Daumic

With the search for oil and gas in the North Sea, significant coal deposits were discovered in this sea (1). The localization of these deposits in a sea-bed prohibits a traditional exploitation by mine. In situ combustion was considered with an aim of generating a combustible gas mixture easier to extract. But this technique was too polluting. There is perhaps another resource to consider: the methane adsorbed in the pores of coal. This type of gas is called CBM for Coal Bed Methane (2). Several data could promise a significant and exploitable gas resource: - the amount of coal present under the North Sea seems significant; the amount of adsorbed methane should be in proportion, - part of these offshore coal deposits are sufficiently close to the coasts of England to be accessible by terrestrial drillings, - the technique of horizontal drilling, already used to recover shale gas, can be employed here to extract gas from coal, - coal is a porous rock, therefore the extraction of gas does not require hydraulic fracturing. (1) https://deepresource.wordpress.com/2018/12/18/north-sea-ucg/ (2) https://en.wikipedia.org/wiki/Coalbed_methane#

Perhaps the mining of shale deposits isn’t the enemy of energy transition. Since 2005, hydraulic fracturing has permit the exploitation of gas and oil confined in shale deposits. These shale deposits produce gas and light liquid hydrocarbons. Some studies have shown the possibility to extract heavy hydrocarbon molecules like paraffin from shale deposits by using supercritical carbon dioxide (1). This fluid is also ideal for the heat extraction from the deep deposit: its high density facilitates the heat transport and its low viscosity eases the circulation in small cracks of the fractured zone. The use of supercritical CO2 on depleted shale wells can associate the extraction of heavy hydrocarbons and geothermal heat. The extraction of heavy hydrocarbons can last some years like the classical extraction of gas and light hydrocarbons in shale deposits. By contrast, the heat extraction can last a very long time. The first test of this heat extraction could be made by the Pittsburgh town in Pennsylvania. This town is surrounded by many wells extracting gas from Marcellus shale deposit (2). This town has also maintained an urban heating network (3). The geothermal heat extracted from the wells located around the town could feed the urban heating network. The geothermal energy has a good reputation as a stable renewable energy but its development is blocked by its high investment cost. If we can associate geothermal energy and hydrocarbon production, the investment cost can be reduced. (1) https://www.researchgate.net/publication/283619903_Extraction_of_Hydrocarbons_from_High_Maturity_Marcellus_Shale_Using_Supercritical_Carbon_Dioxide (2) https://www.fractracker.org/map/us/pennsylvania/pa-shale-viewer/ (3) https://apps.pittsburghpa.gov/mayorpeduto/District_Energy_in_Pittsburgh_DOE_Power_Point_AL.pdf

Unsupported optimism : this comment can be applied on high temperature geothermy. This sort of renewable energy is a promise never realised because its costs are too high. The extraction of gold or other high value metals in deep wells can help the financing of geothermy. Yes, it is speculative. Why not ? In hydrothermal deposits, gold is more often associated with molybdenum or platinum than zinc.

If gold mining by fracking is possible, the gold value can amortize quickly the high cost of drilling and fracturing. After the gold extraction, the drills and fractured zone remain for another use, like geothermal energy. T Finally, the great value of gold can facilitate the development of geothermal energy.

I have a preference for the first method. As you noticed, this method permit to choose precisely the zone of interest.

The great part of the ore is made of silicate that is not dissolved by pyridinethiol. If pyridinethiol extracts other transition metals like zinc, it is not a bad thing, it can add a value to the extraction.

The use of pyridinethiol is for chelating gold and not for dissolving ore.

In few years hydraulic fracturing has revolutionized the world of energy by the production of shale gas and shale oil. It is perhaps possible that fracking can reach another resource in the depth of the Earth: gold. A new theory established by geochemists (1) describes a transport of gold by trisulphide ion in hydrothermal deposit. Trisulphide ion chelates gold and facilitates its transport towards the ground surface by water. But the stability of trisulphide ion depends of temperature and pressure. Trisulphide ion decays at a depth of some kilometres and leaves a first deposit of gold. According to this theory, a second transport by chloride and sulphide ions explains the gold deposits near the surface. We can imagine a deep gold deposit under each hydrothermal gold deposit. The deep gold deposits are probably more massive than the upper deposits because the transport by trisulphide ion is more efficient than the transport by chloride and sulphide ions. These deep gold deposits are not accessible by classical process of mining. These deep deposits are perhaps accessible by hydraulic fracturing. A depth of some kilometres is not a problem. The shale oil deposits of Permian Basin exploited in Texas by fracking have an equivalent depth of some kilometres. How can we extract gold? Perhaps by the following process: - two vertical wells to reach the deep layer of deposit, - horizontal drill between the vertical wells with a hydraulic fracturing, - circulation of water with gold chelatant in the fractured zone, for example pyridinethiol (2). If this process works, gold extraction by fracking can be the beginning of a new chapter of fracking industry: the deep mining. (1) Sulfur radical species form gold deposits on Earth (https://www.pnas.org/content/112/44/13484) (2) Pyridinethiol‐Assisted Dissolution of Elemental Gold in Organic Solutions https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201810447