I've made a post on this a while back. I'm far from an expert, but my understanding is it all depends on geology of the rock they are fracing. If it was a really permeable rock, then low density may do the trick, because it would just crack the target. At the depths of the hosston, high density fracing would hopefully aim to bust the rock with millions of tiny fractures spaning hundreds of metres. They do this by firing explosive charges in the target zones in order to breach the casing. They then pump extremely high pressure fluids into the well, but do this in an on-off-on-off… manner so that the pressure is not constant but rather huge, then none, then huge, then none, etc. When the pressure is turned on and off like this it fracs the rock more effectively due to constant change in stress. While pumping high pressure fluids into the well they also add sands which help fill these cracks and hold them “open” while still enabling flow once the fracing is complete. As you could appreciate this is no overnight job. I have seen coal seams that have been pressure frac’d to help release methane and when you go down to mine the coal afterwards, it falls apart like talc, so the process can be very effective. Hope this helps… & makes sense.