Miscellaneous Subsurface Methods 617 



The major advantage of the method is excellent recovery. Some disad- 

 vantages are that (1) it usually requires more rig time, (2) it requires an 

 appreciable amount of special equipment, (3) it is generally adaptable 

 only to formations that are consolidated enough to stay intact as they are 

 washed to the surface, (4) it may increase the circulation-loss problem, 

 and (5) it usually requires installation of casing at or near the top of the 

 formation to be cored. 



Side-Wall Coring 



Side-wall coring techniques were developed to obtain a sample from 

 the wall of a previously drilled hole at desired intervals. In general no 

 surface equipment is required in addition to that used in some of the other 

 coring methods, except possibly with the electrically motivated de- 

 vices, which require a regular logging truck. Figure 319^ illustrates a 

 hard-formation, electric-motor-driven side-wall coring device. In the same 

 figure B and C show two general types of side-wall-coring tools, in which 

 the core tubes are driven into the formation by an electrically detonated 

 charge. The electrically driven devices are lowered into the hole on an 

 electric-logging line, and these devices are usually run into the open-hole 

 section without requiring installation of drill pipe. The core tubes are 

 generally driven into the formation either directly or indirectly by an 

 electrically detonated powder charge or an electrically driven motor and 

 are retrieved by hoisting the assemblies. 



Wire-line side-wall samplers, as shown in B of figure 320, are run on 

 an ordinary wire line, and the core tubes are deflected to the formation by 

 suitable assemblies, which were previously attached to the drill pipe. The 

 core is obtained usually by lowering the drill pipe until the core tube has 

 penetrated the formation and then retrieving the core barrel, using the 

 wire line and a suitable overshot assembly. The bit of a one-wire side- 

 wall-coring assembly, shown in A of figure 320, is drilled into the side- 

 wall formation by rotation of the drill pipe. A third type of side-wall- 

 coring device, shown in figure 321, is run on drill pipe, and the core tubes 

 are projected into the formation by mud pressure. These cores, obtained 

 by the hydraulic method, must be retrieved by pulling the drill pipe. 



The general advantages of the side-wall-sampling techniques are (1) 

 that a sample can be obtained from the wall of a previously drilled hole, 

 generally at any desired interval, and (2) that the method can be a val- 

 uable aid in confirming electric-log interpretations. Disadvantages are (1) 

 that samples are usually too small for ordinary laboratory core analysis, 

 and (2) that samples usually have been subjected to considerable flushing 

 action of filtrate from the drilling mud. 



In rotary coring it is very important to have a circulating fluid in the 

 hole that has very good physical characteristics in order to reduce to a 

 minimum the flushing by mud filtrate of the section to be cored. The treat- 

 ment of water-base muds to filtration-rate values in the very low ranges 



