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ii
TRANSIENT HYDRAULICS AND MULTIPHASE KICK TOLERANCE STUDY
TO IMPROVE DESIGN OF NARROW MARGIN WELL
GOO JIA JUN
A project report submitted in partial fulfilment of the
requirements for the award of the degree of
Master of Petroleum Engineering
Faculty of Chemical and Energy Engineering
Universiti Teknologi Malaysia
JANUARY 2018
iv
To my beloved mother and father
v
ACKNOWLEDGEMENT
In preparing this thesis, I was in contact with many people, researchers,
academicians, and practitioners. They have contributed towards my understanding and
thoughts. I wish to express my sincere appreciation to my main thesis supervisor,
Associate Professor Issham Ismail, for his endless encouragement, guidance, critics
and friendship.
I am also very thankful to Schlumberger Wellog, Vestigo Petroleum,
PETRONAS Carigali and Shell Malaysia for providing the data required and software
in my research study. Librarians at UTM also deserve special thanks for their
assistance in supplying the relevant literatures.
My fellow postgraduate classmates should also be recognized for their support.
My sincere appreciation also extends to all my colleagues and others who have
provided assistance at various occasions. Their views and tips are useful indeed.
Unfortunately, it is not possible to list all of them in this limited space. I am grateful
to all my family members.
vi
ABSTRACT
Hydraulic and well control studies are the essential parts of well construction
planning, especially for drilling of complex and challenging wells with narrow drilling
margins. However, the complete applications of dynamic hydraulic analysis and
multiphase kick tolerance studies in well design are scanty, which result in ineffective
mud pressure management and extra cost spent on unnecessary casing strings, due to
excessive emphasis on previous practices (steady-state model) with liberal sprinkling
of safety factors. This research project was set out clearly to improve the well design
for narrow margin field, in terms of hydraulics and well control. A deductive
quantitative method constitutes major part of the research methodology, in which
simulation of real case studies and interpretation were conducted. The dynamic
hydraulics simulated equivalent circulating density (ECD) was compared with steady-
state results in terms of accuracy and extensiveness in providing a good well design.
In addition, the single bubble kick tolerance results which are commonly used by the
industry in spreadsheet format were compared with the multiphase model results.
Sensitivity studies were performed to understand the effect of each of the operational
or well design parameters towards primary and secondary well control. As compared
to steady-state hydraulics, transient model covers important parameters like pressure
and temperature dependent fluid properties, thermophysical properties, detailed
geometry description and operational effects, thus it is more representative to the
operational ECD. Meanwhile, multiphase kick model is proven to be more effective
for the evaluation of kick tolerance as it is able to provide the information of pressure
development during a well control operation, from initial influx and shut-in until influx
is circulated out of the well at the surface. This includes all phase transitions including
dissolving of a gas kick in oil based mud and breakout of free gas when the gas
contaminated mud reaches the bubble point at shallower depth in the well. The flow
model is much more accurate and reliable than the over-conservative traditional single
bubble theory.
vii
ABSTRAK
Kajian kawalan telaga dan hidraulik ialah komponen yang penting dalam
perancangan untuk membina telaga terutama bagi penggerudian yang rumit dan mencabar
dengan margin tekanan yang sempit. Namun begitu, tahap kesedaran yang rendah tentang
kepentingan penganalisisan hidraulik dinamik dan toleransi tendangan multifasa dalam
kerja mereka bentuk telaga telah menjejaskan keberkesanan pengurusan tekanan lumpur.
Akibatnya, kos tambahan diperlukan untuk pemasangan rentetan selongsong berikutan
penekanan terhadap model keadaan mantap yang sentiasa memberikan margin
keselamatan secara berlebihan. Kajian ini bertujuan untuk memperbaik reka bentuk telaga
yang menghadapi margin tekanan yang sempit, dari segi kawalan telaga dan hidraulik.
Kajian secara kuantitatif and deduktif ini melibatkan penyelakuan kes telaga sebenar dan
pentafsirannya. Ketumpatan peredaran setara (ECD) yang diperoleh daripada penyelakuan
hidraulik dinamik telah dibandingkan ketepatan dan rangkumannya dengan keputusan
daripada model keadaan mantap bagi menghasilkan reka bentuk telaga yang baik. Di
samping itu, keputusan toleransi tendangan gelembung tunggal yang menjadi amalan
industri turut dibandingkan dengan hasil daripada model multifasa. Kajian kepekaan
dilaksanakan untuk memahami kesan setiap parameter operasi dan reka bentuk telaga
terhadap kawalan telaga utama dan sekunder. Model hidraulik dinamik yang mencakupi
parameter penting, misalnya sifat bendalir yang peka terhadap suhu dan tekanan, sifat
termo-fizikal, huraian geometrik telaga, dan kesan operasi, didapati mampu memberikan
keputusan operasi ECD yang lebih praktikal berbanding model keadaan mantap. Selain
itu, model tendangan multifasa adalah lebih jitu dalam penilaian toleransi tendangan
kerana model terbabit mampu memberi maklumat tentang perubahan tekanan ketika
operasi kawalan tekanan, bermula daripada berlakunya tendangan gas dan kurungan,
hingga ke bendalir tendangan dikeluarkan di permukaan. Maklumat turut mencakupi
perubahan semua fasa termasuk keterlarutan gas tendangan di dalam lumpur dasar minyak
dan pelepasan gas bebas bila lumpur yang dicemari gas mencapai takat gelembungnya
pada kedalaman yang dangkal. Model aliran ini ternyata lebih tepat dan boleh dipercayai
berbanding teori gelembung tunggal yang konservatif.
viii
TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGCMENTS
ABSTRACT vi
ABSTRAK vii
TABLE OF CONTENTS viii
LIST OF TABLES xi
LIST OF FIGURES xiii
LIST OF EQUATIONS xvii
LIST OF ABBREVIATIONS xviii
LIST OF APPENDICES xix
1 INTRODUCTION 1
1.1 Background 1
1.2 Problem Statement 6
1.3 Hypothesis 9
1.4 Objectives 9
1.5 Research Scope 10
1.6 Significance of Study 12
1.7 Chapter Summary 12
2 LITERATURE REVIEW
2.1 Well Planning Cycle 13
2.2 Wellbore Stability Analysis 15
2.3 Wellbore Instability in Narrow Margin Drilling 20
2.4 Steady State Hydraulics and Dynamic Hydraulics 21
ix
2.5 Parameters Effecting Variation of ECD 24
2.5.1 Mud Pressure, Volume, Temperature (PVT)
Dependencies
24
2.5.2 Cutting Loadings 27
2.5.3 Mud Rheology 28
2.5.3.1 Bingham Plastic Model 29
2.5.3.2 Power Law Model (Ostwald–de
Waele Relationship)
29
2.5.3.3 Robertson-Stiff / Herschel Buckley
Model
30
2.5.4 Well Thermodynamics 30
2.5.5 Annular Flow Path 32
2.6 Single Bubble Theory 33
2.7 Application Software 37
2.7.1 WellPlan Hydraulics 38
2.7.2 Drillbench Dynamic Hydraulics 38
2.7.3 Sysdrill Hydraulics 39
2.7.4 WellPlan Well Control 40
2.7.5 Drillbench Well Control 40
2.7.6 Sysdrill Well Control 41
2.8 Chapter Summary 42
3 RESEARCH METHODOLOGY
3.1 Introduction 43
3.2 Research Design 43
3.2.1 Research Approach 44
3.2.2 Research Strategy 45
3.3 Detailed Research Flow Chart 46
3.4 Methodology 47
3.4.1 Data Request, Gathering and Validation 47
3.4.2 Hydraulics Simulation Model Building and
Sensitivity Study
50
x
Operators Well Control Standards and Single
Bubble Kick Tolerance Spreadsheet Review
52
Well Control Simulation Model Building and
Sensitivity Study
53
3.5 Well Data Input 56
3.5.1 Well 1 Data (Pressure Ramp Scenario) 56
3.5.2 Well 2 Data (Deep water, high temperature
wild cat drilling scenario)
60
3.5 Chapter Summary 64
4 RESULTS AND DISCUSSIONS
4.1 Comparison between Steady-state and Dynamic
Hydraulics Model
65
4.2 ECD Variations 66
4.2.1 Mud Pumping Rate 66
4.2.2 Wellbore Temperature 67
Surface Torque 68
Mud Weight 69
Rate of Penetration 70
Annular Flow Area 71
4.3 Dynamic Hydraulic Results 72
4.4 Hydraulic Results vs. Real Time Operational Data 75
4.5 Well Control Simulation Results 78
4.6 Model Applications, Benefits and Limitation 85
4.7 Chapter Summary 86
5 CONCLUSIONS AND RECOMMENDATIONS 87
REFERENCES 92
APPENDICES 99
GANTT CHART 101
xi
LIST OF TABLES
TABLE NO. TITLE PAGE
1.1 Data input and parameters for research 11
2.1 Common well design modules 14
2.2 Mud weight window limits 16
2.3 The wells drilled in the US Gulf of Mexico deepwater and
shallow water, wellbore instability accounted for 34% to
56% of nonproductive time.
20
2.4 List of oil PVT correlation established between 1947 and
1998
24
2.5 List of water PVT correlation established between 1944
and 1987
25
2.6 Affecting parameters on temperature distribution profile of
a well
31
2.7 Common drilling hydraulics software 37
2.8 Common well control software 37
3.1 Data requirement 47
3.2 Data Quality Assessment Checklist (DQA) elements 49
3.3 Drilling parameters for sensitivity study 51
3.4 Example of kick tolerance requirement for different hole
section
52
3.5 Example of kick tolerance spreadsheet assumption and
formulas
53
3.6 Well control parameters for sensitivity study 56
3.7 Geothermal gradient 56
3.8 Well trajectory 56
3.9 Wellbore stability 57
xii
3.10 Casing profile 58
3.11 12-1/4in section drillstring 58
3.12 Drilling mud rheology 59
3.13 Geothermal gradient 60
3.14 Well trajectory 60
3.15 Wellbore stability 61
3.16 Casing profile 62
3.17 12-1/4in section drillstring 63
3.18 Drilling mud rheology 63
4.1 Dynamic and steady state hydraulic model comparison 66
xiii
LIST OF FIGURES
FIGURE NO. TITLE PAGE
1.1 Normal drilling margin (a) and narrow drilling margin
<1ppg (b)
1
1.2 Increasing water depth and under-compaction narrow the
drilling margin
2
1.3 Equivalent static density (a) and equivalent circulating
density (b)
3
1.4 Well design plot 4
1.5 Steady-state hydraulics results 7
1.6 The single bubble gas kick expands when circulated out of
well and create pressure upsurge
8
1.7 Research scope 10
2.1 Well planning workflow, starting from pre-drill well
design (blue) and while-drilling drilling monitoring (green)
13
2.2 ESD and ECD must be within the weight window (green)
to ensure wellbore stability is achieve. Four limits: pore
pressure, breakout, mud loss and breakdown are
established as operating envelops.
15
2.3 Compressional failure 16
2.4 Schmidt-pole graph which indicates the azimuth of 90° and
270° has minimum breakout pressure at depth 6353 ft.
(recommended direction)
17
2.5 Leak off test pressure profile 18
2.6 Schmidt-pole graph which indicates the azimuth of 90° and
270° has maximum breakout pressure at depth 6353 ft.
(recommended direction)
18
xiv
2.7 High mud weight causes tensile failure 19
2.8 (a) Example of mud weight window produced by
geomechanics expert before well design. (b) Mud weight
window vs azimuth chart is shown to investigate the best
azimuth for well directional drilling by courtesy of
Schlumberger.
