A Discrete Element Simulation for the Effects of In-situ Stresses on the Mechanical Specific Energy of PDC Drill Bits

In the drilling of oil and gas wells, confining pressure or the pressure at the well bottom is one of the effective factors affecting the rate of drilling and the amount of specific energy (the energy needed to remove a single volume of rock). In this operation, the rock cutting process is a combination of two modes of failures, i.e. brittle and ductile. Each of these failure modes has a different effect on the specific energy and structure of the crushed material, and thus on the rate of drilling. In this paper, the distinct element method is used to understand the relationship between rock fracture and confining pressure and its effect on the specific energy. For this purpose, a particle flow code is used that numerically simulates the mechanical behavior of the granular materials such as rocks. Based on the results obtained in condition of no confining pressure, the force applied to the cutter blade causes failure of the inter-granular connections in a single failure plane. But under confined pressure conditions, a different mechanism is taking place, and the difference in pressure created during the cutting action of PDC drill bits keep the crushed rock on each other and increase the mechanical specific energy of the rock cutters. Also, up to a pressure of about 26 MPa, with increasing tension, the specific energy has a relatively linear increase during the cutting action of the bits. But after this pressure due to the increased confining stresses and near-hydro-static conditions, the incremental increase in the mechanical specific energy decreases as the drilling depth increases.