• Question: Does oil extraction leave a cavity?

    Asked by higglebiggle to David, Luna, Probash on 24 Mar 2011 in Categories: .
    • Photo: Probash Chowdhury

      Probash Chowdhury answered on 24 Mar 2011:

      You’re right, when oil rigs extract oil there is a danger of leaving a cavity and the cavity collapsing. However, they pump salt water (usually sea water for off-shore rigs) back into the cavity to fill the void and avoid a collapse.

    • Photo: David Pyle

      David Pyle answered on 24 Mar 2011:

      No. Rocks are not very strong, and cavities inside the Earth tend to collapse, unless there is something to hold them open – like in mine shafts, for example. At the depths where oil is stored (a few hundreds of metres to kilometres below the sea-floor) cavities in rocks are usually small (mm- to cm-sized ‘pore’ spaces), and filed with fluids – oil, gas, water. One way to get oil out of a reservoir rock like a porous sandstone is to inject water into the base of the rock unit. This will displace the oil – which will float – out of the reservoir (and into the well), and fill the pore-spaces in its place.

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