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Title: Investigation of the cutting process in sandstones with blunt PDC cutters
Author: Jose Raimundo, Almenara Chau
Awarding Body: University of London
Current Institution: Imperial College London
Date of Award: 1992
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The forces acting on the tools when cutting rock are not only important for the design of the cutters but also because they can be utilized to characterize the rock being cut. To understand these forces, it is necessary to have a good insight into the mechanisms that govern the failure of the rock. This thesis reports on studies of the cutting process with blunt - Polycrystalline Diamond Compact (PDC) - cutters. It presents a critical review of previous analytical, experimental and numerical models for the cutting mechanisms of single cutters. Associated mining and tunnelling studies suggest that the main rock cutting mechanism is tensile failure followed by fracture propagation. The work here provides evidence that these mechanisms are not applicable when drilling sedimentary rocks under pressure with PDC cutters - where it is more likely that the mechanisms of failure is different in nature, being ductile. A cutting bit response model (Detournay and Defourny, 1992), which is based on ductile failure and considers the drilling process as a combination of a pure cutting action at the cutter face and a frictional contact at the wear flat, was selected for the present research. The model predicts that there is a linear relation between specific energy, £, and the drilling strength, S, which are two quantities with dimension of stress that are respectively defined as the horizontal and vertical force divided by the cross-sectional area of the groove traced by the cutter. The two processes, the cutting and the friction at the interface, are then studied by means of finite difference simulations with a computer programme Fast Lagrangian Analysis of Continua (FLAC). The numerical simulations are compared to the upper and lower bound plasticity solutions for this problem to determine the validity of the code. This code allows one to model large deformations and also friction at the tool/rock interface. The simulations were performed to determine: • the validity of assuming that the two processes are independent; and • to establish whether there is a linear relation between £ and S by modelling different depths of cut. An experimental programme of single cutter tests was undertaken to corroborate the numerical and analytical models. Results of cutting tests on three different sandstones using blunt PDC cutters are presented and analyzed. The experimental data support the theoretical prediction that there is a linear relation between the specific energy £ and the drilling strength S. Various quantities such as the cutting parameters (e and £), the friction coefficient (fi) and the contact strength (< r) are estimated for each of the rocks tested. The thesis also contains discussion on how these basic parameters of the drilling process are related to the geomechanical characteristics of the rocks tested. A discussion of the influence of small imperfections along the cutting edge of a "sharp" cutter on the determination of e and is also presented. The main contribution of this research therefore is the verification of the cutting model, which in turn will enable the state of wear of PDC cutters to be established form the forces measured on site.
Supervisor: Hudson, J. A. ; Detournay, E. Sponsor: Imperial College London ; Schlumberger Cambridge Research Ltd
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral