USA

That’s okay. If they secede, the US government can simply resort to blunt force like we often do when oil is involved. I don’t see The Republic of Texas joining OPEC.

Texas does indeed have the most, largest refiners in the US, but not all of them. However, things would get complicated fast. The biggest refinery in the US is the Motiva refinery and it is owned and wholly controlled by Saudi Aramco as of 2017. I'd give it maybe a week tops before Saudi Arabia indicated that the refinery be in US control or they'd shut the plant down. So Texas would have a very hard call on their hands because an attempt by an "independent" Texas to control the refinery would be an invasion of Saudi Arabia and trigger all kinds of "fun" international ramifications.

The next biggest are Beaumont Refinery and Galveston Bay Refinery. Beaumont is owned by Exxon and Galveston is owned by Marathon. They would have similar asks… Because let's face it, none of the oil companies want to get involved in a US/Texas spat. But as I was saying they would have similar asks to keep operating under the US as Exxon is HQ in New Jersey and Marathon is HQ in Ohio, which if they're not joining Texas in the whole leaving part, then Texas controlling the refineries is about the only way they get to make life bad for the US via oil.

Now taking control of those two isn't quite the affair that taking Motiva would be. But both companies would absolutely have legal claim to challenge Texas' hostile takeover which, highly likely, Texas would largely ignore any kind of legal summons to answer for. The entire point is that Texas doesn't get to keep the refineries as most of them are private property and Texas would have to (since we're talking about an independent Texas here) nationalize them. Which would trigger all kinds of havoc that Texas wouldn't be able to surmount. So Texas would (hopefully they're smart enough to understand this, but I don't give them that much credit) understand that any attempt to monopolize the oil would basically be a scorched Earth approach and not a "let's use the oil to get rich!".

And if we're wondering how the US would enforce that, it's a pretty simple approach. If the US cannot have them, Texas sure as shit doesn't get to keep them. It's not like their position is unknown to the US, and because of their nature, all of them are easily located on the Gulf Coast easily within SM-2 range. It would be an average Thursday for the Navy to level all of them in a few hours to very fiery piles of metal. It would take an independent Texas a lifetime to rebuild them, especially since them attempting to, what amounts to, stealing them would nix them off the International oil markets.

Also the lack of access to the USD would hurt their ability to trade the oil on the market. They would need bilateral trade deals. And it wouldn't be like their current situation is some big secret, every bilateral deal would absolutely not be in their favor because every other nation would know they're selling the oil because their hurting. That's literally the position Russia is current in with their oil trades, which is why India is buying it up like a fat kid in a free chocolate cake store. India was buying at $80/bbl when the G7 set caps at $60/bbl which obviously had India come November to start getting Russia oil well below the $60/bbl set by the G7. That would be Texas, but way worse.

So Texas would have two calls, let US companies continue to operate the oil under the US (which would still hurt consumers but not quite as much) or attempt to take them over and no one gets them (which would really hurt consumers). There's not a third option there. Texas would never be allowed to take profit on the oil and no nation that's an ally of the US would buy from them.

So even if the worse happened and we had to blow up the refineries in Texas, the US has the capital to build them elsewhere, Texas does not. And yes, prices would sky rocket because the refineries in California, Louisiana, and New York running at full blast would provide nowhere near enough gasoline for everyone, building more ports has always been on the schedule for the US and now they would have great reasons to pull those plans out. And the oil companies would enjoy the new grants to build new facilities. Texas would be bankrupt trying to rebuild anything near the size of Motiva. Not to mention, Motiva's current input isn't even North American oil, it's input is 100% OPEC oil and mostly Saudi Arabian oil, they literally boat it from the other side of the world to that site for processing.

Texas wouldn't have a lot of say on oil and would have a lot to lose if an "independent" Texas didn't at least play economic ball with the US while the US settled the spat in court. The upside is that it would likely increase demand for EVs and remove a lot of barriers to Lithium mining in the US (likely to the destruction of wildlife RE: McDermitt Caldera).