Abstract

Drillability or drilling efficiency of rock depends on three main parameter groups: rock characteristics, drill rig parameters and operational parameters. This paper describes the methodology of rock cuttings collection, preparation and their analysis for evaluation of the drilling efficiency and the performance of a passive Vibration Assisted Rotary Drilling (pVARD) tool in particular. The pVARD tool utilizes rock bit interactions to create axial vibrations in order to improve rock penetration. Analyses showed the correlation of such parameters as rate of penetration and weight on bit to cuttings size distribution. Several parameters such as mean particle size and coarseness index were used for numerical representation of cuttings size. Also a new particle size distribution bar diagram was proposed for cuttings samples comparison.

1. INTRODUCTION

One of the purposes of an effective drilling fluid system is to convey the cuttings from around the bit to the surface [1].

Several studies have been carried out on drilling cuttings as these are commonly used for the geological description of wells. Also studies on the petrophysical properties of formations, such as porosity and permeability have been conducted [2,3]. These studies have outlined the importance of cuttings analysis.

However, few studies have focused on studying cuttings as a mean of understanding the cutting action of the drilling tools. By analyzing the way that the cutter affects the different drilled formations and establishing relations between drilling parameters, drilling performance and cuttings, it is more feasible to make assumptions on the rock-bit interaction models for different drilling techniques.

Drilling performance is often defined as the drillability of a rock, that is, how fast a certain formation can be drilled. It is characterized by the rate of penetration (ROP). The parameters that define the drillability of a rock are usually separated in three groups: Rock characteristics (physical, mechanical, and micro-structural properties of the drilled formation), machine parameters (rotation, force, cuttings removal, etcetera.) and operating processes (drilling techniques, state of the equipment, etcetera.) [4].

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