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Casing Design Methodology for Casing While DrillingWhile Drilling By

Karunakar Charan Nooney

in partial fulfilment of the requirements for the degree of

Master of Science

at the Delft University of Technology,

to be defended publicly on Wednesday December 16, 2015 at 04:00 PM.

Supervisor: Prof. Dr. Ir. J.D. Jansen

Thesis committee: Prof. Dr. Ir. J.D. Jansen

Prof. Dr. W.R. Rossen,

Prof. Dr. A.V. Metrikine

Author(s) : Karunakar Charan Nooney

Date : December 16, 2015

Department of Geoscience & Engineering

Delft University of Technology

Telefax : (31) 15 2781189

All rights reserved.

Stored in a retrieval system, or transmitted,

In any form or by any means, electronic,

Mechanical, photocopying, recording, or otherwise,

Without the prior written permission of the

Section for Petroleum Engineering

ii

Abstract In the current plans of Delft Aardwarmte Project (DAP), it is considered to perform the drilling operation by using pipes which remain in the well after drilling thus acting as a casing, the so-called “Casing While Drilling” (CwD) technique. Due to the absence of drill pipe tripping prior to casing the well, this technique results in reduced drilling time compared to conventional drilling. Additionally, potential downhole problems due to drill pipe tripping are precluded. This thesis presents a simulation based approach to selecting casing steel of suitable grade capable of withstanding typical loads encountered while drilling of the well and during its producing life. The developed algorithm is then used in the design of the casing string for the proposed DAP geothermal producer well. The algorithm first considers the effect of uni-axial stresses on casing due to defined burst and collapse pressure loads encountered due to loss of well control while drilling or in the production phase to make a preliminary selection. The effect of axial stress due to buoyed weight of casing and the bending stress due to wellbore curvature is then used to re-evaluate the design against the same collapse and burst loads. This is performed by using a bi-axial approach for the former and the Von- Mises triaxial stress criteria for the latter. A ‘Johancsik’ torque and drag model developed in MATLAB is used to predict drag values during tripping. The bi-axial and Von-Mises stress analysis approach is repeated to include the effect of the computed pull-out drag forces. The associated torque values are used to compute torsional stresses and to identify casing connections of appropriate torque capacity. The final step in the algorithm is to simulate the loads occurring during drilling and calculate the equivalent Von Mises stress values throughout the casing string. Typical drilling loads considered include torque and casing lateral vibration which induce torsional and bending stresses respectively. It was identified that rather than bending stress due to whirling or buckling, torsional stress was more likely to cause casing string failure. This is due to its relatively higher magnitude and the weaker maximum torque capacities of conventional casing connections. Additionally, the MATLAB tools developed for analysing buckling and whirling are used to compute the critical load for inducing sinusoidal buckling as a function of wellbore inclination and the critical rotary speed at surface to induce lateral vibration for varying weight on bit (WOB) respectively. From the generated mode shapes, the bending stress magnitude at each node and therefore the points of maximum stress occurrence for bucking and whirling are also identified.

iii

Acknowledgements

First and foremost, I would like to thank my parents without whom none of this would have been

possible. Their unconditional love and support is the foundation on which I have built my career

thus far. Thanks are also due to my elder brother whose keen insight from having completed a

Master’s degree himself benefited me tremendously in the planning and execution of various

activities throughout my MSc studies.

I am extremely grateful to my thesis supervisor, Professor Jan-Dirk Jansen for accepting me as his

student. I am constantly amazed by how he finds time to fulfil his responsibilities as thesis supervisor

inspite of the numerous demands on his time. Without his understanding and guidance, this project

would never have reached completion.

I would also like to thank Professor W.R Rossen, Professor A.V Metrikine & Eduardo Barros for

consenting to form part of the thesis assessment committee and evaluate this project.

I would be seriously remiss not to acknowledge the moral support of my friends here in Delft, Anand

Sundaresan, Akshey Krishna, Bharadwaj Rangarajan, Jeyakrishna Sridhar & Saashwath Swaminathan.

Thanks for everything guys, I couldn’t have done it without you!

Last but not the least, I would like to thank the management at Cost Engineering Consultancy,

Zwijndrecht for their cooperation in agreeing to defer the starting date of my employment so that I

could complete this thesis.

1.1.1 Non-Retrievable BHA ...................................................................................................... 1

1.1.2 Retrievable BHA .............................................................................................................. 2

1.1.4 Casing Pipe Connections for CwD ................................................................................... 3

1.2 Thesis Outline .......................................................................................................................... 4

2 Design Algorithm ............................................................................................................................. 5

3.1 Implementation of Torque and Drag Model ......................................................................... 10

3.2 Model Validation ................................................................................................................... 11

4 Static Deflection ............................................................................................................................ 13

4.2 Results ................................................................................................................................... 16

4.2.1 Deflection ...................................................................................................................... 16

4.3 Slope & Bending Moment for various Weight-on-Bit Conditions ......................................... 18

5 Buckling Analysis ........................................................................................................................... 19

5.1 Governing Equation .............................................................................................................. 19

