Report EP 94 - 1130 April 1995
CONFIDENTIAL
CORE ANALYSIS MANUAL by
H.H. Yuan SIPM EPD/222 and B.A. Schipper KSEPL RR/37
Contributions by R.M.M. Smits KSEPL RR/37
J.G. Maas KSEPL RR/44
PETROPHYSICS AND RESERVOIR ENGINEERING
This document is confidential. Neither the whole nor any part of this document may be disclosed to any party without the prior consent of Shell Internationale Petroleum Maatschappij B.V., The Hague, the Netherlands. The copyright of this document is vested in Shell Internationale
Petroleum Maatschappij B.V., The Hague, the Netherlands. All rights reserved. Neither the whole nor any part of this document can be reproduced, stored in any retrieval system or transmitted in any form or by any means (electronic, mechanical, reprographic, recording or otherwise) without the prior written consent of the copyright owner.
SHELL INTERNATIONALE PETROLEUM MAATSCHAPPIJ B.V., THE HAGUE EXPLORATION AND PRODUCTION
The authors appreciate the comments and review of the following people: P.van Ditzhyijzen, SIPM- EPD/22
A.J.T. Grimberg, SIPM- EPD/21 A.B. Graper, SIPM- EPD/21 H.Niko, SIPM- EPD/221 P.R.A. Betts, SIPM- HTRH/52 P.M.T.M. Schutjens, KSEPL- RR/37 K.A. Heller, SIPM- EPD/21
J.P. van Hasselt, EPX/43 E.C. Thomas, SOC F.R. Bradburn, SOC
Many petrophysicists at PDO, EXPRO, NAM, SSB, SVEN
SUMMARY
Core analysis is the acquisition of experimental data measured on core material for determining parameters used for developing and managing a hydrocarbon reservoir from initial discovery to mature field development. There are two main reasons for core analysis. Firstly, core analysis data are used by petrophysicists to calibrate wireline logs in the determination of hydrocarbon reserves. Such data include routine core analyses as well as special core analyses such as measurement of electrical parameters for resistivity log interpretation. Secondly, reservoir engineers use core analysis measurements such as relative permeability and pore volume compressibility to provide input parameters for reservoir computer simulation. Core analysis data are also used by other disciplines such as for production technologists to determine injectivity and well performance and for explorationists in quantifying acoustic rock properties. Geological core analysis (the subject of a manual in preparation) is done to establish the geological framework of a reservoir.
Careful planning of a core analysis programme requires the involvement of an integrated team of petrophysicsts, geologists, reservoir and production engineers and explorationalists to ensure that core measurements meet critical data needs. Since optimum analysis programmes require multi-disciplinary input, the manual is prepared in such a way to assist teams of petroleum engineers to develop core analysis programmes. The contribution of each PE discipline is highlighted. An appendix on application of value of information concepts as applied to core analysis is given to provide a clear method for evaluating and justifying core analysis projects. The various parameters which can be obtained from the analysis of core material are discussed briefly. The available measurement techniques are detailed and discussed briefly. The available measurements techniques are detailed and recommendations are made concerning the reliability of the techniques and how best to obtain quality results. Core sampling guidelines that allow easier application of core data, proper core preparation procedures, core screening methods for obtaining representative cores, wettability considerations and ancillary measurements that ensure quality and data applicability are described in detail.
GENERAL RECOMMENDATIONS
• A core analysis programme should be assembled with input from all PE disciplines to ensure that the right data are measured with the proper procedures on appropriate core material. The core analysis programme allows proper planning in the multi-disciplinary environment and assists in the management of the core analysis programme.
• Value of information concepts should be used for core analysis programmes and in programmes justification.
• High quality data can be obtained with careful core selection, core screening and core preparation steps. Proper core screening is necessary to obtain relevant core data. • Extensive special core analysis programmes can be discussed with SIPM EPD/22 and/or
KSEPL RR/37 to assist in decisions as to where work should be carried out.
• SIPM recommends that special core analyses be carried out in-house using facilities at KSEPL. Bellaire Technology Center, Houston, and Calgary Research Center can be used as alternatives with sufficient prior planning and available capacity. If Shell E&P laboratories are not available, core contractors approved by SIPM can be used.
• If contractor laboratories are used it is recommended that KSEPL be requested to carry out duplicate special core analysis measurements on a small number of samples in order to verify the performance of the contractor. A review of any extensive core analysis programme is recommended and will be provided by SIPM upon request.
• To date quality assessments have been made on the techniques used at Core Laboratories, Simon Petroleum Technology, Poroperm-Geochem, GAPS Geological consultants, and Corex (Aberdeen). SIPM recommends regular quality assessment of any core analysis contractor involved in Shell work.
• All measured data, procedures and equations used should be requested from the analysis laboratory, including any data used in calculating final results such as raw data.
