Open Hole Sidetrack 8 ¾” Mill Head

Modification

Dull Analysis and Modification Options to Reduce

Cutter Damage and Improve Performance

Prepared by:
Chung Lee
Technical Engineering Advisor
Technical Service
Nov 21, 2008
Table of Contents
1.  Definitions and Nomenclature ................................................................................ 3 
2.  Mill to Whip Face Interaction ................................................................................. 4 
2.1.  Initial Cut-out Ramp (15°) ................................................................................... 4 
2.2.................................................................................................................................... 4 
2.3.  Full Gauge Length (0°) ........................................................................................ 5 
2.4.  Following Section (3°, 1 1/2°) ............................................................................. 6 
2.5.  Mid Ramp (15°) ................................................................................................... 7 
3.  Dull Observations ................................................................................................... 9 
3.1.  Little Wear or Damage on Last Cutter ................................................................. 9 
3.2.  Critical area of Damage...................................................................................... 10 
3.2.1.  Impact Damage ........................................................................................... 10 
3.2.2.  Heat Checking ............................................................................................. 11 
3.2.3.  Transition Cutter ......................................................................................... 13 
3.3.  Effects of PDC Cutters ....................................................................................... 14 
4.  Big Bevel Cutter Placement .................................................................................. 15 
5.  Cutter Exposure Protection ................................................................................... 15 
6.  Summary ............................................................................................................... 17 

Appendix A – Constant Slide Whip Stock……………………………………………… 19

Appendix B – Modified Mill Head for 8 ¾” Open Hole……………………………….. 21
Appendix C – Modified Mill Head for 7” 26# Casing…………………………………. 22

Disclaimer:
None of the theories and conjectures presented in this report has been verified by
laboratory or FEA analysis. They have all been derived from studies of operating
procedures, field operating parameters and dull observations.

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1. Definitions and Nomenclature

TCI cutter
Cylindrical cutting element composing solely of tungsten carbide

PDC cutter
Cylindrical cutting element composing of tungsten carbide substrate and a
polycrystalline diamond (PCD) table.

Bevel
Chamfer on the cutting edge of a PDC diamond table at set at 45°

SB (Standard Bevel)
Bevel chamfer of 0.012”

MB (Medium Bevel)
Bevel chamfer of 0.016”

BB (Big Bevel)
Bevel chamfer of 0.025”

Millhead orientation
Following orientation will be used in this report

Gauge End Nose End

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2. Mill to Whip Face Interaction

For an open hole sidetrack operation, the ideal whip stock would have a constant angle
slide with the profile of the mill designed to match the whip stock (see Appendix A).
With a constant angle whip stock slide, the mill design can be optimized to provide even
loading across its blade profile during its contact with the whip face. With constant and
distributed loading, cutter impact damage can be minimized.

However due to the multi-angle design of the current Trackmaster whip stocks, each
different angled section of the whip stock will impart different forces and area of contact
against the mill head cutting structure.
3 Degree 1 ½ Degree 15 Degree 3 Degree
15 Degree 0 Degree

Full Gauge Mid Ramp

Length Length

Figure 1. Standard Trackmaster Plus Whip stock

2.1. Initial Cut-out Ramp (15°)

Mill head has a 15° profile to match the bevel of the whip face. During the initial
cutout, the mill head will slide along the 15° whip face with all the cutters of the
mill, except those in the nose, contacting the whip face at one point (Figure 2).
Cutters will be loaded evenly along the length of the blade and no significant
damage should occur during this process if proper procedure had been followed.
15°

2.2.

Figure 2. Mill Head at the Initial Cut Out Ramp

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 It must be noted that even if the shear bolt had been sheared properly, there will
be at least ½ inch of shear bolt protruding from the whip face. This extra metal
must be milled out properly before the mill head can precede further. If the mill
is ran in too fast or hard, the cutters on the nose or shoulder of the mill can break
or shatter leading to ring out (Figure 3).

Figure 3. Shoulder Damage on Mill Head

2.3. Full Gauge Length (0°)

Most of the impact damage to the gauge cutters will occur as the mill head slide
along the 0° section of the whip stock. In this section, the mill will be reaming
the formation with the full length of the mill’s tapered cutting structure.
However, only the gauge cutters of the mill will be in contact with the whip stock.

During the reaming process, reactive tangential forces from the formation will be
acting on the mill head. This will lead to opposing forces to be concentrated at
the few gauge cutters in contact with the whip face (Figure 4) causing point
loading. During this stage, any increase in weight on mill will cause a
corresponding increase in point loading on the gauge cutters. High RPM can also
lead to increased impact damage due to the minimal number of cutters which are
in actual contact with the whip face.

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 Tangential forces
exerted from formation

Point loading on gauge cutters

Figure 4. Mill Head at Zero Degree Slide

2.4. Following Section (3°, 1 1/2°)

Not as much impact damage will occur at the gauge cutters during milling along
these sections. Tangential side forces as well as the point loading on the gauge
cutters will be decreased. The load against the whip stock will be more evenly
distributed along the cutters on the blade taper. Point loading on the gauge cutters
will be greatly diminished (Figure 5).

The gauge cutters will still be responsible for maintaining a full gauge hole.
Damages to the gauge cutters during the initial milling process will greatly
decrease ROP. Most operators try to compensate and increase the ROP by
increasing the weight on mill. However, this can lead to even greater adverse
effect on the gauge cutters. The increase in weight can flex the mill mandrel,
running tool and the flex joint causing the gauge cutters to once again experience
point loading along the face of the whip.

Although the load on the cutters is more distributed along the length of the taper,
cutter to metal contact still exists. In most instances, part of the whip stock will
be milled along with the formation. With proper parameters and precaution this
can be minimized.

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 Figure 5. Mill Head at 3 Degree Slide

2.5. Mid Ramp (15°)

The mill head ‘s profile will once again match the profile of the whip stock along
the mid-ramp. Once again the cutters will be loaded evenly along the length of
the blade and no significant damage should occur during this process if proper
procedure had been followed (Figure 6). However, unlike during the initial cut
out, the nose of the mill will begin to engage the formation. As the mill crosses
the mid ramp and the nose of the mill begin cutting formation the milling process
should not be significantly affected if the nose of the mill has not been damage in
the previous section (Figure 7).

Figure 6. Mill Head at Mid-Ramp

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Figure 7. Nose of Mill Begins Cutting Formation.

As the mill crosses the mid-ramp the mill head will begin to get wedged into the
hole between the formation and the whip stock. In most instances part of the mid-
ramp will be milled out, leaving less than 15º profile at the mid-ramp. Heat
checking is also likely to occur on the surface of the carbide cutters at this point
due the mill being wedged between two surfaces

The gauge cutters will not only maintaining the hole gauge but also will be
actively cutting the formation. Any previous damages to the gauge cutters will
have the greatest affect on the ROP at this point.

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3. Dull Observations
Careful evaluation of the dull conditions of the mill head will help determine the type of
damage experienced by the mill as well as the mechanism causing it. Most mill heads are
not uniformly damaged across the cutting surface; different areas experience different
types of damages. Two types of damage we are most concerned about are damaged due
to impact or wear. Evaluation of the dull characteristics can help determine the areas on
the mill which are susceptible to either types of damage. Characterization of these areas
can lead to the understanding of the possible mechanism causing the damages. Only by
understanding the mechanism causing the damage can we provide an adequate solution.

3.1. Little Wear or Damage on Last Cutter

Most of the dulls observed had little to no wear on the last cutter. The cutters on
the last row are protected below the profile of the gauge cutters and will
experience minimum contact with either the whip face or the formation under
normal milling conditions. Most of the damages to these cutter will be due to
backreaming operations.

Figure 8. Little or no wear or damage on the last cutter despite the damages on
the adjacent cutters

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3.2. Critical area of Damage
Critical area of damage can be narrowed down to the last 5 cutters from the gauge
end of the mill excluding the last cutter
2 3 4 5 6

Figure 9. Location of Maximum Cutter Damage

3.2.1. Impact Damage

Observations of the dulls show that the impact damages on the mill head are seen
predominantly on cutters 2-5 (Figures10). These are the cutters that were
determined to have undergone high loading against the face to the whip stock.

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Figures 10. Impact Damages on Cutters 2-5

3.2.2. Heat Checking

Heat checking were also observed along the top of the cutter 2-5. These may be
the result of the mill being wedged between the whip stock and the formation.
The mill is most likely milling part the whip stock during this process. The
contact with the formation and the whip stock can generate high heat which will
lead to heat checking.

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Figures 11. Heat Checking on Cutters Top

When PDC cutters are used along side of TCI cutters, there may be instances
where the heat checking appears only on the PDC cutters. Although the heat
checking is more sever closer to gauge, heat checking has most likely occurred
along the whole length of the blade. The abrasiveness of the formation would
polish the top of the carbide cutters giving an appearance of a smooth surface.
The diamond table of the PDC cutter would be able withstand the abrasive
formation better and not wear as much. Careful observation will show signs of
heat checking on the TCI cutters (Figure 12).

Heat Checking Signs of Heat Checking Polished Surface

  (PDC)  (TCI)  (TCI) 

Figures 11. Polished Heat Checking on Cutters Top

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3.2.3. Transition Cutter

Transition from damage caused by impact to wear has been observed to occur at
cutters 5 and 6. Investigation of dull mill heads show cutters in these positions
can have both impact and wear damages (Figures 12). This mainly due to the
staggered positioning of the cutters along the profile of the blade resulting in
varying distance of cutters from the end of the mill. However, this can also be
due to formation or run parameter causing excessive impact or wear.

Figures 12. Impact and Wear on Cutters 5 and 6

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3.3. Effects of PDC Cutters
Diamonds have higher wear resistance than tungsten carbide. In high abrasive
formation, the use of PDC cutters will allow the cutters to last longer and drill
further. This fact has always been known from the numerous field and lab test on
PDC cutter. Cutter tests in mill for open hole have also confirmed this
observation. Figure 13 shows the result of cutter test where the PDC cutters were
layed out in a overlapping spiral pattern around the mill head, giving a pattern of
every third cutter being a PDC along the length of the blade. (See Appendix XX
for explanation of the angular wear seen on the TCI cutters)

PDC PDC PDC

Figures 13. Wear Comparison of PDC and TCI Cutters

Diamond’s superior wear resistance has an adverse effect on its impact resistance.
Diamond cutters are more brittle than TCI and more susceptible to chipping and
breakage with impact than TCI (Figure 14). The leached (HOT) cutters used in
the past has shown to have up to 4 times greater impact resistance than normal
PDC cutters. Leached cutters are no longer allowed to be used on mills due to a
licensing agreement.

Figures 14. Impact Comparison of PDC and TCI Cutters

Note: Complete delamination of PCD layer indicates overheating of the cutters
during brazing causing graphitization of the diamond. Graphitized
diamond table will delaminate with any impact.

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4. Big Bevel Cutter Placement
One option to prevent impact damage is to use big bevel cutters. Due to recent
improvements in cutter technology as well as the proliferation of the leaching process,
oilfield industry has moved away from the use of big bevel cutters. New cutters have
greater impact resistance and thicker diamond table than those of the past. Most bit
manufacturers have since moved to cutters with smaller bevel size to achieve higher
ROP.

Since ROP is not the major concern in milling operations, the larger bevel will allow the
cutters to withstand higher impact. Although the exact value of the increase in impact
resistance has not yet been quantified for the big bevel GV4 cutters, they should provide
a significant increase in impact resistance over the standard bevel cutters. The big bevel
cutters should be placed in the area with greatest amount of impact damage (cutter 2-5)

5. Cutter Exposure Protection

Another option to decrease impact damage is to reduce cutter exposure to impact.
Protecting the cutters and reducing its exposure will limit the amount of cutter susceptible
to impact as well as prevent weight stacking.