6 Natural Frequency of Lateral Vibration for Rotating Casing ......................................................... 22

6.1 Derivation of Governing Equation ........................................................................................ 23

6.2 Results ................................................................................................................................... 24

6.2.1 Comparison with Analytical Solution ............................................................................ 24

6.2.2 Variation of Natural Frequency with Applied Weight on Bit ........................................ 25

7 Application of Developed Concepts to DAP Producer .................................................................. 27

7.1 Initial Well Data ..................................................................................................................... 27

7.2 Stresses Considered .............................................................................................................. 29

7.2.1 Axial Stress .................................................................................................................... 29

7.2.2 Bending Stress ............................................................................................................... 29

7.2.3 Torsional Stress ............................................................................................................. 30

7.3 Power Law Fluid Rheology Model ......................................................................................... 31

7.4 Casing Design Load Cases...................................................................................................... 33

7.5.1 Surface Casing ............................................................................................................... 36

7.5.2 Production Casing ......................................................................................................... 36

7.6 Axial Loading ......................................................................................................................... 38

7.6.1 Wellbore Trajectory ...................................................................................................... 38

7.6.2 Surface Casing ............................................................................................................... 38

7.6.3 Intermediate Casing ...................................................................................................... 39

7.6.4 7” Liner .......................................................................................................................... 39

7.7 Combined Loading ................................................................................................................ 40

7.7.2 Von Mises Analysis for Burst Loading ........................................................................... 42

7.8 Torque & Drag Analysis ......................................................................................................... 42

7.8.1 Axial Loading due to Drag Forces .................................................................................. 43

7.8.2 Torque Analysis ............................................................................................................. 45

7.10 Drilling Loads ......................................................................................................................... 47

7.10.2 9 5/8” Casing – Bending Stress Due to Buckling ........................................................... 49

7.11 Von Mises Analysis of Drilling Loads during 7” Liner Section ............................................... 51

8 Conclusions ................................................................................................................................... 52

8.1 Recommendations ................................................................................................................ 52

vi

Appendix E. Torque Analysis & Corresponding Selection of Casing Connection .............................. 73

Appendix F. Fluid Hydraulics – Frictional Pressure Losses ................................................................ 75

vii

Figure 1 Non Retrievable BHA [1] ........................................................................................................... 2

Figure 2 Retrievable BHA. Combination image generated from [2] & [3] .............................................. 2

Figure 3 Retrievable BHA for drilling with Liner [5] ................................................................................ 3

Figure 4 BTC with Torque Shoulder [36] ................................................................................................. 4

Figure 5 Typical Buttress Threaded Connection [10] .............................................................................. 4

Figure 6 Overview of the Casing Design Process .................................................................................... 5

Figure 7 Downhole Forces on Casing [11] ............................................................................................... 7

Figure 8 Force Balance on Drill String Element [13] ............................................................................... 8

Figure 9 Discretization of Drill String into Nodes [7] .............................................................................. 9

Figure 10 Snapshot of EXCEL spread sheet used for accepting Input String Data for T&D Model ....... 10

Figure 11 Sample Output Plots for Surface Hook Load & Cumulative Surface Torque ........................ 10

Figure 12 12 1/4" OH Section Hook Load Measurements for Well South Sangu-4 .............................. 11

Figure 13 8 1/2" OH Section Hook Load Measurements for SS-4 ......................................................... 11

Figure 14 Forces acting in radial direction [19]..................................................................................... 13

Figure 15 Variation of axial load in drill string [19] ............................................................................... 14

Figure 16 Plot of Deflections at Inclination of 10 Degrees ................................................................... 16

Figure 17 Clamped beam with Pinned End on which is exerted a constant lateral load and axial

compressional force. [20] ..................................................................................................................... 17

Figure 18 Verification of Numerical Model ........................................................................................... 18

Figure 19 Variation in Bending Moment for Hold Inclination of 10 Degrees ....................................... 18

Figure 20 Casing initially resting on lower side of wellbore ................................................................. 19

Figure 21 Mode shapes at 0 degrees Inclination .................................................................................. 21

Figure 22 Plot of numerical & analytical critical loads as a function of wellbore inclination ............... 21

Figure 23 BHA Whirl [25] ...................................................................................................................... 22

Figure 24 Section of BHA rotating about axis [19] ................................................................................ 23

Figure 25 Mode shapes for the first natural frequency of Lateral Vibration ........................................ 24

Figure 26 Dependence of natural frequency on the applied Weight on Bit ......................................... 25

Figure 27 Drilling Window for Wellbore Fluid Gradient ....................................................................... 28

Figure 28 Planned Trajectory of Producer ............................................................................................ 28

Figure 29 Graphical representation of Design Loads vs. Rated Strength ............................................. 37

Figure 30 Measurement of Key Wellbore Trajectory Parameters ........................................................ 38

Figure 31 Plot of Design Load vs. Rated Strength for Collapse ............................................................. 41

Figure 32 Plot of Pick Up Drag Forces When Tripping Out of String ..................................................... 42

Figure 33 Design Collapse Load vs. Strength for DAP Producer ........................................................... 44

Figure 34 Increased FOS for Burst Loading Due to Effect of Drag Forces ............................................. 44

Figure 35 Cumulative Torque Observed at Surface for Various Drilled Depths ................................... 45

Figure 36 Flow Path for Drilling Fluid in 7" Liner Drilling Phase............................................................ 46

Figure 37 Fluid Pressure Distribution for 7" Liner Drilling Phase .......................................................... 46

Figure 38 Whirling Mode Shapes .......................................................................................................... 47

Figure 39 Natural Frequency of Lateral Vibration vs. WOB .................................................................. 47

Figure 40 Bending Stress Due to Whirl ................................................................................................. 48

Figure 41 Buckling Mode Shapes and Dependency of Critical Load on WOB ....................................... 49

Figure 42 Bending Stress due to Buckling in 9 5/8" Intermediate CSG ................................................ 50

Figure 43 7" Liner Von-Mises Stress Analysis for Drilling Operation .................................................... 51

Figure 44 Discretization of Drill String [19] ........................................................................................... 58

Table 1 Error Percentages for Simulated Hook Load Results ............................................................... 12

Table 2 Flow Rate for Minimum Annular Velocity [30]......................................................................... 31

Table 3 Factor of Safety [31] ................................................................................................................. 33

Table 4 Production Casing Specifications ............................................................................................. 37

Table 5 Summary of Required Material Properties based on Design Loads......................................... 37

Table 6 Surface Casing Properties ......................................................................................................... 38

Table 7 Intermediate CSG Properties ................................................................................................... 39

Table 8 Liner Hold Section Properties ................................................................................................... 39

Table 9 Build Up Section Properties ..................................................................................................... 39

Table 10 Power Law Input Viscometer Measurements ........................................................................ 46

Table 11 Casing String Design for DAP Producer Geothermal Well ...................................................... 52

Table 12 Von Mises Stress Analysis at Survey Points for Burst Loading Scenario ................................ 66

Table 13 Simulated Pick-Up Drag Values at Survey Points for 7” Liner Drilling .................................... 68

Table 14 Von Mises Analysis at Various Survey Points for 7'' Deviated N80 Liner Section .................. 69

Table 15 Simulated Bending Stress due to Whirling for 7” Liner Stand (30 M) .................................... 70

Table 16 Von Mises Tri Axisal Stress Analysis of 7" Liner for Drilling Conditions at Survey Points ...... 71

Table 17 Bending Stress due to Buckling for 9 5/8" Intermediate CSG Stand (30 metres) .................. 72

Table 18 Torque Analysis at Survey Points for 7" N80 Liner Section .................................................... 73

Table 19 Connection Make-Up Torque Capacities from Manufacturer Catalogue .............................. 74

Table 20 Pressure Loss Computed By Power Law Rheology ................................................................. 75

1.1 Casing While Drilling

The primary motivation for this thesis is the plan of the Delft Aardwarmte Project (DAP) to drill the producer geothermal well by employing the CwD technique. Due to the absence of drill pipe tripping, this technique results in reduced drilling time compared to conventional drilling whilst also precluding numerous downhole problems. The simulation based approach presented in this thesis is designed to select a suitable grade of casing steel for the DAP geothermal producer well but this algorithm can also be extended towards composite materials as well. The reduced weight of composite materials in comparison to steel would allow DAP to use rigs of smaller draw-works capacities without reduction in target depths thereby resulting in significant cost savings. The CwD has the following advantages in comparison to conventional drilling:

Eliminates the need for tripping of drill string prior to casing the well. The primary benefit of

doing so is the reduction in rig operating time. Additionally, potential downhole problems

such as loss of well control, surge and swab pressures when the casing is being run in or hole

collapse due to presence of unstable zones in the wellbore can be avoided.

Problematic ‘thief’ zones which absorb drilling fluid completely, thereby causing a loss of

primary well control can be bypassed in this technique due to the “plastering effect” of the

casing. Due to the low annular clearances between casing and borehole wall, the casing

effectively smears the drilling fluid particles into the borehole wall creating a superior filter

cake thereby bypassing these zones.

Low annular volumes enables higher flow velocities which facilitates hole cleaning.

It is not without its disadvantages however and these are:

Low annular clearances lead to higher frictional flow losses which necessitate the use of

higher capacity pumps for same drilled depths in comparison to conventional drilling

In order to execute a CwD operation, the rig hoisting equipment (usually the top drive) has

to be modified to accommodate the casing. Additionally, other special equipment such as

torque rings, modified elevators and tongs have to be employed which result in a further

increase in costs

There are two variations in bottom hole assemblies (BHAs) used for this technique which are applied

based on the well trajectory and target depth requirement.

1.1.1 Non-Retrievable BHA

This technique is used for shallow depth vertical sections in formations with soft to medium

hardness levels only. Typically, the top hole section is drilled using this BHA. No drill collars are used

however stabilizers may be used in regular intervals of one stand. Other cementing equipment such

as float shoe and collar may also be used. Typically, special polycrystalline diamond compact (PDC)

bits which are soft enough to be drilled through by conventional bits used in the next drilling phase

are preferred. The flow path for circulating of drilling fluid is identical to conventional drilling

operations.

