Root Cause Analysis_selective pages

Root Cause Analysis Approach to Bit Failures during Under-Balanced Drilling

operations in -37

Carried out by Eng. Fawaz A. M. Saleh, Lead Project and Interface Engineer

CONFIDENTIAL
Summary of Bits Performance

The 6” hole section took nineteen bit runs to reach the final TD depth of 3151m MD. Total
hours of 263.5 on bottom were needed to drill or mill the complete section length from
2565m MD to 3151m MD.

At the start of this section, drilling performance was good. Only 3 bits were used to drill
from 2565m MD to 2777 m MD. Afterwards, there were 5 bits were used to drill from 2777m
MD to 2966m MD. While drilling ahead from 2966m MD, bit life dropped drastically and took
11 bits to drill just 185m till 3151m MD where TD was called. All sorts of bits including NOV
FuseTek were used. The following figure shows the performance of each bit used:

Figure 1: -37 Bit Records and Performance

Root Cause Analysis Approach to -37 Bit Failures Page 1

1. Introduction

The following reasons have attracted my attention to investigate these bits overall poor

performance, consecutive failure incidents and premature bit wear phenomenon, which

took place during UBD implementation in -37:

1. Spent 50 days to drill 494m by 19 bits

2. No similar bit failures were experienced in field with likelihood of occurrence

and magnitude of consequence
CONFIDENTIAL
3. Symptoms of bit failures were consequently present in all bits used in the UBD mode,

except the first bit which is considered as a model, in order to understand what went

wrong with the other bits

4. As a result of these bit failures, further UB drilling activities were suspended and an

early well TD (at 3151mMD) was called , instead of preplanned well TD at 3,700mMD
5. High Cost associated with these drill bit failures , estimated *,***KUSD

6. Such bit failures jeopardize the success of UBD campaign and deviate from its preset

objectives

7. Recurrence of identical bit failures in -41a

2. Data Collection, Observations and Analysis

In order to investigate these bit failure incidents, the relevant data on each of the bit failures
were collected and analyzed, through which sequence of failure events were tracked and
bits performance graphs were plotted, sources of these data are :

1. Daily Drilling Reports
2. Daily Mud Logging Reports
3. Daily Directional Drilling Reports
4. Daily Mud Reports
5. Bit Records
6. UBD End of Well Report
7. Mud Log

3. Observations

Observation is intended to locate any abnormal indication, failure sign and cause in reports
during the UBD implementation, which are outlined as below:

Root Cause Analysis Approach to -37 Bit Failures Page 2

4. Analysis

In order to get a good understanding of what wrongly occurred and what caused these
bit failures of poor performance , premature worn out and severe damage, we have to
get a knowledge of UBD drilling environment (Aerated fluids) related cause i.e.,
Corrosion as the root cause, its contributing factors (Agents) , factors affecting corrosion
rates and failure modes caused by corrosion:

4.1 Analysis of Basement Characteristics, UBD Environment and Drilling Fluid
Prosperities

CONFIDENTIAL
4.1.1 Basement Characteristics
Characteristics of the Basement: The fractured Proterozoic Basement is an extremely
heterogeneous reservoir consisting of granite, quartzite/gneiss, amphibolites, epidotic-
quartz breccias and volcanic lithologies, therefore, Basement is categorized as Hard and
Abrasive formation. While drilling the basement, it was observed a high content of quartz
and chlorites.

Chloride is a major anion in the geothermal fluid matrix that is derived naturally from the
dissolution of rocks with hot fluids. Anions such as chlorides increase the electrical
conductivity of aerated drilling fluids. Since most corrosion processes involve
electrochemical reactions, the increased conductivity gives enhanced corrosion rates.
When stresses are present in a chloride-containing environment, steels are prone to
chloride-induced stress corrosion cracking (SCC).

Abrasive formation’s effect can severely causes bit wear when it’s associated with
abrasive drilling fluids and high impact of thrust force (WOB)

4.1.2 UBD Environment

The basement reservoir was drilled using an underbalanced annulus environment while

drilling. The 6" hole section was drilled using a 2-phase drilling fluid to reduce the

bottom hole circulating

pressure (BHCP) while

drilling through the

Shuqra, Kohlan and

Basement formations. The

technique used to reduce

the BHCP was drill pipe

gas injection, achieved by

injecting membrane

generated nitrogen as the

gas phase, to a water

based mud.

