Haven't read the rest of the thread, so I'm probably repeating people, but this is a pretty interesting topic considering the country isn't even a nation, but a royal estate writ large. Saudi Arabia is a clusterfuck only held together by money, hired guns, and its role funding the playboy lives of the House of Saud.
Around 1/5th of their population are Salalfi or Wahhabi, the extremely hardline Islamic sect that wants everything older than Muhammad and even any Islamic shrine or tomb torn down, considers any other Muslim sect to be apostates, and from which pretty much most of the jihadis get inspired from. Another 1/10th are Shia.
Their population is hilariously inbred, over half of the population are in incestual marriages. Even other Arabs consider them inbred, and Arabs as a whole are inbred and proud, so that's saying something.
So once Abdullah and Ahmed can no longer fly to America and fuck prostitutes while doing crack then come back to Arabia and piously bang their head into the ground five times a day, things are going to collapse faster than a Saudi flying a plan near a skyscraper. A huge chunk of their population is another ISIS waiting to happen and is only kept from such by bribery. Once the money is gone, they'll turn to religion and fighting, and the Sauds will soon realise they can't have their playboy lifestyle anymore, leading to the entire house fracturing, if they don't get akbared by their own people for being neck-deep in sex, drugs, and alcohol.
So to answer, a civil war is inevitable, yup. Saudi Arabia isn't a country, it's a family estate, even the name ('the bit of Arabia owned by the House of Saud') points that out. It's like an even worse and more fundamentally broken Austria-Hungary. The only thing that's kept them together has been money and religion balanced precariously. Religion giving a common unifier, and money keeping them happy and paying for hired guns to get rid of anyone not happy while the royal family jets around the world to crash new expensive cars and rape new exotic women. But it's money above all else that stabilises the country, and religion is becoming a danger because Islam's love of killing shit coupled with increasing conservatism and hardline stances that see anyone even slightly divergent from fundamental Islam to be fair targets to kill, not just infidels. Said hardline Islamists aren't going to laugh off the fact that the royal family does more drugs and drinks than the rest of the country combined and are making a mockey of Islam. Now add in all the Shia, 1/10th of the population, that Iran will arm immediately so it can finally destroy its biggest rival and help cement its place as the dominant Islamic country (and advance Shiite Islam at the same time).
I don't know how it'll affect OPEC and the rest of the world, but Saudi Arabia collapsing is inevitable and inescapable. It's a medieval kingdom and royal playground in the modern world held together solely by oil money. Once the oil runs out, I think it's immediately over for them, even borrowing won't be easy because they have nothing else to offer, I'd say. I'd imagine most of the royal family will flee rather than seek to take the succession, they seem to just want to wallow in wealth and luxury more than actually rule, and all they'd need to do is escape to the West and live out their lives with their comfy bank accounts. I suppose it depends on just how badly the country collapses, whether it seems possible to reunify under the Sauds or if it's completely over, considering that the Saudis are not a nation or people, just Arabs living under a dynasty and so won't have the impetus to form another Saudi state.