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Does all commercially available helium come from natural gas wells?
Is it theoretically possible to have a pure helium well?
Helium is produced by the radioactive decay of primordial uranium and thorium. It should not be strongly associated with non-primordial 'fossil' hydrocarbons.
Helium is produced by the radioactive decay of primordial uranium and thorium. It should not be strongly associated with non-primordial 'fossil' hydrocarbons.
The first statement is correct. The second is not. There are several reasons that helium should be strongly associated with non-primordial hydrocarbons. Both are fluids found predominately in the crust due to the highly lithophilic nature of uranium and thorium in the case of helium and the fossil origin in the case of hydrocarbons. Being a gas, helium is somewhat soluble in hydrocarbons. Like most gases, helium solubility in hydrocarbons increases with pressure. Unlike many gases, helium solubility in hydrocarbons increases with temperature.
Trapped hydrocarbons thusly act as a trap for helium. In places where hydrocarbons aren't present, helium might be trapped by some capping rock, or it might escape through the tiniest of pores. As the smallest gas (even molecular hydrogen is larger), helium is wont to find a way to escape to the surface -- unless temporarily trapped by a trapped liquid.
David Hammen's answer explains why He is extracted from natural gas. But, it is not found only there. Helium exists just about everywhere on earth. You find it in volcanoes, in subseafloor hydrothermal vents and even just slowly leaking away from the ground in U and Th rich zones. As with all extraction of natural resources, it comes down to economics. How much is there, can we extract it, and how much does it cost to extract it?
Extracting the He from natural gas sources is easy: it's relatively concentrated, and we're drilling the thing anyway to get the gas out. Might as well get any by-products that are there, one of them being He. This is just like extracting indium from zinc ores, or extracting rhenium from molybdenum ores. We don't have indium mines or rhenium mines, they're extracted as by-products from other deposits.
The helium in natural gas wells is created from uranium and thorium (as you correctly state) contained in the underlying granite basement rock or radioactive black shales that allows the natural gas to be trapped and contained. It's not that the helium is created from the fossil hydrocarbons, but that it comes from the rocks that trap those hydrocarbons (source).
Nat gas is the lowest cost source. The He ( alpha particles) is from decay of elements from silver and up ( atomic weight > 106 per Wiki ). For a price it can be extracted from air. For example atmospheric argon is a significant nuisance in chemical plants that use air as a raw material , such as ammonia production. The recycle stream must have a constant bleed to prevent excessive concentration of Ar. I don't believe they analyzed for He . The bleed for Ar must have also reduced the He.
