boring

chappies,
though it may be a bit of a bore (heh heh) i think it is time to clear up some misconceptions regarding well casing and why it is done, even though the running of which takes up expensive rig time. with no criticism of previous posters intended, it has nothing to do with protection of aquifers from invasion from well bore fluids, or the protection of well fluids from incursions of formation fluid. although it will of course serve these purposes as well.
there are two principal reasons. the first is that formations open to the well bore may not be particularly competent, and hence could collapse into the bore hole, sticking the drill pipe and leading to the total loss of the well. as a minor example, remember the bridging of the Heron well which killed the production flow test of the Plover.
the second reason is that drilling mud is circulated in the hole. this mud brings the drill cuttings to surface, but is also engineered to be a higher density than water, typically with a specific gravity of maybe from 1.2 to 1.6 or so but sometimes higher if high downhole pressures are anticipated. the pressure of this mud prevents formation fluids from entering the bore hole, which is useful if you do not want uncontrolled oil or gas flow at surface. however, if the formations open to the well bore are not particularly competent, then the pressure of this mud may in fact fracture the formations and be injected into the rock. hence it disappears from the well bore and downhole pressure is lost, which can lead to loss of control of the well. cased hole obviously is not prone to this hazard.
which is why it is done, costly in terms of rig time and steel pipe though it may be. and in a wildcat area where little is known of downhole conditions, it is prudent to plan a conservative casing scheme.