Reducing cutter exposure can be achieved by either building up the back of the cutter
pocket on the blade top with additional metal or by the addition of SRT to the top of the
blade. Addition of extra metal to the blade top would give a similar design feature as the
Geodiamond’s M feature (Figure 15). The amount (thickness of extra material) and the
degree (protection at individual cutter pocket vs. across the whole blade top) of extra
metal to be applied will be further determined by welders based on the ease of which the
powder metal can be applied.

Figures 15. M-Feature on a PDC (Option 1)

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The addition of SRT to the blade back would be similar to the Geodiamond’s V feature
(Figure 16). This option may be easier for rebuild purposes but will require additional
design changes to the current mill design. The current design does not allow for enough
spaced on the blade top to allow a placement of an insert.

Figures 16. V-Feature on a PDC (Option 2)

Besides limiting cutter exposure, both options will also provide additional bearing surface
for the mill against the whip face. As the milling progresses the abrasive formation will
be able to wear down extra protection to expose new cutter surface, always allowing a
sharp cutting surface to be available. For this purpose a material softer than the cutters
must be used or new cutting surface will not be exposed. For blade top build up the
softer eutalloy power should be used. For the SRT option, we are currently limited to
tungsten carbide inserts which may to be too hard for this application.

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6. Summary

Due to the highly abrasive formation encountered in the open hole whip stock operations,
PDC cutters are required to provide a longer wear life. Most obvious solution is to run a
Geotrack or a hard formation mill which have 100% PDC cutters and are designed for
high compressive strength formation. However due to the current difficulties in
manufacturing and the political nature of this industry, the Fastrack mills remain the only
viable option available for these jobs at the current time.

FasTrack mills have originally been design for casing exits and are not optimized for
open hole side tracking jobs in hard abrasive formation. Drilling through these
formations require the wear resistance properties of PDC cutter. However the FasTrack
mills have not originally been designed to accommodate PDC cutters on gauge. The
brittle and low impact properties of these cutters further contribute to the damages done
at gauge due to impact against the whipface and the formation. The damages to the
gauge cutters can be minimized by either more impact resistant cutters or by limiting the
depth of cut thereby reducing their exposure.

Cutters 2 to 6 have shown to have suffered the most damage during the milling
operations. Cutters 2 to 5 are especially susceptible to impact damage and should be
protected.
Transition
zone

1 2 3 4 5 6

Little Impact Mostly Wear

damage Some Impact

Two options proposed in this report is the use of big bevel cutter or limiting cutter
exposure. Both options should be tested separately from each other and the advantages
and disadvantages of each be investigates.

This report has focused primarily on minimizing the impact damage on the gauge cutters.
Damages to the nose cutters are not as critical for open whips tock operations but should
be considered for increasing the durability of the mill as a whole. Theoretically the nose
inserts should not drill any formation until it crosses the mid ramp and suffer no impact
damage against the whip stock. However, due to the relatively small quantities of the
cutter in the nose and the lack of cutter pocket around each cutter, these cutters are
susceptible to damage due to vibration and impact. Highly abrasive formation may also
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wear the cutter pockets exposing enough metal to allow cutters to fall out. Building up
the cutter pockets in the nose should not hinder the mills ability to drill but should protect
the nose cutters from vibration and impact damage and cutter loss.

One aspect not examined in this report but should be investigated more thoroughly is the
effect of the follow mill in the milling operations. Currently the millheads for the 8 3/4 “
hole section from OKC facility are dressed to 8.52” and the follow mill dressed to 8 ¾”.
This in effect causes the lead mill to drill a pilot hole and the follow mill to ream the hole
to the correct diameter. This may account for the dramatically decrease in ROP as the
lead mill goes undergauge causing the follow mill to do more work. The lack of cutting
structure on the follow mill makes it inefficient for drilling formation.

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Appendix A – Constant Slide Whip Stock

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Appendix B – Modified Mill Head for 8 ¾” Open Hole

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Appendix C – Modified Mill Head for 7” 26# Casing

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