TAML Multilaterals Guidebook July 1999

TAML Multilaterals Guidebook

Note: This document was prepared by the TAML network (Technical Advancement of Multilaterals see www.taml.net for more information) in July 1999, as a web-based tool. As the TAML website from that time is no longer active, the tool has been re -compiled in January 2004 as a pdf document for downloading and usage by TAML members.

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Table of Contents

TECHNICAL ADVANCEMENT OF MULTILATERAL WELLS......................................................................................1

CHAPTER 1. INTRODUCTION ....................................................................................................................................................7 1.1 PRELIMINARIES............................................................................................................................................................................7 1.2 OBJECTIVES ..................................................................................................................................................................................7 1.3 SCOPE OF THE GUIDE AND DATABASE .....................................................................................................................................7 1.4 GUIDE AND DATABASE DEVELOPMENT ...................................................................................................................................8 1.5 FORMAT FOR THE GUIDE ............................................................................................................................................................8

CHAPTER 2. BASIC FACTS ON MULTILATERALS ........................................................................................................ 10 2.1 DEFINITION AND DESCRIPTION ..............................................................................................................................................10

2.1.1 Definition ......................................................................................................................................................................... 10 2.1.2 Geometry of Multi-lateral and Multi-Branched Wells ....................................................................................... 10

2.2 HISTORICAL REVIEW ................................................................................................................................................................12 2.3 CLASSIFICATION SYSTEM .......................................................................................................................................................13

2.3.1 Complexity Ranking .................................................................................................................................................... 14 2.3.2 Functionality Classification ....................................................................................................................................... 14 2.3.3 Classification Code Examples ................................................................................................................................. 17

2.4 OVERVIEW OF CURRENT STATUS .........................................................................................................................................19 2.4.1 Statistics when, where, how, how many .......................................................................................................... 19 2.4.2 Proprietary Systems and their TAML Classification ........................................................................................ 21 2.4.3 Future Adaptations and Developments................................................................................................................ 22

2.5 BUSINESS CASE FLOWCHART ...............................................................................................................................................24 2.5.1 List of Benefits .............................................................................................................................................................. 24 2.5.2 Cost and Risk implications ....................................................................................................................................... 25

CHAPTER 3. DECIDING ON A MULTILATERAL............................................................................................................. 30 3.1 GENERAL CONDITIONS FAVORING ML SOLUTION ...............................................................................................................30 3.2 BUSINESS BENEFIT ANALYSIS.................................................................................................................................................30

3.2.1 Process for determining business benefit .................................................................................................................... 30 3.2.2 Case studies for business benefits................................................................................................................................. 32

3.3 SCREENING PROCESS.................................................................................................................................................................37 3.3.1 Introduction...................................................................................................................................................................... 37 3.3.2 High level objectives....................................................................................................................................................... 38 3.3.3 Data gathering: reservoir and fluid characteristics.................................................................................................. 38 3.3.4 Well objectives and design criteria............................................................................................................................... 41 3.3.5 Evaluation and comparison of options........................................................................................................................ 41 3.3.6 Recommendation and final decision............................................................................................................................. 48

3.4 QUANTITATIVE RISK ASSESSMENT (QRA) APPROACH .......................................................................................................48 3.4.1 Introduction...................................................................................................................................................................... 48 3.4.2 QRA Methodology for Well Construction, Installation, Maintenance and Operation ........................................ 49 3.4.3 QRA Results ...................................................................................................................................................................... 51 3.4.4 Case Studies: Risk Related Issues................................................................................................................................. 52

3.5 ROLES AND RESPONSIBILITIES.................................................................................................................................................54 3.6 MODELING OF MULTI-LATERAL PERFORMANCE ..................................................................................................................55

3.6.1 Analytical Models............................................................................................................................................................ 55 3.6.2 Inflow for Vertical Wells................................................................................................................................................. 56 3.6.3 Inflow for Horizontal Wells............................................................................................................................................ 57 3.6.4 Flow in Multi-lateral Wells............................................................................................................................................ 58

3.7 TRADEMARKS.............................................................................................................................................................................71

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3.8 REFERENCES...............................................................................................................................................................................71

CHAPTER 4. DESIGNING A MULTILATERAL.................................................................................................................. 74 4.1 INTRODUCTION...........................................................................................................................................................................74

4.1.1 Cost Benefit Onshore/Offshore...................................................................................................................................... 74 4.1.2 Re-entry versus New Well............................................................................................................................................... 74 4.1.3 Creating Laterals............................................................................................................................................................. 75 4.1.4 General Considerations (with respect to drilling and completing ML/MB wells) ............................................... 75

4.2 JUNCTION....................................................................................................................................................................................76 4.2.1 TAML Classification Scheme......................................................................................................................................... 76 4.2.2 Level 1-Open Hole Trunk and Laterals. ...................................................................................................................... 76 4.2.3 Level 2- Cemented Trunk and Open Lateral ............................................................................................................... 78 4.2.4 Level 3- Cased Hole Trunk, Mechanically Supported............................................................................................... 78 4.2.5 Level 4- Mother-Bore & Lateral Cased & Cemented............................................................................................... 79 4.2.6 Level 5- Pressure Integrity at the Junction................................................................................................................. 79 4.2.7 Level 6- Pressure Integrity at the Junction................................................................................................................. 80 4.2.8 Level 6s- Downhole Splitter: Surface or Downhole.................................................................................................. 80 4.2.9 Kick Off Methods........................................................................................................................................................... 80 4.2.10 Casing Exit Options...................................................................................................................................................... 80 4.2.11 Section Milling Vs Window Milling............................................................................................................................ 80 4.2.12 Section Milling Advantages and Disadvantages................................................................................................... 81 4.2.13 Window Milling Advantages and Disadvantages................................................................................................. 82 4.2.14 Casing Exit Selection Guide. ....................................................................................................................................... 83 4.2.15 Types of Junctions and Isolation Design................................................................................................................... 84 4.2.16 Cemented Junction Level 4....................................................................................................................................... 85

4.3 LATERAL PLACEMENT ..............................................................................................................................................................87 4.3.1 Drilling Flat Wellbore Trajectories.............................................................................................................................. 88 4.3.2 Accurate Wellbore Placement....................................................................................................................................... 88 4.3.3 Directional Profile Planning......................................................................................................................................... 88 4.3.4 Number of Completion Zones. ....................................................................................................................................... 89

4.4 JUNCTION INTEGRITY................................................................................................................................................................90 4.4.1 Lateral Entry Nipple System (LEN).............................................................................................................................. 91 4.4.2 Selective Re-Entry Tool (SRT)....................................................................................................................................... 92 4.4.3 Lateral Seal and Control System................................................................................................................................... 93 4.4.4 Mechanical tieback to the main casing string............................................................................................................ 93 4.4.5 Hydraulic seal of lateral from the main casing string............................................................................................... 94 4.4.6 Local Formation Damage.............................................................................................................................................. 95 4.4.7 Formation Characteristics at the Lateral Bore Kick -Off Junctions....................................................................... 95 4.4.8 Differential Pressure at the Junction........................................................................................................................... 95 4.4.9 Junction Tie-Back/Seal ................................................................................................................................................... 95 4.4.10 Junction Integrity and Risks........................................................................................................................................ 96 4.4.11 Potential Sand Production from the Junction........................................................................................................... 96

