Considering that they're talking about extracting it from what are almost certainly old oil and gas wells (the depth, as well as the Smackover formation referenced in the article, as well as the fact that ExxonMobil is the one proposing this), I'd assume that a big part of this is "well, it's not producing oil and gas anymore; is there anything else valuable down there we can get to?" The sort of operation that you wouldn't be able to make profitable if starting from scratch, but since you already have a lot of the infrastructure and experience in place, it might make sense.