Rock Strength Determination Based on Rock Drillability Index and Drilling Specific Energy: Numerical Simulation Using Discrete Element Method

Abstract

In the present research, two real-time rock strength determination models based on the drilling parameters were studied. Firstly, a discrete element software named Particle Flow Code in 2 Dimensions (PFC2D) was used to analyze the applicability of rock drillability index and drilling specific energy, in which graded particle assemblies were created to simulate the rock behaviour. In the numerical models, Weibull distribution was used to randomize the bonds between particles and make the assembled rock model with different heterogeneity. The drilling process of a PDC (polycrystalline diamond compact) cutter was modelled in two steps: horizontal linear rock cutting and vertical pressing-in progress. From the horizontal linear rock cutting process, the peak cutting forces were obtained and vertical pressing-in process outputs the relationship between normal force and cutting depth. Subsequently, the methods for calculating rock drillability index and drilling specific energy were proposed in the discrete element model. Then, the relationship between the calculated indexes and rock strength was investigated (supported with the regression analysis). Moreover, the effect of rock heterogeneity, along with error comparison between the above two indexes, were discussed. The results showed that the rock drillability index is more accurate than drilling specific energy in rock strength assessment (The error is about 10% smaller).