"As damp in a stone foundation" seems odd to me. I'm used to "damp" as an adjective, not a noun.
That said, you need to consider the oil underground like it's inside of a balloon and someone is squeezing the balloon. When you first drill the hole, the oil shoots out because the pressure of the oil underground is higher than the pressure of the air outside.
So the oil flows with no effort or work put into it.
However, like a balloon you puncture, the pressure is released because the material (substance) inside is being released. Eventually enough oil has come out that the pressure underground has lowered to the point where it is equal to the pressure of the air above ground. There is now no force that naturally brings the oil up through the drill hole.
Now, you have to use energy to pump the oil out of the hole.
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