and the other three cut the bottom and. leave the core. The short cutter assembly 

 is welded to a sub that contains the threads and fluid passages. This cutter as- 

 sembly can be interchanged with a bladed-crown type for coring soft formations 

 (bottom illustration of fig. 33-3) . The blades of this bit are coated with tungsten 

 carbide to provide more abrasion resistance. 



WIRE-LINE CORING Early in the use of conventional core barrels, 



two inherent difficulties stimulated oil-field 

 equipment personnel to design a coring device that could be brought to the 

 surface without pulling the entire drill string from the hole. One difficulty was 

 that in easy penetration, the entire string would have to be pulled to the surface 

 when the core barrel was filled, even though the bit was capable of further 

 penetration. The second difficulty arose when, during regular drilling operations, 

 an interesting strata was encountered. The inconvenience of pulling the entire 

 drill string in order to run in a core barrel before the bit was dulled often re- 

 sulted in by-passing a critical core. 



The wire-line coring and drilling equipment was devised to overcome these 

 difficulties. In this type of core barrel (fig. 33-4), a drill bit that leaves the core 

 in the center is attached to an outer body or drill collar, and the core barrel is 

 dropped into the assembly. The core barrel is generally of the protruding type 

 and sticks through the center of the bit, thus penetrating ahead of the bit cutters. 

 This type, which is mounted on a spring arrangement that allows the head to 

 float in and out, drills ahead of the main bit in softer formations and retracts 

 into the main bit when hard formations are encountered. When coring is com- 

 pleted, an overshot is lowered through the drill pipe on a wire line, hooks onto 

 the barrel, unlatches it, and pulls it to the surface. If drilling is to be continued 

 without taking cores, another barrel equipped with a center bit is used. This bit 

 drills up the core left by the drilling bit. By this means, the main bit can remain 

 in the hole as long as it is sharp, and as many cores may be taken as are desirable. 

 If a core is not needed, the center bit allows normal drilling to proceed. How- 

 ever, at any time, the center bit may be quickly pulled out and the core barrel 

 dropped in place to accommodate unexpected drilling breaks. 



As Figure 33-4 shows, the upper part of the barrel consists of the driver 

 sub whose upper end carries a conventional tool-joint thread attached to the 

 drill string. On the lower end a special pin thread connects the main barrel. 

 On the inside of this pin is a special box thread that holds the core-barrel driver. 

 The main outer barrel is connected to the driver sub and connects at the lower 

 end to the drilling bit. The drilling bit may be either a hard-formation type 

 with rolling cutters or a soft-formation bladed type. The wire-line barrel is 

 dropped into this assembly. The lower part of the barrel contains the cutter 

 head, which trims the core to the size of the barrel. Inside this head is a core 



701 