19
2.9 Governing equations for hydraulic analysis 22
2.10 Steady-state hydraulic computation workflow 22
2.11 Dynamic hydraulic computation workflow 23
2.12 Parameters effecting variation of ECD 24
2.13 PVT relationship for a drilling mud by courtesy of MI
Swaco
25
2.14 Drilling cuttings increases ECD 27
2.15 Drilling fluid rheology models 28
2.16 (a) Shearing of liquid due to resistive force due to viscosity
(b) Shear rate
28
2.17 Heat transfer schematics across well center to/from
formation
31
2.18 Increase of ECD due to (a) rough flow surface (b)
drillstring tool joints
32
2.19 Example hydraulic analysis results by courtesy of
Schlumberger
33
2.20 Single bubble and multiphase flow regimes 36
2.21 Screenshot of WellPlan steady-state ECD results 38
2.22 Screenshot of Drillbench ECD results 39
2.23 Screenshot of Sysdrill ECD results 39
2.24 Screenshot of WellPlan Well Control simulation 40
2.25 Screenshot of Drillbench Well Control simulation 41
2.26 Screenshot of Sysdrill Well Control simulation 41
3.1 Deductive Research Approach 44
3.2 The case study approach 45
3.3 Research flow chart 46
3.4 Drilling operational hydraulics modeling 50
xv
3.5 Drillbench Dynamic Hydraulics module version 2016.1.1 50
3.6 WellPlan version R5000.1.14 51
3.7 Interactive mode input of Drilbench Dynamic Hydraulics 52
3.8 Example of single bubble kick tolerance spreadsheet 52
3.9 Drillbench Dynamic Well Control module version
2016.1.1
54
3.10 Surface equipment delay input in Drillbench Dynamic
Well Control
54
3.11 Drillbench Dynamic Well Control operational control
parameters
54
3.12 Well Control operation sequence modeling 55
3.13 (a) Well schematics (b) well geothermal profile 58
3.14 (a) Well schematics (b) well geothermal profile 62
4.1 Steady-state and dynamic simulation scope 65
4.2 ECD vs. pump rate 66
4.3 ECD vs. mud inlet temperature 67
4.4 ECD vs. surface torque or RPM 68
4.5 ECD vs. mud weight used 69
4.6 ECD vs. ROP 70
4.7 ECD vs. drillpipe diameter 71
4.8 The trend of ECD at bit while drilling 72
4.9 The trend of bottomhole temperature while drilling 73
4.10 The trend of ECD at shoe while drilling 74
4.11 Steady state vs dynamic hydraulics results 75
4.12 Actual standpipe pressure (blue dotted line) vs dynamic
hydraulics results (black line)
76
4.13 Actual downhole temperature while drilling (blue dotted
line) vs dynamic hydraulics results (black line)
76
4.14 Actual ECD while drilling (blue dotted line) vs dynamic
hydraulics results (black line)
77
4.15 Single gas bubble and multiphase influx model 79
4.16 While single bubble kick tolerance modeling by using
spreadsheet neglect the dynamic effect and will provide
80
xvi
over-conservative results. It uses simplified and idealistic
model, Boyle’s Law to calculate the gas expansion which
is the model will never happen and make the drillable
depth to be shallower and more casings are needed.
4.17 Choke pressure profile generated by transient model, the
backpressure by choke is adjusted to maintain constant
bottomhole pressure (overbalance by 100psi)
80
4.18 Downhole pressure in time manner, the well pressure
overbalances pore pressure (denoted with dotted line) by
100psi during killing operation
81
4.19 Multiphase kick tolerance will provide realistic results and
is aimed to maximize the safe drilling and reduced the
number of casing
81
4.20 Surface pit gain profile. 50 barrels (required kick tolerance
value) of influx observed at surface before shut in. Pit gain
increase when free gas expands and reaches surface. Pit
gain back to zero when well killing is completed.
82
4.21 Gas flow rate out during killing operation. The top of gas
first reaches annular surface at 233rd minute. Maximum
gas rate out to be checked with degasser capability to
ensure it can divert the gas safely out of the well.
