While there can be a great deal of pressure pushing the oil up the well, there is also resistance to that flow up a narrow pipe (perhaps 4 inch pipe). The balan

While there can be a great deal of pressure pushing the oil up the well, there is also resistance to that flow up a narrow pipe (perhaps 4 inch pipe). The balance of forces can be very close but the momentum can keep the flow going. In some fluids, there can be methyl hydrates (or wax) that can cause problems with maintaining flow. There can also be particles that are moving in the fluid (sand), slowing or stopping the flow can cause these particles to settle, preventing future fluid movements. There is a temperature and pressure gradient which can allow dissolved gas to bubble out the fluid as it ascends the well which can lead to added complexity. And we haven’t yet discussed oil recovery methods like gas/water injection or electrical sub surface pumps. Getting a well restarted after shutdown, may not be possible. I worked with some petroleum engineers, getting the best from your wells can be a very complex simulation problem followed by expensive engineering solution.