry-drilling from the surface, using gas jets such as hydrogen, steam or carbon monoxide, and/or mechanical drill bits, with the cuttings processed by a unit above the drill section, which burns the cuttings, and sprays the remaining molten metals onto the walls of the shaft?
Essentially, the oxygen is freed, reducing the volume of the cuttings by almost half.
The sole advantage of this method would be that there would be a gaseous exhaust, but not a flow of water that might become problematic as pressures and depths increased. Since 45% of basalt rock is oxygen, the hope would be that a high enough heat and possibly other compounds incorporated in the gas cutting mixtures, would bring them to the surface as carbon dioxide or oxygen or other gas mixtures, if not to a porous carbon or resin skip repeatedly dropped into the shaft.
Daisy-chained gas pumps and long sections of vertical pipe could be used, to supply the MAPP and other gases used for the dry-drilling and oxygen-processing of the machine. Ideally, as mentioned, the metals -- Si, Al and Mg -- would stay in the shaft, while the carbon dioxide and other waste gases would rise up as exhaust.
Further informationj gathering would be the first stage of the project.
Experiments with basalt, ultrasound, hydrogen jets, and other of the-shelf drilling technologies would be next. Samples would be ground, processed for their oxygen, measured and sprayed.
A large diameter very shallow shaft would be the next stage of the project, ideally on land, though ocean-going drilling has many good things to recommend it, according to the literature. The design calls for a drill bit much larger than the width of the shaft above it, which may introduce other issues to resolve. The design should also accomodate telerobotics of some kind, though an advantage of air drilling is that a worker might be able to drop down, and inspect and conduct repairs, once the heat and drill were stopped.
It's possible that complete oxygen reduction is not possible, so that the "cuttings to diameter ratio" may be much lower, 5% or 10% versus the 45% of mass which oxygen occupies. This might affect the drill bit arrangement as well.
An unconventional drill bit might also be experimented with at this stage. Thermite-like reactions might work to reduce metals like magnesium and silicon of their oxygen, according to cursory review of the literature. The aluminum oxide might be purged by carbon monoxide or mixed ligand gas. This might greatly help the heat demands of the process, since thermite heats reach approximately 2000 C, though it's unclear how much heat is really needed to free basalt oxygen to gaseous form. Dry catalytic processes for oxygen removal it is hoped would make these ideas low priority. Pumping a ligand mixture containing alumina might prove as awkward as pumping cutting from a depth of three miles using water.
Current literature about hydrogen jet reducing drilling suggests compounds like sulfuric acid may be produced on the inside of the shaft, as some of the incidental issues to this and similar methods.
Nevertheless, given a reducing method like hydrogen gas, and/or a fuel gas like MAPP, acetylene, or hydrogen, and metal oxides like those found in crustal rocks, and some means of safely depositing the molten metal free of oxygen inside the drill hole, and/or rendering it gaseous; the potential to drill a very deep hole without pumping water for cuttings is alluring.