4.5 COMPLETION AND PRODUCTION CONTROL ............................................................................................................................96 4.5.1 Desirable goals for the design of ML Completion Systems...................................................................................... 98 4.5.2 Completion systems ......................................................................................................................................................... 98 4.5.3 Completion requirement................................................................................................................................................. 99 4.5.4 Technical Criteria............................................................................................................................................................ 99 4.5.5 Completion Options.......................................................................................................................................................100 4.5.6 Completion Selection Parameters...............................................................................................................................100 4.5.7 Completion Design........................................................................................................................................................101

4.6 SELECTIVE ISOLATION.............................................................................................................................................................102 4.7 LATERAL COMPLETION...........................................................................................................................................................103

4.7.1 Possible lateral completions........................................................................................................................................103 4.7.2 Sand control/stimulation re-entry requirement for laterals linked to junction/completion specs. ..................104 4.8.1 Screening Process..........................................................................................................................................................106 4.8.2 Risk data, based on operational experience..............................................................................................................108

CHAPTER 5. CURRENT MULTILATERAL TECHNOLO GY ......................................................................................111 5.1 INTRODUCTION TO VENDORS AND SPONSORS.....................................................................................................................111 5.2 DRILLING ..................................................................................................................................................................................111

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5.2.1 Window Milling/Whipstocks and Associated Equipment........................................................................................111 5.2.2 Junction Integrity...........................................................................................................................................................124 5.2.3 Selective Access..............................................................................................................................................................125

5.3 LATERAL MONITORING AND CONTROL................................................................................................................................128 5.3.1 Vendors............................................................................................................................................................................128 5.3.2 Completion Tools and Control Sleeves......................................................................................................................128 5.3.3 Monitoring and Measuring Systems...........................................................................................................................131 5.3.4 Smart/ Intelligent Completions....................................................................................................................................139 5.2.5 Reeled Completions and Systems................................................................................................................................143 5.3.6 Reeled Systems ...............................................................................................................................................................146 5.3.7 Expandable Tubular Technology................................................................................................................................148

5.4 REFERENCES.............................................................................................................................................................................148 5.4.1 Contact Information......................................................................................................................................................148 5.4.2 Other Documents ...........................................................................................................................................................151

CHAPTER 6. PLANNING MULTILATERAL WELL .......................................................................................................152 6.1. CONTRACTING CONSIDERATIONS........................................................................................................................................152 6.2. CONTRACTING SEQUENCE.....................................................................................................................................................152 6.3 PRE-QUALIFICATION...............................................................................................................................................................152

6.3.1. Evaluation Notes: .........................................................................................................................................................154 6.3.2. Pre-Qualification -- Definitions.................................................................................................................................154 6.3.3. Pre-Qualification -- Example Questionnaire...........................................................................................................155 6.3.4. Pre-Qualification Technical....................................................................................................................................161 6.3.5. Pre-Qualification Review.........................................................................................................................................162 6.3.6. OPTIONAL Pre-Qualification Financial Considerations..............................................................................164 6.3.7. Pre-Qualification Financial Considerations - Example.....................................................................................164

6.4. COMMERCIAL TENDER...........................................................................................................................................................169 6.4.1. Multi-Well Contract .....................................................................................................................................................171

6.5. SPECIFICATIONS AND EVALUATION DATA.........................................................................................................................177 6.5.1. Evaluation Data............................................................................................................................................................177 6.5.2. During the Contractor Tender Preparation.............................................................................................................178

6.6. EVALUATION...........................................................................................................................................................................178 6.7. QRA APPLICATIONS...............................................................................................................................................................179

6.7.1 QRA Applied to Evaluation of Multilateral Equipment and Services Vendor....................................................179 6.7.2 QRA Applied to Incentive Contracts: .........................................................................................................................179

CHAPTER 7. ESTABLISHING MULTILATERAL WELL .............................................................................................180 7.1 GENERIC MAIN PROCEDURES................................................................................................................................................180 7.2 CONTINGENCY PLANNING......................................................................................................................................................180 7.3 MAKING A JUNCTION..............................................................................................................................................................181

7.3.1 Example Generic Procedure for Placement and Orientation................................................................................181 7.3.2 Example Detailed Basic Procedure for Constructing an Isolated Junction........................................................181 7.3.3 Specific procedures for Anadrill Rapid Access Multilateral..................................................................................183 7.3.4 Debris Management......................................................................................................................................................183 7.3.5 Junction Consolidation.................................................................................................................................................184

7.4 CASE STUDY: MULTILATERAL WELL INSTALLATION ON TROLL FROM A FLOATING RIG............................................184 7.4.1 Technology Requirements............................................................................................................................................184 7.4.2 Effect of Rig Heave on the Multilateral Operations................................................................................................184 7.4.3 Weight on Bit and Depth Positioning.........................................................................................................................185 7.4.4 Placement of Downhole Equipment on Depth..........................................................................................................185 7.4.5 Horizontally Placed Junction......................................................................................................................................185 7.4.6 Unconsolidated Reservoir Sand..................................................................................................................................185 7.4.7 Field Installation Results..............................................................................................................................................185

7.5 SAFETY......................................................................................................................................................................................186 7.5.1 Well Control Procedures..............................................................................................................................................186 7.5.2 Technical Safety.............................................................................................................................................................186 7.5.3 Procedural Safety ..........................................................................................................................................................187 7.5.4 HSE Management (specific to ML wells) ..................................................................................................................187

7.6 CHECKLIST FOR PLANNING AND EXECUTION OF MULTILATERAL WELLS......................................................................188

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CHAPTER 8. OPERATING A MULTILATERAL WELL................................................................................................190

8.1 STARTING UP THE MULTILATERAL WELL ...........................................................................................................................190 8.1.1 Completion Integrity .....................................................................................................................................................190 8.1.2 Clean-up..........................................................................................................................................................................190 8.1.3 Junction Integrity...........................................................................................................................................................191

8.2 MAINTENANCE AND TESTING.................................................................................................................................................192 8.2.1 Full Access Capability..................................................................................................................................................192 8.2.2 Intervention.....................................................................................................................................................................192 8.2.3 Well Treatments .............................................................................................................................................................192 8.2.4 Workovers .......................................................................................................................................................................192

8.3 RESERVOIR MONITORING, PRODUCTION ALLOCATION AND CONTROL ..........................................................................192 8.3.1 Well Testing and Logging.............................................................................................................................................192 8.3.2 Production Logging.......................................................................................................................................................192 8.3.3 Pressure Build Up Testing ...........................................................................................................................................193 8.3.3 Production Allocation...................................................................................................................................................193 8.3.4 Performance Prediction................................................................................................................................................194 8.3.5 Branch Flow Control ....................................................................................................................................................194

8.4 ABANDONMENT .......................................................................................................................................................................195 8.4.1 Abandonment Objectives..............................................................................................................................................195 8.4.2 Testing of Abandonment Plugs....................................................................................................................................195 8.4.3 Abandonment of Lateral Branches.............................................................................................................................196 8.4.4 Methodology of Abandonment.....................................................................................................................................196 8.4.5 Cement Types to be used for Abandonment ..............................................................................................................197 8.4.6 References on Abandonment........................................................................................................................................197

8.5 SAFETY PARTICULAR TO ML WELL ......................................................................................................................................197 8.6 EVERY DAY OPERATION..........................................................................................................................................................197

8.6.1 Production Allocation...................................................................................................................................................198 8.6.2 Crossflow during Shut in..............................................................................................................................................198

APPENDIX 1. RISK-BASED TIMES AND COSTS RESULTS. ......................................................................................199 A1.1 SUB SEA MULTI-LATERAL WELL......................................................................................................................................199

A1.1.1 Rig On Location..........................................................................................................................................................199 A1.1.2 Set Template.................................................................................................................................................................199 A1.1.3 Drill 36" Hole and Set 30" Casing (103 m - 165 m) .............................................................................................200 A1.1.4 Drill 17 1/2" Hole and Set 13 3/8" Casing (103 m - 1900 m) .............................................................................201 A1.1.5 Drill 12 1/4" Hole.......................................................................................................................................................202 A1.1.6 Run and Cement 9 5/8" Casing (100m - 2900m)..................................................................................................205 A1.1.7 Drill 8 1/2" Hole (Reservoir Section) (3000 m - 4500 m)...................................................................................207

A1.2 LOGGING 8 1/2" RESERVOIR SECTION (MAINBORE)......................................................................................................210 A1.2.1 Pipe Conveyed Combo (3000 m - 4500 m)...........................................................................................................210 A1.2.2 Pipe Conveyed RFT (3000 m - 4500 m) .................................................................................................................212 A1.2.3 Run and Cement 7" Liner (2900 m - 4500m) ........................................................................................................214 A1.2.4 Bottom Completion in Mainbore..............................................................................................................................215 A1.2.5 Run Pipe Conveyed CBL/CET (2900 m - 4500 m) ...............................................................................................217

A1.3 BUILD JUNCTION...................................................................................................................................................................219 A1.3.1 Set Whipstock ...............................................................................................................................................................219 A1.3.2 Set Whipstock ...............................................................................................................................................................220 A1.3.3 Drill 8 1/2" Lateral (Reservoir Section) (2870m - 3900m) ...............................................................................222

A1.4 LOGGING 8 1/2" RESERVOIR SECTION...............................................................................................................................225 A1.4.1 Pipe Conveyed Combo (2870m - 3900m) ...............................................................................................................225 A1.4.2 Pipe Conveyed RFT (2870m - 3900m) ....................................................................................................................227 A1.4.3 Recover Mainbore.......................................................................................................................................................228 A1.4.4 Run Top Section of Completion................................................................................................................................230

APPENDIX 2. ACTIVITY FLOWCHARTS ..........................................................................................................................232 A2.1 SUB SEA ML WELL..............................................................................................................................................................232 A2.2 DRILL SUB SEA WELL..........................................................................................................................................................233 A2.3 DRILL 12 1/4 HOLE (BASE CASE - SILICATECONVERTED TOOLS).............................................................................234 A2.4 SET 9 5/8 CASING ..............................................................................................................................................................235

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A2.5 DRILL 8 1/2 HOLE (RESERVOIR SECTION).....................................................................................................................236 A2.6 SET 7" LINEAR (ACROSS RESERVOIR)...............................................................................................................................237 A2.7 COMPLETE MAINBORE.........................................................................................................................................................238 A2.8 BUILD JUNCTION...................................................................................................................................................................239 A2.9 RECOVER MAINBORE...........................................................................................................................................................240

APPENDIX 3. CASE STUDIES ..................................................................................................................................................241 A3.1 AERA, TAML LEVEL 6........................................................................................................................................................241 A3.2 ANETH FIELD UTAH, TAML LEVEL 1...............................................................................................................................242 A3.3 BRUNEI. TAML LEVEL 1 5 .................................................................................................................................................250 A3.4 DUNBAR.................................................................................................................................................................................257 A3.5 IDD EI SHARGI NORTH DOME,. TAML LEVEL 4.............................................................................................................260

A3.5.1 History:.........................................................................................................................................................................260 A3.5.2 Objectives:....................................................................................................................................................................260 A3.5.3 Well design requirements:.........................................................................................................................................260 A3.5.4 Sperry-Sun RMLS characteristics:...........................................................................................................................260 A3.5.5 Well Construction:......................................................................................................................................................260

A3.6 OSEBERG C-12, TAML LEVEL 5........................................................................................................................................262 A3.6.1 The Business Driver. ...................................................................................................................................................262 A3.6.2 Well Objectives............................................................................................................................................................262 A3.6.3 System Description, Technical Requirements........................................................................................................262 A3.6.4 Well Completion, Production Performance. ..........................................................................................................263

A3.7 PRUDHOE BAY ......................................................................................................................................................................264 A3.8 TROLL, TAML LEVEL 4: MULTILATERAL WELL INSTALLATION FROM A FLOATING RIG ........................................265

A3.8.1 The Business Driver....................................................................................................................................................265 A3.8.2 Technology Requirements..........................................................................................................................................265 A3.8.3 Effect of Rig Heave on the Multilateral Operations. ............................................................................................265 A3.8.4 Weight on Bit and Depth Positioning......................................................................................................................265 A3.8.5 Placement of Downhole Equipment on Depth.......................................................................................................265 A3.8.6 Horizontally Placed Junction. ..................................................................................................................................266 A3.8.7 Unconsolidated Reservoir Sand...............................................................................................................................266 A3.8.8 Field Installation Results...........................................................................................................................................266

A3.9 SOUTH FURIOUS FIELD. TAML LEVEL 4.........................................................................................................................267 A3.9.1 Objectives:....................................................................................................................................................................267 A3.9.2 Well Design Considerations: ....................................................................................................................................267 A3.9.3 Junction Seal/Integrity:..............................................................................................................................................267 A3.9.4 Well Construction:......................................................................................................................................................267 A3.9.5 Problems and Lessons Learned:...............................................................................................................................267

APPENDIX 4. REFERENCES ....................................................................................................................................................269

Chapter 1. Introduction Page 7 of 277

Chapter 1. Introduction

1.1 Preliminaries

Multi-lateral wells are not a new concept but their successful application has increased over the last decade. Multi-laterals represent an alternative well construction strategy to complement vertical, inclined, horizontal and extended reach well trajectories.

Multi-laterals can be utilised as a strategy for both new wells, as well as existing wells in oil and gas reservoirs. A range of geometrical configurations are available to provide the optimum economic benefit in specific reservoir scenarios. The complexity of the technology is also variable, depending upon well requirements ranging from simple commingled side tracks, to complex multiple laterals, each offering individual pressure isolation, flow control and intervention capabilities.

Whilst new technologies such as multi-laterals offer considerable benefits in certain scenarios, they do introduce greater complexity in terms of the reservoir management / exploitation and the drilling / completion of the wells. To date, many companies have applied multi-laterals but the diversity of applications and engineering systems deployed, has created a variable learning curve for the industry.

1.2 Objectives

The objectives of the Guide are to promote the Advancement of Multi-lateral Technology (TAML) through the share and dissemination of methodologies, technology and experience in relation to multi-laterals. It is not intended that the Guide be prescriptive, but rather provide a framework for petroleum or project engineers evaluating, designing or implementing multi-laterals in new or existing petroleum reservoir developments.

The Guide therefore provides a focal point for the operating and service sectors to collate, review and disseminate knowledge, and experience with multi-laterals. The form of the Guide utilises both conventional media as hard copies, as well as electronic access, using a web-site to allow rapid updating and access to the most current information available.

This should result in the site being the major source of data and information and hence a focus to drive forwards the successful development and application of multi-laterals.

The organisation and deliverables of the TAML Project are shown in Figure 1.1.

* Project Manager/Lead Contractor

** Sub-contractor

OperatingCompanies

ServiceCompanies

and Vendors

Field Data

Product Info

Design Procedures

Design Data

Contractors

Well ServiceTechnology A/S *

Advanced WellAssociates Ltd **

Deliverables

Design Guide

ML Database

TechnologyTransfer

Objectives

Efficient deployment of multilaterals

Efficient technologyProving/Development

Greater understandingof the benefit/risk

Figure 1.1 TAML Project Structure, Deliverables and Objectives

1.3 Scope of the Guide and Database

The Guide recognises the need for a broad range of considerations and inputs for the successful design and implementation of multi-laterals, and hence is multidisciplinary in perspective.

Chapter 1. Introduction Page 8 of 277

It covers the processes and criteria for:

Initial screening / validation of the application of a multi-lateral.

Assessment of the business case in terms of benefits, cost and risk.

The design process, criteria, options and considerations.

Multi-lateral project management and planning.

Operational implementation to drill and complete the multilateral.

Operation of the multi-lateral well through the entire life cycle.

The scope of the Guide is depicted in the chronological process is shown in Figure 1.2.

Screeningof the

Application

Developmentand

Assessmentof the

BusinessCase

DesignFormulation

andConfiguration

ProjectPlanning

andManagement

Implementationof the

Multilateral Well

Operationand

Controlof the

Multilateral

Figure 1.2 Chronological Sequence of ML Project

1.4 Guide and Database Development

The development and maintenance of the multi-lateral Guide and Database is being conducted on behalf of a consortium of operating and service companies by Well Service Technology a/s (WST). WST is lead contractor, Project manager and ultimately will act as site custodian of the web-site for the TAML Guide and Database.

Advanced Well Associates Ltd (AWA) is a partner on the project, subcontracted to WST to contribute its expertise to the design guide, database and web-site, as well as having responsibility for document production.

1.5 Format for the Guide

It is essential that the Guide and its associated database be continuously updated and accessible to sponsors, to provide them with the greatest potential benefit from the commercial application of multi-laterals. The Guide and its associated database will be available in both hard copy and electronic format, as shown in Figure 1.3.

The Guide will be periodically reviewed and updated by the project contractor over the period 1998 - 2000. Bound copies will be distributed to sponsors as updated. The Guide will also be available on a read-only basis through the web-site. Access to the Guide through the web-site will be restricted to project sponsors.

The database will be formulated by WST and posted on the web-site. Sponsors will have the mechanism to search and add to the entries on the database.

The mechanisms for data supply and access are depicted in Figure 1.3. The web-site will possess a notice board to promote inter-company dialogue and information exchange.

Control of access, manipulation of data, quality and confidentiality will be implemented by the site custodian.

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FieldData

ProductInfo.

PublicDomain

Info.

OperatingCompanies

Vendors/Service

Companies

TAMLProject

Contractors

ProjectCoordinator/

SiteCustodian

MLDesign

Guide & Database

WebSite

TechnicalSteering

Committee

Hard CopyUpdates

Deliverables

Database

Design Guide

Noticeboard

Data Source TAML Project Delivery Formats Figure 1.3 TAML Project Input and Delivery Mechanisms

Chapter 2. Basic Facts on Multilaterals Page 10 of 277

Chapter 2. Basic Facts on Multilaterals

2.1 Definition and Description

2.1.1 Definition

The general definition of a multi-lateral well is one in which there is more than one horizontal or near horizontal lateral well drilled from a single side (mother-bore) and connected back to a single bore.

Generally a multi-branched well is one which has more than one branch well drilled from a single site connected back to a single bore. The branch may be vertical horizontal, inclined or a combination of the three. A multi-lateral well is always a multi-branched well however the reverse is not necessarily true.

Multilateral completions improve reservoir drainage by allowing access to fractured or thin layer reservoirs from existing wellbores without drilling new wells. Scenarios can range from simple barefoot multilaterals to sophisticated multibranch, selective re-entry systems.

2.1.2 Geometry of Multi-lateral and Multi-Branched Wells

Generally the following naming convention is used to describe the well geometry of multi-lateral wells

The well configuration may be described as stacked, planar or opposed. For more complex configurations a physical description may be used e.g. Y-well or herring bone. The number of laterals may be described as dual-lateral, tri-lateral, quadrilateral etc.

Table 2.1 contains some of the well configurations that are possible in multilateral and multibranch drilling.

Stacked Dual and Tri-Lateral

Dual-Opposed Lateral and Stacked Opposed Quadrilateral

Planar Dual-lateral or planar Y-well

Planar Tri-Lateral


Planar Offset Quadrilateral

Planar Opposed Quadrilateral or Herring-Bone Pattern

Stacked/ Inclined Tri-Lateral

Radial Quadrilateral

Radial Tri-lateral Extending from a primary vertical wellbore

Stacked Radial Quadrilateral

Table 2.1: Various well configurations


2.2 Historical Review

The principle of initiating new wellbores from existing wellbores is not a new one. Russian engineers experimented with multilaterals in the 1950s as a development of horizontal drilling practices. A Russian engineer Alexander Grigoryan drilled the first Russian multilateral well. The well had nine branches emanating from the mother and subsequent mother bores. A diagram of the well is depicted in Figure 2.1. Another multilateral well was drilled in 1968 in East Siberia with no more cases documented until the mid-1980s. These early forms of branched well drilling evolved from open-hole sidetracking techniques that were used to avoid obstacles encountered in the main wellbore. But in the intervening years, it became apparent that the drilling of several branches into a reservoir from a main wellbore could result in additional benefits in terms of improved drainage, productivity and well economics.

At the present time there are two major areas where multi-lateral drilling is used significantly; Russia and America. In addition, the technique is being increasingly adopted in the Middle East, South America, Canada and Europe for both new well applications and as a re-entry technique in existing wells.

Figure 2.1: Russias first multilateral, 1953

In the Austin Chalk region of Texas, USA, ML wells have grown in popularity since the 1990s. A number of operators have drilled open hole sidetracks from the main wellbore, with a view to increasing production and exploiting irregular shaped leases. Between 1987 and mid 1995, 315 multilateral wells were drilled in the USA. The majority of these wells were in the Austin Chalk region, although multi-laterals have also been drilled in Wyoming, California, New Mexico, and Michigan. The growth of multilateral wells drilled in this period is illustrated on Figure 2.2:

0102030

405060708090

100

1987 - 1990 1991 1992 1993 1994 1995* (6/95)Years

Nu

mb

er of W

ells

Figure 2.2 Rate of US Multilateral Drilling (period 1987-1995). Total Wells: 315

In Western Canada, where ML activity has been increasing, many level 1 and 2 completion systems have been installed.


In Europe, a number of Multi-Lateral wells have been drilled and completed since the mid-1990s. Elf Aquitaine completed one of Europes first multi-lateral wells in 1984 in the Paris Basin, France. This well was a three-legged multi-drain well. More recently, in 1995, the UKs North Seas first tri-lateral was completed by Phillips Petroleum.

Recently, the first successful deployment of a level 5 system was performed on Norsk Hydros Oseberg field in the North Sea. Table 2.2 shows a historical summary of multi-lateral development.

In recent years drilling and completion methods have become good enough to allow an every increasing number of wells to be completed as multilaterals. The question that should be asked is not whether a multilateral system is available, rather what type of multilateral, if any, is best suited to the reservoir and production needs.

Year Operator Field Type Milestone

1953 Bashikiria, Russia Onshore

1957 Borislavneft, Ukraine Onshore

1950s Chernomorneft, Russia Onshore

1968 Markova, Eastern Siberia Onshore

1984 Elf Aquitane Eschau, France Onshore

1988 Louisiana, USA Onshore 10 laterals from one single horizontal wellbore

1989 Arabian Oil Co. Khafji, Saudi Arabia Offshore

1992 Maersk Kraka, Denmark Offshore First in North Sea

1993 ADCO Abu Dhabi Onshore

1993 Texaco Austin Chalk, USA Offshore

1993 Unocal Dos Cuadras, USA Offshore

1993 Maersk Dan, Denmark Offshore

1994 Mobil Galahad, UK Offshore First in UKCS

1995 Phillips Alison, UK Offshore First tri/quad lateral in UKCS

1996 Petronas Bokor, Malaysia Offshore First tri-lateral well in Asia (level 5)

1996 Norsk Hydro Oseberg, Norway Offshore First level 5 completion installed

1997 PDO Shuaiba, Oman Onshore Record dual/tri-lateral wells

Table 2.2: Summary of Multilateral Development

2.3 Classification System

The TAML classification is split into two tiers Complexity ranking Functional classification


2.3.1 Complexity Ranking

An indication of the ML/MB junction complexity is defined by a number between 1 and 6. In a well that has more than one junction, the most complex one would be referred to. These complexity ratings are illustrated in table 2.3.

2.3.2 Functionality Classification

The second tier is sub-divided into two sections Well description Junction description

The functionality classification provides more technical detail on the major ML/MB well attributes. The system combines elements of the tier one classification to describe critical functionality characteristics of the well.

Well Description

The well description is broken down into four major categories

1. New/Existing Well. Two distinct applications where issues such as the method of casing exit and the ability of achieving pressure integrity at the junction require different approaches

2. Number of Junctions. Important to a wells complexity. Currently the majority of wells are drilled dual lateral however as the technology advances and experience with the technology is gained the average number of laterals drilled will increase.

3. Well Type (Producer with or without artificial lift, Injector or Multipurpose). The functionality requirements of a producer are different from that of an injector, particularly the levels of pressure integrity required at the junction and pressure exerted during well shut-in.

4. Completion Type (Single, Dual or Concentric Bore). Describes the completion above the production packer, which will in turn have an impact on the type of equipment required at the junction.

Table 2.4 gives the alphanumeric classification code used to describe the well.

Level Description Illustration

1 Open/ Unsupported Junction

Barefoot mother-bore & lateral or slotted liner hung-off in either bore

2 Mother-bore Cased and Cemented

Lateral Open

Lateral either barefoot or with slotted liner hung-off in open hole


3 Mother-bore Cased and Cemented Lateral Cased but not Cemented

Lateral liner anchored to mother-bore with liner hanger but not cemented

4 Mother-bore and Lateral Cased and

Cemented

Both bores cemented at the junction

5 Pressure Integrity at the Junction

Straddle packers or (integral) mechanical casing seal

.(Cement is not acceptable)

6 Pressure Integrity at the Junction

Achieved with the casing

(Cement is NOT acceptable)

6S Downhole Splitter

Large main well bore with 2 (smaller) lateral wellbores of equal size

Table 2.3: TAML Completion Complexity Rankings

Well Status Number of Junctions Well Type Completion Type

New 1 PA Producer with artificial lift

S Single Bore

PN Producer with natural lift

Existing 2 IN Injector D Dual Bore

3 MP Multipurpose C Concentric bore

Table 2.4: Well Description

Junction Description

The second area is the junction description, which focuses on the following


1. Connectivity. For a dual lateral, this indicator would be the same as that included in the Tier 1

ranking. Wells with more than one junction would have a unique level indicator for each junction, which may or may not be similar. The most complex junction would determine the overall well complexity ranking. In addition to level, a pressure rating would also be included where applicable (e.g. Level 5 5000 psi)

2. Accessibility (No Selective Re-entry, Re-entry by Pulling Completion or Through Tubing Re-entry). Describes the level of re-entry, which is catered for during the life cycle of the well. Although window apertures can be re-entered on a trial and error basis by utilising bent joints, if there is no fixed datum from which the aperture can be easily located the lateral is deemed to have no re-entry capability. Table 2.5 illustrates the accessibility options.

3. Flow Control (None, Selective, Separate, Remote Monitoring or Remote Monitoring and Control). Describes the degree of control over the production or injection fluid flow across the junction. Monitoring includes any of the following: Pressure Temperature Flow Sand production Scale deposition Saturation profile Seismic SCSSSV status Well integrity Corrosion

Table 2.6 illustrates the flow control options. The junction description is given by the alphanumeric classification codes using the letter or number highlighted, in Table 2.7.