82
4.22 Standpipe pressure of around 350psi during shut in
signifies that the downhole pressure is underbalanced
350psi. Pump pressure of 350psi + 100 overbalance +
frictional mud pressure lost is needed during for well
killing
83
4.23 During killing operation, most of the influx gas (methane
in this case) is dissolved in the oil based mud. It is
circulated up until the top of dissolved gas reaches 400m
MD, the free gas starts to pop out at certain rate and
continues to travel out of borehole. Pit gain will increase
significantly when free gas reaches surface.
84
5.1 Flow simulation for blowout control from relief well 90
xvii
LIST OF EQUATIONS
EQUATION NO. TITLE PAGE
2.1 Volume of cutting 27
2.2 Bingham plastic rheology model 29
2.3 Power Law rheology model 29
2.4 Robertson-Stiff rheology model 30
2.5 Pressure at bit depth 34
2.6 Pressure of mud column in the open hole 34
2.7 Wellbore pressure at the assumed casing setting depth 34
2.8 Height of the gas bubble 34
2.9 Kick tolerance volume 34
4.1 ECD calculation 69
xviii
LIST OF ABBREVIATIONS
AMSL - Average Main Sea Level
BHA - Bottom Hole Assembly
BHP - Bottom Hole Pressure
BHT - Bottom Hole Temperature
BOP - Blowout Preventer
DP - Drill Pipe
ECD
ESD
-
-
Equivalent Circulating Density
Equivalent Static Density
FBHP - Flowing Bottom Hole Pressure
gpm - Gallons per minute
GUI - Graphic User Interface
ID - Inner Diameter
IWCF - International Well Control Forum
KT - Kick Tolerance
MD - Measured Depth
OBM - Oil Based Mud
OD - Outer Diameter
PI - Productivity Index
ppg - Pounds per Gallon
RKB - Rotary Kelly Bushing
SDM - Synthetic Based Mud
TD - Total Depth
TFA - Total Flow Area
TOC - Top of Cement
TVD - Total Vertical Depth
WBM - Water Based Mud
xix
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Non-disclosure Terms 98
1
CHAPTER 1
INTRODUCTION
1.1 Background
Well construction activities are executed within a pressure window bounded
on the lower side by reservoir pressure (is also known as pore pressure) and on the
upper side by the fracture pressure of the formation in an openhole (Figure 1.1). As the
drilling continuously expands into harsher and high complexity terrains, the pressure
window can be very narrow as less than 1 ppg, requiring the operations to be performed
by walking the tight rope between these limits.
(a) (b)
Figure 1.1 Normal drilling margin (a) and narrow drilling margin <1ppg (b)
2
Thus, conventional drilling methods typically become strained when
attempting to drill wells with narrow operational windows (Li et al., 2012). For
instance, non-productive time to drilling operational problems, such as kicks, loss
circulation, and stuck pipe events are commonly mount and result in hefty cost
(Noniface and Marcus, 2014).
Several subsurface settings can contribute to narrow margin, such as increasing
of water depths and widely varying formation pressures (Figure 1.2). Particularly in
deepwater environment, the high water depth alters the subsurface pressure profiles by
decreasing the overburden stress on the formation, which eventually translates to a
reduced fracture gradient in the vicinity of the wellbore. Meanwhile the pore pressure
gradient is typically determined by hydrostatic head of overlying fluid, therefore it is
not affected by the water depth (Rocha et al. 2004).
Apart from that, the presence of abnormal pore pressure pockets (where the
pore pressure is greater than hydrostatic pressure) further reduces drilling margin.
These pockets are originated from tectonic movements, salt dome effects, under-
compaction, as well as chemical and thermal actions (Freire et al., 2010). The ensuing
challenges share a commonality relating to safe navigation between the drilling
margins.
Figure 1.2 Increasing water depth and under-compaction narrow the drilling margin
3
The hydrostatic pressure created by the drilling fluid column is commonly
expressed as equivalent static weight (ESD), by relating the pressure at a specified
depth to its corresponding fluid density. During circulation conditions, the mud creates
an equivalent circulating density (ECD) which is higher than ESD. Referring to IADC
Lexicon, ECD, in the unit of ppg, is defined as the summation of fluid’s hydrostatic
pressure, cuttings, and annular friction pressure loss divided by vertical depth and by
0.052 (API RP59, 2012). ESD and ECD (Figure 1.3) must be kept between pore
pressure and fracture pressure gradients throughout the drilling process. Once the
wellbore pressure falls below the pore pressure, an influx of wellbore fluids can lead
to a kick or even an underground blowout. Similarly, excessive hydrostatic pressures
must also be avoided, as it can initiate and propagate fractures, which will cause loss
of circulating fluid into the formation (Stave, 2014).
