posted on Jan, 1 2005 @ 08:01 AM
The only time any type of fluid is pumped back into a reservoir is when the driving pressure (the reservoir pressure) has decreased so much from
production that the well can no longer produce at a pressure that will drive the oil up the wellbore. Then, if the oil company deems it profitable,
they will do gas-drives (use gas to drive the oil), steam drives, water drives, and sometimes even a fire-drive. The oil company will not use salt
water if the reservoir is not stable to salt water - they'll use fresh water in that case. But if it is a reservoir conducive to salt water then
they'll use salt water.
There is no vast "void" left behind when oil is taken from a reservoir. Reservoirs are not big holes filled with oil. The oil is in the
interstitial spaces of the permeable rock. (It is like sucking water out of a sponge.) So the only voids left behind are the microscopic holes in
the rock. And if it is not a permeable zone, then reservoir is fractured so that the oil can have the fracture as a path to flow to the well. Along
with fracturing there are several other production enhancement techniqes that can be used. For instance, acidizing will eat the matrix of the rock to
cause an apparent increase in the permeability so that the oil trapped in the rock can flow out.
Now typically, what happens in a depleted oil well, is that the earth itself fills the tiny voids in the rock. You see on a typical oilwell you'll
have the fluids stacked in the rocks just as they would be if they were sitting in a glass...lighter down to heavier. So you'll have a salt dome or
shale that "caps" everything below it, then you'll have your gas zone, then you'll have your oil zone, and below that will be the water. As an
oil zone is depleted the water below it starts rising...eventually the well starts making too much water to be profitable anymore, and then it is
plugged and abandoned. By the way, this is what is starting to happen in the mideastern oilfields...they are beginning to have problems with water