Devaluing their currency makes their oil the cheapest on the planet.
And destroys the Russian economy for decades. Russia cannot win a oil price war it is a lose lose end result.
The one thing Russia does have that is economically important is their natural gas exports, but watch Putin screw that too.
As others have expressed this may result in Putin starting a war in the middle east.
Which means that their oil revenue will have the lowest return on the planet. You need to learn something about economics.