the circulating fluid. Later a very simple type of coring tool was used which 

 consisted of a joint of pipe with the lower end slotted to form teeth. This pipe, 

 called a Texas type or "Poor Boy" core barrel, was fairly effective in recovering 

 cores from the softer formations. To prevent washing away the core by circula- 

 tion coming through the drill pipe, an opening was prepared in the side of the 

 pipe a short distance from the anticipated level of the core. The opening per- 

 mitted the drilling fluid to pass into the annulus of the hole. When the core 

 was cut, the rotation of the barrel at high speed and at an increased weight caused 

 the teeth on the bottom of the pipe to bend inward, cut off the core, and hold it 

 in the barrel. 



Today's highly complex coring bits and barrels are in considerable con- 

 trast to the earlier types. Drilling depths and other factors have made the cut- 

 ting of cores a much more complicated and professional operation. 



Core equipment can be placed in two general categories: (a) core-drilling 

 bits, which cut a core and advance the main well bore at the same time, and (b) 

 side-wall coring tools, which cut a core from the wall of a previously drilled 

 bore. Core-drilling bits can be further classified as, (a) conventional core bits 

 in which the cores are recovered by bringing the tool to the surface, and (b) 

 wire-line core bits in which the core barrel can be pulled to the surface by a 

 cable and another substituted without removing the drill string from the hole. 



Side-wall coring tools also come in many types and can be classified gener- 

 ally into two main groups. The first type requires running of the drill string 

 into the hole with a deflector shoe attached to the bottom. Coring tools that are 

 then run on a wire line either drill or punch the core from the hole wall. Because 

 the coring barrel is run on a wire line, the deflector shoe may be positioned at 

 different points in the hole and successive cores taken where desired. The other 

 type of side-wall tool is run on a wire line and has a device which punches a core 

 from the side wall by means of a hollow bullet fired from a gun. Side-wall 

 coring produces a relatively small-diameter core that penetrates only a short 

 distance into the wall of the hole. 



CONVENTIONAL CORING A modern conventional coring barrel is il- 

 lustrated in Figure 33-1. This type of barrel 

 can be used without special surface equipment because it is attached to the 

 lower end of the drill string and run into the hole. After the core is cut, the 

 drill string is pulled with the core barrel. As Figure 33-1 shows, the outer casing 

 of the tool consists of a connection at the top which is attached to the bottom- 

 tool joint and the barrel. The outer barrel is a tube connecting the drill string 

 and the drill bit. The drill bit, consisting of a cutter-head body, is attached to 

 the cutter-head crown. On the inside of the tool, the core catcher is positioned 

 between the core barrel and the cutter head. Two different types of core catchers 



696 



