Salty Mud 



If one assumes now that the mud resistivity is equal to 0.1, R x0 is approxi- 

 mately equal to 3. 



In oil, the values found for R a are respectively 19.6, 15.4, and 11.4 against 

 the true value, 24. When the formation is waterbearing, the values of R 

 are 1.8, 2.1, and 2.4 against 1.5, the true value. The Laterolog thus reads 

 below the true resistivity for oil-bearing and above for water bearing beds. 

 The relative errors are very small for shallow invasion and, though appreciable, 

 are still acceptable in deep invasion. At any rate, the qualitative discrimination 

 between oil-bearing and water-bearing beds is quite clear. 



It appears from the preceding that conditions favorable to the use of the 

 Laterolog are obtained when the resistivity of the mud is low; the Laterolog, 

 therefore, finds an excellent field of application in those instances where, in 

 contrast, the conventional log is the most adversely affected. 



It seems worthwhile mentioning that exceedingly conductive muds have 

 some drawbacks, the most important of which is the reduction of the SP curve 

 to an almost flat line that is thus of no value for the discrimination between 

 shale and permeable beds. Taking into account all the aspects of the interpreta- 

 tion, one can say that the optimum mud resistivity should be selected at about 

 3 to 5 times the connate-water resistivity. 



Effect of Bed Thickness 



The Laterolog, because of its focusing system, is better adapted for the 

 investigation of thin beds than the conventional devices. In order to illustrate 

 this point, the Laterolog 7 will be compared to a normal device in a schematic 

 example of a thin bed of high resistivity, non-invaded and bounded by thick 

 formations of low resistivity, with the thickness of the bed being slightly greater 

 than that of the sheet of current (fig. 14-18). 



The figure is divided into two parts by a vertical dash-dot line that co- 

 incides with the axis of the borehole. A power electrode A is shown on the 

 axis midway between the boundaries of the bed. The distribution of the current 

 emitted from electrode A without any focusing system — a normal device — is 

 represented qualitatively on the left part of the figure. This distribution, cor- 

 responding to the use of a focusing system, is shown on the right. 



For the device without a focusing system the current lines diverge from 

 A in all directions and are definitely attracted upward and downward by the 

 adjacent formations, more conductive than the bed, so that the resistance 

 offered by the bed to the current is, to a great extent, by-passed. The apparent 

 resistivity read opposite the bed is, therefore, much lower than the true resis- 

 tivity of the bed. With the Laterolog, all the current lines flow within the 

 boundaries of the bed, at least over a large distance from the borehole, so 



309 



