Bottom Hole Assembly Design for downhole drilling of oil and gas wells

Bottom hole Assy

1. BOTTOM HOLE ASSEMBLY
DESIGN

2. BOTTOM HOLE ASSEMBLY DESIGN
CRITERIA
!
Purpose
!
D.H Tools to be included
!
Stabilization
!
Bit selection
!
Limitations
!
Cost per foot
!
Motors

3. BHA DESIGN
PURPOSE
The purpose for BHA choice may be limited to a simple
Build‐up or Tangent, or it may need to be designed as a
combinatorial BHA. The requirements of BHA'svary
immensely.
Multi‐tasking BHA'sare the most complex and are usually
those used to enter the reservoir, comprising a suite of
Logging‐While‐Drilling (LWD) tools above a motor or Rotary
Steerable drilling System (RSS). A motor or RSS is able to
make quick directional changes to the well profile at the
DD's command based on Real‐Time data being transmitted
through the fluid column to the surface.
LO13

4. BHA DESIGN
PURPOSE
Rotary BHA'sare the most unpredictable and, once in the
hole, there are few choices on altering their tendency to
either build, drop or turn. However, experienced DD's with
local field knowledge can often save the client hundreds of
thousands of dollars with their use.*

5. BHA DESIGN
PURPOSE Example: BHA is designed to Build angle from vertical to 60 °through
problematic shales.
A rotary BHA should be seriously
considered here. Problematic shales
could be subject to swelling, caving
and instability. Therefore, the risk of
getting stuck in hole due to "packing
off“is relatively high and the client
does not want to lose expensive
tools in the hole.
Tricone bit
String stabilizer
under‐gauge (UG)
Drill collar(s)
Down force
Up force

6. BHA DESIGN
TOOLS TO BE INCLUDED !
Is a motor required?
Yes, if the well has to be steered.
No, if there is a tangent section and hole problems are
expected. !
Is an RSS required?
Yes, if hole tortuosityand/or hole cleaning is a potential
problem.
Yes, if LWD tools require continuous rotation to give
the best logs.

7. BHA DESIGN
TOOLS TO BE INCLUDED
No, if sliding will not interfere with log data quality or
data not critical.
No, if well cost is a consideration, (RSS is still costly). !
Does the client require gamma, resistivity,
neutron density / porosity, or sonic logs?
!
Is there a need for Annular Pressure While Drilling
(APWD), formation PWD, continuous inclination
and azimuth or DH temperature?
TOOL DECISIONS HUGELY AFFECT THE COSTS
!
Is an RSS required? (ctd)

8. BHA DESIGN
MOTORS !
Early motors were run with Bent Subs ‐where the axis of
the lower connection was slightly offset from vertical.
!
The Bent Sub offset was scribed to the surveying device ‐
usually a UBHO ‐a Universal Bottom Hole Orienting Sub
just above the motor.
!
A key‐seat was aligned with the scribe mark and locked in
place by tightening two or three allenscrews.

9. BHA DESIGN
MOTORS !
The survey tool would then be run with a muleshoestinger
screwed to the bottom, above which would be a spring to
absorb the impact when the survey tool "seated" in the
key‐seat.
!
A lead "tattle tale" in a small hole at the tip of the stinger
would be dented as the stinger aligned in the key ‐seat,
confirming a good seat.

10. BHA DESIGN
MOTORS
Muleshoe stinger
Keyseat
Allen screw
The muleshoestinger is
shaped so that it will turn and
lock into the keyseataligned
with the bent sub. Lead tattle tale dents on impact
with key.
If the tattle tale is not
dented on return to
surface the survey tool
should be rerun.
Orienting survey tools with bent subs and motors

11. BHA DESIGN
The PDM
Courtesy of
Schlumberger
MOTORS

12. BHA DESIGN
The PDM
Courtesy of
Schlumberger
The Power section of a Positive Displace ‐
mentMotor converts the muds' hydraulic
energy into rotation used the turn the bit.
Using the reverse Moineauprinciple, mud
is forced down a sealed, helical shaft,
causing it to rotate (hence the Rotor)
within the seal, the Stator. The Stator is
connected to the drill string and will only
rotate from surface.
MOTORS

13. BHA DESIGN
The PDM
Courtesy of
Schlumberger
Hence the energy is transmitted along a
drive shaft to the bit box, below which is
the bit, enabling the bit to ratateindep ‐
endentlyof the drill string.
MOTORS

14. Geometric DLS
MOTORS
L = Length Bit to upper stab.
L1 = Length between UBHS &
upper stabilizer.
L2 = Length between UBHS
& Bit
BHA DESIGN L1
R
L2
Top stabilizer
Bottom stabilizer
Bit
Tilt
Arc produced by system in
oriented mode assuming full
gauge stabilizers.

