Using New Computational Fluid Dynamics Techniques to Improve PDC Bit Performance
Authors: Watson G.R. , Barton N.A. , Hargrave G.K. 
Published: December 2007

The performance of the PDC bit used to drill the 8 1/2" section in the North Sea Armada project has been significantly improved using recent advances in Computational Fluid Dynamics (CFD). This paper details the successful application of new CFD techniques to optimise the hydraulic design of a PDC bit. The resulting performance improvements are demonstrated in pressurised drilling tests and successful field applications. CFD had previously been applied to drill bits but computing and meshing limitations precluded the use of a highly detailed bit model or modeling of the individual cutters. Optimisation for the Armada project applied new mesh generation techniques and proved that CFD can be a practical and flexible design tool. Detailing of the model, including the shape of the waterways and modeling the individual cutters, was shown to be very important to the accuracy of the predicted flow pattern. The results of the CFD work were verified experimentally, using Laser Doppler Velocimetry and sheet laser lighting in conjunction with digitally processed video recording. There was strong agreement between the CFD predictions and actual flow patterns.

Introduction

Even though PDC drill bit technology is now twenty years old, the pace of design improvements to PDC drill bits is faster than ever. This is fueled by the greater understanding of drilling mechanics, the increasing number of applications throughout the world and higher expectations from the drill bit - today's milestone drilling achievements are tomorrow's routine performance. To satisfy the increasing array of applications in ever shorter lead times the bit designer is constantly developing more effective and efficient ways to design bits.

Simulation techniques, such as Computational Fluid Dynamics (CFD), have existed for many years but their application to PDC drill bits has been limited. The length of time required to model the geometric complexity of a drill bit and the computing power required has meant that CFD has been used for research projects rather than as a design tool. However the capabilities of CFD have been steadily increasing. The development of the bit for the 8 1/2" section of the Armada field in the North Sea provided an opportunity to use the latest generation of CFD codes and provided the opportunity to assess whether CFD had developed into an effective and realistic tool for the PDC bit designer.

Influence Of Bit Hydraulics On Performance Although reduced cost per foot is the overall benefit, drill bit performance is usually described in terms of penetration rate, life and directional response. Whilst much attention is rightly focused on the cutting structure and cutter technology, another aspect that makes a significant contribution to drill bit performance is hydraulics. Optimum performance can only be obtained by making the best possible use of the available hydraulic energy. This is especially relevant when drilling long or extended reach wells where system losses reduce both the mechanical and hydraulic energy available at the bit.

Bit Cleaning. Rate of penetration (ROP) is greatly enhanced by the rapid removal of cuttings from the bit face to the annulus. In addition the mechanical efficiency of the bit is increased by minimising the energy expended on reworking cuttings that remain at the bit face. Therefore maximising the rate of cuttings removal will maximise the ROP potential of the bit.

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