STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

966 Drilling and Well Completions
4
€ 3
E
0
0
0 0.5 1 2 5 10 20 50 70
Gas in mud, o/o
of Volume
Figure 4-266. Bottomhole mud resistivity in the annulus as a function of the
gas content of the mud. (Courtesy Petroleum Engineer lnternational [108].)
Example 12: Drilling Parameters-Downhole
Weight-on-Bit and Torque
We want to measure the bottomhole weight on bit and torque with a sensing
collar sub 5-in. OD and 3-in. ID. The differential pressure across the three %-in.
bit nozzles is 1,000 psi. We want to use platinum strain gages with a resistance of
50 a and a gage factor of 4. The Young modulus of the steel sub is 30 x lo6 psi,
the shear modulus is 12 x lo6 psi.
1. Compute the end effect due to the internal differential mud pressure.
Should we correct for this effect?
2. Compute the force acting on the drill collar for a weight-on-bit of 0, 30,000
and 100,000 lb.
3. The gages are stuck on the sub and connected to a Wheatstone bridge.
What is the change in resistance from no load and no pressure for a weighton-bit
of 0, 30,000 and 100,000 lb?
4. Show that with two gages conveniently placed on the sub the bending strain
compensates.
5. The bridge is supplied with 10 V, balanced for 0 lb. What is the unbalance
for 100,000 lb?
6. Trace the response versus the weight-on-bit with
(a) no differential pressure,
(b) 1,000 psi differential pressure.
7. The maximum torque to be measured is 20,000 ft-lb. Using the same type
of gages, properly placed on the sub, show that with two gages conveniently
oriented the differential pressure strain and weight-on-bit strain do not
register on the bridge.
8. Compute the resistance variation for each gage due to torque.
9. Compute the maximum signal using a 10-V supply.