Subsurface Logging Methods 393 



INDUCTION LOGGING AND ITS APPLICATION TO LOGGING OF 

 WELLS DRILLED WITH OIL-BASE MUD 



H. G. DOLL 



The measurement of the resistivity of the formations traversed by 

 drill holes has become standard practice in oil-well drilling during the 

 last twenty years. The technique used requires that direct contact be made 

 with the mud filling the bore hole by means of electrodes connected to the 

 insulated conductors of the supporting cable. A current of constant in- 

 tensity is generally made to flow in the surrounding medium through one 

 or two of these electrodes called "power electrodes." It produces in the 

 surrounding medium, by ohmic effect, potential differences which are pro- 

 portional to its average resistivity. These potential differences are picked 

 up by one or more measuring electrodes and are recorded continuously 

 at the surface of the ground, giving the resistivity log. 



There are cases, however, where a direct contact between the elec- 

 trodes and the drilling mud is not possible; for instance, in holes drilled 

 with cable tools, which are generally dry, or in holes where nonconductive 

 oil-base mud is used in rotary drilling. The conventional electric-logging 

 method then requires scratcher electrodes, which are forced by springs 

 on the wall of the hole to make direct contact with the formations. In 

 some cases the results are fairly satisfactory, but sometimes, particularly 

 in wells drilled through hard formations, the measurements are not re- 

 liable because of poor contacts with the formations. It is particularly for 

 that reason that a new method of electric logging, known as "induction 

 logging," has been introduced for resistivity measurements in oil-base 

 mud. 



The induction-logging system does not require any direct contact 

 with the mud or with the ground. As indicated by the name of the method, 

 the formations surrounding the logging apparatus are energized by in- 

 duction. To that effect, alternating current of appropriate frequency is 

 made to flow through a coil, referred to as the "transmitter," which is sup- 

 ported by an insulating mandrel. The alternating magnetic field thus 

 created generates eddy currents, which follow circular paths coaxial with 

 the hole and the coil system, in the formations surrounding the hole. These 

 eddy currents create a secondary magnetic field, which induces an electro- 

 motive force in a second coil, referred to as the "receiver," mounted on 

 the same nonconductive mandrel at a certain distance called "spacing" 

 fr©m the transmitter. 



If the transmitter current is maintained at a constant value, the in- 

 tensity of the eddy currents is proportional to the conductivity of the 

 ground. Thereby, the conductivity of the ground determines the secondary 

 field created by the eddy currents and the signal generated in the receiver. 



As in regulai logging with electrodes, the signal is recorded con- 

 tinuously at the surface of the ground while the apparatus is moved along 