2

1.1.2 Retrievable BHA

Figure 2 Retrievable BHA. Combination image generated from [2] & [3]

The retrievable BHA as the name suggests can be retrieved by wireline after the section has been

drilled prior to cementing. The primary components are the pilot bit for drilling the initial hole, an

under reamer for subsequently enlarging it, rotary steerable or mud motors, “Measurement While

Drilling” (MWD)/ “Logging While Drilling” (LWD) tools and the drill lock/latch assembly. The DLA is

locked in such a way to prevent relative axial and torsional motion between the BHA and the casing

[4]. Seals used in the DLA as shown in Figure 2 prevent the flow of drilling fluid into the casing and

instead divert into the BHA [4]. Any type of formation can be drilled at any depth without placing

any restrictions on wellbore inclination & depth or formation hardness.

3

1.1.3 Liner While Drilling

In “Liner while Drilling” operations, the drill pipe used to run in the liner includes the liner hanger for

supporting the weight of the liner and the BHA for drilling. Consequently, the circulation path for

drilling fluid is entirely through the inner drill string as a result of which, the entire liner is not

exposed to internal fluid pressure as shown in Figure 3.

Figure 3 Retrievable BHA for drilling with Liner [5]

1.1.4 Casing Pipe Connections for CwD

Typical casing connections are designed for one time use only where the casing connection is made

up at surface and run into the hole. They are not designed for handling torsional and alternating

stresses induced by rotation of the string. Therefore special casing connections have to be designed

with high torque transmission capability. As per [1], [6], [7], [8]& [9] the preferred connections used

are the API Buttress Threaded Coupled (BTC) connections with metal to metal seal. Depending upon

the specific well trajectory, these connections may also provide insufficient torque capacity.

Consequently, special metal torque rings are inserted between casing threads to further increase the

maximum torque capacity by providing a positive torque shoulder which prevents thread crushing

by over-torqueing and acting as a spacer between the two threads.

4

1.2 Thesis Outline

Chapter 2 presents the design algorithm to be followed for the selection of casing steel grade with a

brief discussion of each step in the sequence.

Chapter 3 introduces the torque and drag model starting with a brief discussion of the need for

calculating wellbore torque and drag forces and the subsequent implementation of the ‘Johancsik’

model in this thesis by using a combination of an MS EXCEL spread sheet for accepting input data

and MATLAB for computing actual forces and plotting the results. Steps taken for checking the

accuracy of the model are also presented.

Chapters 4, 5, and 6 are concerned with the theoretical derivation of the governing equations or

Eigen value formulations for buckling and whirling analysis. The starting point is the derivation for

the deflection of the string under the influence of the external load applied at surface. In addition to

the theoretical derivation, the governing equations are solved using the finite difference method in

MATLAB for which the details are contained in Appendix A, Appendix B and Appendix C. Each model

is validated by comparing with analytical solutions from literature.

Chapter 7 focuses on the application of the algorithm and supporting MATLAB tools developed, to

the DAP producer well casing design. The implementation of the previously defined algorithm along

with the analysis of results obtained is described comprehensively. Supporting data for bending

stress due to Buckling & Whirling, Torque, Drag & Von Mises stress obtained from the MATLAB tools

is tabulated in Appendix D & E for reference. Appendix E also presents a simple example of selecting

a suitable buttress casing connection from a manufacturer catalogue on the basis of rated make-up

torque capacity. Implementation of the hydraulic model based on the “Power law” rheology model

for calculating the wellbore pressure distribution is reported. Supporting hydraulic calculations are

summarized in tabular format in Appendix E.

Lastly, in Chapter 8 the conclusions derived from the thesis work are presented and suggestions are

made towards extending specific aspects of this thesis.

Figure 4 BTC with Torque Shoulder [36]

5

6

In this section, the overall process of selecting the casing steel for the CwD application is conveyed in

the form of an algorithm displayed in Figure 6 above. The starting point of the algorithm is the

formation strength and fluid pressure gradient data (section 7.1) needed for calculating bottom hole

pressure to maintain hydrostatic balance with formation fluid. The fluid properties of plastic

viscosity, yield point and shear rate are used to calculate the wellbore dynamic fluid pressures on

the basis of the power law rheology model as shown in Appendix F. In the next step, the collapse

and burst loads are estimated for making the preliminary selection of casing on the basis of the

uniaxial loading criteria (section 7.4). The selected grade of casing steel is also verified for axial

loading due to buoyed weight and bending due to wellbore curvature (section 7.6).

As axial loads reduce collapse resistance of the casing string, a combined load analysis for hoop and

axial loads is performed (section 7.7.1) along with a tri-axial stress analysis for the burst scenario

(section 7.7.2). If in each of these cases, the ratio of material yield strength to the equivalent stress is

found to be greater than the design factor of safety, than the design is considered to be effective.

