54 
FLORIDA GEOLOGICAL SURVEY-I 4 TH ANNUAL REPORT 
wells producing oil at a depth of from 4,500 to over 5,000 feet. In 
drilling, the cost increases greatly with depth, and hence only those wells 
that are large producers repay the expense of deep drilling. Con¬ 
versely, small wells, if obtained at a shallow depth, may be very profit¬ 
able. 
methods of drilling 
. It is not the purpose in this paper to describe methods of drilling oil 
wells. It may be well to state, however, that two common methods of 
well drilling are in use, one by rotary and one by cable tools. Rotary 
drilling is practicable where the formations to be drilled through are 
relatively soft, but operates at a disadvantage in hard-rock formations. 
The cable tools are used to advantage where the formations are prevail¬ 
ingly hard. In the cable outfit, drilling is by means of a heavy drill 
which is alternately lifted and dropped, thus breaking or pounding its 
way through the rock. The broken or pounded up material is removed 
from the well by a bailer, the rock material being suspended in water. 
For a large cable rig such as is necessary for deep wells, a derrick is 
built at the well site by which to support and handle the machinery. 
On the market also are a number of well-drilling rigs of small size, which 
do not require a derrick, but operate by means of a portable center sup¬ 
port. 
The method of drilling by rotary is entirely different from that by 
cable tools. In the rotary drill a string of pipe extends from the top to 
the bottom of the well. This pipe revolves and, in doing so, cuts into 
the rock. The pieces of rock loosened in this operation are floated to 
the surface by means of a stream of water which is conveyed into the 
well through the drill stem and flows back around, that is, on the outside 
of the line of pipe. 
The cable tool method of making a hole is usually regarded as desir¬ 
able in drilling in a new territory. The character of rock drilled into 
is more readily determined when drilling by cable than by rotary. Also 
the mud carried by the rotary is likely to obscure small showings of oil 
such as might be detected by the cable method. On the other hand, 
where the formations are prevailingly soft, the cost of drilling by rotary 
is much less than by cable. 
The possible use of the diamond drill is worthy of consideration. This 
method as at present developed is specially adapted to hard-rock forma- 
