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.
The figure 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 29 V, 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 222 inches long and utilizes 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.