Origin of SP 



It is commonly accepted that the electromotive forces generating the SP 

 current arise from two types of phenomena: electrochemical and electrokinetic. 



The electrochemical phenomenon occurs at the contacts beween the 

 drilling mud and the connate water in the pores of the permeable beds and the 

 adjacent shales. Figure 14-6 shows schematically a permeable bed situated be- 

 tween the shales and penetrated by a drill hole filled with mud. The three 

 media involved are the mud and/or the mud nitrate, the connate water, and 

 the shales. These three media are separated by three boundaries across which 

 three electromotive forces arise. As each medium is considered fairly ho- 

 mogeneous, each of the electromotive forces is uniform along its corresponding 

 boundary. 



Theory and laboratory experiments have shown that in clean formations 

 the sum of the electromotive forces generated by the electrochemical phenomenon 

 can be represented by the formula E c = — K log 10 R„,//R. !C , where K w is the 

 resistivity of the connate water contained, R,„/ is the resistivity of the mud 

 nitrate, and K is a constant. When the mud and the connate water contain 

 essentially sodium chloride, factor K has been found theoretically equal to 80 

 at 150F, with E being expressed in millivolts. This result has been supported 

 by field experience in many regions. 



When the waters are very saline or contain other salts in solution, the K w 

 computed from the above formula will be too low. This value is called the 

 equivalent resistivity, (R w ) e ; it can be corrected to true K w by use of the inset 

 chart. In formations containing shaly material, the above relations are no 

 longer valid. This fact will be discussed later. 



The electrokinetic phenomenon is caused by the infiltration of the mud 

 filtrate into the permeable beds. This infiltration causes an electromotive force, 

 Efc, to appear primarily where the pressure differential is at a maxium, i.e., 

 across the mud cake. Since for a given formation the electromotive force 

 depends on the nature of the filtrate and of the filter (mud cake) and on the 

 pressure differential, it will be uniform all along the contact mud, permeable 

 formations traversed by a drill hole. 



Field experience has shown that in these regions where high salinity 

 connate waters are the rule, as in the Gulf Coast, the amplitude of the electro- 

 kinetic emf (electromotive force) is generally small enough with respect to the 

 electrochemical emf to be negligible. 



Circulation of SP Current 



The emf's of various origins add their effects to generate the SP current 

 which follows the paths represented schematically in Figure 14-6 by solid lines. 

 Each current line necessarily crosses the three boundaries. In the usual case 

 where the resistivity of the mud is higher than the resistivity of the connate 



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