15. BHA DESIGN
The PDM ‐advantages !
More power to the bit.
Energy from the circulation system rather than the
column.
!
Ability to change well trajectory if desired.
Rotary BHA'shave zero influence on direction.
!
Drill string rotation can be minimized to protect casing
strings.
Wear due to drill‐pipe rotation can cause casing failure.
MOTORS
!
Less rig power is used.
Also less wear and tear on the Top Drive system.

16. BHA DESIGN
The PDM ‐disadvantages !
PDM'scan fail, necessitating a trip OOH.
Any kind of PDM failure means a trip, whether it be
bearings or rotor.
!
The DLS of a required curve may not be met, even with
full‐stand slides.
The Bent Housing can only be adjusted at surface.
!
The pressure drop through a motor is significant.
This can be detrimental to drilling wells where the
system pressure is already high, perhaps due to the
length of open hole.
MOTORS

17. BHA DESIGN
Care of motors Motors can be made to stall when: !
Too much WOB is applied in an effort to force ROP.
The hole will load up with cuttings and pack off.
MOTORS
!
A fault is encountered.
A suddentchange in formation or unconformity.
!
The bit becomes unevenly dulled or damaged.
A lost cone or broken teeth will cause problems.
!
An unusually hard formation is encountered.
The right bit choice is important here.

18. BHA DESIGN
Types Low Speed High Torque (0.11 rpm/gal)
Used for very soft formations where minimum washing is
required (such as spudding30" casing into a soft seabed),
or in situations where the bit is difficult to turn, (very hard
formations, aggressive PDC bits with large cutters to
maximize ROP. Good for rock bits in shales.
MOTORS

19. BHA DESIGN
TypesMOTORS
Medium Speed Medium Torque (0.28 rpm/gal)
Used for medium soft formations where ROP can be
maximized. Goodoptionfor use with Insert and PDC bits
that can withstand higher rotation speeds. Medium cutter
size will perform better.

20. BHA DESIGN
TypesMOTORS
High Speed Medium‐Low Torque (0.45 rpm/gal)
Used for medium hard formations where ROP is slow.
PDC bits withsmallcutters or diamond bits are preferred.

21. BHA DESIGN
Drilling techniques
DIFFERENTIAL PRESSURE.
The pressure required to overcome the
resistance caused by the bit.
MOTORS
Uneven pressure fluctuations
can indicate bit or motor problems.
A continuous off‐bottom pressure decrease
indicates a bearing pack failure as more flow is
allowed through them.

22. BHA DESIGN
Drilling techniques MOTORS
Zero increase in torque on bottom.
This could indicate a broken drive shaft or balled
up bit.
A sudden total loss of motor pressure off bottom.
Usually indicates a broken motor. This is rare but
always has a cause that can be identified.

23. BHA DESIGN
Bent Housing
Bit offset
Muleshoe stinger seats in orienting sub
Motor
Here, the muleshoehas been lined up
physically at surface with the motor High
Side ‐the direction of the Bent Housing
Now the DD has a reference
direction and will be able to
orient the motor with a gyro
Intended well path direction.
Drive Shaft
Bearing Housing Upper Bearing Housing Stabilizer (UBHS)
Scribing
LO14

24. BHA DESIGN
STABILIZATION Influences:
!
BHA direction (principally inclination)
!
Drill string harmonics (BHA stability)
!
Torque and drag
!
ROP
!
Hole conditioning
!
Magnetic interference

25. BHA DESIGN
STABILIZATION BHA direction !
There are many stabilizer options.
!
There is no set rule for any particular application.
However, there are accepted BHA types known to
work predictably.
!
BHA'sfor larger hole sizes are stiffer.
Therefore, stabilizers need to be placed farther apart.