Description Illustration

No selective re-entry

Re-entry by pulling completion

Through-tubing re-entry

Table 2.5 Examples of Accessibility Options


Description Illustration

Commingled

Selective

Separate

Table 2.6: Examples of the Flow Control Options

2.3.3 Classification Code Examples

Table 2.8 contains 2 examples of the classification codes.

Connectivity Accessibility Flow Control

Same categorisations as Tier 1 ranking system (plus pressure rating if applicable)

Level 1

Level 2

Etc.

NR No selective re-entry

PR Re-entry by pulled completion

TR Through tubing completion

NON None

SEL Selective

SEP Separate

REM Remote monitoring

RMC Remote monitoring and control

Table 2.7: Junction Description Alphanumeric Classification. Each junction is described from bottom to top.


Complexity Rating Well Description Junction Description

Example 1: Level 2; Ranking N-1-PN-S/ 2-TR-SEL

Level 2

Mother-bore cased and cemented; lateral open

N-1-PN-S

New well

One junction

Producer-Natural lift

Single bore completion

(above production packer)

2-TR-SEL


Through tubing re-entry

Selective Production

Example 2: Level 5; Ranking E-2-IN-D/2-PR-NON/5 (3,000 psi)-TR-SEP

Level 5

Pressure integrity at the (upper) junction, achieved with the completion

E-2-IN-D

Existing well

Two junctions

Injector

Dual bore completion

2-PR-NON


Re-entry by pulling completion

No flow control

5 (3,000 psi)-TR-SEP

(Upper junction)

Pressure integrity at the junction (3,000 psi)

Through tubing re-entry

Separate production

Table 2.8 : Examples of TAML Classification


2.4 Overview of Current Status

2.4.1 Statistics when, where, how, how many

Currently, the number of multilateral wells of level 3 complexities and above makes up only a small percentage of the total number of multilaterals drilled worldwide. The majority (>1000) are of the more simple level 1 and 2 completions. This can be seen on figure 2.3, which illustrates this graphically.

When we look more closely at the break up of the level 3 to 6, figure 2.4, it is clear that the more complex ML systems are not in wide spread usage. This clearly indicates that operators are nervous about utilizing multilateral completions of the higher complexity.

0

200

400

600

800

1000

1200

Wel

l Nu

mb

ers

Level 1-2 Level 3-6 Figure 2.3 Comparison of Level 1 -2 Completions versus Level 3-6 Completions.

Illustration of ML Drilling Activity Worldwide up to 1997

0

20

40

60

80

100

120

Level 3 Level 4 Level 5 Level 6

Nu

mb

ers

of W

ells

Figure 2.4 The Number of Worldwide Level 3 to Level 6 Completions. Illustration of ML

Drilling Activity Worldwide in 1997

Examples of Level 3 and 4 Completions

1. Weyburn Unit, Saskatchewan, Canada (1995)

This field was discovered in 1954 however by 1992 several factors ruled out drilling more vertical wells. Experiments with single laterals drilled underbalanced but many proved uneconomical especially in thinner bedded, lower permeability reservoirs.


Operator PanCanadian next step was a program of dual lateral wells with the eventual aim of using quad laterals in the lower quality Marley pay. In all, 25 quad laterals were drilled underbalanced. They estimated that production was 40% better than the single lateral programme, which more than made up for the 20% increased cost.

PanCanadian then proceeded to a quad-lateral well campaign, and the region selected was the east end of the Weyburn formation. They chose this region due to the low production and the fact that dual laterals proved uneconomical. Their plan was to drill two opposing build sections, 180o apart. The well was termed a multiple-build system, which required Mechanical simplicity Access to either build section without sophisticated or expensive equipment Running and cementing liners across each build section and back to the host wellbore Full diameter entry to vertical well bore after cementing the liners over the build section

For these reasons, PanCanadian chose a system from Sperry-Sun called the Retrievable Multi-Lateral System (RMLS). It has since been used on a further 16 quad laterals in the Weyburn Unit.

2. Peace River, Alberta, Canada (1995)

The Peace River oil sands deposit in Alberta, Canada contains viscous, 7.5o API Crude produced by expensive thermal recovery processes. During the 1990s, horizontal wells were utilised to increase the economics of the field.

Shell Canada decided to try to use a pre-milled window in a vertical well, through which the horizontal well could be drilled. They felt that this would provide a straight, vertical sump section below the junction for artificial lift.

For these reasons Sperry-Suns Lateral Tie-Back (LTBS) system was chosen, the system contained an orientable, pre-milled window and was installed in April 1995. The technology was so successful that over the next 12 months Shell Canada installed a further 11 LTBS systems.

3. Idd El Shargi North Dome Field, Qatar (1995)

The field has been producing since 1964. In 1995 the new field operator, Occidental Petroleum of Qatar, and Qatar General Petroleum Corp. decided to drill multilateral wells. They planned to re-enter each well bore selectively for reservoir management, and for this reason Sperry-Suns RMLS system was chosen.

The first well that Occidental planned to use the Sperry-Sun system, cementing and tool retrieval technologies on was to field test all the equipment. The well was completed as a single horizontal producer.

The plan for the second, IS 75, was to complete it as a water injection well. The upper lateral was drilled through the window into the Shuaiba A formation. However, production from the well was so good that Occidental decided to complete the well as a producer. The option still remained to complete the lower lateral through the main well bore.

The third well in the campaign to be completed was IS 76. This well fully exploited the multilateral window system. IS 76 was completed as a bi-planar multilateral with medium reach horizontal well bores in to both the Shuaiba A and B formations. The well was completed as a 5 single producer with the ability to selectively re-enter either lateral. Each lateral was independently stimulated with coiled tubing.

The following 2 wells were competed following the basic plan of IS 76. The costs were. However, reduced because casing scraper runs, cement bond logs, clean outs and junk basket trips were eliminated. Through Occidentals experience they conclude that the cost of re-entering the lateral was about 30% of the cost of a single horizontal well. They also found


that long-term maintenance costs were favourably affected since both production horizons could be assessed through the common wellbore.

4. Bokor Field, offshore Malaysia (1996)

In September 1996, PETRONAS Carigali drilled the first tri-lateral horizontal well in South East Asia. Baker Hughes completed the TAML-level-5 well using their Cemented Root system. The system was chosen because it met the following requirements Selective production from each lateral Full isolation of each lateral wellbore from the overburden formation Sand exclusion

The Bokor well has two production tubing strings from the formation to the surface, with the option of production from the third layer in one of the strings. Redundant pressure isolation was achieved by running two strings from the upper and lower laterals to the surface. This was to allow future coiled tubing operations in either of the two branches.

