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EM MWD

The Model AP250 Electric Dipole System measures and transmits downhole data to the surface, enabling the directional drilling of a borehole. Data transmitted typically includes the inclination and azimuth angles of the borehole, the drilling system toolface (roll angle), and the tool temperature. The system can also be configured to transmit accelerometer and magnetometer sensor values, which can then be used to calculate the borehole angles.

Figure 1 shows the downhole parts of the electric dipole system. At the top of the system, a non-magnetic drill collar with an electrically insulated gap sub is used. To generate electromagnetic signals that propagate to the surface, a low frequency phase modulated signal is applied across the insulating gap. An upper electrode and the bolt in dipole head are used to apply the signal across the gap.

The gap sub signal is generated by the power driver electronics unit mounted in a pressure barrel just below the dipole head.

Typically, two 29V, 29 Amp-hour batteries encased in pressure barrels are used to power the electric dipole system, although use of three or more batteries is possible for situations where high power output for a long period is required.

The borehole angular orientation and drill string toolface are measured by a directional sensor mounted in a pressure barrel below the batteries. A gamma sensor is mounted directly below the system directional sensor.

The Electric Dipole System is typically approximately 150 inches long and utilize 1-7/8" diameter pressure barrels for system electronics and batteries. The same basic system can be used with 4-¾", 6-½", 8" and 9" drill collar sizes. Mounting spacers enable the use of drill collar sizes greater than 4-¾".

To stabilize the tool string in the drill collars, rubber finned centralizers are used to connect the various system pressure barrels.

  • Underbalanced Drilling
  • High Speed Drilling where fast data rates are required
  • Low Pressure Formations or Lost Circulation Situations
  • Proven design with high reliability and an MTBF above 2500 hours
  • Data transmission from depths of 3000+ meters in favorable conditions
  • Real time Annulus Pressure, Drill Collar Internal Pressure and Gamma

Downhole System Specifications

Frequencies 2-10 Hz, user selectable
Baud rate One half of the transmission frequency. For instance, for a transmission frequency of 9Hz a 4½ baud rate is achieved.
Power Batteries typically consist of two Double D stacks of 8 moderate rate cells each, producing 29V at 29 ampere-hour capacity. Battery lifetime depends upon transmit power selected.
- At 10 Watts power level, battery life is 80.
- At 20 Watts power level, battery life is 40 hours.
- At the maximum power level of 40 Watts, battery life is 20 hours.
Mechanical  
Gap Sub Sizes 4¾" OD X 67.375 L; 3½ IF pin at bottom, box at top 6½" OD X 72.0” L
Tool string approximate length with 2 batteries 222”
750 Directional Sensor  
Angular accuracy (0-150°C)
- Azimuth (latitude <55°C)
- Toolface (roll)
- Inclination
±0.3°
±0.1°
±0.1°
Operating Temperature Range 0 to 150°C
Shock 1000G 1ms half sine wave
Vibration 20G rms 5-1000Hz
Digital Interface
- Logic level
- Baud rate
- Protocol

TTL/CMOS
User programmable up to 38400 baud
User selectable ASCII, binary, or APS .NET format
Connector MDM15SH003P (ITT Cannon)
Mating Connector MDM15PH003L (ITT Cannon)
751 Gamma Sensor  
Scintillation Crystal 1" diameter X 5" long mounted SST Case
Photomultiplier Tube Hamamatsu Model R1288
Sensitivity (in a 1-7/8" Beryllium Copper pressure barrel) 1 Count per API
Accuracy ±5%
Thin Bed Resolution (8" hole diameter)
- Operating Temperature Range
- Storage Temperature Range
6" (236mm) Temperature Range
0°C to 150°C
-25°C to 160°C
Shock 1000 G 1ms half sine wave
Vibration (random) 10 G rms, 50-250Hz

Uphole Component Specifications

Model 560 preamplifier/filter system  
Preamp gain selectable 0 to 42
Amplifier gain selectable 0 to 96
Power 115V @ 1A
220V @ 0.5A
Size 19"W x 13"D x 3 ½"H (rack mountable)
High Pass Filter 0.22 to 11 Hz roll off, user selectable
Low Pass Filter 3 to 21 Hz roll off, user selectable
Model 574 Downlink System  
Main power 115V @ 10A
220V @ 2.5A
Size 16”W x 12”D x 7½”H
Laptop Computer Off-the-shelf unit with 16-bit A/D installed. One RS 232 port required for Driller’s Display.