Study of DTH Bit–Rock Interaction Numerical Simulation Method and DTH Bit Properties Evaluation

Abstract

A systematic bit–rock interaction model was built by analyzing the borehole bottom lithomechanics of air hammer drilling. Moreover, based on the balance laws from mechanics of continuum, a damage evolution equation was constructed which considers the strength deterioration of rock elements caused by original cracks, a rock damage model for strength reduction and damage accumulation was constructed. By using static tester and split Hopkinson pressure bar, the constitutive parameters for the sampled rocks were acquired by coupling the “strain-softening” damage evolution equation with dynamic HJC constitutive and then embedding them into LS-DYNA. By simulating the rock breakage performance by Down the Hole bit, the well-distinguishable fluctuation zones and stable zones were found in the curve of impact energy and crushing work ratio, and then the concepts of critical impact energy and critical WOB for air percussive drilling were put forward. From the aspects of rock breakage efficiency, durability and safety, the concepts were proposed about equivalent rock breakage volume and bit energy efficiency coefficient. With the simulation model, the rock breakage performances were compared among the four regular bits. Rock breakage effects by air percussion drilling are distinctive under different structures, with levels of difficulty as follows: reverse fault > strike-slip fault > normal fault. Various types of bits have specific rock breakage performance under different structures, and even the same types of bits inconsistently resulted in different formations.