The next step is the simulation of torque and drag forces induced in the casing string when drilling

and tripping out of the wellbore respectively (section 7.8 ). The drag forces are used to compute an

increased axial load for which the combined loading analysis is…

Karunakar Charan Nooney

in partial fulfilment of the requirements for the degree of

Master of Science

at the Delft University of Technology,

to be defended publicly on Wednesday December 16, 2015 at 04:00 PM.

Supervisor: Prof. Dr. Ir. J.D. Jansen

Thesis committee: Prof. Dr. Ir. J.D. Jansen

Prof. Dr. W.R. Rossen,

Prof. Dr. A.V. Metrikine

Author(s) : Karunakar Charan Nooney

Date : December 16, 2015

Department of Geoscience & Engineering

Delft University of Technology

Telefax : (31) 15 2781189

All rights reserved.

Stored in a retrieval system, or transmitted,

In any form or by any means, electronic,

Mechanical, photocopying, recording, or otherwise,

Without the prior written permission of the

Section for Petroleum Engineering

ii

Abstract In the current plans of Delft Aardwarmte Project (DAP), it is considered to perform the drilling operation by using pipes which remain in the well after drilling thus acting as a casing, the so-called “Casing While Drilling” (CwD) technique. Due to the absence of drill pipe tripping prior to casing the well, this technique results in reduced drilling time compared to conventional drilling. Additionally, potential downhole problems due to drill pipe tripping are precluded. This thesis presents a simulation based approach to selecting casing steel of suitable grade capable of withstanding typical loads encountered while drilling of the well and during its producing life. The developed algorithm is then used in the design of the casing string for the proposed DAP geothermal producer well. The algorithm first considers the effect of uni-axial stresses on casing due to defined burst and collapse pressure loads encountered due to loss of well control while drilling or in the production phase to make a preliminary selection. The effect of axial stress due to buoyed weight of casing and the bending stress due to wellbore curvature is then used to re-evaluate the design against the same collapse and burst loads. This is performed by using a bi-axial approach for the former and the Von- Mises triaxial stress criteria for the latter. A ‘Johancsik’ torque and drag model developed in MATLAB is used to predict drag values during tripping. The bi-axial and Von-Mises stress analysis approach is repeated to include the effect of the computed pull-out drag forces. The associated torque values are used to compute torsional stresses and to identify casing connections of appropriate torque capacity. The final step in the algorithm is to simulate the loads occurring during drilling and calculate the equivalent Von Mises stress values throughout the casing string. Typical drilling loads considered include torque and casing lateral vibration which induce torsional and bending stresses respectively. It was identified that rather than bending stress due to whirling or buckling, torsional stress was more likely to cause casing string failure. This is due to its relatively higher magnitude and the weaker maximum torque capacities of conventional casing connections. Additionally, the MATLAB tools developed for analysing buckling and whirling are used to compute the critical load for inducing sinusoidal buckling as a function of wellbore inclination and the critical rotary speed at surface to induce lateral vibration for varying weight on bit (WOB) respectively. From the generated mode shapes, the bending stress magnitude at each node and therefore the points of maximum stress occurrence for bucking and whirling are also identified.

iii

Acknowledgements

First and foremost, I would like to thank my parents without whom none of this would have been

possible. Their unconditional love and support is the foundation on which I have built my career

thus far. Thanks are also due to my elder brother whose keen insight from having completed a

Master’s degree himself benefited me tremendously in the planning and execution of various

activities throughout my MSc studies.

I am extremely grateful to my thesis supervisor, Professor Jan-Dirk Jansen for accepting me as his

student. I am constantly amazed by how he finds time to fulfil his responsibilities as thesis supervisor

inspite of the numerous demands on his time. Without his understanding and guidance, this project

would never have reached completion.

I would also like to thank Professor W.R Rossen, Professor A.V Metrikine & Eduardo Barros for

consenting to form part of the thesis assessment committee and evaluate this project.

I would be seriously remiss not to acknowledge the moral support of my friends here in Delft, Anand

Sundaresan, Akshey Krishna, Bharadwaj Rangarajan, Jeyakrishna Sridhar & Saashwath Swaminathan.

Thanks for everything guys, I couldn’t have done it without you!

Last but not the least, I would like to thank the management at Cost Engineering Consultancy,

Zwijndrecht for their cooperation in agreeing to defer the starting date of my employment so that I

could complete this thesis.

1.1.1 Non-Retrievable BHA ...................................................................................................... 1

1.1.2 Retrievable BHA .............................................................................................................. 2

1.1.4 Casing Pipe Connections for CwD ................................................................................... 3

1.2 Thesis Outline .......................................................................................................................... 4

2 Design Algorithm ............................................................................................................................. 5

3.1 Implementation of Torque and Drag Model ......................................................................... 10

3.2 Model Validation ................................................................................................................... 11

4 Static Deflection ............................................................................................................................ 13

4.2 Results ................................................................................................................................... 16

4.2.1 Deflection ...................................................................................................................... 16

4.3 Slope & Bending Moment for various Weight-on-Bit Conditions ......................................... 18

5 Buckling Analysis ........................................................................................................................... 19

5.1 Governing Equation .............................................................................................................. 19

6 Natural Frequency of Lateral Vibration for Rotating Casing ......................................................... 22

