Chapter 16 

 NEW TYPES OF WELL RESISTIVITY-METERS 



E. A. POLYAKOV 



The specific electrical resistance of the drilling fluid (water solution of 

 salts or a mud suspension ■which fills a bore-hole) is normally determined 

 with the aid of a well resistivity -meter, of which the principal part consists 

 of a small logging probe. The measurements are made according to a pro- 

 cedure analogous to that used in the electrical resistance logging. The specific 

 electrical resistance q of the drilling fluid is determined according to the 

 following formula 



Q = K^, (1) 



Where / is the current strength, passing through the feeding electrode A ; 

 Av is the potential measured between the electrodes M and N and K is the 

 resistivity -meter coefficient. 



It is usually assumed that the resistivity -meter coefficient remains con- 

 stant, which is not true in the existing resistivity-meters, since their probe 

 coefficient varies depending on the specific resistance of the driUing fluid, and 

 on the conditions of measurement, and is generally unstable. In determining 

 the specific resistance of the drilling fluid, this circumstance leads to in- 

 accuracies, often reaching 50 per cent. 



CAUSES OF VARIATIONS IN THE RESISTIVITY-METER COEFFICIENT 



Variations in the resistivity -meter coefficient can be evoked by the follow- 

 ing causes. 



(1) Changes in the mutual positions of the constituent parts of the resist- 

 ivity-meter and in the dimensions of the probe. 



It is obvious that any change in the mutual positions of the constituent 

 parts of the instrument and of the distances between them leads to a redis- 

 tribution of the electric field produced by the current electrodes of the 

 probe, consequently causing a change in the resistivity-meter coefficient 

 and therefore errors in the results of measurements. Owing to the shortness 



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