STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

956 Drilling and Well Completions
6 ft per 3,000 ft drilled at no deviation to 24 ft per 3,000 ft drilled at 55" of
deviation. The reference position is given by the inertial Ferranti platform FINDS
[lll]. A large dispersion is noted on the 102 wells surveyed.
When the borehole is vertical and a kickoff must be done, a mud motor and
bent sub are generally used. To orient the bent sub in the target direction, the
gravity toolface is undetermined according to Equation 4-176. Up to about 5"
or 6" of deviation the magnetic tool face is used. The magnetic tool face is the
angle between the north vertical plane and the plane defined by the borehole
(vertical) and the mud motor or lower part of the bent sub if the bent sub is
located below the mud motor. After reaching 5" or 6" of inclination the surface
computer is switched to the gravity tool face mode.
Drilling Parameters
The main drilling parameters measured downhole are:
weight-on-bit
torque
bending moment
mud pressure
mud temperature
Strain gages are usually used for the first four measurements.
Strain Gages. Strain gages are used to measure the strain or elongation caused
by the stress on a material. They are usually made of a thin foil grid laid on a
plastic support as shown in Figure 4-259. They are the size of a small postal
stamp and are glued to the structure to be stressed.
The sensitive axis is along the straight part of the conducting foil. When
elongated, this conducting foil increases in resistance. The change in resistance
is very low. Two gages are usually used and mounted in a Wheatstone bridge.
Two more gages not submitted to the strain are also used to compensate for
temperature variation. The change in resistance for one gage is given by
F*R.o
AR = (4-192)
E
where AR = resistance change in R
F = gage factor
R = gage resistance in R
E = Young modulus in psi or Pa
o = stress in psi or Pa
Demonstfation. The gage is a platinum gage with F = 4, R = 50 a. The structure
measured is steel with E = 30 x lo6 psi. If the stress is 1000 psi, the resistance
change is
AR = 0.0067 R
For constantan, the gage constants are usually F = 2, R = 100 R.
Weightsn-Bit. Weight-on-bit is usually measured with strain gages attached to
a sub subjected to axial load. The axial load is composed of three parts: