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I Water Well Pressure

  1. Apr 15, 2016 #1
    Imagine there is a water well underground. The pressure at the surface of the water will be the atmospheric pressure+ the pressure due to the weight of the rock. If i drill a hole i assume the pressure differential will push the water up the hole. Is the mass of water displaced equal to the maximum weight that the force produced by differential pressure could lift? (Sorry for this poor English )
    When the water rises is there a vacuum at the bottom of the well?

    Is there any situation where all the water from the well could be drained without the need of a pump?

    Thank you very much
     
  2. jcsd
  3. Apr 15, 2016 #2
    No vacuum otherwise the water would not flow. When drilling for a well they drill into and below the water table, not a large cave or chamber holding water. Not that I'm saying that they never do hit a chamber. Either way, you could never drain a well unless you drained all the water below the depth drilled to. And you will need a pump. Even if it was a chamber if water got in it will keep flowing in.
     
  4. Apr 15, 2016 #3
    I'm not interested in how they do it in reality. With this conditions, how would things play out if they opened the hole?
     
  5. Apr 15, 2016 #4
    Depends how big the hole is. If the hole is big enough, then the atmospheric pressure would keep said water table in place. But if the hole is small, less atmospheric pressure vs. Pressure of water and land, then the water will rise up the hole. It would depend on the pressure difference to how high that water would rise. But both water pressure and land mass pressure are working together against atmospheric pressure so the more water coming out the less water pressure you would have to the two pressures would eventually equalize to the point that you would need a pump
     
  6. Apr 15, 2016 #5
    So as the water rises is there a vacuum on the bottom ?what happens to the bottom of the well, if no more water is coming in ?
     
  7. Apr 15, 2016 #6
    If it were a chamber? It drains and becomes and empty space, air and other natural gases would be present. It would never be a vacuum like space.
     
  8. Apr 15, 2016 #7
    Yes if it was a chamber where no air or gases could enter from the bottom. The only entrance would be the hole on the top
     
  9. Apr 15, 2016 #8
    If everything was air tight then yes a vacuum would form that would stop the flow of water according to the pressure pushing the water up and the vacuum pulling back.
     
  10. Apr 15, 2016 #9
    So, are you saying that no water would spill ?
     
  11. Apr 15, 2016 #10

    CWatters

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    I don't believe your question can be answered without more information...

    Is that a question or an assertion? It's not necessarily true, it depends how the rock above is supported. Should we assume that the roof is held up by the water pressure alone?

    Suppose you pumped all the water out. Would the roof fall/sink? If not then it must be supported by something other than the water pressure. So after a little water is allowed to escape the pressure might fall dramatically.

    If the roof does fall/sag then the amount it does so will matter. If it can collapse fully then all the water might be displaced.
     
  12. Apr 15, 2016 #11
    I've thought of that and that is where my doubts come from. Imagine that instead of water we have oil. When the hole is opened and if the earth is supported by the oil then we expect the earth to fall until the chamber is totally empty..

    But if the earth is supported only partially by the oil, when we open the hole the oil would spill in a very short burst, because the weight of the earth would stop being supported by the oil and instead be dissipated trough the adjacent earth.

    But my question then is: how can we have oil wells bursting for long minutes ?
     
  13. Apr 15, 2016 #12

    CWatters

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    Somewhere between these two extremes lays reality.
     
  14. Apr 15, 2016 #13
    So, on a oil well where there are bursts of oil that last for a long time, should we a expect to see a change in the altitude of that land
     
  15. Apr 15, 2016 #14
    The pressure of the water in the pores of the rock does not even come close to supporting the overlying rock. The rock matrix for the most part supports itself as an integral solid structure, but the hydrostatic pressure of the water in the pores perturbs the state of stress in the rock and the pressure causes the pores (and the rock itself) to swell a little. Under ordinary circumstances, the pressure of the water in the pores is in hydrostatic equilibrium. If we sink a well into an unconfined aquifer and pump water out of the rock, there is a "drawdown" effect that occurs in the vicinity of the well, with a localized depression in the water table. This reduction in hydraulic head near the well provides the driving force for water to flow horizontally through the porous rock to the well.
     
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