Recommendations for multi-disciplinary core analysis planning
A core analysis programme should address all issues in core analysis from justification, core acquisition, well-site handling, core preparation to core measurements and data application. Planning should include the following:
• Justification (see Chapter 2 and Appendix 1) and clearly stated objectives of the core analysis programme (Chapters 3 and 4)
• Core acquisition considerations including type of coring bit, core barrel, overbalance, drilling fluids, well-site handling (see Core Handling Manual); core transport and fluid sampling considerations
• Multi- disciplinary input ensuring proper utilisation of core material and representatives of the samples to be used in the core analysis programme
• Core analysis considerations including types and scope of the core analysis, numbers of samples, core sample screening methodology, core preparation methodology especially cleaning, experimental conditions (confining pressure, temperature, pore pressure, fluids to be used, experimental duration, etc), wettability conditions and so on
• Fluid analysis considerations focusing on types of fluid analyses that can be used to support interpretation of core data. It is a frequently overlooked aspect of formation evaluation • Detailed measurement sequence defining expected measurements on each core sample.
Scheduling is important so that the performing organisation can meet the deadlines required for core analysis data
• Costs and value of information concepts used in programme justification
• Finalised core analysis programme allowing each discipline to contribute and to agree to the goals and methods and allows for better project management.
Acknowledgements
Many people have contributed to the preparation of this manual. Staff at SIPM and KSEPL have extremely reviewed the manual and helped in ways too numerous to detail. Interest and review from Opcos has also encouraged us to make the manual as useful as possible. The information contained in this manual has been collated from a number of previous SIPM and KSEPL publications. Especially useful in the writing of this report were:
EPD/22/23
SIPM Coring Series Bulletin l: Core Justification
EP 88-1465 EPD/22/23
SIPM Coring Series Bulletin IIl: Core Analysis
EP 89-0105
Rock Characteristics Research – Special Core Analysis
KSEPL, brochure 1991.
B.A. Schipper, R.J. van den Oord, and S.J. Adams
Petrophysical Core Analysis Contractors - Procedures and Quality Assessment .
EP 92-1355
S.J. Adams and R.J. van den Oord
Capillary Pressure and Saturation Height Functions
EP 93-0001
This manual completes a series of three manuals dealing with aspects of coring, core handling and core analysis. The first two manuals are:
J.A. Okkerman and L.C. van Geuns
Core Handling Manual
EP 93-2200
L.C. van Geuns and J.A. Okkerman (in preparation)
Geological Core Analysis
Contents
SUMMARY I
GENERAL RECOMMENDATIONS II
Recommendations for multi-disciplinary core analysis planning III
Acknowledgements IV
List of Figures (p) denotes photo X
List of Tables XIV
1. Introduction 1
1.1 About this manual 2
1.2 Problems solved by core analysis 6
1.3 SIPM/KSEPL recommendation on core analysis 8
1.4 Availability of other Shell E&P Laboratories for core analysis 9
1.5 Quality in core analysis 10
1.6 Literature 12
2. Economics of core analysis 13
2.1 Value of Information (VOl) 14
2.1.1 VOl Nomenclature 15
2.1.2 Value of prospect screening - Summary 16
2.1.3 Value of project optimisation - Summary 17
2.1.4 Value of correct core analysis data - Summary 18 2.2 Value of information examples as applied to core analysis projects 19
2.2.1 Example 1 - Prospect Screening (Unconsolidated Sandstone) 20
2.2.2 Example 2 - Ekofisk 22
2.2.3 Example 3 - Project optimisation 24
2.2.4 Example 4 - An Opco VOl Example 26
2.3 Core analysis aspects of VOl 28
2.4
Literature 30
3. Planning a core analysis programme 31
3.1 Planning in an integrated PE team 34
3.1.1 Core analysis programme development 35
3.2 The core analysis programme 36
3.2.1 An example of a core analysis programme 39
3.3 Considerations for major lithologies 43
3.4 Where to perform the core analysis programme 46
3.5 Literature 47
4. Core and fluid analysis considerations 48
4.1 Scope of a core analysis programme 49
4.2 Multi- disciplinary considerations 50
4.2.1 Petrophysics 50
4.2.2 Geology 51
4.2.3 Reservoir engineering 53
4.2.4 Other disciplines 54
4.3 Core measurements 55
4.3.1 Basic core analysis 55
4.3.2 Special core analysis 58
4.4 Coring considerations and well-site planning 61
4.5 Core handling 64
4.5.1 At the well-site 64
4.5.2 Upon arrival at the Laboratory 65
