STUDY OF THE METHOD OF CLASSIFICATION OF DRILLABILITY OF ROCKS IN THE COAL-BEARING FORMATIONS

The criteria system of classification of drillability of rocks in the coal-bearing formations, as well as the analytical and testing methods, using cemented carbide drilling method are studied.In addition, the portable analytical and testng instruments, equipments and tools used in the field condition are manufactured.Among the 7 items of criteria system of classification, the indentation strength, the grindability, the plasticity coefficient and the drilling rate measured using the mini-needle cemented carbide bit are first applied in the classification of drillability of rocks.Especially, two items using mall sample to test the "indentation strength" and using the mini-needle cemented carbide bit to measure the "drilling rate" can reflect the category of classification of rocks in soft coal-bearing formations effectively.Using the mini-needle cemented carbide bit can make up for the defects that using min-iimpregnated diamond bit can not drill in soft coal-bearing formations below the 6th category or is difficult to reach the stipulated criterion of "drilling rate".