1040 EXPLORATION GEOPHYSICS 



Potentials Due to Electrochemical Forces 



Electrochemical potentials occur at the contact or boundary between two 

 solutions of dissolved salts. For example, an electrochemical potential is 

 created when the sweet water of a bore hole comes in contact with the salt 

 water of a porous formation. 



Figure 643 shows schematically a permeable layer, for example, a salt-water sand, 

 situated between impervious formations such as clays or shales. The three media, the 

 salt-water sand layer, the adjacent clay formations, and the mud, are separated by 

 boundaries A, B and C, as indicated on the figure, and electromotive forces a, b and c 

 of electrochemical origin exist at the corresponding boundaries. As each medium is 

 considered fairly homogeneous, each of the electromotive forces is uniform along the 

 corresponding boundaries.! 



Laboratory experiments^ have shown that the sum E of the electromotive forces 

 a, b, and c generated by the electrochemical phenomenon can be represented by the 

 formula : 



E = K\og£^ 



where />«, is the resistivity of the connate water contained in the layer ; pm the resistivity 

 of the mud ; and K a constant factor which depends on the constitution of the clay 

 formation and the chemical composition of the fluids in contact. The sign of E changes 

 according to whether pm is greater or smaller than p«,. When pm = />«> the E.M.F. is 

 equal to zero. 



At boundary A both electrokinetic and electrochemical phenomena are present, so 

 that the total electromotive force a, which occurs along this boundary, is actually 

 equal to the algebraic sum of electromotive forces contributed by the two different 

 phenomena. 



Circulation of S.P. Current. — The three electromotive forces a, b, and c, add their 

 effects to generate the S.P. current which follows the paths represented on Figure 

 643B by solid lines, each line corresponding to a line of flow. In the usual case where 

 the pressure of the mud is higher than the formation pressure of the sand layer, and 

 the resistivity of the mud is higher than the resistivity of the connate water, the current 

 circulates in the direction of the arrows (from inside the bore hole towards the 

 permeable bed). 



Each current line must necessarily cross the three boundaries A, B, and C. Fur- 

 thermore, that part of the current generated by each of the three E.M.F.'s, a, b, and c, 

 follows the same path. In other words, the intensity of the current circulating in the 

 mud of the drill hole depends only upon the algebraic sum of all the partial E.M.F.'s 

 in the circuit, and does not depend upon the allocation of these partial E.M.F.'s to each 

 boundary, provided that each E.M.F. is uniform everywhere on its corresponding 

 boundary. 



This combination of three electromotive forces at the boundaries, producing a 

 current along a closed path traversing all three media, is termed a three-link chain 

 E.M.F. 



Along its path, the S.P. current has to force its way through a series of resist- 

 ances, both in the ground and in the mud. In so doing, it produces potential differences. 

 Along a given line of flow, the potential decreases continuously in the direction of the 



t In order to simplify the explanation, boundary A, between the mud and the original fluid in 

 the layer, is considered to coincide with the wall of the hole. The conclusions would be approxi- 

 mately the same for the case in practice where this boundary is located at a certain distance beyond 

 the wall of the hole. 



t C. and M. Schlumberger and E. G. Leonardon, "A New Contribution to Subsurface Studies 

 by Means of Electrical Measurements in Drill Holes," A.I.M.E. Geophysical Prospecting, 1934, 

 pp. 273-288. 