4.6 Core screening 66
4.7 Core sampling 67
4.7.1 Sampling for basic core analysis 67
4.7.2 Sampling for special core analysis 68
4.7.3 Sampling considerations 69
4.8 Core sample preparation 70
4.9 Core sample screening for special core analysis 71
4.10 Core preservation 75
4.11 Fluid measurements 76
4.11.1 Brine measurements 76
4.11.2 Oil measurements 77
4.12 Fluid handling considerations 78
4.13 Sequencing and scheduling 79
4.14 Costs 79
4.15 Economic impact and justification 79
4.16 Project reporting 80
4.17 Project review 83
4.18 Literature 85
5. Core preparation 86
5.1 Plug drilling 87
5.1.1 Drilling consolidated samples 87
5.1.2 Drilling unconsolidated samples 88
5.2 Core cleaning 90
5.2.1 Cleaning consolidated samples 90
5.2.2 Cleaning unconsolidated samples 91
5.3 Core drying 93
5.3.1 Oven drying 93
5.3.2 Critical Point Drying (CPD) 94
5.3.3 Humidity controlled drying 97
5.4 Review of some contractor preparation procedures 98
5.5 Literature 101
6. Basic core analysis 102
6.1 Porosity and grain density 103
6.1.1 Bulk volume by buoyancy in mercury 103
6.1.2 Bulk volume by mercury displacement 104
6.1.3 Bulk volume by caliper 106
6.1.4 Pore volume by liquid saturation 107
6.1.5 Grain density by pycnometer 108
6.1.6 Grain volume by buoyancy 110
6.1.7 Grain volume by Boyle's law porosimetry 112
6.2 Steady- state gas permeability 114
6.2.1 Air permeability 114
6.2.2 Probe permeability 118
6.3 Fluid saturations 120
6.3.1 Fluid saturations by Dean-Stark extraction 120
6.3.2 Retort method or summation of fluids 122
6.4 Literature 123
7. Porosity and permeability at stress and whole core analysis 124
7.1 Stressed Porosity 125
7.1.1 Stressed pore volume by liquid saturation 125
7.1.2 Stressed pore volume by Boyle's Law porosimetry 126
7.2 Stressed permeability 127
7.2.1 Stressed steady-state permeability 127
7.2.2 Pulse decay permeability 129
7.3 Whole core analysis 131 7.3.1 Whole core porosity by Boyle's Law porosimetry 132
7.3.2 Whole core steady-state gas permeability 133
7.3.3 Other whole core measurements 135
7.4 Literature 137
8. Capillary pressure 138
8.1 Mercury/air capillary pressure 141
8.1.1 Mercury/air capillary pressure by high pressure injection - Autopore 9200, 9220 142 8.1.2 Mercury/air capillary pressure by pressure equilibrium 145
8.1.3 Stressed mercury/air capillary pressure 147
8.2 Oil/water capillary pressure 149
8.2.1 Oil/water capillary pressure by centrifuge 150
8.2.2 Oil/water capillary pressure by pressure equilibrium 152
8.3 Gas/liquid capillary pressure 154
8.3.1 Gas/liquid capillary pressure by centrifuge 155 8.3.2 Gas/liquid capillary pressure by porous plate vessel 156
8.4 Literature 157
9. Electrical properties 158
9.1 Formation Resistivity Factor, FRF, and cementation exponent, m 159 9.2 Resistivity index, I, and saturation exponent, n 161
9.2.1 Resistivity index by pressure equilibrium 163
9.2.2 Resistivity index by continuous injection 165
9.2.3 Resistivity index by porous plate vessel 168
9.2.4 Resistivity index by rapid desaturation 169
9.3 Cation Exchange Capacity (CEC) and Qv 170
9.3.1 Qve by membrane potential 171
9.3.2 Qv by multiple salinity measurements, Co-Cw 174
9.3.3 CEC by conductometric titration 176
9.3.4 CEC by absorbed water correlation 178
9.4 Literature 179
10. Wettability and interfacial tension 180
10.1 Wettability 181
10.1.1 Cleaned-state samples 183
10.1.2 Restored-state samples (aging) 184
10.1.3 Fresh-state samples 185
10.1.4 Preserved-state samples 186
10.1.5 Pressure-retained core samples 186
10.1.6 Restored state vs native state 187
10.2 Wettability determination 189
10.2.1 Amott 190
10.2.2 United States Bureau of Mines method (USBM) 192
10.2.3 Other wettability determination methods 193
10.3 Interfacial tension 194
10.3.1 Interfacial tension by 'Pendant Drop' 195
10.3.2 Surface tension by 'du Nouy balance' 197
10.3.3 Interfacial tension by spinning drop tensiometer 199
10.4 Literature 200
11. Relative permeability 202
11.1 Steady-state measurement 204
11.1.1 Relative permeability by steady-state 204
11.2 Centrifuge measurement 207
11.2.1 Oil/water relative permeability by centifuge 207
11.2.2 Gas/liquid relative permeability by centrifuge 211
11.3 Unsteady-state measurement 213
11.3.1 Oil/water relative permeability by unsteady state displacement 213 11.3.2 Gas/liquid relative permeability by unsteady-state displacement 216
11.4 Relative permeability at reservoir conditions 219
11.4.1 Restored-state (see section 10.1.2) 219
11.4.2 Native-state (see section 10.1.3, 10.1.4, 10.1.5) 219
11.5 Literature 220
12. Mechanical rock properties 222
12.1 Compressibility 223
12.1.1 Uniaxial compaction 224
12.1.2 Hydrostatic compaction 230
