After R m/ /R TO is obtained from the chart, R™ is easily derived from the 

 knowledge of R m /. Quite often K, nf is measured directly on actual samples of 

 mud. If not, R m / can be selected from average statistical values, as is shown on 

 page A-4 of the Schlumberger Log Interpretation Charts. 



In complex muds, the value of R^/ may be quite variable; and a direct 

 measurement of R m / is advisable. 



In shaly sands, the SP deflection gives the pseudostatic SP which for the 

 same values of K mf and R w is smaller than the static SP. 



The application of the SP formula, using the pseudostatic SP, instead of 

 the static SP, would give too high a value for R w . This too-high value is called 

 apparent connate-water resistivity and designated as R Mffi . It should not be used 

 as a basis for shaly sand analysis. 



The reduction factor, the ratio Pseudostatic SP _ a ? is a conven ient 



Static Sr 



factor, which enters the formula for quantitative analysis of shaly sands. 



Resistivity-Measuring Devices 



If a single electrode A in the hole and a surface electrode B are connected 

 across a source of direct or low-frequency alternating current, a current will 

 flow between these electrodes through the hole and formations. If the hole 

 electrode is surrounded by a homogeneous and isotropic medium, the current 

 will flow radially from electrode A equally in all directions. This current will 

 give rise to equipotential surfaces which are spheres centered around the 

 electrode. 



If another electrode M is introduced in the vicinity of electrode A and 

 the potential measured between this electrode and another remote electrode N, 

 the measured potential will be directly proportional to the resistivity of the 

 medium surrounding the hole electrodes. This system is basically what is 

 known as the normal resistivity. 



If the remote-measuring electrode N is placed close to M relative to the 

 AM distance, the potential difference between M and N is again proportional 

 to the resistivity of the surrounding medium. This is basically the lateral 

 resistivity. 



The proportionality factors to convert the potential readings to resistivity 

 are constant for any given electrode arrangement. Figures 14-9 and 14-10 

 illustrate the actual circuits used to measure the resistivity curves. In the normal 

 resistivity circuit the current return electrode B is actually in the borehole for 

 technical reasons. In the lateral circuit both arrangements shown will give 

 the same results based on the theory of reciprocity. The latter, or MAB, 

 system is the one used in actual practice. 



285 



