Drilling Bit

QAB3012 DRILLING AND PRODUCTION TECHNOLOGY

DRILLING BIT

LESSON OBJECTIVES  To describe the basic types of drill bit and their differences  Select drill bit for specific application  Describe the bit evaluation process and IADC system  Describe the techniques to evaluate bit performance  Calculate cost per foot of a bit run  Select a bit on a basis of a previous bit run data

CONTENTS  INTRODUCTION  TYPES OF DRILLING BIT  BIT SELECTION

 BIT EVALUATION  BIT PERFORMANCE

INTRODUCTION Drill Bit

INTRODUCTION  Drill bit is the cutting or boring tool which is made up on the end of the drill string  It drills through the rock by scraping, chipping, gouging and grinding  How well a bit drills depends on:  Type and condition of the bit  The weight applied to make it drill

 The rate at which it is rotated  The type of drilling fluid used for circulation  The kind of rock/formation it is drilling

TYPES OF DRILL BIT DRILL BITS FIXED CUTTER

ROLLER CONE

FISHTAIL MILL TOOTH

TUNGSTEN CARBIDE INSERT

PDC (POLYCRYSTALLINE DIAMOND COMPACT)

NATURAL DIAMOND

ROLLER CONE BIT 

The cones are mounted on bearing pins, or arm journals, which extend from the bit body



The cones rotate and drill hole with a crushing and/or gouging action



Require high WOB to fail the rock with high compressive loads



Major advances:  Jet nozzles to improve cleaning  Tungsten carbide for hardfacing and gauge protection  Sealed bearings Faiza MN – July07

NATURAL DIAMOND BIT 

Employ natural diamonds set into tungsten carbide matrix body



Cutting action is by scraping/grinding



Very resistant to erosion and abrasion and are especially effective in harder formations.



Expensive and care must be exercised when running in the hole since they are not resistance to impact loading or drilling on junk.



Normally requires high RPM for better performance due to the limited depth per cut per rev

PDC BIT 

Use small discs of synthetic diamond



Use shearing or grinding action



In relatively plastic, sedimentary rocks as shales, limestone, and weakly cemented sandstones the shearing action is most efficient cutting mechanism requiring the least energy to drill.



The PDC cutter’s self sharpening effect results in long bit life and high rates of penetration.



Thermally Stable Polycrystalline (TSP) is similar to PDC bits but are tolerant of much higher temperatures

BIT SELECTION  Must consider both mechanical and geological factors  Useful to have condition of previously used bit in that hole or in the same section of an earlier drilled hole MECHANICAL FACTORS

 Cutters - offset, journal angle, tooth or insert bits

 Fluid Circulation - regular, jet fluid, air cooled

 Bearing Assemblies

GEOLOGICAL FACTORS

 Soft Fmn – soft clay, unconsolidated

 Medium Fmn – shales, gypsum, salt, chalk, siltstone, limestone, sandstone

 Hard Fmn – limestone, dolomite, lime, hard compacted sand, quartzitic

BIT SELECTION (cont’d) BIT OPERATING PARAMETERS Formation Type

WOB (per in of bit diam)

Rotary Speed (rpm)

Flowrate

SOFT

Low 125 - 150 (3000-5000 lb/in)

High since ROP high (500 – 800 gpm)

MEDIUM

Low (3000 – 6000 lb/in)

150 - 250 (shales) 100 - 150 (chlk/soft ss)

High

HARD

High (6000 – 10000 lb/in)

40 - 100

Not critical

BIT SELECTION (cont’d) 

Many variations in the design of drill bits



IADC has developed a system of classifying drill bits, based on formation type and design variations.



One for Roller Cone and one for Fixed Cutter

Series # - Type – Features – Additional Example: 1-2-4-E 5-1-7-X

ROLLER CONE IADC CHART Mill Tooth

TC insert

Faiza MN – July07

IADC - example A Smith F2 bit has an IADC classification of 517X: 

5 indicates that the Smith F2 has tungsten carbide inserts,



1 The bit is designed for use in soft formations with low compressive strength;



7 indicates that the cones on this bit have sealed friction bearings, and that the bit is designed for protection against gauge wear;



X indicates that the inserts have a chisel tooth configuration (as opposed, for example, to a conical shape) – there are 10 other characteristics.

BIT EVALUATION  As each bit is pulled from the hole, its physical appearance is inspected and graded according to the wear it has sustained  Why evaluate bit?  To improve future bit type selection  To identify effects of WOB, RPM etc.  To improve ability of personnel to recognise when a bit should be pulled due to wear  To improve bit design  Using IADC Dull Grading system

IADC BIT DULL GRADING Inner / Outer Cutting Structure

Location of Dull Characteristics

BIT PERFORMANCE 

A good drill bit should gives:  Good Rate-of-Penetration (ROP)  Longest possible number of rotating hours  Drill hole the same as the bit (true-to-gauge)  Lowest cost per foot of hole drilled



ROP is affected by:  WOB  Rotary Speed (RPM)

 Mud Properties  Hydraulic Efficiency

BIT PERFORMANCE (cont’d) Cost per foot of hole drilled



C



Cb

Rt Tt Cr F

C

= Overall cost per foot ($/ft)

Cb

= Cost of bit ($)

Rt

= Rotating time with bit on bottom (hrs)

Tt

= Round trip time (hrs)

Cr

= Cost of operatiing rig ($/hrs)

F

= Length of the bit run (ft)

The equation can be used to:  Post drilling analysis to compare one bit run with another in a similar well  Real-time analysis to decide when to pull the bit – theoretically when the cost per ft is at its minimum

BIT SELECTION FROM OFFSET WELLS  The process requires several assumptions:  The lithology encountered in the offset bit runs must be similar to that expected in the proposed well  The depth of the offset bit runs are similar to that in the proposed well

 The bit runs in the offset wells were run under optimum operating conditions  Having made the assumptions, the best bit will be selected on the basis of footage drilled, ROP and Cost-per-foot of bit run