That will not happen. Even the southern branch of the Druzhba pipeline is still carrying some Russian crude through Ukraine to Hungary and Slovakia. This is all mirage. Russian oil gets mixed in India and then gets back to EU. Only U.S., Canada, the EU, the UK, Japan, South Korea, Australia, and a few others sanctioned Russia. So over 140 countries did not sanction Russia and can buy oil from them without any mixing and outside the Western price cap system. Cutting Russia off completely from the global oil market would send prices through the roof, and most countries don’t want that. That’s why Ukraine has reportedly been reminded behind closed doors not to hit Russian oil exports too hard.

Ukraine is targeting refineries rather than Crude production/supply. This impact refined oil exports and domestic supply and is causing Russia pain. Oil refineries are also very soft target.

They can't cut off the Druzhba pipeline because they need to keep Hungary and Slovakia happy.