BORE-HOLE INVESTIGATIONS 1087 



compared to bullets fired, is at present about 70 per cent. In general, the 18-shot gun 

 may be expected to return 12 to 15 cores, and this loss of a few cores is relatively 

 unimportant because another attempt is always possible and because the method is 

 extremely rapid. The loss of bullets in the formation by breaking the fastener wires 

 is about 2 per cent. 



Figure 678 illustrates the discovery of a new sand in an old field from 

 the combined data provided by multi-electrode recording and side wall 

 sampling. The normal and third curves indicate a higher resistivity in the 

 region from 5571 to 5622 feet. The fourth curve verifies the resistivity 

 indication in this section and indicates further that the sandy shale from 

 5622 to 5632 feet is also oil-bearing. Cores, taken before the casing was 

 set, are shown. The loose sand proved to be a medium grain oil sand of the 

 Marginulina with a very good odor. Casing was set and perforated from 

 5596 to 5606 feet with ten shots, resulting in an initial yield of five barrels 

 per hour, 



THE SECTION GAUGE 



The hole calipering or section gauge is an instrument for measuring 

 the effective diameter of a bore hole. As it is moved in the hole, the effective 

 size of the hole at the depth of the instrument is logged continuously by a 

 recorder at the surface. 



The measurement of the size of a drill hole permits the computation of 

 the hole volume and of the quantities necessary in cementing operations. 

 It is useful in the selection of points for setting packers, and for determin- 

 ing the amount of gravel to be used for gravel-packing a well. Another use 

 is for determining the efifects of nitroglycerin shots and the efifects of 

 acidizing. 



A most important use is the more accurate interpretation of the various 

 types of electrical logs : the determination of the true resistivities by means 

 of the departure curves requires a knowledge of the hole diameter. Gener- 

 ally, the S.P. log resembles the caliper log, because the shale sections wash 

 away, enlarging the hole. 



The operation of one type of instrument may be understood by reference to the 

 schematic diagram of Figure 679. The measuring system of the instrument comprises 

 three long flexible springs which belly out until they touch the sides of the bore hole. 

 The bottom ends of the springs are fastened to a plunger which telescopes into a 

 cavity in the upper part of the instrument. Relative distance of insertion of the plunger 

 into the cavity depends on how much the flexible springs are constricted, which in 

 turn depends on the size of the hole. 



The position of the plunger determines the degree of inductive coupling between 

 an energizing coil and a pick-up coil contained in the upper part of the instrument. 

 Through the energizing coil is passed an alternating current of constant amperage. 

 The electromotive force induced in the pick-up coil depends on the position of the 

 plunger, and therefore on the size of the hole. The design of the coils and the plunger 