12.1.3 Oedometer compaction test 231
12.2 Rock strength parameters 232
12.2.1 Rock strength by triaxial testing 233
12.2.2 Brinell Hardness Number (BHN) 234
12.2.3 Thick-Walled-Cylinder strength test (TWC) 236
12.2.4 Unconfined Compressive Strength test (UCS) 239
12.3 Acoustic properties 240
12.3.1 Acoustic Travel Time (ATT) 240
12.4 Literature 241
13. Supplementary tests 244
13.1 Rock analyses 245
13.1.1 Grain size by laser diffraction 247
13.1.2 Grain size by sieve analysis 249
13.1.3 Grain size by image analysis 250
13.1.4 Source rock analysis 251
13.1.5 Cap rock/seal analysis 253
13.2 Fluid analyses 254
13.2.1 Counter Current Imbibition (CCI) 254
13.2.2 Oil and gas analyses 256
13.2.3 Formation water and core water analysis 258
13.3 Rock-fluid compatibility 260
13.3.1 Compatibility flood 260
13.4 Miscellaneous tests 263
13.4.1 Acid response test 263
13.4.2 Solvent flushing - for wax removal 266
13.5 Literature 267
14. Core analysis research activities 268
14.1 Rock characteristics 269
14.1.1 Ultrasonic Velocity Cell (UVC) 269
14.1.2 Acoustic transmission anisotropy 271
14.1.3 Apparatus for Pore Examination (APEX) 273
14.1.4 Resistivity 275
14.2 Fluid flow 277
14.2.1 Capillary Pressure and Resistivity Index by Continuous Injection, CAPRICI 277 14.2.2 Relative Permeability at Reservoir Conditions (3-phase), REPARC-3 279
14.2.3 Critical gas saturation 281
14.3 Supplementary 283
14.3.1 Nuclear Magnetic Resonance, NMR 283
14.4 Literature 287
APPENDIX 1 Value of Information 289
A1.1 Value of prospect screening 290
A1.2 Value of project optimisation (reducing uncertainty) 293 A1.3 Value of correct core analysis data (Shell EP Laboratories vs contractors). 297
APPENDIX 2 Core screening techniques 301
A2.1 X-Ray Computer Tomography scanning (CT) 302
A2.2 Core gamma ray 306
A2.3 X-ray fluoroscopy 308
A2.4 Coreslab inlarging 309
A2.5 Literature 311
APPENDIX 3 Petrophysical data from geological analysis 312
A3.1 Microstructure/Petrography 313
A3.1.1 Petrography from Scanning Electron Microscopy (SEM) and Enhanced Image
Analysis (IA) 314
A3.1.2 Petrographic image analysis from thin sections 319
A3.2 Mineralogy 321
A3.2.1 X-ray diffraction 322
A3.2.2 Energy Dispersive X-ray analysis (EDX) 323
A3.2.3 Mineralog 324
A3.3 Literature 325
APPENDIX 4 Core analysis on small cores, sidewall samples and cuttings 326
A4.1 Small core samples from slim holes 327
A4.1.1 Analysis of a 13/4" diameter core 328
A4.1.2 Analysis of a 25/8" diameter core 329
A4.1.3 Further slim hole core analysis 330
A4.2 Sidewall samples 331
A4.2.1 Rotary drilled samples 332
A4.2.2 Percussion sidewall samples 333
A4.2.3 Sidewall sample measurement techniques 334
A4.3 Cuttings 335
A4.3.1 Collection/sampling 335
A4.3.2 Measurement techniques used in cuttings analysis 337
A4.4 Literature 339
APPENDIX 5 Sponge core analysis 340
A5.1 Oil-Wet sponge analysis 341
A5.1.1 Sponge analysis by gas chromatography 341
A5.1.2 Other oil-wet sponge analysis techniques 342
A5.2 Water-wet sponge analysis 343
A5.3 Literature 344
Appendix 6 Conversion from hydrostatic to uniaxial strain conditions 345
Points 348
Index 349
List of Figures (p) denotes photo
2.1 Economic choices for example 1 - prospect screening 2.2 Economic choices for example 2 - Ekofisk
2.3 Economic choices for example 3 - project optimisation 2.4 Economic choices in Opco example
3.1 Flow diagram for core analysis planning
4.1 Recommended flow diagram for basic core analysis 4.2 Recommended flow diagram for special core analysis (p) 4.3 Longitudinal CT-scans (tomograms) of a core plug (p) 4.4 Flow diagram highlighting core analysis data review 5.1 Drilling plugs with liquid nitrogen (p)
5.2 Unconsolidated sample cleaning apparatus at KSEPL (p) 5.3 A conventional drying oven (p)
5.4 Illustration of the principle of critical point drying 5.5 A sample after CPD (p)
5.6 A sample after air drying (p)
6.1 Bulk volume by buoyancy in mercury at KSEPL (p) 6.2 Pycnometer at KSEPL (p)
6.3 An automated pycnometer (p)
6.4 Grain volume by buoyancy at KSEPL (p) 6.5 Schematic of a typical Boyle's law porosimeter 6.6 A typical Hassler-type core holder
6.7 Capability for permeability anisotropy and air permeability measurements at KSEPL (p) 6.8 Air permeameter at KSEPL (p)
6.9 Schematic of a probe permeameter 6.10 Dean-Stark apparatus at KSEPL (p) 7.1 Schematic of stressed brine permeability 7.2 Schematic of pulse decay permeameter
7.3 Schematic of flow paths in whole core horizontal permeability measurements 7.4 Schematic of flow paths in whole core vertical permeability measurements 7.5 Whole core stressed porosity and FRF at CRC
8.1 Capillary pressure curve parameters 8.2 An Autopore 9220 (p)
