trusion of the cutter head is checked with the barrel pushed against the stop. 

 If the core-cutter head does not stop within a quarter inch of the adjacent bit 

 tooth, the protrusion may be adjusted by inserting washers under the retracting 

 spring-retaining bolt. The outer barrel and bit are then run in the hole several 

 pipe stands, and the core barrels are dropped into place and retrieved to make 

 certain all is in proper working order. In this operation, one must be sure that 

 the bit and outer barrel are submerged in the drilling mud, because dropping 

 a core barrel into a dry tool may damage the landing ring. If the hole is caving 

 or full of contamination, a core barrel equipped with a center bit is put into the 

 tool when it is run in the hole. After the bit is run into the hole, it is stopped 

 three or four feet off bottom and the center bit is retrieved. The kelly is attached 

 and the tool is circulated for 10 or 15 minutes to clean the barrel and the bottom 

 of the hole. The connection then can be broken and the core barrel dropped. 

 It can be allowed to fall freely, but it is usually faster to connect the kelly and 

 to pump the barrel down. When it seats properly, as indicated by an increased 

 mud pressure, the core barrel is latched by rotating it slowly and feeding the bit 

 to the bottom. It is rotated for a minute with enough weight to retract the core 

 barrel; then it is lifted off bottom and lowered again. Coring can then be started. 

 If at any time during the coring" operation the drill string" must be raised, as 

 when making a connection, the core barrel is pulled and coring is resumed with 

 a new one after the connection is made. 



Another occasion for changing the core barrel arises in coring broken 

 sand and shale. As soon as a shale streak is penetrated, a new core barrel should 

 be substituted to prevent loss of the sandy core, because this core probably 

 would not be sufficiently strong to push the sticky shale up the barrel. Upon 

 completion of coring, the core is detached by raising the bit off bottom. In 

 very hard formations, extra weight and rotation may be necessary to break the 

 core cylinder. Before the core barrel is retrieved, the tool is circulated for 10 

 or 15 minutes to clean out any material that might jam the barrel. After latching 

 onto the barrel with the overshot, one should not pull the assembly to the 

 surface too rapidly, because of a swabbing action that might pull a soft cored 

 material from the barrel. If the core barrel cannot be retrieved from the tool, 

 the overshot is released by letting the trip tube slide down the hoisting line and 

 opening the latching dogs. The overshot then can be equipped with a jar to 

 break the core barrel loose. The jar should be used at all times when coring in 

 deep wells. 



Wire-line barrels also have been furnished with diamond cutting heads. 

 In this instance, however, a seating-in head-type barrel is used because a forma- 

 tion suitable for diamond drilling is too hard for a retractable barrel to work 

 satisfactorily. Other specialized uses are made of the wire-line equipment. 

 Special surveying tools are mounted on dummy barrels and dropped from the 

 drill string. These tools protrude through the cutting bit and are able to measure 



704 



