I'm actually not sure what the purpose of this is. All this will accomplish is increase the supply of Rubles to ensure a price ceiling for the value of the Ruble. Anytime the Rubles value increases, relative to other currencies, people will buy gold, sell to central bank for Rubles and then sell Rubles increasing the supply of Rubles and ensuring the value of Rubles doesn't go up too high.

The only possible explanation I can think of this is that Russia expects the change to requiring O&G to be paid for in Rubles will drive the value of the Ruble up significantly which may hurt exports so this is to cap the value of the Ruble. This should hurt imports and help exports presuming countries buy from Russia. Seems like Russia is trying to make themselves less reliant on everyone else and make everyone else more reliant on them.