5. Eldfisk Field, Norwegian Sector, North Sea (1996)

During September 1996, Phillips Petroleum of Norway completed a bi-planar multilateral well using a system with selective re-entry and isolation capabilities. The well had been abandoned in 1984 until August 1996 when the exiting casing was section-milled at about 1,195 ft to allow re-drilling of the bi-planar well.

The system chosen by Phillips was a combination of Sperry-Suns and Dress Oil Tools multilateral system primarily for selective isolation and shut in capabilities while maximizing the production intervals. By using both systems, it allowed increased reservoir exposure while permitting non-rig intervention and independent flow control. The combined system also provided access to the primary casing, maintaining full bore access to the lateral, which may be re-entered at any time during the life of the well.

The system made possible The first commingled multilateral production in the Norwegian North Sea First Norwegian North Sea well to utilise a pre-drilled lateral casing window Economic exploitation of a tight formation Successful isolation of the upper lateral while performing a 10,000 psi acid fracture

stimulation in the lower section Coiled tubing post-completion lateral re0entry and stimulation

Example of Level 5 Wells

1. Oseberg C platform, Norwegian Sector, North Sea (1996)

From the Oseberg C platform Norsk Hydro identified more field targets than remaining platform slots allowed. In May 1996, in Oseberg C-12 they decided to install Halliburtons newly developed System 3000 using Weatherfords casing exit and whipstock technology. This was the first well in the world which provided full bore re-entry access to a hydraulically isolated lateral with a single casing size reduction, while maintaining the integrity of the main wellbore.

The second well, Oseberg C-10, has been producing from the lateral since October 1996 and commingled production started in March 1997. The third well, Oseberg C-7, was completed in March 1997 and commingled production began shortly thereafter.

2.4.2 Proprietary Systems and their TAML Classification


The table below illustrates the current multilateral systems available and how the systems cross reference with the TAML classification:

Level Baker-

Hughes TIW Sperry-Sun

Halliburton Anadrill

Schlumberger

Smith Red Baron

1 Open-hole retrievable whipstock

COBRA

Selective/ Non-Selective Lateral Entry (SLE/ NSLE) with open-hole/ inflatable anchor

2 No name yet

MLAS MOLE TTR

LTBS & RMLS

RAPID Connect

SLE with big bore prod. packer thru-tubing ML system Hydraulic set full bore ML system

3 Flow past whipstock FORMation junction Hook Hanger

MLAS MOLE TTR

LTBS & RMLS System 3000 System 4501 w/o cement

perforated ML whipstock hollow whipstock ML Selective/ Non-selective ML Liner system thru-tubing ML liner system

4 Cemented root Cemented hook hanger

MLAS MOLE TTR

LTBS & RMLS System 4501, 4502, 4503

RAPID Access, MLPS

hydraulic set full bore ML system SLE with big bore prod. packer Thru-tubing ML system

5 SRT System SHD System

As above MSCS System 5000

PCE MLR dual bore ML system single selective ML system

6 FORMation junction

As above

6s Splitter system, Deepset Splitter

Table 2.9: Proprietary Systems and their TAML classification

Notes:

TIW Systems relate primarily to the anchor and/or packer used in all cased hole situations and, therefore, can be used with any completion of level 2 and above.

2.4.3 Future Adaptations and Developments

Recent developments, which will spur multilateral use, include

1. Latex cement. Due to its high tensile and flexural strength, durability and crack resistance makes latex cement suitable for cementing the junction in multilateral wells. It provides a better seal than normal cement. Also, due to better rheology, less shrinkage and reduced permeability latex cement is better for gas migration control than regular cement.


2. One trip milling systems. One trip milling systems with more accurately centred

windows and more effective management of debris will avoid problems later on in the well life.

3. Intelligent completions. Intelligent completions will provide complete control and monitoring of reservoir performance during a wells life with a minimum of intervention operations.

4. Lateral stimulation and workover. Little has been done so far to address the problem of stimulating a multilateral well. Several companies provide tools for re-entering completed laterals with conventional drill string, but they have found little use in the field. Only coiled tubing has been used to date for stimulating. To date, TIW, PCE, Dresser Oil Tools, and Weatherford have developed multilateral selective re-entry systems. This consists of an isolation plug in the main well bore and a selective whipstock, the MLR system allows lateral bore access with CT string for logging, perforating, cleaning and stimulating. Baker oil tools have also developed several systems for re-entering the lateral and main bore through production tubing using coiled tubing or wireline. Multilateral jobs with these systems are planned for the future

The future for multilateral technology is undisputed. Future challenges for ML wells, which will increase their applicability. 1. Reliable multilateral equipment

2. Downhole lateral flow control and measurement

3. Data transmission

4. Application of ML technology to a wider variety of hole sizes

5. Fully cemented liner systems for total zonal isolation.

6. Formation damage must be minimised through the adoption of suitable drilling and completion practices since this will optimise the productivity between/along laterals. If the formation damage is minimised and / or the impairment mechanisms is overcome or removed, the clean-up process has a greater chance of success.

7. Downhole separation equipment. Separation of gas from a lower lateral leg, right above the junction, and inject it into an upper lateral for a downhole gas drive, will be one of the more important ultimate aims of downhole separation technology.

8. Simpler systems as in Level 6

As multilateral technology coupled with intelligent completions, look-ahead look aside drilling and coiled tubing drilling together with 4D seismic profiling, the ability to manage a reservoir like a process plant will be attained. Drilling in real time will enable reacting pro-actively rather than in retrospect, which is often too late.


2.5 Business Case Flowchart

The overall process to generate the business case for ML/MB wells:

Validate Applicationof ML/MB in Concept

Pre-Design Screening

Analytical Model

Simulation

Economic Evaluation

Risk Weighting

Decision TreeAnalysis

Geometry forParticular Reservoir

Type

Rank AlternativesIdentify Critical

Features

Rates and Sensitivityto Geometry and

Reservoir Parameters

Production Profile

NPV DCF

Productivity NPVDistribution

Operational andContingency

Planning

2.5.1 List of Benefits

A ML/MB well is defined as a well that has more than one branch drilled from a single site and connected back to a single wellbore. Drilling ML/MB wells into a reservoir from a main wellbore may offer various potential benefits such as a reduction in the number of wells, improvements in both productivity and ultimate recovery, leading to enhanced economics in both new and mature fields.

Nevertheless, the prime objectives are to access the recoverable reserves without incurring large well development costs. The cost objectives may be difficult to satisfy with


conventional vertical or horizontal well technology when reservoir zones are thin or contain unfavorable permeability barriers or faults. Because ML/MB technology uses the best aspects of both conventional and horizontal well technology, many fields could benefit from its application.

