BORE-HOLE INVESTIGATIONS 1029 



For the determination of true formation resistivities, it is useful to 

 record curves having large radii of investigation. This is particularly true 

 in the case of permeable formations which have been deeply invaded by 

 the mud filtrate and which therefore have, encircling the hole, a zone 

 whose resistivity still differs from the true resistivity of the formations. 



Electrode Combinations for Well Logging. — The relative positions 

 of the electrodes in the sonde or exploring device lowered into the hole 

 govern the type of curve obtained. Depending on the distribution of elec- 

 trodes, a device may, for instance, be good for marking the thin breaks, 

 but may not give a good representation of the thick layers. Another device 

 which permits the determination of fluid content may not be suitable for 

 locating formation boundaries. For this reason, it has been found best to 

 employ several different electrode arrangements simultaneously in order 

 that the several resistivity curves recorded may give a more complete 

 picture of all the formations encountered in the bore hole, however dififerent 

 their characteristics may be. The actual electrode arrangements to be used 

 in any area are dependent on the characteristics of the formations, peculiari- 

 ties of the drilling mud, etc. 



For the soft and usually less resistant formations, a sonde is employed 

 which permits the simultaneous or alternate recording of normal curves 

 and lateral curves of dififerent spacings, using the same set of electrodes. 

 The usual combination is a short normal of AM spacing between 10" and 

 20", a long normal of AM spacing between 20" and 7' , and a lateral of 

 AO spacing from 15' to 30'. 



One electrode arrangement used in modern electrical logging permits, 

 with the use of six electrodes in the hole, the simultaneous recording of 

 an S.P. curve, a \6" short normal, a 64" long normal, and an 18'8" lateral 

 curve. 



Some Properties of Normal and Lateral Devices 



The Normal Sonde. — Figure 636 indicates the shape of curves recorded with 

 normal devices for the case of homogeneous resistive layers sandwiched between beds 

 of low resistivity. The point of measurement of the readings is midway between A and 

 A4 on the sonde. 



It can be seen that the curves are symmetrical with respect to the center planes of 

 the layers. This is a general feature of the normal device. As a matter of fact the 

 same curves are recorded if M is above A, instead of A above M as indicated in the 

 figure. 



The upper part of the figure illustrates the case of a bed thicker than the spacing 

 (bed thickness e = 5d, spacing AM = 3d, d = hole diameter). It is observed that the 

 boundaries of the bed are not sharply indicated on the resistivity log, but tend to be 

 rounded ofif owing to the influence of the drill hole. Moreover the thickness of the bed, 

 as indicated by the distance between the two points of inflection, P and P', on the curve, 

 is less than its actual thickness by an amount equal to the spacing. The error in picking 

 the boundaries of thick resistive beds is small for normal curves of short spacings, and 

 this is one reason for the recording in practice of a short normal. It should be remem- 

 bered, however, that normal curves tend to show resistive beds thinner than they 

 actually are by an amount equal to the spacing. In a similar manner they tend to show 

 conductive beds thicker than they actually are by an amount equal to the spacing. 



