Page 120 - Petroleum Geology

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Fig. 5-3. Drawdown in producing water wells. A: unconfined aquifer. B: confined aquifer.
Static water level indicated by triangle. The drawdown forms a cone of depression.

ing a hole too small to take a full-gauge dressed bit. However, in soft rocks
the hole drilled is larger - sometimes substantially larger - than the bit, due
to the surge of mud past the bit at each stroke.
The hole must be protected from cavings, for these can cause a bit to stick
(and a stuck bit may be freed by an upward blow through the jars). This
protection is given by lining the hole with steel pipe, or casing. At the sur-
face, a few joints of large diameter conductor pipe will normally be cement-
ed in the hole through the soil layer. Below this, when drilling in soft rock,
it is often possible to drive a string of casing with an O.D. (outside diameter)
less than the hole, and an I.D. greater than the bit. This provides almost con-
tinuous protection while drilling in soft rock. Once the casing “freezes”, the
hole has to be drilled ahead without this protection -but the freezing usually
indicates that the borehole has reached more competent rocks. As each string
of casing becomes necessary and is run, so the hole diameter has to be reduced.
In general, the deeper the planned depth of the hole, the larger the diameter
of the bit used to start the hole.
This method of casing a hole is satisfactory for water wells in general, but
it does lead to some waste when artesian water is struck. Because only the
surface conductor pipe is usually cemented (unless artesian water is expected)
artesian water, when confined by well-head installations, can pass outside
the casing and enter porous and permeable beds in which the water is at nor-
mal hydrostatic pressures. Clearly these were very unsatisfactory features of
cable tool drilling when used for drilling for petroleum.
Although the water-well drillers’ techniques could be borrowed by the
petroleum industry with little or no modification, there was another disad-
vantage that was both wasteful and dangerous. That is that the hole during
drilling was necessarily open to the atmosphere. When oil or gas was struck,
there was nothing to stop it flowing out at the surface other than gravity. Oil
would flow at the surface even under normal hydrostatic conditions because
the column of oil in the borehole would not normally balance the column of
water outside the borehole. One trip with the bailer could swab oil into the
hole. Gas coming out of solution in the oil because of the reduction in pressure