Role of Failure Mode on Rock Cutting Dynamics

Abstract

Abstract Rock cutting leads to a combination of brittle and ductile failure, and each failure mode has a different effect on drilling efficiency and the morphology of cuttings produced while drilling. A commercial, two-dimensional Discrete Element Method (DEM) code was used to investigate the link between cuttings morphology and downhole conditions using a single polycrystalline diamond compact (PDC) cutter. The methods described here differ significantly from previous studies in that grain-level forces were spatially averaged to determine the rate of energy dissipation within the rock volume during the entire cutting process. Stress-space methods and plasticity theory were then used to calculate the total energy lost due to brittle and ductile failure modes separately. Results indicate a direct and quantifiable relationship between confining stress, down-hole pressure, drilling efficiency and a transition from chip-like to ribbon-like cuttings morphologies. Introduction The shapes of drill-bit cuttings play an important role in drilling operations. Cuttings morphology impacts the effectiveness of cuttings transport up the annulus and the potential for agglomeration of cuttings around the bit (bit balling), as well as the energy cost of cutting rock under down-hole conditions. While PDC bit cutting mechanisms have been the subject of substantial theoretical and experimental research (e.g., Detournay and Atkinson, 2000), there has been relatively little progress made on linking cuttings morphology to drilling efficiency. Discovering such a link would help provide a solid foundation for drilling advisory systems such as ExxonMobil's Fast Drill Process, which is a drilling operations management workflow designed to maximize rate of penetration (ROP) in every foot of hole drilled (Dupriest and Koederitz, 2005; Dupriest, 2006). The Fast Drill Process makes use of the Mechanical Specific Energy (MSE), originally described by Simon (1963) and Teale (1964) and defined in a later section, to diagnose and correct drilling dysfunctions in real time. The focus of this paper is on two questions:what controls chip morphology, andwhat is the energy cost of creating different morphologies? For the cases studied here, it is argued that cuttings morphology is controlled by the dominant mode of failure—the brittle mode leads to chip-like cuttings and the ductile mode leads to ribbon-like cuttings. The terms brittle and ductile will be defined precisely in the following section. We will show that DEM simulations provide a direct means of calculating the energy used to cut rock in each mode.