What Fills The Space Left In Wells When Oil Is Extracted From The Ground?

Fire Down Below: Oil wells dip 30,000 feet below ground -- still too shallow to hit magma:
Justin Guariglia/Getty Images

From Popsci:

You might guess that magma or tumbling rocks fill the void, but the truth is much more prosaic: water. Petroleum deposits, which are naturally mixed with water and gas, lie thousands of feet below the earth’s surface in layers of porous rock, typically sandstone or limestone. (Contrary to what you might imagine, drilling for oil is more like sucking oil from a sponge with a straw than from a giant pool of liquid.) At such depths, these liquids are under very high pressure. Pump petroleum out, and the pressure in the well drops. Water in the surrounding rock, which is also packed under high pressure, then pushes its way into this low-pressure pocket until the pressure reaches equilibrium. “It’s just like digging a hole at the beach, where water in the sand around it flows into the lower pressure zone of the hole,” explains Chris Liner, a professor of petroleum seismology at the University of Houston.

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