What happens to the cavity created by emptying an oil reserve?

Question

What happens to the cavity created by emptying an oil reserve?

Answer 1

Depends on the nature of the reservoir rock.  Most reservoirs are sand and the oil is held in between the grains.  If there is clay in the matrix and water too, then subsidence can happen, like it has at the Goose Creek Oil Field in Texas, or the Willmington Oil Field in Long Beach, CA.  Subsidence usually occurs in relatively shallow fields.  Shallow being less than 2000 feet below the ground surface.

For the most part, not much happens to the pore space other than the opportunity for different subsurface fluids being able to migrate into the voids left behind.  These fluids could be fresh water, brine (not so fresh), natural gas, and yes, even more oil.

There are lots of folks here in Texas making lots of money extracting oil from oid oil fields that had those pesky cavities from production 20 to 60 years earlier.  This is only because of the large increase in the price of crude in the past year or so.

Answer 2

Oil does not fill these large,cavernous holes underground like some people seem to think. Oil s found in the pore spaces between the grains of the reservoir rock. These pore spaces are quite small.

When the petroleum is removed some subsidence is expected, but usually the drilling muds and brines are pumped back into the reservoir to limit subsidence.

Answer 3

We fill them up again with toxic waste?

Answer 4

It just remains a cavity unless something else fills it up or it collapses, but in general it's a relatively small space.

Answer 5

When the oil is removed 2/3s of it remains unless it is water flooded. The oil comes out of the porous sand or rock because of pressure it has on it underground. When the pressure is gone the oil ceases to flow to the well bore. Water flooding repressures the oil bearing rock and causes it to flow out with the water which is pumped again and again into the oil zone. When it is no longer profitable water is left behind.

If no water is pumped in an oil zone may over much time regain some small amount of pressure from seepage for further out in the oil zone or from somebody injecting water into the zone as a disposal method. Old wells have been known to produce again for short periods of time. Sometimes oil is pushed over from elsewhere down there and more oil can be pumped, but in the vast majority of wells they are plugged and never produced again. The oil taken is gone forever.

There is no large empty cavity though there can be a small one around the bottom of an old sand well that has been fracked and sand pumped in the past to clean out the well bore.