Chap. U] GEOPHYSICAL WELL TESTING 827 



been used or proposed. While in resistivity surface mapping a simple 

 circuit with but two electrodes would not be suitable because of the pre- 

 ponderance of the contact resistance at the terminals, such an arrangement 

 is permissible in electrical coring, since the distance between electrode and 

 wall is small enough for the formation resistivity to affect the contact 

 resistance of an electrode suspended in drilling mud of generally uniform 

 resistivity. 



Fig. 11-1 illustrates various electrode arrangements in increasing order 

 of electrode number. The measuring devices indicated are not necessarily 

 limited to the electrode arrangement with which they are shown and may 

 be interchanged. Scheme a is used by the Halliburton Oil-Weil Cementing 

 Company (Blau patent) and in the Karcher system. In the latter, the 

 electrode is the (insulated) bit at the end of the drill pipe. Scheme (6) 

 is a Wheatstone bridge arrangement; it is used by Lane-Wells in a modified 

 form, the upper electrode being represented by the cable sheath. The 

 two-electrode scheme of Fig. 11-lc is well suited for separate self -poten- 

 tial and resistivity measurements. The three-electrode arrangement 

 shown in Fig. 11-ld (Schlumberger) is now in most extensive use. The 

 D.C. source indicated in the current circuit may be replaced by an A.C. 

 generator, or a commutator may be provided, together with an A.C. meter 

 in the potential circuit to record resistivity. One of the potential elec- 

 trodes is then switched by a second commutator to a D.C. meter and a 

 grounded electrode as in scheme (c) to give the self -potential record. Fig. 

 11-le (Hummel) shows a modification with potential electrodes down; 

 Fig. 11-1/ is the conventional four-terminal arrangement with commutator, 

 adapted to well surveying. 



The spacing of the electrodes in schemes h and c is usually ten to twenty 

 times the diameter of the hole; in schemes d and e, the distance CPi is 

 ordinarily about 60 feet and PiPz is about 15 feet. These distances may 

 be decreased and increased to obtam two curves of different side penetra- 

 tion. Because of the uniform constitution of the drilling fluid, it is gen- 

 erally satisfactory to use ordinary metal electrodes in the hole; however, 

 some companies use porous-pot electrodes. Two- or three-electrode 

 arrangements usually have the form of a weighted insulated bar, the weight 

 at the bottom representing the energizing electrode and the rings at the 

 other end representing the potential electrodes. The electrode assemblies 

 are lowered on cables that contain as many heavily insulated conductors 

 as there are electrodes, and that protected by stranded flexible steel wire 

 and surrounded by a steel sheath on the outside. 



Current is straight D.C, commutated D.C, or, most often, low- 

 frequency A,C. (20 to 100 cycles) as indicated above. The current can 

 be held sufl&ciently constant so that a recording potentiometer, whose drum 