(a) (b)
Figure 1.3 Equivalent static density (a) and equivalent circulating density (b)
4
Figure 1.4 illustrates the drilling window of a hypothetical well, the pressure is
represented by the step profile created when a successively heavier mud is used
(vertical line) and the wellbore is drilled to a maximum depth in which mud pressure
approaches pore pressure (horizontal line) which a casing is set.
In addition to ECD and ESD computation, the well design has to take
consideration of kick effect in which every section must be able to withstand the well
killing pressure without exceeding the fracture pressure. The maximum volume of gas
kick in barrels, known as kick tolerance is usually set by operator as a well design
standard which measure the integrity of the openhole to successfully shut in during
kick event and circulate the kick out of the hole without breaking the weakest
anticipated formation straight in wellbore (usually at previous casing shoe) (Redmann,
1991).
Figure 1.4 Well design plot
5
For instance, the section of 8.5 in hole requires 25 barrels kick tolerance as per
well design standard of the operator. Based on the bottom-up casing seat design
method, by using 11.1 ppg mud, the previous casing shoe can be set at 5000 ft.,
however, due to kick tolerance compliance, additional casing is set at 8000 ft. before
drilling to total depth (TD) at 12000 ft..
Generally, the challenges of the narrow margin drilling can be divided into two
main categories:
1. Drilling mud pressure variation
The series of standard drilling operations comprise of start and stop pumping
intervals as drill pipe connections are made during tripping and drilling operations.
When circulation stops, static conditions apply in the wellbore, and bottom-hole
pressure (BHP) is determined solely by the mud column hydrostatic pressure.
Restarting the pump will bring the well back into a dynamic state, thereby increasing
downhole pressure and re-establishing ECD. Such variances in bottomhole condition
can present added challenges, especially when seeking to maintain wellbore pressure
within a window.
Moreover, mud pressure changes based on well depth, thus there is a high delay
time of changing wellbore pressure when required as mud must first be mixed on the
surface and circulated downhole. In addition of relying on the MWD data, pressure
simulator is used to model and predict the downhole pressures, in order to give a quick
look ahead to gauge the hydraulics condition and precise control of ECD.
6
2. Casing string usage
In order to reach the target depth in narrow margin condition, the excessive use
of casing string is required. This is necessary to prevent mud losses as mud density is
incrementally elevated to contain wellbore pressure. Furthermore, the compliance to
kick tolerance worsens the case as additional casings are necessary to ensure well
integrity during well control events (Leblanc and Lewis, 1968). Further to heightened
expenses and drilling time, extra casing string can affect the production performance
as the flow area becomes smaller.
1.2 Problem Statement
As mentioned in the earlier section, the narrowing of the operating margin
available within the drilling window increases the technical challenges associated with
drilling operations. Strategic planning always yields huge improvement potential and
provides the highest impact on well performance without compromising the safety
aspect. First and foremost, in terms of drilling hydraulics, the need of accurate
computation of ESD and ECD is important in this case as it would optimize the well
design and determine the safe drilling and casing depths.
The steady-state hydraulics model is widely used by the well engineers during
planning phase which the magnitude of ECD is constant with time throughout the
entire domain analyzed at certain bit depth. Figure 1.5 shows the results of steady-state
hydraulics study which the ECD and ESD values are usually computed when the
drilling bit reaches TD. These results do not change in time, in which symbolize that
the bit reaches TD with certain rate of penetration (ROP) and circulation rate, drilling
operation is performed continuously until the ECD reaches static condition. This
condition is impossible as during drilling operation, the ECD has been observed to be
sensitive to cuttings and temperature changes, and also it does not explain how the
ECD varies starting with the operations of drilling out cement, making-up connection,
continuing drilling until TD, and hole cleaning. Multiple runs of computation have to
be performed in order to understand the whole well construction hydraulics effect.