The mud used was (Bio-

Root Cause Analysis Approach to -37 Bit Failures Page 5

Factors Affecting Corrosion Rates

pH
It is a scale measuring hydrogen ion concentration in solution. Since the pH scale is
logarithmic, each pH increment of one unit represents a ten-fold change in hydrogen ion
concentration. Values of pH towards 1.0 are increasingly acidic and pH values toward
14.0 are increasingly alkaline. In the presence of dissolved oxygen, the corrosion rate of
steel in water is relatively constant between pH 4.5 and 9.5.

Temperature

Corrosion is an electrochemical reaction that causes the alteration and degradation of

material by its environment. In the aerated drilling environment, the material altered or

degraded is usually the steel components of drill pipes or tubulars and to some extent
CONFIDENTIAL
the casing. The principal corrosive agents affecting drill stem materials in aerated fluids

are oxygen from air and gases from geothermal fluids, chlorides. The degree of elevation

of the temperature and pressure in mixed geothermal/aerated fluids also play a

significant role in the degree of corrosion. Corrosion rates increase with increasing

temperature.

Increase in pH, Increase Corrosion

Increase in Temperature, Increase Corrosion Rate affecting Rate affecting bits from no. 13 to 19

bits from no.3 to 19

High Stresses
Highly stressed areas may corrode faster than areas of lower stress. During the slide
mode , bit undergoes higher stresses and high bending moments due to combined
effects of excessive thrust impacts (WOB) and hole abrasiveness on the front (insert bits
and cones ) and side (gage and shirttail ) surfaces . To be discussed thoroughly under
failure mode no.2 Chloride-induced stress corrosion cracking (SCC)

Root Cause Analysis Approach to -37 Bit Failures Page 7

4.1.3 Drilling Fluid Prosperities

The mud density was maintained constant at 8.5 ppg during the entire drilling operation

from 2565m MD to 3151m MD, where it was decided to call TD. The Plastic Viscosity was

in a range between 5-10 cP, while the Yield Point between 15-21 lb/100 ft2 with a

maximum solids content of 2%. Regarding the pH, it was maintained at the outset below

9.5 and then increased (10.5) in order to mitigate corrosion concerns as suggested by

Weatherford. The above graph summarizes the properties for the mud used in -

37 UBD operations.

Plastic Viscosity (PV)

A high PV of the

drilling fluid results in Decreases in Plastic Increase in Plastic
Viscosity, Increase Viscosity, Decreases
ROP ROP and cause tensile
CONFIDENTIALincreased equivalent

circulating density

(ECD) caused by

increased pump

pressures needed for

pumping such a fluid.

A fluid with a high PV

also has a detrimental

effect on the rate of

penetration (ROP), as

an increase in number

of solids in the fluid

slows down the

penetration rate and

might cause tensile

stress cracks along

the highest horizontal

principal stress direction.

Yield Point (YP)
Yield Point (YP) is
resistance of initial
flow of fluid or the
stress required in
order to move
the fluid .The YP
indicates the ability of
the drilling mud to
carry cuttings to
surface. As YP
increases, there is
usually an increase in
ECD.

