Coring in the Oklahoma City Field

The author has made a statistical analysis of the relations between recoveryand length of cores, diameters of cores and of drill pipe, and cost of coringunder different contractual conditions. The data are presented in tabularform. The length of the core appears to playa minor part in the percentage recovered, with a tendency to lower percentages with increased core length; approximately50 per cent of the length cored is normally recovered if the cores are morethan 2 in. in diameter. The diameter of the core has a more pronounced influence on the recovery; 13/8-in. cores average 29.1 per cent, 2 1/8-in. core 53.4 per cent and 3 1/8-in.cores with 6 5/8-in. drill pipe 68.5 per cent. Where 37S-in. cores were takenwith smaller diameter drill pipe, i.e., 4Y2-in. drill pipe in 8 1/4-in. hole, the recovery dropped to 49.8 per cent, indicating that the more rigid drillpipe favors good core recovery, a conclusion also borne out by the resultsobtained in using large drill pipe for smaller diameter cores. Surprisingly good recoveries were obtained from both the Johnson and Simpsonsands; shale, limestone and dolomite each gave less than averagerecoveries. No serious fishing jobs resulted from coring operations. Attention is called tothe need of a core drill which will make it possible to drill 3 full-sized holewhile coring, and thus reduce the cost of cores. Data pertaining to core drilling should be consistently recorded, such as, diameter of drill pipe and collar, diameter of core, recovery, size of cutterheads, type of core-barrel, coring time, speeds of rotation and of pumps, weight on core-barrel, etc.