Subsurface Logging Methods 441 



still have the bore hole tend to stand up. Nevertheless, numerous prob- 

 lems connected with rotary drilling and oil producing were a direct result 

 of hole caving in rotary holes. The exact nature and extent of this caving 

 was unknown. Every driller and every geologist had a theory, but there 

 was no exact knowledge. Subsurface bore-hole caving was likened to 

 surface erosion, but owing to the different character of the sediments and 

 the greatly accelerated subsurface erosional forces, this parallel could 

 not be drawn too far. 



Present knowledge indicates three major reasons for differential hole 

 size in a hole drilled with rotary tools: (1) action of the drilling fluid, 

 (2) action of the bit, and (3) action of the drill pipe. Of these, it is 

 believed that the action of the drilling fluid is most important. 



The hole change caused by mud or any conventional drilling fluid is 

 due either to a chemical effect (hydration) or a mechanical effect (attri- 

 tion or dissolution) . Of these two, it is believed that the former is the 

 more important cause. 



Water-base muds must have a tendency to cause many shales to swell 

 and heave or to disintegrate. Many shales disintegrate beyond the 32- 

 inch range of the modern caliper tool. Interestingly enough, a mud cake 

 may be built up on the face of a formation, causing a hole to caliper 

 smaller than bit size. 



In order to reduce hole change, many muds other than water-base 

 muds are used. Oil-base, oil-emulsion, silicate-base, and salt-base muds 

 are all commonly used to prevent the caving of shales or soluble forma- 

 tions. It is not the purpose of this section to discuss the merits of various 

 types of muds. The effect of these muds on hole size has been clearly 

 shown to the industry by caliper logs. Figure 205 shows caliper logs on 

 offset wells, one drilled with oil-base mud and one with water-base mud. 



In 1932 M. M. Kinley made a successful attempt to caliper a hole. 

 A few years later R. B. Bossier measured the enlargement of a hole caused 

 by shooting. These early tools measured a short section of the well and 

 were used mostly in shallow areas. The original Kinley caliper had four 

 separately actuated arms, each arm giving a separate record. A stylus- 

 recorded strip chart was obtained. 



The present modern caliper used by the oil industry was developed 

 from the original M. M. Kinley tool by the Halliburton Oil Well Cement- 

 ing Company in 1940. 



The present caliper (figs. 206 and 207) is a chrome-plated tool three 

 inches in diameter and approximately five feet long. It consists of an 

 oil-filled chamber containing the electrical components, the four caliper 

 arms, and the releasing mechanism. The tool itself is standard equipment 

 on an electrical-service truck and is run in a well on a five-sixteenths-inch 

 logging line. 



The four spring-actuated arms of the tool contact the walls of the 

 bore hole when they are released. The motion of these arms is trans- 