8.3 Penetrometers for an Autopore 9200/9220 8.4 Schematic of mercury injection apparatus
8.5 Schematic of stressed mercury/air capillary pressure apparatus 8.6 Schematic of coreholder for high-speed centrifuge
8.7 Schematic of pressure equilibrium cell 9.1 Schematic of formation resistivity factor cell
9.2 Typical I-Sw relationships
9.3 Hysteresis in the I-Sw relationship
9.4 View of the cell for resistivity index by pressure equilibrium method, and oil/water capillary pressure curves (p)
9.5 Schematic of resistivity index by continuous injection
9.6 Multiple resistivity index by continuous injection cells at KSEPL (p) 9.7 Schematic of Qv by membrane potential
9.8 Membrane potential measurement at KSEPL (p) 9.9 Co as a function of Cw for a shaly sandstone
9.10 CEC by conductometric titration at KSEPL (p) 10.1 Wettability concepts
10.2 Initial water saturation on primary and secondary drainage -water-wet system
10.3 Diagram showing the difference between initial water saturation on primary vs secondary drainage
10.4 Amott and USBM wettability indices
10.5 Interfacial tension by pendant-drop apparatus (p) 10.6 De Nuoy balance at KSEPL (p)
11.1 Relative permeability curves
11.2 Effect of wettability on relative permeability 11.3 Schematic of steady-state apparatus at KSEPL
11.4 Schematic of core holder for centrifuge relative permeability measurements 11.5 Interior of centrifuge apparatus for relative permeability at KSEPL (p)
11.6 Comparison of centrifuge and steady-state method (first drainage with n-decane/nitrogen, Berea sandstone)
11.7 Schematic of unsteady-state apparatus at KSEPL (p)
11.8 Comparison of steady-state and unsteady-state (Welge) methods
12.1 Schematic of first triaxial compaction apparatus at KSEPL 12.2 Schematic of second triaxial compaction apparatus at KSEPL 12.3 Radial displacement transducer
12.4 Typical compaction curve showing axial displacement as a function of pore fluid pressure during uniaxial compaction under pore pressure depletion conditions.
12.5 Brinell Hardness equipment at KSEPL (p)
12.6 Pressure cell for thick-waIled-cylinder (TWC) strength test 12.7 Samples after thick-waIled-cylinder (TWC) testing (p) 12.8 Unconfined Compressive Strength (UCS) at KSEPL 13.1 A typical grain size analysis report
13.2 Grain size by laser diffraction (p) 13.3 Sample is immersed in toluene
13.4 Weight change with time indicates residual saturation
13.5 Typical residual-initial curve from counter current imbibition measurements 13.6 Automated compatibility flooding set-up
13.7 Typical acid response curve 14.1 Schematic of UVC cell at KSEPL 14.2 Acoustic transmission anisotropy 14.3 A representation of APEX data 14.4 Schematic of EMPRESS at KSEPL 14.5 Schematic of CAPRICI
14.6 CAPRICI at KSEPL (p)
14.7 REPARC-3 equipment at KSEPL (p)
14.8 Critical gas saturation experiment at KSEPL (p) 14.9 NMR spectrum from a rock sample
14.10 NMR spectrometer at KSEPL (p)
A1.1 Economic choices for prospect screening core analysis A1.2 Economic choices for optimisation core analysis
A1.3 Economic choices in selecting core analysis laboratory A2.l CT-scanner at KSEPL (p)
A2.2 Scanning of core material using CT-scanning A2.3 Natural core gamma ray scanner (p)
A2.4 A coreslab image
A3.l An SEM secondary electron (SE) image A3.2 An SEM back scattered (BSE) image A3.3 An SEM cathodoluminescence (CL) image A3.4 Quantitative analysis from SEM
A3.5 A thin-section image
A3.6 Analysis of the thin-section image
A4.l Autopore penetrometers used for drill cuttings analysis
List of Tables
4.1 Information from geological evaluation
4.2 Basic core analysis parameters and their uses 4.3 Information derived from core preparation 5.1 Laboratory comparison for core preparation 5.2 Summary of cost and timing in core preparation 10.1 Definitions of wettability
12.1 Brinell hardness number suggested loading schemes A3.1 Image Analysis Regression Statistics
1. Introduction
Core analysis encompasses techniques used to derive formation properties from core material taken from the well-bore. The techniques generally involve measurement on plug samples of the core material. In most cases, the sample should be maintained in or restored to a state that would be representative of the state of the material in the formation and may, for example, necessitate the application of appropriate stresses and/or temperature. In other cases, measurements are made on the matrix material itself without regard to representative state. Measurements range from the simplest determinations of porosity to the most complicated measurement such as three phase relative permeability measurement at reservoir conditions.
