Miscellaneous Subsurface Methods 615 



Wire-Line Coring 



Advances in the development of coring apparatus eventually resulted 

 in obtaining a wire-line core barrel, primarily to eliminate the necessity 

 of pulling the drill pipe from the hole to recover each core. In wire-line 

 coring, a suitable hoisting assembly, shown in figure 317, including a wire- 

 line reel, a wire line, a prime mover, and a sheave, is needed in addition to 

 the usual surface drilling equipment. Additional subsurface equipment in- 

 cludes a special core-drill collar and bit, a core barrel and bit, and a wire- 

 line guide and overshot, all of which are shown in figure 318. The core- 

 drill collar and bit are run into the well on the bottom of the drill pipe. 

 Routine drilling may be done by dropping a bit plug inside the drill 

 pipe to shut off the main core-barrel passage through the bit and drilling 

 the formation in the center of the hole. When it is desired to take a core 

 of the formation, the bit plug is removed by means of an overshot that is 

 run inside the drill pipe on a wire line. The core barrel, with cutter head 

 and core-catcher assembly on the bottom, is then dropped inside the drill 

 pipe and automatically latched into place in the drill collar. After the 

 core has been cut in lengths up to twenty feet, depending upon local condi- 

 tions and requirements, the core barrel, with core inside, is removed with 

 the same overshot and wire line used to recover the bit plug. Consecutive 

 cores may be cut until the core bit is dulled. Weight on bit, rotary speed, 

 and circulation rate are varied somewhat from those used during routine 

 drilling. The advantages of wire-line coring include these: (1) that consec- 

 utive cores may be cut until the core bit has been dulled, without the 

 necessity of pulling the drill pipe from the hole to recover each core; (2) 

 that coring and drilling may be done intermittently, until the bit is dulled, 

 without the necessity, of making a round trip with the drill pipe; and (3) 

 that a lower coring cost usually is realized. The disadvantages include (1) 

 the requirement of an appreciable amount of additional surface equipment, 

 (2) the limit of the use to coring relatively soft formations, (3) the use of 

 cores comparatively smaller than conventional and diamond-cut cores, and 

 (4) the usually lower core recovery than in conventional and diamond 

 coring. 



Reverse-Circulation Coring 



Reverse-circulation coring is a very specialized type of coring and 

 can be used only in limited cases. Usually casing must be set at or near 

 the top of the section to be cored, and a special drilling-head assembly 

 and swivel arrangement must be used along with regular surface drilling 

 equipment. The subsurface equipment generally includes a regular con- 

 ventional or diamond-core bit, which is attached to a special interval- 

 flush-drilling string. Mud circulates down the drill-pipe-casing annulus 

 through the core bit, up the drill pipe, and back to the surface. Cores are 

 continuously circulated up the drill pipe to the surface and caught in a 

 screen basket, through which all returns from the well bore are directed. 