Root Cause Analysis Approach to 37 Bit Failures Page 11

4.2 Analysis of PowerPak motor’s Failure Modes

• Rotor and Stator Failure Mode (Motor Stalling & Chunking)
Motor Stalling is the condition in which the torque required to turn the bit is greater than
the motor is capable of producing.
When a motor stalls, the rotor is pushed to one side of the stator and the mud is pumped
across the seal face of the opposite side of the rotor. The lobe profile of the stator must
deform for the fluid to pass across the seal face. This causes very high fluid velocity across
the deformed top of the stator lobes and leads to chunking. Chunking caused by motor
stalling when sliding has a straight path along one wall of the stator tube. Chunking caused
by motor stalling with surface rotation can be uniform or follow a spiral path.
Running for excessive hours in an abrasive drilling fluid (in presence of chlorides), chlorides
can severely corrode the Chrome plating on rotor.
In addition to the damage caused to rotor by corrosion, the rough edges left on rotor lobes
damage the stator by cutting the top off the elastomer in the stator lobe profile. These
cuts reduce the effectiveness of the rotor/stator seal and cause motor to stall (chunking
the stator) at low differential pressure.
CONFIDENTIAL
• Mud-lubricated Bearing Failure Mode
When high weights are applied on a motor, they accelerate the wear of the balls and races
of the axial bearings.
The races in the mud-lubricated axial bearings are case-hardened. Their wear rate
is not linear, because the amount of wear increases once they have worn past the
hardened area. Just because a motor has only minor wear after a long run does
not mean that the bearings are capable of repeating the same run time.
The WOB limits (the rated maximum weights on bit) for A475M motor is 25 klbs.
Running a motor at or near the rated maximum WOB decreases the life of the
axial bearing, a limit of 80% of the rated maximum WOB is recommended for the
long runs.
Bit pressure drop is the amount of force acting to push mud through the radial
and axial bearings. The fluid flow passing through the bearings must be at a high
enough rate to cool and lubricate them, but too much flow will wash out the
bearings. The recommended flow rates range is 100-250 gpm for A475M motor.
The minimum bit pressure drop is 250 psi for standard bearings.
Highly abrasive muds with solids of more than 2% can cause excessive wear on
the entire bearings pack (radial and axial).
Bearing failure can be determined by measuring the distance between the lower part of the bearing section and the
top of the bit sub. The difference between the two distances is the amount of axial bearing clearance.
The maximum allowable clearance for both the pre-installation check (before the bit is installed) and after running
for A475XP is 5mm.

Indications of Motor Failure: Secondary Observation
Unchanged flow rate
Observation Torque increases
ROP drops

Pressure surges

Root Cause Analysis Approach to 37 Bit Failures Page 13

4.4 Dull Grading Analysis of Bit Failures (Severe Characteristics)

In addition to previous analysis of Analysis of Drilling Fluids Aeration in UBD

Environment in which corrosion was the dominant root cause of all bit failures

experienced in -37. This part of analysis focused on severe dull grading

characteristics of these bit failures, aims at finding other potential causes which might

attribute with corrosion and cause severe wear and damage to these bits.

The below table outlines the potential causes of severe dull grading characteristics:

Dull Grading Potential Causes
Characteristic
FormationCONFIDENTIAL
Excessive
WOB
Junk
Damage
Drilling
Environme
nt
Inadequate
Hydraulics
Excessive
Hydraulics
Vibrations
Cored (CR)

Lost Cones (LC)

Cracked Cones
(CC)
Erosion (ER)

Analysis of bits dull grading is based on Smith Roller Cone Dull Grading Manual

By analyzing the dull grading characteristics of these damaged bits, Potential causes can be
classified as follows:

• Corrosion
• Drilling Motor Failure
• Junk Damage
• Improper drilling parameters
• Improper drilling practices

4.5 Drilling Log Analysis of Each Bit Failure

By applying the effects of these potential causes on all eighteen bits, we can get a good
understanding which of these potential causes is root cause(s) of specific bit failure or bit
failures.

Root Cause Analysis Approach to -37 Bit Failures Page 15

Failure Mode:

Chloride-induced stress corrosion Small Rubber pieces as
cracking (SCC) indication of motor weak

Cause of Bit Failure: condition (elastomer
Root Cause: Corrosion swelling)

Secondary Cause: Improper Drilling Parameters

(Reaming slide interval with High RPM)