Core analysis measurements are of interest to a wide range of disciplines in EP from
petrophysics, geology and reservoir engineering to drilling, production and exploration. Because core analysis has so many customers, it must be a focus of the integrated efforts of PE teams throughout Shell.
1.1
About this manual
The core analysis manual presents a strategy and plan for obtaining the highest quality data and maximising the value from core analysis measurements. All too often core analysis fails to get the engineering attention that it deserves. Frequently, core analysis planning is done poorly, if at all, and the results of such efforts in terms of data acquired can often be confused and contradictory. Yet core analysis remains an important source of critical information for quantifying reservoir models and calibrating formation evaluation tools like wireline logging. As the construction of reservoir models becomes more sophisticated, the demand on acquiring properly measured formation properties using core analysis becomes that much more important.
This manual is about the many facets of core analysis, but the authors have taken the approach that business processes involving core analysis need to be "re-engineered" to reflect the
business needs of Shell EP companies in the '90's. Two important business processes begin this manual and they are:
• evaluating economic impact of core analysis and • planning a core analysis programme.
Chapters 1 to 4 address economic and planning issues of core analysis.
While core analysis can be expensive, the value of core analysis is generally much greater. Obtaining proper value for a given expenditure is key to hydrocarbon resource management. The economics of core analysis is the subject of Chapter 2 and is based on value of information concepts. Several examples are given to allow PE staff to realise the value of their own core analysis projects.
Once core analysis economics are assessed, a detailed core analysis programme should be assembled. The subject of Chapter 3 addresses the needs of core analysis planning in an integrated multi-disciplinary environment. As part of the planning process consultation with core analysis experts at SIPM/KSEPL is recommended. Proper planning leads to the delivery of high-quality core analysis data for the development of the hydrocarbon resource.
Chapter 4 discusses in more detail the issues to be addressed in getting started with a core analysis project. Chapter 4 should be used as a guideline for those who are starting afresh with a core analysis project. Every issue mentioned in Chapter 4 merits attention although the critical items deserve the most attention.
The remainder of the manual describes the most commonly used core analysis techniques performed today. The methods are included to ensure that PE staff are acquainted with general methodology of core analysis. The method descriptions are aimed to be sufficiently detailed to assist with planning, to allow optimisation of core measurements and to provide a means of ascertaining quality and consistency. Beginning with Chapter 5, a reasonably comprehensive survey of core analysis preparation techniques is given. From Chapter 6 onwards the progression from basic analysis to aspects of special core analysis is presented. Chapter 13 addresses supplementary analyses and chapter 14 presents an overview of research activities that
represent a glimpse into the probable future of core analysis activities. By necessity, brevity has been imposed in order to allow the manual to be of reasonable length. Each measurement technique is described by the following scheme:
• principle - brief description of how the measurement is made
• points - remarks that highlight critical aspects of the measurement. An assessment of every measurement is made whether the technique is recommended, acceptable or not
recommended.
The not recommended assessment is not to say that the technique always produces incorrect data but that the technique has inherent tendencies which make obtaining reliable data more difficult. General issues such as limitations, advantages and disadvantages are also addressed, including possible data handling issues.
• precision/accuracy - an estimate of uncertainty in measurement accuracy. The estimate is either expressed in measurement precision, repeatability or uncertainty in true value of a measured parameter.
• price/timing/number of samples - is designed to understand approximate costs, how long an experiment can take and an appropriate number of samples balancing cost and time involved.
Prices are meant to be approximate and can vary significantly (by up to 40%) between regions depending on market competition and local factors.
