关键词: 钻井工程, 岩石, 可钻性, 分级, 可钻性指数, PDC微钻头, 牙轮微钻头

Abstract: The grading of rock drillability can provide an important basis for guiding the optimization of drilling technology and improving drilling efficiency, which is of great significance to promote the progress of the engineering technology of drilling. In the past few decades, the standards for grading rock drillability have been continuously improved. However, with the advancement of industry and the continuous increase of drilling depth, the standards for grading rock drillability have gradually become unapplicable to the increasingly advanced drilling technology; moreover, there are certain difficulties in identifying the strata that are difficult to drill and guiding the optimization of drilling technology. Therefore, it is necessary to improve the drillability grading methods to adapt to the development of current and future technologies. The author reviewed the development history and limitations of the methods for grading rock drillability, conducted a large number of drillability tests covering three types of lithology, and performed statistical analysis. The analysis results indicate that the grading standards can not be established based on the statistical distribution of drillability test results. There is a poor correlation between the test results of the two types of microbits. The drillability test results of PDC microbits has good repeatability, so it is more scientific to use PDC bit for the drillability test method; by exploring the calculation model of rock drillability, it is suggested that in the future, the standards for grading drillability should be improved in terms of identifying hard strata; the concept and calculation methods of drillability index are proposed for PDC microbit and roller microbit; it is recommended to classify the effective drilling time of PDC microbits 0-60 s, 60-360 s, 360-600 s, and >600 s as being "soft", "medium-lower+medium+medium-upper", "hard-lower+hard+hard-upper", and "extremely hard", respectively, corresponding to the drillability indexes of 0-10, 10-60, 60-100, and >100; it is recommended to classify the effective drilling time of roller microbits 0-40 s, 40-280 s, 280-400 s, and >400 s as being "soft", "medium-lower+medium+medium-upper", "hard-lower+hard+hard-upper", and "extremely hard", respectively, corresponding to the drillability indexes of 0-10, 10-70, 70-100, and >100;

Key words: drilling engineering, rock, drillability, grading, drillability index, PDC microbit, roller microbit