Research on rock breaking mechanism of rotary-percussion drilling in marine hard rock strata and the influence of engineering and tool parameters on ROP

Abstract

Rotary-percussion drilling technology was used to improve drilling efficiency in marine deep hard rock formations, but the compatibility among the engineering & tool parameters, PDC cutter, and the hard rock formation has not been sufficiently studied. In response, a theoretical model of the axial impact hammer motion mechanism under drilling fluid driving conditions was established, and the speed and frequency of reciprocating motion of the hammer were analyzed under different drilling fluid and tool structure parameters. A numerical model of stress waves generated by the axial impact hammer on the base was established, and the variation of stress wave parameters (peak value, duration, etc.) with impact velocity was analyzed. Based on the calculation mentioned above, a numerical model of PDC drilling teeth cutting hard rock formations under the coupling effect of dynamic and static loads during rotary-percussion drilling was established, and the changes in penetration depth, rock damage, and rock debris size under different engineering and tool parameter conditions were analyzed. The research results indicated that as the drilling fluid displacement and density increased, the impact velocity and reciprocating frequency presented linear and nonlinear growth trends, respectively, which was beneficial for increasing the penetration depth, but it would increase the size of rock debris. The impact velocity and reciprocating frequency exhibited a nonlinear decreasing trend as the nozzle diameter increased, leading to decreased penetration depth and reduced size of rock debris. As the diameter of the pressure-bearing ring increased, the penetration depth increased initially and subsequently reduced. The findings could offer a reference for the engineering and tool parameter optimization during rotary-percussion drilling.