BORE-HOLE INVESTIGATIONS 1073 



water at a minimum because this reduces the pumping, treating, and 

 disposal costs, and in addition reduces corrosion damage to casing, pump, 

 rods, and tubing. Also, in many fields, the maintenance of reservoir 

 pressure is dependent upon the formation water, and a minimum water 

 production will insure a longer flowing life for many wells. f 



Repair operations to stop the flow of water into a well must be preceded 

 by a means for locating the point where the water is entering the well. 

 Several geophysical methods have been developed for determining the 

 depth at which water is entering the hole. Usually, proper interpretative 

 technique, based on production experience, will show the cause of water 

 entrance and its remedy.* 



In contrast to the previously described electrical logging methods, the 

 methods under discussion utilize means for causing the water to enter 

 the drill hole from the formations, as well as means for locating the point 

 of entry. Thus, for these methods, the hydrostatic pressure must be re- 

 duced by bailing, in order to allow the formations to produce into the hole. 



Resistivity Methods. — In contrast to the use of resistivity methods 

 for structural studies, this application requires that the sphere of measure- 

 ment be confined substantially within the bore hole. The method of Figure 

 634 may be applied by reducing the lengths of ri and r^ to values some- 

 what smaller than the radius of the bore hole. 



During resistivity measurements, the hole should be conditioned with 

 fresh water if the formation fluid is salty and with conductive salty fluid 

 if the water source produces fresh or relatively non-conductive fluid. 

 Immediately after the conditioning, a survey is made to verify that the 

 column of fluid in the bore hole is of uniform resistivity. Following this, 

 the well is bailed and a second survey made. The entire procedure 

 (bailing and temperature surveying) is repeated until a change of resistivity, 

 produced by the entrance of water into the well, is noted. 



Early work along these lines was done by Schlumberger, HuberJ and 

 Elliott.§ 



Natural or Self Potential Method. — The spontaneous potential 

 method utilizes the electrochemical potentials previously described. Ob- 

 viously, if a well is conditioned with a fluid of different electrolytic con- 

 centration than that of the formation fluid and if water is then caused 

 to flow into the hole from the formation, a potential will exist at the 

 boundary between the entering fluid and the conditioning fluid. 



This method uses two electrodes, situated only a short distance 

 apart, and suspended on a cable. The parameters measured, therefore, 

 are the potential gradients. Conditioning of the well is efifected by circu- 



t P. T. Amstutz, Jr., and E. A. Steplianson, "Optimum Producing Rates for Arbuckle Lime- 

 stone Wells," Univ. of Kans. Research Foundation, Bull. 1, Sept. 1944. 



* H. G. Abadie, "Method and Apparatus for Locating Leaks in Wells." U. S. Patent 2,383,455, 

 Aug. 28. 1945. 



tF. W. Huber, U.S. Patents 1.536,007; 1,555,800; 1,555,801; 1,555,802; and 1,555,803, 



§ R. D. Elliott, U. S. Patent 1,537,919. 