Bit Failure Analysis: After drilling for one hour and a half
in slide mode (mechanical stress) on hard formation (high chlorides contents) in abrasive fluids
causing stress corrosion cracking wear on the bits cones due to overheating and the second
contributing factor was reaming the slide interval with high RPM which caused dislodgment of
the cones .
Cause of Motor Failure: Running for excessive hours in Abrasive drilling fluids (in presence of
chlorides). Chlorides can severely corrode the Chrome plating on rotor.
In addition to the damage caused to rotor by corrosion, the rough edges left on rotor lobes
damage the stator by cutting the top off the elastomer in the stator lobe profile. These cuts
reduce the effectiveness of the rotor/stator seal and cause motor to stall (chunking the stator) at
low differential pressure.
CONFIDENTIAL
3500 Indication of motor Indication of Bearing/seal 25.00
3000 stalling with pressure failure with erratic pressure Irregular pressure , Irregular
2500 surge and drop in ROP and steady torque increase torque & ROP drop are
indications of a bit running
on20a.j0u0nk (its lost cone)

2000 15.00 SPP
Flow

1500 Slide Mode 10.00 ROP
1000 5.00 WOB
Torque
500

0 0.00
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91

Bit Record:

Bit Depth Depth Bit Serial# Bit Motor Drilling Footage ROP Motor Bit Dull Grading
# In (m) Out Type RA5250 Conditio serial# Hours Condition
(m) n 8-8-LC-G-8-16-CC-
2 2657 XR50Y N7363 14.8 44.8 3.04 New PP
2702 New

Root Cause Analysis Approach to -37 Bit Failures Page 16

Failure Mode: Centermost cutting elements
Chloride-induced stress corrosion are lost indicators of:
cracking (SCC) Junk Damage

Cause of Bit Failure:
Root Cause: Junk Damage
Secondary Cause: Improper Drilling Practice (inefficient
milling operation

Bit Failure Analysis: After rotary drilling for six hours by rotary
assembly with high WOB and RPM on junk (cone left in hole from previous bit without
performing mill operations) causing stress corrosion cracking wear on the bits cones due to
overheating and Fluid Hammer’s mechanism which resulted in fluctuations in pressure & torque
and drop in ROP
CONFIDENTIAL
Indication of Indication of coring bit after bit Reason for POOH was
running on junk: cone broken and left in hole: drop in ROP despite of
Erratic torque and Erratic torque and pressure applying constant WOB
pressure

3200 30.00

3100 25.00
3000 20.00
2900

2800 15.00

2700 10.00
2600 5.00
2500

2400 0.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Spp Torque ROP WOB

Bit Record:

Depth Depth Bit Serial# Bit Motor Drilling Footage ROP Fluid Bit Dull Grading
Bit In (m) Out Type RA4788 Condition serial# Hours 1.76 Hammer 8-7-BC-A-N-1-CR-PR
# (m) Condition
XR50Y New 3739 4.5 8
9 2966 2974 Used 2nd run

Root Cause Analysis Approach to 37 Bit Failures Page 23

Failure Mode: centermost cutting elements
Chloride-induced stress corrosion are lost indicators of:
cracking (SCC) Junk Damage

Cause of Bit Failure:
Root Cause: Junk Damage
Secondary Cause: Improper Drilling Practice (no milling
operation)

Bit Failure Analysis: After rotary drilling for six hours by
rotary assembly with high WOB and RPM on junk (cone left
in hole from previous bit without performing mill operations) causing stress corrosion cracking
wear on the bits cones due to overheating which resulted in fluctuations in pressure & torque
and drop in ROP
CONFIDENTIAL
Indication of Applying excessive Indication of coring bit after bit Reason for POOH was
running on junk: WOB and High RPM cone broken and left in hole: drop in ROP despite of
Erratic torque and Erratic torque and pressure applying excessive WOB
pressure

3700 200.00

3600 180.00 Psi
3500 160.00 Torque
3400 140.00 ROP
120.00 WOB
RPM
3300 100.00

3200 80.00
3100 60.00
3000 40.00
20.00

2900 0.00

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33

Bit Record: Bit Serial# Bit BHA Type Drilling Footage ROP Bit Dull Grading
Type E195550 Conditio Hours 17 2.83 5-0-CR-C-X-I-LN-PR
Bit Depth Depth n Fluid
# In (m) Out FUSE Hammer 6
TEK New
(m) FT711

14 3030 3047

Root Cause Analysis Approach to -37 Bit Failures Page 28

Bit Failure Mode Casual Factors Potential Causes
Run# (Effect of Failure) (Root Causes)