Timing addresses the length of time a measurement takes which should assist in the scheduling and timing of a core analysis project.
Recommended number of samples is the recommended minimum number to ensure reasonable characterisation of a rock type in a core analysis programme. More samples should be considered if the justification through Value of Information shows the measurement value to be very great, as might happen with any special core analysis measurement.
• peripheral measurements - necessary ancillary measurements which quantify
measurement consistency, check quality and assist in data interpretation. Without peripheral measurements, core analysis data are difficult to apply. Peripheral measurements deserve considerable attention and are the key to establishing a quality core analysis programme.
The Appendices contain information that may be useful relating to CT-scanning and CT-scanning interpretation. While the manual addresses measurements made from samples taken from whole core, core analysis can also be made on other material sources such as sidewall cuttings and drill cuttings.
There are many specialised core analysis techniques which are not included in this manual such as coal bed methane analysis. The authors have not attempted to provide an exhaustive manual on core analysis but to address the core analyses used in day-to-day Shell operations.
This manual completes a sequence of manuals on core, which are Core Handling, EP 93-2200, and Geological Core Analysis, (in preparation).
1.2
Problems solved by core analysis
Core analysis can be used to answer many questions related to the production of hydrocarbons from the subsurface. Besides providing the opportunity to directly see and describe the rock formations of interest, core analysis also provides:
• Detailed description of geological environment and setting including variation of lithology, rock composition and rock type along the length of the core.
• Important petrographic information can be obtained from microscopic examination of the core material using techniques such as thin sections, scanning electron microscopy, X-ray diffraction.
• Non-destructive imaging using the CT-scanner or core gamma scanner. • Values of routine petrophysical formation properties as a function of depth:
- porosity; - permeability; - grain density;
- oil and water saturations.
• Log interpretation parameter values: - Archies lithologic exponent, m; - Archies saturation exponent, n;
- Waxman-Smits parameters, clay conductivity, m* and n*; - Grain and fluid densities.
• Distribution of fluids within the hydrocarbon column from capillary pressure measurements. • Grain size distribution data for engineering application in well completion programmes and for
geological application in assessing heterogeneity and depositional environment.
• Values of special formation properties for reservoir engineering such as: - wettability;
- relative permeability:
- effective permeability to oil, - effective permeability to water; - initial water saturation;
- residual oil saturation.
• Values of exploration formation properties: - shear and compressional acoustic velocity; - acoustic impedance.
• Values of rock mechanical parameters used in production engineering and platform design: - rock strength;
- compressibility; - compaction;
- waterflood sensitivity.
• Tests for non-reservoir rock, seal analysis and source rock analysis.
• Fluid measurements are also important to provide a complete picture of the downhole environment. Brine properties such as composition and conductivity, oil properties such as viscosity, acid and base number and identification of gas/oil using High Pressure Liquid Chromatography (see HPLC manual).
• Measurement of the interfacial tension measurement between oil and water used in scaling mercury/air capillary pressure curves to oil/water systems.
1.3
SIPM/KSEPL recommendation on core analysis
Core analysis remains one of the critical sources of formation data necessary for proper
development planning and field management. Careful multi-disciplinary core analysis planning in the integrated PE environment is necessary for acquisition of high-quality core analysis data, avoiding sub-optimal data acquisition and improving data application. Proper economic analysis, including "Value of Information" concepts, are important in appreciating the role core analysis plays in field development and hydrocarbon resource management. SIPM recommends the application of Shell core analysis technology which has been developed at Shell E&P laboratories world-wide. Accordingly, critical special core analysis measurements should be performed at KSEPL, Rijswijk, where possible, or at a core contractor recommended by SIPM/KSEPL. At the very least, major core analysis programmes should involve SIPM/KSEPL, who will develop and maintain a strategy of core contractor quality assurance. Consultation with SIPM EPD/22 and KSEPL RR/37 (who will act as focal point for KSEPL) is recommended for any core analysis questions including core data interpretation.
1.4
Availability of other Shell E&P Laboratories for core analysis
1.4.1 Bellaire Technology Center (BTC), Houston, Texas
SIPM has reached an agreement with SOC which allows Bellaire Technology Center (formerly known as Bellaire Research Center) to provide core analysis services for SIPM when mutually convenient. Should KSEPL be unavailable to perform any critical core analysis, it is now possible to arrange, via SIPM, for work to be performed at Bellaire Technology Center, Houston, when facilities there are available. Bellaire Technology Center has a long history of excellence in core analysis and has developed numerous Shell standard techniques over the years. SIPM regards BTC as a source of high quality core analysis data which incorporates Shell technology.
Arrangements should be made through EPD/222. Consult with EPD/222 for further information.
1.4.2 Calgary Research Center (CRC), Calgary, Alberta
Calgary Research Center has developed considerable expertise in whole core analysis (see chapter 7). These capabilities can be used for Group whole core analysis when mutually convenient and should be arranged by SIPM EPD/222.