2.5.1.1 Technical and economic advantages

The technical advantages can be summarised as follows: Reservoir exposure can be increased for production or injection strategies, especially in

heavy or viscous-oil, depleted naturally fractured and tight reservoirs. Areal connectivity can be increased to reduce coning and cusping effects, reduce

sanding potential, increase vertical and horizontal sweep efficiencies and enhance gravity drainage in reservoirs that have a high fluid-density mobility contrast

More efficient exploitation of complex geological features Delineation of field periphery

Economic advantages: Re-entry: cost of original well, casing and surface location have already been written off

or may be acquired at reduced cost Offshore ML/MB strategies can provide more production/pay-section exposure for each

platform/template slot Permit and planning costs can be reduced in comparison to operating from multiple sites

utilising conventional technology ML/MB technology offers the potential to reduce CAPEX within a field development

strategy The application of ML/MB can be used to increase productive reserves

2.5.1.2 Risks and disadvantages.

The application of any new technology carries elements of economic risk and technical complexity.

Technical disadvantages: Well intervention requirements Reservoir monitoring and management Drilling risk and reservoir description uncertainty Issue of well control during drilling and production operations Impairment and clean-up of individual lateral or branch

Economic disadvantages: Concentrated investment and economic risk (i.e. the potential economic cost of losing

the mother wellbore or a lateral/branch in an ML/MB system) Dependent on relatively new technology OPEX is not well defined due to the risk element

2.5.2 Cost and Risk implications

Nowadays, completing wells using ML/MB technology may be achievable from a technical point of view. However, when looking at the economic feasibility of the project, the risks and costs associated with this emerging technology are still difficult to quantify e.g. the inherent costs associated with the implementation (Capex) and maintenance (Opex) of the whole system may be considerably higher than conventional completion strategies.


Capex/Opex considerations:

Planning resource requirements for a multi-lateral project is extensive compared to conventional projects, since the technology is more complex and is still in its infancy. The key point is to find out, what the operators want in terms of system design features to provide added value to the field development. Service companies have developed level 3 and level 4 ML/MB hardware kits but these need to be customized to withstand specific downhole reservoir conditions and situations. As a result the development costs of such systems will increase significantly.

In addition to this, operational experience with respect to these highly complex completion systems is just starting to be acquired however there is still a lot to be learned to improve the current installation and maintenance procedures.

A critical assessment to reduce overall well costs can be summarized as follows:

Identify the reservoir parameters and conditions that are most likely to affect the operational state of the system components so that design precautions can be taken to resolve these potential problems at an early stage (e.g., near-wellbore effects, downhole vibrations, production of abrasive solids, corrosion, scaling)

Identify and simulate the critical static and dynamic reservoir conditions that are most likely to cause component failure (e.g., severe fluctuations in flow, pressure and temperature regime)

Quantify the associated risk in terms of system (component) reliability by defining the function of each downhole component and stating its relevance to the overall system and well objectives (areas to focus on are: intervention and servicing requirements and back-up functions)

Identify the additional components that are required to safely produce a ML/MB well in a specific field application (this depends for a great deal on the attitude towards risk management). Initiate the development of an industry-shared database to steadily improve the selection, design and implementation techniques of ML/MB completion systems.

Finally, unless a system component or material is not critical in its function to provide additional value to the overall production system it should be regarded as one component more to possibly fail.

To demonstrate and complement the issues mentioned above, ML/MB drilling development costs will eventually drop by reducing the number of wells to drain a reservoir. Table 1 below shows the decrease in cost per lateral for single horizontal wells, dual, quad and six laterals in Texacos Greater Aneth field.

Number of Laterals Total Cost ($) Cost per Lateral ($)

Single 385,000 385,000

Dual 505,000 253,000

Quad 700,000 175,000

Six 950,000 158,000


Dual laterals have more than doubled the production in the past, and quads have produced nearly five times more oil. Whereas horizontal wells increase reserves by a factor of 1.5 to 1.7, multi-lateral wells increase probable reserves by a factor of nearly 2.3.

Risk assessment

As stated before the implications of costs for ML/MB wells during its production life are to a great extent dependent upon the planned intervention and servicing operations. As the number of ML/MB wells increase the issues of reservoir monitoring, maintaining adequate well productivity or performing remedial operations become more important. The ability to perform these operations is constrained by two major parameters: internal-diameter restrictions and lateral/branch accessibility.

Level of risk

One can identify different levels of risk:

Task-performance risk1: High risk areas are the actual physical building of the junction and drilling of the lateral for level 3+ ML/MB wells (e.g.: whipstock setting and milling, cementing of junction and drilling lateral)

Task-performance risk2: High-risk areas include hydraulic fracturing, screen placement, and through tubing operations once the ML/MB completion is in place. Lower risk areas include operating sliding sleeve doors and setting plugs

Location risk: High-risk areas include operations performed in the upper lateral/branch because production from the entire system is potentially at risk. Operations performed near the junction in any of the laterals/branches are high risk because hydraulic integrity may be a problem and production from all laterals/branches is potentially at risk.

Intervention risk and strategy

Once risk level is established the assessment process should determine whether intervention risks can be eliminated or minimized by use of preventive measures or by adopting a different intervention strategy to reduce associated cost. The following (technical) points should be taken into account:

Is intervention really necessary or alternatives available? If reliable data are available for the reservoir or well, the use of preventive measures or built-in options should be considered at an early completion design phase to avoid any downtime at a later stage.

If intervention cannot be avoided can the extent of intervention be minimized? It may be possible to reduce or simplify the number of operations required and thereby minimize non-productive time with an alternative completion system or by adopting different intervention techniques.

Are complex operations or new technologies required? If non-standard equipment procedures and practices are adopted, the degree of risk and non-productive time incurred will be affected, especially if the system is a prototype.

Can equipment be operated by CT, wireline or remote control? If equipment can be manipulated without a full workover or by remote control, the intervention requirements will be minimized.

Are back-up systems available? Any risk assessment should consider providing a back-up system in the event of primary system failure and the consequent implications for access to laterals/branches through a back-up system

Could problems with the regulatory authorities and resulting cost implications arise? Specific safety requirements may need to be satisfied during the course of ML/MB intervention work.


How many laterals and or branches are required for a specific reservoir candidate? Take into account geological and petro-physical constraints in conjunction with technical/economic production objectives, since they tend to resolve the drainage or injection strategy, hence the number of laterals required.

What level of complexity is required? Assess the lateral accessibility, isolation constraints.

What is the position of the lateral in the reservoir? Especially with respect to:

Depth limitations for coiled tubing and wireline operations.

Complexity and the tortuosity of the re-entry path.

Type of re-entry operation performed.

Type of re-entry equipment used in the well.

Physical properties and compatibility of the produced fluid from individual zones. Desirability or feasibility to commingle*

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Chapter 3. Deciding on a Multilateral Page 30 of 277

Chapter 3. Deciding on a Multilateral

3.1 General Conditions Favoring ML solution

As described previously the benefits of using a multi-lateral well are directly linked to the ability to reach multiple targets with a single well of multiple dimensionalitie s.

The conditions that favour or re