7
Therefore, steady state hydraulics does not represent the operation condition
and most of the time the steady state model does not take into account parameter such
as fluid thermal effect and temperature profiles, cuttings (slip) effect, mud gel strength
effect, pressure-volume-temperature (PVT) relationship of the mud, etc. (Rommetveit
and Bjorkevoll, 1997). A good well design with narrow margin requires an in-depth
understanding of the transient hydraulics during all phases of the operation. The
drilling process is usually highly dynamic and complicated to model, thus, transient
modeling of drilling hydraulics has traditionally been neglected, although the model is
able to replicate a real drilling operation and provide a more accuracy prediction than
steady-state models.
Figure 1.5 Steady-state hydraulics results
Admittedly, kick tolerance is a key element when establishing a well
design. Apart from the mud weight design, the number of hole sections also relies on
the kick tolerance results computed using simple spreadsheet, which certain volume of
kick must be bearable by each sections (Wessel and Tarr, 1991). NORSOK (2004)
states that the recommended kick tolerance for each section size and the operators have
their own standard values which governs their well integrity, as a measure to prevent
underground blowout. However, neither the American Petroleum Institute (API) nor
International Association of Drilling Contractors (IADC) provides the method for
computing kick tolerance.
8
Generally, the kick tolerance is very well defined in open literature, the kick
models consider single gas bubbles (Figure 1.6) with simplifications regardless the
effect of frictional pressure loss during killing operation, equipment handling delay,
gas migration rate and gas solubility in oil based fluid (Mosti et al., 2017). The
assumptions are made too conservatively to ensure strict compliance for safety and
simplicity (Denney, 2012). The single gas bubble model does not characterize the
actual downhole behavior during a kick scenario, and the condition of single bubble is
impossible to happen (Santos et al. 2011). Conservative designs may increase costs or
prohibit drilling due to safety concerns, it does not mean the narrow margin well is
impossible to drill. A comprehensive optimization kick tolerance study is achievable
by multiphase kick modeling without compromising the safety and drilling standards.
Figure 1.6 The single bubble gas kick expands when circulated out of well and create
pressure upsurge: (a) A well control event starts with a gas influx, with pressure
gradient of 0.1 psi/ft. at the bottom of the well, (b) Gas bubble expands while traveling
up, (c) The casing shoe depth experiences the highest pressure when the top of gas
reaches the casing shoe depth, (d) The overall annular pressure profile during well
killing is greater than mud hydrostatic pressure, larger influx yields higher pressure.
(a) (b)
(c) (d)
9
1.3 Hypothesis
The detailed modeling of hydraulics in transient mode and multiphase kick
tolerance can assist in understanding better the degree of drillability of the narrow
margin hole which subsequently can improve the well planning. Hypothetically, an
accurate dynamic hydraulics model combines transient effects with a detailed
specification of fluid properties and geometry allows engineers to view the ECD in
time and develop, in order to promote effective pressure control and efficiently reach
the well’s objectives. ECD rises with the increase of mud pumping rate, mud weight
used, fluid viscosity, circulation time, and rate of penetration. Contrariwise, it declines
with the increase of temperature, rotational speed of the drill string, annular flow area,
and drilling depth.
During gas migration, the influx interacts with the mud and dissolves in the oil
phase of the mud until it reaches the bubble point condition at shallow depth. At bubble
point, it pops out from the oil and starts to expand. In the water-based mud, the influx
does not dissolve, instead it expands once it enters the well, with certain liquid hold up
(Umar et al., 2014). The single-bubble models cannot represent these dynamic
processes and often provide unrealistic results with higher casing shoe pressure.