26. BHA DESIGN
STABILIZATION BHA direction !
BHA'sare constantly evolving with the use of new
technology.
New LWD tools often require their own stabilization.
!
Stabilizer placement is a CRITICAL aspect of the DD's job.
Good stabilization saves time (and cost) correcting
the trajectory.
!
Stabilizer placement is the most important part of BHA
design.
Influnceswhirl, BHA harmonics, shock & vibration,
"stick and slip".

27. BHA DESIGN
Bottom Hole Assembly types ‐1. Although BHA classification is subjective, some sort of order
is required.
Rotary BHA's:
Threebasic types can be classified:
!
Packed Hole.
!
Fulcrum.
!
Pendulum.
STABILIZATION

28. BHA DESIGN
1)The Packed Hole Assembly
STABILIZATION
60 ft (18m)
30 ft (9m)
30 ft (9m)
30 ft (9m)
Bit
NB Roller Reamer
Stabilizer
Stabilizer
Stabilizer
Stabilizer
Tendency to maintain angle

29. BHA DESIGN
2)The Fulcrum Assembly
STABILIZATION
60 ft (18m)
30 ft (9m)
Bit
NB Stabilizer
UG Stabilizer
FG Stabilizer
Tendency to build angle

30. BHA DESIGN
3)The Pendulum Assembly
STABILIZATION
30 ft (9m)
30‐45 ft (18‐24m)
Bit
FG Stabilizer or RR
FG Stabilizer
Tendency to drop angle

31. BHA DESIGN
Bottom Hole Assembly types ‐2. Motor BHA'shave no recognized classification, so this is
the first!
Motor BHA's:
Threebasic types can be classified: !
Build / Turn & Hold.
!
Drop & Turn.
!
Performance Drilling / Horizontal geosteering.
STABILIZATION
Question: why do you think motor BHA'shave a dual
classification?

32. BHA DESIGN
STABILIZATION
a) Build / Turn & Hold
This assembly could complete a Build ‐up (from the 9⅝"
shoe, for example) to horizontal, entering the reservoir and
drilling ahead.
Stabilizer would be slightly under ‐gauge (UG).
BH set 1.15° Slightly UG stabilizer
Motor
Sleeve
Stabilizer
Would have a small build tendency during when rotating.
Mitigates the need to slide.
Why?

33. BHA DESIGN
STABILIZATION
b) Drop & Turn
This assembly could drop in rotary mode.
BH set 1.15° In‐gauge stabilizer
Motor
Sleeve
Stabilizer (1/8" UG)
Caused by pendulum effect caused by in ‐gauge
stabilizer.
Insertion of a "pony" 3 ‐4m “pony”DC would increase the
effect.

34. BHA DESIGN
STABILIZATION
b) Drop & Turn ‐ctd
Answer:motors of 8" OD and smaller tend to be more
flexible, so placing the stab ilizer farther back may cause
the motor to flex and actually build angle! This is where the
DD's job gets complicated.
Question:to what extent does the angle already in the
hole play a part in the performance of this BHA?

35. BHA DESIGN
STABILIZATION
b) Drop & Turn ‐ctd
Experience has shown that shorter motors tend to react
better to FG stabilization at angles greater than 30 °. The
extral‐long (XL) and extra‐power (XP) motors of Baker
Inteqand Schlumberger are more likely to flex.
Question:what would you do if you desired a drop
and saw that the BHA was building angle?

36. BHA DESIGN
STABILIZATION
c) Performance Drilling / Horizontal Geosteering
Although these two applications are different, they have
one thing incommon: RATE OF PENETRATION (ROP).
Performance drilling is a term used to describe maximum
desirable ROP, either because there are no consraints
caused by the inclusion of LWD tools in the BHA*, or
because the target tolerance is generous** and the need
to slide will be non ‐existent or at worst, infrequent.

37. BHA DESIGN
STABILIZATION
c) Performance Drilling / Horizontal Geosteering
Sometimes, the motor is set with the BH at 0 °which makes
it useless for orienting to change the wellpath. Baker
Hughes monitored motorperformancein various regions
of Oman with BH at zero and with anoptionto slide (a small
setting) and found no difference in ROP trend.

38. BHA DESIGN
STABILIZATION
c) Performance Drilling / Horizontal Geosteering
Horizontal drilling will be discussed in detail later, but the
use of a motor in horizontal applications is to keep the bit
in the reservoir.
Geosteeringis the term used to correct the wellpathin a
timely manner based on real‐time data arriving at the
surface from the LWD tools in the BHA.

