Drilling Bit

QAB3012

DRILLING AND PRODUCTION

TECHNOLOGY

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

CONTENTS



INTRODUCTION



TYPES OF DRILLING BIT



BIT SELECTION



BIT EVALUATION

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:

 The weight applied to make it drill

 The rate at which it is rotated

 The type of drilling fluid used for circulation

TYPES OF DRILL BIT

Faiza MN – July07

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

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

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

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

 Fluid Circulation - regular, jet fluid, air cooled

 Bearing Assemblies

 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)

Formation Type WOB (per in of bit diam) Rotary Speed (rpm) Flowrate SOFT Low (3000-5000 lb/in)

125 - 150 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

Faiza MN – July07

ROLLER CONE IADC CHART

Mill Tooth

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

IADC BIT DULL GRADING

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

BIT PERFORMANCE (cont’d)

 Cost per foot of hole drilled

F C T R C C b t t r

C = Overall cost per foot ($/ft) C_b = Cost of bit ($)

R_t = Rotating time with bit on bottom (hrs) T_t = Round trip time (hrs)

C_r = 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