"So Sasol could provide a replacement for oil that only costs 10-20$ per barrel?"
With sufficient natural gas feed stocks, yes.
As to why it's not being used now, until the very recent fracking revolution the supply of gas had been pretty tight, at least at where it's needed for consumption. See e.g. Russia's regular fights with the Ukraine.
Probably the only places where it might make sense to build GTL plants are in relatively unstable oil producing nations that are currently flaring off their gas instead of doing anything else.
There's also the issue of your confidence in predicting the future. These are expensive plants, and if e.g. a world-wide recession eventually leads to a crash in demand then you may not be able to service your debt on what you're able to sell your product at (there are plenty of people who suspect oil might crash to $10 a barrel, e.g. what if the PRC's demand sharply drops?).
With sufficient natural gas feed stocks, yes.
As to why it's not being used now, until the very recent fracking revolution the supply of gas had been pretty tight, at least at where it's needed for consumption. See e.g. Russia's regular fights with the Ukraine.
Probably the only places where it might make sense to build GTL plants are in relatively unstable oil producing nations that are currently flaring off their gas instead of doing anything else.
There's also the issue of your confidence in predicting the future. These are expensive plants, and if e.g. a world-wide recession eventually leads to a crash in demand then you may not be able to service your debt on what you're able to sell your product at (there are plenty of people who suspect oil might crash to $10 a barrel, e.g. what if the PRC's demand sharply drops?).