39. BHA DESIGN
STABILIZATION
c) Performance Drilling / Horizontal Geosteering !
LWD tools measure an array of formation properties.
!
Drilling of wells has become more efficient due to global
communications.
!
Client able to monitor operations in remote areas from his
office and exchange RT data on ‐site.

40. BHA DESIGN
STABILIZATION
c) Performance Drilling / Horizontal Geosteering
Factors enabling Geosteeringtechniques: !
Technological advances in tool
design.
!
Increased transmission rates from the d.h. tools to
surface.
!
Improved communication and data transmission
security.

41. BHA DESIGN
STABILIZATION
c) Performance Drilling / Horizontal Geosteering
Factors enabling Geosteeringtechniques:
Advent of the Internet has enabled Real ‐Time participation
by all the major players in the drilling operation, from
Drilling Engineers to Senior Geologists in any location on
the globe. This has incr ‐eased the decision‐making
efficiency (and hence, the overall success) of most wells
drilled today.

42. BHA DESIGN
BIT SELECTION
Correct Bit selection is critical to BHA performance.
Threebasic types can be classified:
!
Roller Cone.
!
Polycrystalline Diamond Compact (PDC).
!
Diamond.

43. BHA DESIGN
BIT SELECTION !
Are still the most commonly used bits worlwide.
Roller Cone bits.
!
Can comprise one, two or three cones ‐triconescommon.
!
Cones have steel teeth or tungsten carbide inserts.
!
Work with a grinding or chipping action.
!
First roller cone bit introduced by Hughes in 1909 ‐bicone.
!
First triconebits in use in 1930's.

44. BHA DESIGN
BIT SELECTION
Roller Cone bits.
Advances since early bits include:
!
Use of jet nozzles to clean bit.
!
Use of tungsten carbide hard ‐facing to protect cones
and gauge.
!
Introduction of sealed bearings to prevent failure due
to mud corrosion & abrasion.
!
Widespread use of 3 cones achieves more even
weight distribution, improved stability and longer bit life.

45. BHA DESIGN
BIT SELECTION
Roller Cone bits.
There are now two distinct types of Roller Cone Bit: !
The Mill Toothhas integral steel or welded teeth on the
cone.
!
Latest generation milltoothbits are extremely strong and
reliable.
!
Perfect for breaking up claystones(shales).
!
Capable of drilling 1000m+.
!
Options include tungsten carbide hardfacingon key
parts of the bit (teeth, gauge and shirttail).

46. BHA DESIGN
BIT SELECTION
Roller Cone bits.
There are now two distinct types of Roller Cone Bit !
Uses tungsten carbide inserts of various shapes in a
steel matrix, depending on the application.
The Insert
!
Today's insert bits are extremely robust and reliable,
making them hugely competitive to PDC's.

47. BHA DESIGN
BIT SELECTION
Roller Cone bits.
MilltoothInsert
Roller Cone bits.
(Courtesy of Hughes
Christensen).

48. BHA DESIGN
BIT SELECTION
Roller Cone elements Elements of Roller Cone
bits. (Courtesy of Hughes
Christensen).

49. BHA DESIGN
BIT SELECTION
Roller Cone bearings
There are three types of bearings in Roller Cone Bits: !
Roller bearings form the outer assembly supporting the
radial load.
!
Thrust bearings that resist longitudinal or thrust loads
and secure the cones to the journals.
!
A friction bearing in the nose assembly that helps to
support the radial loading. it is a bushing pressed to
the cone nose.

50. BHA DESIGN
BIT SELECTION
Roller Cone bearings
Sealed bearings were introduced in the 1950's. !
Abrasive solids cannot enter the bearing.
!
Bearings are lubricated by grease fed from a
pressure‐equalized reservoir.

51. BHA DESIGN
BIT SELECTION
Journal bearings
Journal bearings do not have rollers. !
The cones are mounted directly onto the journal.
!
This provides a larger contact area over for the load
on the cone.
The cone is treated with alloys to increase wear
resistance.
!
Only minimal lubrication is required.

