CEMP-E
TI 814-01
3 August 1998
APPENDIX C
DRILLED WELLS
C-1. METHODS. Drilled wells are normally constructed by one of the following methods:
- Hydraulic Rotary
- Cable Tool Percussion
- Reverse Circulation Rotary
- Hydraulic-Percussion
- Air Rotary
These methods are suitable for drilling in a variety of formations. Diameters may be as large
as 60 inches for wells constructed by the reverse circulation method. Smaller diameter wells
may be constructed by drilling to depths of 3000 or 4000 feet. For a detailed discussion of
these methods, see Ground Water and Wells by Johnson Division, UOP Inc. The first two
methods listed are the most common in well construction and a brief description of each
follows:
a. In the hydraulic-rotary method of drilling, the hole is formed by rotating suitable tools
that cut, chip, and abrade the rock formations into small particles. The equipment consists of a
derrick, a hoist to handle the tools and lower the casing into the hole, a rotary table to rotate
the drill pipe and bit, pumps to handle mud-laden fluid, and a suitable source of power. As the
drill pipe and bit are rotated, drilling mud is pumped through the drill pipe, through openings in
the bit, and up to the surface in the space between the drill pipe and the wall of the hole,
washing the drill cuttings out of the hole at the same time. The borehole is kept full of a
relatively heavy mud fluid. Due to its viscosity, this fluid exerts a greater pressure against the
walls of the hold than the water flowing in from the water-bearing bed. Therefore, the mud
tends to penetrate and seal the pore spaces in the walls, and prevents caving. Water under
low hydrostatic pressure (pressure exerted by the weight of the water in the water zone)
cannot force its way into the hole.
b. In the cable tool percussion method of drilling, the hole is formed by the pounding and
cutting action of a drilling bit that is alternately raised and dropped. This operation is known as
spudding. The drill bit is a club-like, chisel-type tool, suspended from a cable. As the bit is
raised and lowered, the cable unwinds and rewinds, which gives the bit a grinding motion as
well as a chisel-type action. It breaks hard formations into small fragments and loosens soft
formations. The reciprocating motion of the drilling tools mixes the loosened material into a
slurry that is removed from the hole at intervals by a bailer or sand pump.
C-1
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