farther to the side and penetrate the formation. During this operation, if the 

 weight on the tool exceeds 30,000 pounds, the core barrel may be damaged. In 

 softer cores, the weight may not be apparent, and the operator must put a refer- 

 ence mark on the drill stem and lower it not more than 2i/ 2 or 3 inches. This 

 procedure permits the core barrel to punch a core out of the side wall. The 

 pipe then is lifted, and the core barrel returns to its original position. The core 

 barrel now may be recovered by an overshot on the wire line. The body then 

 can be positioned at a different depth and another core barrel run to take a new 

 core. This tool must be used on formations that are not too hard because the 

 cores are taken by main force; however, a core can be taken in formations in 

 which a rock bit will penetrate at a rate of 2 or 3 feet an hour. The obtained 

 cores are 1% inches in diameter in the larger sizes and ll/g inches in diameter 

 in the smaller sizes. They are from 4 to 6 inches in length, which is considered 

 generally to be long enough to penetrate the wall cake and the invaded zone at 

 the side of the hole, and to secure a typical sample of the formation. 



This procedure of side-wall coring is somewhat obsolete due to several 

 inherent difficulties. It is a slow method because it requires the deflecting body 

 to be run in on a drill string with the consequent expense of round-tripping. 

 The core barrel is subject to damage when used in hard formations. Finally, 

 the core barrel penetrates formations by pivoting around a hinge point. This 

 action cuts a curved core that is forced into a straight tube, thus causing dis- 

 tortion and cracking that frequently reduces the value of the sample. 



Rotary Side-Wall Coring Tool 



A more recent tool, the rotary side-wall coring device, is shown in Figure 

 33-5. This tool, which is not limited to formation hardness, produces a larger 

 and longer core than other types of side-wall coring mechanisms. As shown in 

 Figure 33-5, these improvements were obtained at the cost of considerable com- 

 plexity. The upper part of the tool rotates and drives the core barrel with the in- 

 ternal splines. A seal below the splines prevents mud flow past the coring barrel. 

 The lower end is attached to the whipstock section by a set of swivel bearings that 

 permit rotation of the drill pipe without turning the whipstock. The whipstock 

 has an enclosed body with a windoAV through which the deflector plate guides 

 the core barrel into the formation. A fluted shoe mounted on the outside of 

 the body opposite the window is selected to make the assembly fit snugly in the 

 bore hole. The core barrel is dropped into the bottom hole assembly from the 

 surface and is retrieved with an overshot on a wire line. The overshot latches 

 onto a spear point at the top, which in turn retracts the driving key and permits 

 the barrel to be raised. The core barrel is rotated by turning the drill pipe so 

 that the splines in the upper body drive the core-barrel key. Since mud circulation 

 is prevented from flowing past the barrel by the seal, it is diverted into the 



707 