52. BHA DESIGN
BIT SELECTION
Cutting structure !
The height and spacing of the teeth or inserts on a roller
cone bit form the cutting structure.
!
Soft formation structures:
!
Moderate to hard formation structures:
Roller Cone bits
!
Teeth are long & thing, widely spaced to prevent bit
balling.
!
Designed for heavier loads. Typically have shorter,
wider teeth that scrape and gouge. Spacing must
allow for efficient cleaning .

53. BHA DESIGN
BIT SELECTION
Cutting structure ‐ctd !
Hard formation structures:
Roller Cone bits
!
Rely on chipping rather than tooth penetration. Teeth are
short and stubby with high density to share high loads.
!
Spacing less important as cuttings are smaller & ROP
tends to be slower.

54. BHA DESIGN
BIT SELECTION
Cutting structure ‐ctd !
Tungsten carbide hardfacingis applied to the teeth of soft
formation bits to increase bit life. Hard formation bits have
similar protection applied to the gauge of the bit to prevent
the bit from going UG.
Roller Cone bits
!
Bearing may evetuallyfail if the bit goes UG, due to
the modified distribution of thrust forces.

55. BHA DESIGN
BIT SELECTION
Bit fluid circulation !
Takes place through tungsten carbide nozzles placed
strategically around the bit body.
!
Nozzle selection is a critical aspect of BHA hydraulics,
bit performance and bit life. Many sizes are available.
Roller Cone bits

56. BHA DESIGN
BIT SELECTION
Other design aspects
Roller Cone bits !
Cone design
!
Degree of meshing between cones.
!
Cone slippage
!
Amount of drag due to radial cone angle.
!
The journal angle
!
Amount and offset axes

57. BHA DESIGN
BIT SELECTION
PDC bits
Major aspects of PDC bit design are: !
Cutter material
!
Body matrix material
!
Cutter size and distribution / density
!
General bit profile
cutter exposure, shape and rake.
!
Fluid circulation pattern

58. BHA DESIGN
BIT SELECTION
PDC bits
Courtesy of
Hughes Christensen

59. BHA DESIGN
BIT SELECTION
PDC bits
Cutter Material !
Polycrystalline Diamond is a synthetic material of 90 ‐95%
pure diamond, (PCD)
!
Polycrystalline Diamond is formed in a two‐stage, high
temperature / high pressure process.
!
The material is then manufactured into compacts that are
set into the body of the matrix, hence the name:
PolycrystallineDiamond Compact

60. BHA DESIGN
BIT SELECTION
PDC bits
Cutter Material Stages
1)Artificial diamond crystals are formed by exposing
graphite, in the presence of cobalt, nickel and iron or
manganese catalyst to 600,000 psi. Diamond crystals
form. !
As a result there is volume shrinkage, which causes the
catalyst to flow between the crystals, preventing them
from bonding, thus producing only the diamonecrystal
powder.

61. BHA DESIGN
BIT SELECTION
PDC bits
Cutter Material Stages ‐ctd
2)The second stage forms the PCD cutter by a liquid phase
sintering operation. The diamond powder is mixed with a
binder at 1,400°C and 700 psi.

62. BHA DESIGN
BIT SELECTION
PDC bits
Cutter Material Stages ‐ctd !
Sintering dissolves the diamond crystals at the edges.
This is followed by growth of a crystal layer on crystal
(epitaxial growth) on faces of low contact angle between
the crystals.
!
The regrowthforms true diamond‐to‐diamond bonds.
The result is a continuous network of pores coexisting
with diamond in a concentration of 90 ‐97% by volume.

63. BHA DESIGN
BIT SELECTION
PDC bits
Thermally Stable Polycrystalline (TSP) !
The answer was to leach out the cobalt binder with acid,
leaving only the bonds between the adjacent diamond
particles, retaining 50 ‐80% of the compacts' strength.
!
Note that leached PCD will degrade in the presence of
oxygen at 875°C`and the sacrifice of the immensely
strong tungsten carbide bonding substrate leaves the
PCD weaker. Consequently, the TSP diamond is
restricted to small sizes.