Replace damaged bit

9 Stress Corrosion LC-Lost Cone Junk (Lost Cone) left in Hole
Cracking (SCC) CR-Cored Bit Improper Drilling Practice (Ineffective
Trip Out Milling Operation from previous run)
10 Stress Corrosion Run junk milling operation
Cracking (SCC) Replace damaged bit Junk (Lost Cone) left in Hole
Improper Drilling Practice (No Milling
11 Erosive Wear CR-Cored Bit Operation)
Short Bit Life
12 Stress Corrosion CONFIDENTIAL Trip Out Junk (Lost Cone) left in Hole from previous
Cracking (SCC) Replace damaged bit run
Improper Drilling Practice(no mill operation
13 Stress Corrosion Erratic Pressure was undertaken)
Cracking (SCC) Erratic Torque
ROP Drop Improper Drilling Parameters (High WOB)
14 Stress Corrosion Short Bit Life Corrosion
Cracking (SCC) Trip Out
Replace damaged bit Junk (Lost Cone) left in Hole
15 Stress Corrosion Improper Drilling Practice (No Milling
Cracking (SCC) CR-Cored Bit Operation)
Short Bit Life
16 Erosive Wear Trip Out Junk (Lost Cone) left in Hole
Replace damaged bit Improper Drilling Practice (No Milling
17 Erosive Wear Operation)
CR-Cored Bit
18 Erosive Wear Short Bit Life Improper Drilling Parameters (High WOB)
Trip Out Corrosion
19 Stress Corrosion Replace damaged bit
Cracking (SCC) Junk (Lost Cone) left in Hole
CR-Cored Bit Improper Drilling Practice (No Milling
Short Bit Life Operation)
Trip Out
Run junk milling operation Improper Drilling Parameters (High WOB,
Replace damaged bit High RPM)
Corrosion
CR-Cored Bit
Short Bit Life Improper Drilling Parameters (High WOB,
Trip Out High RPM)
Replace damaged bit Corrosion

Pressure Drop
Erratic Torque
Short Bit Life
Trip Out
Replace damaged bit

Pressure Increase
Erratic Torque
Short Bit Life
Trip Out
Replace damaged bit

Pressure Drop
Erratic Torque
Trip Out

LC-Lost Cone Corrosion
Short Bit Life Improper Drilling Practice (high RPM after
Trip Out Slide interval)
Run junk milling operation

Root Cause Analysis Approach to -37 Bit Failures Page 35

6. Recommended Actions

• Drilling Fluid properties:

1. Keep Viscosity range from 36 to 40 s/qt
2. Keep Plastic Viscosity range between 8-9 cP
3. Keep Yield Point at 18-20 lbf/100f2
4. Keep Gel 10s at 5 lbf/100f2
5. Keep Gel 10m at 7 lbf/100f2
6. Keep pH at 10.5 (maximum)

• Drilling Parameters:

7. Apply 15 klbs WOB on Roller Cone bit and less than 10 klbs WOB on PDC bit, taking into
consideration adjusting the Weight Indicator Gauge (Martin Decker) to zero so as to reflect the
actual WOB.

8. Apply RPM 50-60 revs
9. Keep Flow rate at 200 gpm

• Drilling Practices:

10. Monitor motor performance and to be laid out on first indication of motor stalling by observing
pressure surge and drop in ROP

11. When POOH to change bit: Check axial bearing clearance of the motor (maximum allowable
Axial Bearing clearance for A475 is 5mm)

12. Minimize sliding mode by stabilization technique : BHA tendency to be agreed with Directional

Driller

• To build the angle: Motor with Slick Sleeve and Full Gauge string stabilizer
• To hold the angle: FG Motor sleeve and FG St Stabilizer
• To drop the angle: FG motor sleeve and UG St Stabilizer

13. In case of need to slide to correct the Azimuth: Apply WOB as low as possible
14. To ream/ backream the slide interval: apply 20-30 revs RPM
CONFIDENTIAL

Root Cause Analysis Approach to -37 Bit Failures Page 36