Reply #12: Water pressure at 5000 feet is a little over a ton per square inch

[FarCenter]

About 2165 psi IIRC.

If the crude oil has a specific gravity of 0.82 (and different crudes have varying spedific gravities) then the pressure at the bottom of a 5000' pipe filled with crude would be 0.82 x 2165 or 1775 psi.

Suppose however that the oil is at the bottom of a hole 8000 feet below sea level under 3000 more feet of rock with a specific gravity of 2.5. Now the total pressure of sea water plus rock overburden at the bottom of the hole could be:
2165 + (3/5)*2.5*2165 = 5412 PSI

However, the oil in the 8000 foot well would have a pressure of 8/5 * 1775 = 2840 PSI.

So there is a pressure differential of up to 5412 - 2840 = 2572 PSI available to push the oil up the well. Hence the blowout.

Now the above is not exact, since the pressure of the oil flowing out of the sand at the bottom of the well is not as much as the simple calculation would suggest, given that the rock is stiff and there is resistance to flow through the sand or rock to reach the well.

But that is the general idea.

During drilling, the well is supposed to be filled with a dense "drilling mud" in order to overcome the pressure of the oil under the weight of the rock.