64. BHA DESIGN
BIT SELECTION
PDC bits
Body Matrix Material
There are two types:
1) Steel, into which the cutters are interference ‐fitted as
studs, often with tungsten carbide buttominserts to
provide gauge protection. 2)
Steel shell, with tungsten carbide matrix surface which
use a cylindrical cutter brazed into a pockerafter the bit
body has been furnacedby conventional techniques.
See note

65. BHA DESIGN
BIT SELECTION
PDC bits
Cutter size and distribution / density !
The process makes the bit erosion and abrasion resistant
with impact resistance provided by the matrix pocket.
However, the raw materials used in their manufacture
are more expensive.
Cutter rakecan be set as back rake or side rake.

66. BHA DESIGN
BIT SELECTION
PDC bits !
Side rake directs cutingsoutwards from the bit and
to the annulus.
!
Small rake angle = larger cuttings…
!
but the cutter will be more exposed to breakage.
The rake angle will determine the size of the cutting.
!
Back rake assists cleaning.

67. BHA DESIGN
BIT SELECTION
PDC bits Cutter size and distribution / density Profile
There are three basic types of crown profile:
!
Flat or shallow cone
!
Tapered or double cone
!
Parabolic

68. BHA DESIGN
BIT SELECTION
PDC bits
Cutter size and distribution / density
Flat or shallow coneprofiles distribute weight evenly over
the entire bit face. This design lacks rotational stability,
but is good for side ‐tracking.
Taperedallows increased distribution of cutters towards
the outerpartof the bit, increasing stability and evenness
of wear.
Parabolic profileallows smooth loading and the largest
contact area. It is the preferred profile for motors and RSS.

69. BHA DESIGN
BIT SELECTION
PDC bits
Cutter size and distribution / density
Cutter densityis the number of cutters per unit area of the
bit face. A higher density will decrease the load per cutter.
To allow for good cleaning, high density cutters should be
smaller.

70. BHA DESIGN
BIT SELECTION
PDC bits
Fluid Circulation Pattern
The fluid circulation across the bit face must be able to:
!
remove cuttings
!
cool the bit
PDC bits commonly use more than three jets. Many bits
have a deicatedjet for each blade. More jets aid bit cleaning
and reducethepressure per jet, thus reducing the chances
of matrix erosion.

71. BHA DESIGN
BIT SELECTION
Diamond bits
Characteristics:
Diamond bits have been used for years, but methods have
changed. !
cutting action is achieved by scraping the rock face
!
diamonds are set in a pattern, bonded to a matrix in a
steel body
!
effective fluid circulation is critical to a sccessfuldiamond
bit run prevents overheating and bit balling

72. BHA DESIGN
BIT SELECTION
Diamond bits
Courtesy of
Hughes Christensen
Typical Diamond bit.

73. BHA DESIGN
BIT SELECTION
Diamond bits
Pros & Cons
"
hardness of diamond makes it an obvious choice for a bit
"
diamonds are more sensitive to shock and vibration
"
diamonds bits cost ten times more than rock bits
"
no guarantee of improved ROP
"
become cost effective where long rotating hours are required
"
have no moving parts to fall off in the hole
"
matrix pattern design makes diamond bits appear self ‐sharpening
"
unused diamonds can be extracted giving bit a salvage value

74. BHA DESIGN
COST PER FOOT
Bit Performance
Measurement criteria
:
!
Footage drilled
!
ROP
!
Cost to run
capital cost of bit plus operating rig cost per foot.
Therefore, the optimum time to pull a bit is when the cost
per foot is at its lowest point, after which time the cost goes
up.

75. BHA DESIGN
COST PER FOOT
Bit Performance
Equation:
((Total Run Time‐Time not drilling ‐tripping/circulating)
x Rig Rate) + (On‐bottom drilling hours + Circulating
hours while drilling) x (MWD + Motor or RSS Rate))
+ Cost of Bit
Note that some operators include tripping time as a cost.

76. BHA DESIGN
BHA LIMITATIONS
Tools
LWD tools have sliding and rotating DL limitations.
Larger size BHA'srequire more crossovers due to the
need for stronger or special connections.
Some tools have RPM limitations that might be a hindrance to
ROP or hole cleaning.
The client will be reticent to run costly tools in problematic
holes.
Influences Well Design
Crossovers are vulnerable to twist ‐off and fatigue failure.

77. BHA DESIGN
BHA LIMITATIONS
Tools ‐ctd
Some tools have flow restrictions that may severely affect
the fluids control and hole cleaning.
High bottom hole temperatures will limit which BHA items
can be run.
Motors and LWD tools can be modified for HT wells.