1.4 Objectives
The objectives of the research cover the hydraulics and well control study
specifically for narrow margin wells:
1. To improve current steady-state drilling hydraulics study to transient model,
which can be a precise representation of operational equivalent circulating
density and offer strong potential to significantly impact the effectiveness of
well design and operations. It delivers quantum value to oil operators in terms
knowledge gained to achieve the operational excellence and reduce non-
productive time. It informs better operational planning and provide more
10
effective means of dealing with the narrow margin challenges in exploration
projects.
2. To model accurate multiphase kick tolerance which can improve the
effectiveness of the single bubble kick tolerance concept. The research
outcome can eliminate the need of unnecessary budget as the well is deemed
undrillable with too many casings or on the other hand an unreliable value that
could put in jeopardy the achievement of the drilling and safety objectives.
1.5 Research Scope
The research scope (Figure 1.7) focuses on the well design using the simulation
of realistic operation, covering the hydraulics changes in operational parameters over
time (circulating, static, drilling, and tripping) and multiphase kick tolerance analysis.
Primary well control: dynamic hydraulics
1. Comparing the steady state hydraulics and dynamics hydraulics results
2. Investigating the transient variation in ESD and ECD and identifying the
operational parameters that affect ECD (Table 1.1)
3. Optimizing drilling parameters which can result in the best hydraulics
condition
4. Verifying the dynamic hydraulics results with real time drilling data
Secondary well control: multiphase kick tolerance
1. Comparing the single bubble spreadsheet method from several oil
operators with multiphase kick tolerance simulation
2. Recognizing the improvement gaps of spreadsheet method
3. Performing multiple simulations for sensitivity assessments (Table 1.1)
Figure 1.7 Research scope
Finalized mud weight
11
Table 1.1 Data input and parameters for research
Simulation cases 1. Shallow water slanted well with pore pressure
ramp (overpressured reservoir)
2. Deep water vertical well with narrow margin
drilling window
Sections 12.25 in. and 8.5 in.
Parameters for ECD study 1. Mud temperature effect
2. Mud viscosity
3. Rate of Penetration / Cutting loadings
4. Drill string rotational speed and torque
5. Annular flow area
6. Mud weight
Kick tolerance sensitivity
study parameters
1. Water based or oil based mud
2. Kick intensity
3. Swabbed kick and drill kick scenario
Exclusions and
assumptions
1. The centrifuge effect of drill string rotational on
cuttings is negligible.
2. Normal drilling scenarios are simulated, managed
pressure drilling and dual gradient drilling
conditions are not studied.
3. Kick tolerance study with influx of pure methane
is presumed to happen at TD of each hole section,
at constant rate of 3 bbl/min.
4. Surge, swab effect and cementing pressure are not
studied.
5. Driller’s method is used for well control modeling.
6. Glasso oil PVT correlation model and Dodson-
Standing water PVT correlation model are used to
model the mud density.
7. Mud rheology model of Roberton-Stiff is used to
represent the correlation of shear rate and shear
stress of the mud.
12
1.6 Significance of Study
The narrow margin well condition creates the need to look beyond the
traditional way of well planning (steady state modeling) to increase operational
margin. Modeling the wells dynamically is essential and strategically important in
order to replicate the real drilling operations and provide accuracy which is not possible
with steady-state models. The transient analysis increases the drillers’ confidence in
accessing to this challenging field safely that would otherwise have been near
impossible to drill without understanding of the pressure control. This rigorous
approach in wellbore pressure management has led the engineers to decision making,
cost saving, and reaching maximum drillable depth safely.
Dynamic modeling in hydraulics and well control of a narrow margin deep
water hole delivers accurate predictions of wellbore pressure and temperature, linking
of safe drilling practices with drilling efficiency. The casing seat and maximum
drillable depths are optimized based on multiphase kick tolerance. The comprehensive
study of drilling hydraulics and kick tolerance thus offers strong potential to positively
impact well planning and operational practices for the operators.
1.7 Chapter Summary
The scope of studies covers primary and secondary well control of narrow
margin well which can be set as the best practice for operators to increase the
confidence in exploring or drilling the tight-margin well safely. The subject
background, objectives, research scope and significance of study were outlined to
realize the proposed research work and to understand the need of author to explore
more in this field of study.
92
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