Chap. 11] 



GEOPHYSICAL WELL TESTING 



829 



The influence of" the 

 drilling fluid on the ap- 

 parent resistivity can be 

 calculated by assuming 

 the mud-filled hole to 

 represent the "cover" of 

 resistivity pi on a layer of 

 uniform resistivity p (two- 

 layer case, page 714). As 

 may be expected from the 

 similarity of electrical log- 

 ging and resistivity map- 

 ping, the drilling fluid 

 decreases in effectiveness 

 with an increase in elec- 

 trode separation. If the 

 resistivity of the forma- 

 tion is ten times greater 

 than that of the drilling 

 fluid, and if the electrode 

 interval CPi is ten times 

 the hole diameter, the 

 apparent resistivity is 75 

 per cent of the true re- 

 sistivity. If the drilling fluid is of high resistivity compared with the 

 formation, the electrode spacing need be but two to three times the hole 

 diameter, to produce an apparent resistivity virtually equal to the forma- 

 tion resistivity (Fig. 11-3). 



Conversely, for measuring mud resistivity, it is necessary to reduce the 

 electrode separation and make the electrode interval less than the diameter 

 of the hole. In that case the electrode intervals are made equal. To 

 suppress any possible wall influence, the electrodes may be enclosed in a 

 nonconductive cylinder, open at both ends. For a still closer approach to 

 true conditions, a third layer representing a zone flooded by drilling mud, 

 may be interposed between the formation and the drill hole in the above 

 calculations. 



As a matter of fact, the replacement of formation water by well water 

 near the hole may be so pronounced as to obliterate formation resistivities 

 and to require the so-called "third" curve, which is a resistivity curve 

 taken with larger electrode spacing and, therefore, greater depth 

 penetration. 



Fig. 11-2. Variation of apparent resistivity with 

 true resistivity and electrode separation in elec- 

 trical logging (after Hummel). 



3 J. N. Hummel and O. Rulke, Beitr. angew. Geophys., 6(3), 265-270 (1937). 



