Miscellaneous Subsurface Methods 609 



CORING TECHNIQUES AND APPLICATIONS 

 H. L. LANDUA 



Since the early days of the oil industry, efforts have been exerted to 

 find better methods of obtaining information about the subsurface forma- 

 tions penetrated during drilling operations. Early techniques included 

 examining (1) the cuttings that resulted from bit action on the formation, 

 (2) a solid piece of the formation obtained by coring, and (3) the cir- 

 culating fluid as it returned from the well bore for possible oil and gas 

 shows. Subsequently numerous methods of formation logging were made 

 available to the industry, such as electric logging, radioactivity logging, 

 permeability-profile logging, and mud-analysis logging. It is the purpose 

 of this section to discuss (1) the various types of coring techniques now 

 being used by the industry, (2) the application of coring to geologic prob- 

 lems, (3) coring in relation to production work, (4) correlation between 

 coring and electric logging, and (5) the limitations of coring techniques 

 and applications to geologic problems. 



Types of Coring 



Two types of general drilling techniques are currently being used in 

 oil-field development. One is rotary drilling, in which a bit is attached 

 to pipe and lowered to the formation to be drilled. Bit cutting action is 

 obtained by turning the string of pipe to which the bit is attached. A 

 suitable circulating system is employed to pump fluid down the drill pipe, 

 through the bit, and back to the surface. Formation cuttings are con- 

 tinuously removed from the hole by circulation, and the bit is kept cooled. 

 The other technique is cable-tool drilling, in which the bit is lowered to the 

 formation on a line of some kind, and drilling action is obtained by rais- 

 ing the bit a short distance off bottom and dropping it against the forma- 

 tion to be drilled. This up-and-down action is carried on continuously 

 and at numerous intervals each minute, usually until too many cuttings 

 accumulate on the bottom to allow further drilling. Then a bailer is run 

 on a line and the cuttings are bailed out, after which drilling is resumed. 



When it becomes necessary to core in rotary drilling, five general 

 types of coring are used: namely, conventional, diamond, wire-line, re- 

 verse-circulation, and side-wall. Conventional-cut cores usually range in 

 diameter from 2f to 3^^ inches; diamond cores, 2|- to 4| inches; wire-line 

 cores, 1 to 2j^ inches; side-wall cores, half an inch to 1^ inches; and re- 

 verse-circulation cores usually are in the same range as conventional and 

 diamond cores, depending upon the size of the drilling string. The max- 

 imum length of core may vary from the small 1^-inch length obtained in 

 some side-wall coring to ninety-foot lengths that may be obtained in some 

 diamond coring. 



Conventional Coring 



For conventional coring, only the surface equipment that is used 

 for routine drilling is necessary. A core barrel, shown in figure 314, is 



