STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

914 Drilling and Well Completions
windings in the opposite direction and generate another voltage pulse of the
same amplitude but of opposite polarity or sign. Because of this voltage pulse
occurring at twice the driving voltage frequency, the flux gate is sometimes
known as "second harmonic magnetometer."
The main advantages of the flux gate magnetometers are that they are solidstate
devices much less sensitive to vibration than compasses, they have uniaxial
sensitivity, and they are very accurate.
Typical specifications are:
Temperature
Operating = 0 to 200°C
Storage = -20 to 200°C
Power requirement, output in impedance, and mechanical characteristics are similar
to the acceleromete sensors.
Electrical characteristics
Alignment = f0.5"
Scale factor = 5 V/G k5%
Bias = k0.005 G @ 25°C
Linearity = f2% full scale
Note: 1 gauss = 1G = tesla
Environmental characteristics
Vibrations = 1.5 cm p-p, 2 to 10 Hz
20 g, 10 to 200 Hz
Shock = 1000 g, 0.5 ms, 0.5 sine
Figure 4-230 shows the photograph of a Develco high-temperature directional
sensor. For all the sensor packages, calibration data taken at 25, 75, 125, 150,
175 and 200°C are provided. Computer modeling coefficients provide sensor
accuracy of fO.OO1 G and fO.1" alignment from 0 to 175°C. From 175 to 200°C
the sensor accuracy is f0.003 G and f0.1" alignment.
Example 1 : Steering Tool Measurements-Tool
Face, Deviation
Single-axis accelerometer systems are used in the steering tools and MWD tools
for inclination and tool face data acquisition. Using a spreadsheet, compute the
current values for each accelerometer in the following cases:
Use a spreadsheet for a three single-axis accelerometer system mounted in a
steering tool or a MWD tool and compute the output current values for each
accelerometer in the following cases:
1. Tool-face angle: 0"
Hole deviation: 0, 15, 30, 45, 60, 75, 90"
2. Hole deviation: 30"
Tool-face angle: -180, -135, -90, -45, 0, 45, 90, 135, 180"
The usual conventions as shown in Figure 4-231 are:
Axis x lines up with the mule shoe key and the tool face.
Axis y is perpendicular to Ox and Oz.
Axis z is the same as tool axis or borehole axis, oriented downward.