6.1 Derivation of Governing Equation ........................................................................................ 23

6.2 Results ................................................................................................................................... 24

6.2.1 Comparison with Analytical Solution ............................................................................ 24

6.2.2 Variation of Natural Frequency with Applied Weight on Bit ........................................ 25

7 Application of Developed Concepts to DAP Producer .................................................................. 27

7.1 Initial Well Data ..................................................................................................................... 27

7.2 Stresses Considered .............................................................................................................. 29

7.2.1 Axial Stress .................................................................................................................... 29

7.2.2 Bending Stress ............................................................................................................... 29

7.2.3 Torsional Stress ............................................................................................................. 30

7.3 Power Law Fluid Rheology Model ......................................................................................... 31

7.4 Casing Design Load Cases...................................................................................................... 33

7.5.1 Surface Casing ............................................................................................................... 36

7.5.2 Production Casing ......................................................................................................... 36

7.6 Axial Loading ......................................................................................................................... 38

7.6.1 Wellbore Trajectory ...................................................................................................... 38

7.6.2 Surface Casing ............................................................................................................... 38

7.6.3 Intermediate Casing ...................................................................................................... 39

7.6.4 7” Liner .......................................................................................................................... 39

7.7 Combined Loading ................................................................................................................ 40

7.7.2 Von Mises Analysis for Burst Loading ........................................................................... 42

7.8 Torque & Drag Analysis ......................................................................................................... 42

7.8.1 Axial Loading due to Drag Forces .................................................................................. 43

7.8.2 Torque Analysis ............................................................................................................. 45

7.10 Drilling Loads ......................................................................................................................... 47

7.10.2 9 5/8” Casing – Bending Stress Due to Buckling ........................................................... 49

7.11 Von Mises Analysis of Drilling Loads during 7” Liner Section ............................................... 51

8 Conclusions ................................................................................................................................... 52

8.1 Recommendations ................................................................................................................ 52

vi

Appendix E. Torque Analysis & Corresponding Selection of Casing Connection .............................. 73

Appendix F. Fluid Hydraulics – Frictional Pressure Losses ................................................................ 75

vii

Figure 1 Non Retrievable BHA [1] ........................................................................................................... 2

Figure 2 Retrievable BHA. Combination image generated from [2] & [3] .............................................. 2

Figure 3 Retrievable BHA for drilling with Liner [5] ................................................................................ 3

Figure 4 BTC with Torque Shoulder [36] ................................................................................................. 4

Figure 5 Typical Buttress Threaded Connection [10] .............................................................................. 4

Figure 6 Overview of the Casing Design Process .................................................................................... 5

Figure 7 Downhole Forces on Casing [11] ............................................................................................... 7

Figure 8 Force Balance on Drill String Element [13] ............................................................................... 8

Figure 9 Discretization of Drill String into Nodes [7] .............................................................................. 9

Figure 10 Snapshot of EXCEL spread sheet used for accepting Input String Data for T&D Model ....... 10

Figure 11 Sample Output Plots for Surface Hook Load & Cumulative Surface Torque ........................ 10

Figure 12 12 1/4" OH Section Hook Load Measurements for Well South Sangu-4 .............................. 11

Figure 13 8 1/2" OH Section Hook Load Measurements for SS-4 ......................................................... 11

Figure 14 Forces acting in radial direction [19]..................................................................................... 13

Figure 15 Variation of axial load in drill string [19] ............................................................................... 14

Figure 16 Plot of Deflections at Inclination of 10 Degrees ................................................................... 16

Figure 17 Clamped beam with Pinned End on which is exerted a constant lateral load and axial

compressional force. [20] ..................................................................................................................... 17

Figure 18 Verification of Numerical Model ........................................................................................... 18

Figure 19 Variation in Bending Moment for Hold Inclination of 10 Degrees ....................................... 18

Figure 20 Casing initially resting on lower side of wellbore ................................................................. 19

Figure 21 Mode shapes at 0 degrees Inclination .................................................................................. 21

Figure 22 Plot of numerical & analytical critical loads as a function of wellbore inclination ............... 21

Figure 23 BHA Whirl [25] ...................................................................................................................... 22

Figure 24 Section of BHA rotating about axis [19] ................................................................................ 23

Figure 25 Mode shapes for the first natural frequency of Lateral Vibration ........................................ 24

Figure 26 Dependence of natural frequency on the applied Weight on Bit ......................................... 25

Figure 27 Drilling Window for Wellbore Fluid Gradient ....................................................................... 28

Figure 28 Planned Trajectory of Producer ............................................................................................ 28

Figure 29 Graphical representation of Design Loads vs. Rated Strength ............................................. 37

Figure 30 Measurement of Key Wellbore Trajectory Parameters ........................................................ 38

Figure 31 Plot of Design Load vs. Rated Strength for Collapse ............................................................. 41

Figure 32 Plot of Pick Up Drag Forces When Tripping Out of String ..................................................... 42

Figure 33 Design Collapse Load vs. Strength for DAP Producer ........................................................... 44

