Breaking cuttings mechanism of cone bits and fractal method for rock drillability

To fulfill the need of high drilling quality and high drilling speed,more rapid and precise observations need to be done on rock drillability.This paper analyzes the rock breaking mechanism of cone bits during drilling and the equiprobable breaking cuttings by using the fractal geometry theory.According to Bond crack theory,this paper sets forth a new rock drillability testing method which takes the detritus from the bottom hole as the research subject.A correlation model of drillability grades and cuttings fractal dimensions is established.The laboratory test shows that there is high correlation between micro-bit drillability grades and cuttings fractal dimensions.Based on this,a predicting index for rock drillability is established,which is shown by the fractal dimension and distribution of the cuttings size.Practically it proves that the fractal method has higher precision to ascertain rock drillability grade.The rock drillability can be reflected from the fractal dimension of upward cuttings during drilling.So fractal dimensions can be used to measure rock drillability.