such things as magnetic field strength without the shielding effect of the sur- 

 rounding drill pipe and collars. High-pressure formation testers also have been 

 made to drop into place in the wire-line barrel in order to obtain pressure 

 samples of producing formations. Modern wire-line core barrels have been 

 made into extremely versatile tools which are particularly adapted to taking 

 continuous cores in soft and medium formations. 



Although conventional and wire-line core barrels differ in operational 

 details, the same general considerations apply to both types. In the process of 

 coring, the principal object is to drill the hole and, at the same time, to obtain 

 a clean unbroken core. Commonly a core will break into sections, sometimes 

 quite thin. This breaking complicates the job of retrieving. Drilling should be 

 done at moderate weights that are sufficient to make the bit penetrate. The 

 weight must be limited to minimize any vibration or jarring, which tends to 

 break and shatter the core. The core barrel must be straight to prevent the bit 

 from gyrating, and for the same reason, the drill pipe just above the barrel 

 must also be straight. It is bad practice to lift the bit off bottom, then go 

 back and resume coring. This procedure causes the core catcher to break the 

 core, and it can jam easily and prevent additional core from entering the barrel. 

 Careful operation with both types of core barrels will insure the maximum 

 recovery of core even in difficult formations. 



SIDE- WALL CORING The foregoing discussion has treated methods 



of taking of cores by tools which drill as 

 cores are being taken. However, the operator may wish to secure a core after 

 drilling has been completed. Either the subsequent logging procedures have 

 indicated a core point that was overlooked originally, or the operator might 

 have elected to drill ahead to total depth, then log and core the well. This type 

 of coring involves side-wall coring, that is, the cutting of a core from the side of 

 the well bore after the hole has been completed. 



Wire-Line Punch-Core Tool 



Figure 33-6 illustrates a wire-line punch-core tool. It consists of an outer 

 body which is run in the hole on a regular drill string, and an inner barrel 

 which is lowered on a wire line. The inner barrel consists of an upper latching 

 device, a center section with an orienting device, and a hinged core barrel at 

 the bottom. The tool is positioned at the proper depth; the kelly is set to one 

 side; and the barrel is lowered on a wire line. Upon reaching bottom, the 

 deflector on the lower part of the body deflects the core tube, which has been 

 held straight by small shear pins so that it bites into the wall of the hole. The 

 operator then carefully lowers the drill pipe to permit the core tube to swivel 



705 



