STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Downhole Motors 899
The hydraulic equivalent diameter is related to the actual nozzle diameters by
de = [ad: + bd,“ + cd,2]’/’’
where a = number of nozzles with diameter d,
b = number of nozzles with diameter d,
c = number of nozzles with diameter d,
d,, d, and d, = three separate nozzle diameters in in.
Nozzle diameters are usually in 32nds of an inch. Thus, if the bit has three
nozzles with $ of an inch diameter, then
de = [(3)(0.4688)2]”2
= 0.8120 in.
The above hydraulic equivalent diameter is close enough to the one obtained
with Equation 4-171. Therefore, the bit should have three +-in. diameter nozzles.
Special Applications
As it becomes necessary to infill drill the maturing oil and gas reservoirs in
the continental United States and elsewhere in the world, the need to minimize
or eliminate formation damage will become an important engineering goal. To
accomplish this goal, air and gas drilling techniques will have to be utilized (see
the section titled “Air and Gas Drilling”). It is very likely that the future drilling
in the maturing oil and gas reservoirs will be characterized by extensive use of
high-angle directional drilling coupled with air and gas drilling techniques.
The downhole turbine motor designed to be activated by the flow of incompressible
drilling mud cannot operate on air, gas, unstable foam or stable foam
drilling fluids. These downhole turbine motors can only be operated on drilling
mud or aerated mud.
Recently, a special turbine motor has been developed to operate on air, gas
and unstable foam [82]. This is the downhole pneumatic turbine motor. This
motor has been tested in the San Juan Basin in New Mexico and the Geysers
area in Northern California. Figure 4-214 shows the basic design of this drilling
device. The downhole pneumatic turbine motor is equipped with a gear reduction
transmission. The compressed air or gas that actuates the single stage
turbine motor causes the rotor of the turbine to rotate at very high speeds (Le.,
-20,000 rpm). A drill bit cannot be operated at such speeds; thus it is necessary
to reduce the speed with a series of planetary gears. The prototype downhole
pneumatic turbine motor has a gear reduction transmission with an overall gear
ratio of 168 to 1. The particular version of this motor concept that is undergoing
field testing is a 9-in. outside diameter motor capable of drilling with a
10 4-in.-diameter bit or larger. The downhole pneumatic turbine motor will
deliver about 40 hp for drilling with a compressed air flowrate of 3,600 scfm.
The motor requires very little additional pressure at the surface to operate
(relative to normal air drilling with the same volumetric rate).
The positive displacement motor of the Moineau-type design can be operated
with unstable foam (or mist) as the drilling fluid. Some liquid must be placed
in the air or gas flow to lubricate the elastomer stator as the metal rotor rotates
against the elastomer. Positive displacement motors have been operated quite