Figure 34 Increased FOS for Burst Loading Due to Effect of Drag Forces ............................................. 44

Figure 35 Cumulative Torque Observed at Surface for Various Drilled Depths ................................... 45

Figure 36 Flow Path for Drilling Fluid in 7" Liner Drilling Phase............................................................ 46

Figure 37 Fluid Pressure Distribution for 7" Liner Drilling Phase .......................................................... 46

Figure 38 Whirling Mode Shapes .......................................................................................................... 47

Figure 39 Natural Frequency of Lateral Vibration vs. WOB .................................................................. 47

Figure 40 Bending Stress Due to Whirl ................................................................................................. 48

Figure 41 Buckling Mode Shapes and Dependency of Critical Load on WOB ....................................... 49

Figure 42 Bending Stress due to Buckling in 9 5/8" Intermediate CSG ................................................ 50

Figure 43 7" Liner Von-Mises Stress Analysis for Drilling Operation .................................................... 51

Figure 44 Discretization of Drill String [19] ........................................................................................... 58

Table 1 Error Percentages for Simulated Hook Load Results ............................................................... 12

Table 2 Flow Rate for Minimum Annular Velocity [30]......................................................................... 31

Table 3 Factor of Safety [31] ................................................................................................................. 33

Table 4 Production Casing Specifications ............................................................................................. 37

Table 5 Summary of Required Material Properties based on Design Loads......................................... 37

Table 6 Surface Casing Properties ......................................................................................................... 38

Table 7 Intermediate CSG Properties ................................................................................................... 39

Table 8 Liner Hold Section Properties ................................................................................................... 39

Table 9 Build Up Section Properties ..................................................................................................... 39

Table 10 Power Law Input Viscometer Measurements ........................................................................ 46

Table 11 Casing String Design for DAP Producer Geothermal Well ...................................................... 52

Table 12 Von Mises Stress Analysis at Survey Points for Burst Loading Scenario ................................ 66

Table 13 Simulated Pick-Up Drag Values at Survey Points for 7” Liner Drilling .................................... 68

Table 14 Von Mises Analysis at Various Survey Points for 7'' Deviated N80 Liner Section .................. 69

Table 15 Simulated Bending Stress due to Whirling for 7” Liner Stand (30 M) .................................... 70

Table 16 Von Mises Tri Axisal Stress Analysis of 7" Liner for Drilling Conditions at Survey Points ...... 71

Table 17 Bending Stress due to Buckling for 9 5/8" Intermediate CSG Stand (30 metres) .................. 72

Table 18 Torque Analysis at Survey Points for 7" N80 Liner Section .................................................... 73

Table 19 Connection Make-Up Torque Capacities from Manufacturer Catalogue .............................. 74

Table 20 Pressure Loss Computed By Power Law Rheology ................................................................. 75

1.1 Casing While Drilling

The primary motivation for this thesis is the plan of the Delft Aardwarmte Project (DAP) to drill the producer geothermal well by employing the CwD technique. Due to the absence of drill pipe tripping, this technique results in reduced drilling time compared to conventional drilling whilst also precluding numerous downhole problems. The simulation based approach presented in this thesis is designed to select a suitable grade of casing steel for the DAP geothermal producer well but this algorithm can also be extended towards composite materials as well. The reduced weight of composite materials in comparison to steel would allow DAP to use rigs of smaller draw-works capacities without reduction in target depths thereby resulting in significant cost savings. The CwD has the following advantages in comparison to conventional drilling:

Eliminates the need for tripping of drill string prior to casing the well. The primary benefit of

doing so is the reduction in rig operating time. Additionally, potential downhole problems

such as loss of well control, surge and swab pressures when the casing is being run in or hole

collapse due to presence of unstable zones in the wellbore can be avoided.

Problematic ‘thief’ zones which absorb drilling fluid completely, thereby causing a loss of

primary well control can be bypassed in this technique due to the “plastering effect” of the

casing. Due to the low annular clearances between casing and borehole wall, the casing

effectively smears the drilling fluid particles into the borehole wall creating a superior filter

cake thereby bypassing these zones.

Low annular volumes enables higher flow velocities which facilitates hole cleaning.

It is not without its disadvantages however and these are:

Low annular clearances lead to higher frictional flow losses which necessitate the use of

higher capacity pumps for same drilled depths in comparison to conventional drilling

In order to execute a CwD operation, the rig hoisting equipment (usually the top drive) has

to be modified to accommodate the casing. Additionally, other special equipment such as

torque rings, modified elevators and tongs have to be employed which result in a further

increase in costs

There are two variations in bottom hole assemblies (BHAs) used for this technique which are applied

based on the well trajectory and target depth requirement.

1.1.1 Non-Retrievable BHA

This technique is used for shallow depth vertical sections in formations with soft to medium

hardness levels only. Typically, the top hole section is drilled using this BHA. No drill collars are used

however stabilizers may be used in regular intervals of one stand. Other cementing equipment such

as float shoe and collar may also be used. Typically, special polycrystalline diamond compact (PDC)

bits which are soft enough to be drilled through by conventional bits used in the next drilling phase

are preferred. The flow path for circulating of drilling fluid is identical to conventional drilling

operations.

2

1.1.2 Retrievable BHA

Figure 2 Retrievable BHA. Combination image generated from [2] & [3]

The retrievable BHA as the name suggests can be retrieved by wireline after the section has been

drilled prior to cementing. The primary components are the pilot bit for drilling the initial hole, an

under reamer for subsequently enlarging it, rotary steerable or mud motors, “Measurement While

Drilling” (MWD)/ “Logging While Drilling” (LWD) tools and the drill lock/latch assembly. The DLA is

locked in such a way to prevent relative axial and torsional motion between the BHA and the casing

[4]. Seals used in the DLA as shown in Figure 2 prevent the flow of drilling fluid into the casing and

instead divert into the BHA [4]. Any type of formation can be drilled at any depth without placing

any restrictions on wellbore inclination & depth or formation hardness.

3

1.1.3 Liner While Drilling

In “Liner while Drilling” operations, the drill pipe used to run in the liner includes the liner hanger for

supporting the weight of the liner and the BHA for drilling. Consequently, the circulation path for

drilling fluid is entirely through the inner drill string as a result of which, the entire liner is not

exposed to internal fluid pressure as shown in Figure 3.

Figure 3 Retrievable BHA for drilling with Liner [5]

1.1.4 Casing Pipe Connections for CwD

Typical casing connections are designed for one time use only where the casing connection is made

up at surface and run into the hole. They are not designed for handling torsional and alternating

stresses induced by rotation of the string. Therefore special casing connections have to be designed

with high torque transmission capability. As per [1], [6], [7], [8]& [9] the preferred connections used

are the API Buttress Threaded Coupled (BTC) connections with metal to metal seal. Depending upon

the specific well trajectory, these connections may also provide insufficient torque capacity.

Consequently, special metal torque rings are inserted between casing threads to further increase the

maximum torque capacity by providing a positive torque shoulder which prevents thread crushing

by over-torqueing and acting as a spacer between the two threads.

4

1.2 Thesis Outline

Chapter 2 presents the design algorithm to be followed for the selection of casing steel grade with a

brief discussion of each step in the sequence.

Chapter 3 introduces the torque and drag model starting with a brief discussion of the need for

calculating wellbore torque and drag forces and the subsequent implementation of the ‘Johancsik’

model in this thesis by using a combination of an MS EXCEL spread sheet for accepting input data

and MATLAB for computing actual forces and plotting the results. Steps taken for checking the

accuracy of the model are also presented.

Chapters 4, 5, and 6 are concerned with the theoretical derivation of the governing equations or

Eigen value formulations for buckling and whirling analysis. The starting point is the derivation for

the deflection of the string under the influence of the external load applied at surface. In addition to

the theoretical derivation, the governing equations are solved using the finite difference method in

MATLAB for which the details are contained in Appendix A, Appendix B and Appendix C. Each model

is validated by comparing with analytical solutions from literature.

Chapter 7 focuses on the application of the algorithm and supporting MATLAB tools developed, to

the DAP producer well casing design. The implementation of the previously defined algorithm along

with the analysis of results obtained is described comprehensively. Supporting data for bending

stress due to Buckling & Whirling, Torque, Drag & Von Mises stress obtained from the MATLAB tools

is tabulated in Appendix D & E for reference. Appendix E also presents a simple example of selecting

a suitable buttress casing connection from a manufacturer catalogue on the basis of rated make-up

torque capacity. Implementation of the hydraulic model based on the “Power law” rheology model

for calculating the wellbore pressure distribution is reported. Supporting hydraulic calculations are

summarized in tabular format in Appendix E.

Lastly, in Chapter 8 the conclusions derived from the thesis work are presented and suggestions are

made towards extending specific aspects of this thesis.

Figure 4 BTC with Torque Shoulder [36]

5

6

In this section, the overall process of selecting the casing steel for the CwD application is conveyed in

the form of an algorithm displayed in Figure 6 above. The starting point of the algorithm is the

formation strength and fluid pressure gradient data (section 7.1) needed for calculating bottom hole

pressure to maintain hydrostatic balance with formation fluid. The fluid properties of plastic

viscosity, yield point and shear rate are used to calculate the wellbore dynamic fluid pressures on

the basis of the power law rheology model as shown in Appendix F. In the next step, the collapse

and burst loads are estimated for making the preliminary selection of casing on the basis of the

uniaxial loading criteria (section 7.4). The selected grade of casing steel is also verified for axial

loading due to buoyed weight and bending due to wellbore curvature (section 7.6).

As axial loads reduce collapse resistance of the casing string, a combined load analysis for hoop and

axial loads is performed (section 7.7.1) along with a tri-axial stress analysis for the burst scenario

(section 7.7.2). If in each of these cases, the ratio of material yield strength to the equivalent stress is

found to be greater than the design factor of safety, than the design is considered to be effective.

The next step is the simulation of torque and drag forces induced in the casing string when drilling

and tripping out of the wellbore respectively (section 7.8 ). The drag forces are used to compute an

increased axial load for which the combined loading analysis is…

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