1.5
Quality in core analysis
The organising principle behind this manual is the central issue of quality in core analysis. Quality is a critical issue. How often does a user of core analysis ponder the question:
How valid are these core analysis data?
The process of formulating an answer to this question begins with the recognition that core material, by nature, is heterogeneous. This fact should be incorporated into every aspect of the acquisition of core analysis data from inception to final data delivery. Without incorporating quality into the performance of the core analysis project, data quality assessment is very difficult to perform. This manual endeavours to build quality into the process of core analysis measurement through prior planning and making the correct suite of core analysis measurements.
These steps are summarised as follows (and more details are provided in the appropriate sections in the manual):
• core analysis planning - much attention is focused on this activity because data
assessment is a multi-disciplinary activity. For example, rock sample selection must involve a geologist for proper attention to rock type.
• sample screening - this step is frequently omitted but all too often core analysis
measurements are performed on samples that are unfit for measurement even though they may appear as perfectly formed cylinders while they may contain internal heterogeneities invisible from the outside.
• peripheral measurements - this manual emphasises measurements that can and should be performed to determine data consistency, to check data quality or to improve data
application. Certain parameters are critical in the determination of core analysis quantities. The most important parameter in much of core analysis is the pore volume because it is the fundamental determinant of the porosity and all saturation measurements are normalised to the pore volume. Thus any error in pore volume is translated directly into errors in saturation.
• core analysis contractors - much of the core analysis performed for Shell companies is done by contractors. Their results suffer from a high degree of variability in quality.
Experience shows that it is important not only to use a reputable contractor laboratory but to use the most reputable personnel within those organisations. It is therefore advisable to pay attention to the individuals performing the core analysis within contractor labs and, where possible, to identify the key personnel who can provide quality data. It is insufficient to only trust the manager or sales representative. In general it is preferable to select a core analysis vendor that is capable of carrying out the bulk of the core analysis programme to reduce handling and transportation that degrades core.
• old core analysis data - many of the techniques presented in this manual can be applied to the examination of old core analysis data. The quality of old core analysis data is difficult to determine when quality assessment planning has been omitted. But the thinking and planning for a quality core analysis programme can assist in determining how old core analysis data may be assessed. Unfortunately, determining the quality of old core analysis is, at best, uncertain.
• application of core data - this manual is designed to allow core analysis measurements to be performed in which quality is assured so that core analysis data can be more easily applied. If core analysis is done with proper regard to geological setting, rock typing and sample screening incorporating proper wettability considerations, then the core data should be suitable for application in formation evaluation or reservoir simulation.
1.6 Literature
SIPM EPD/22/3SIPM Coring and Core Analysis Series Bulletin 1: Core Justification.
EP 88-1465, June 1988.
Advances in EP research 1990-1
SIRM research brief.
The Log Analyst Special Issue - Core Analysis
1991 European Symposia Abstracts - SCA and Formation Evaluation, September-October 1991 van der Grijp KH. and van den Oord R.J.
Well-Site Hydrocarbon Differentation using High Performance Liquid Chromatography (HPLC)
EP 93-0550. Keelan, D.K.,
Core analysis for aid in reservoir description
JPT November 1982.
Maas, J., Boutkan, V., Ligthelm, D.,
Fit-for-purpose basic reservoir data
Production newsletter, February 1993. Skopec, R.A,
Recent advances in rock characterization
The Log Analyst, May-June 1992, p 270. Tannemaat, R.,
Core analysis methods
EP 59259, BSP, April 1983. Haeringen, A. van,
Results of a conventional core analysis contractor comparison exercise.
EP 89-0234.
Schipper, B.A., Aperen, A.E. van, Looyestijn, W.J.,
Quality assessment of core analysis procedures of Core Laboratories Aberdeen.
EP 90-1886.
Schipper, B.A., Aperen, A.E. van, Looyestijn, W.J.,
Quality assessment of core analysis procedures of Poroperm-Geochem Limited, Chester.
EP 90-1901
Schipper, B.A., Hofman, J.P.,
Quality assessment of core analysis procedures of Corex Services Ltd, Aberdeen.
RKTR.93.052, May 1993 (EP 93-1296).
Schipper, B.A, Oord, R.J. van den, Adams, S.A.,
Quality core analysis - essential to our business!
Production Newsletter July/August 1992. Schipper, B.A.
Quality Assessment of the Core Analysis Services of Simon Petroleum Testing, Aberdeen.
RKTR.94.089, May 1994 (EP 94-0974)
2.
Economics of core analysis
Core analysis projects are subject to the necessity and rigor of economic justification. It is
important that each project be judged by its economic impact and not just solely on cost. Careful economic justification generally shows that core analysis projects have a far greater economic value than their cost. By recognising the economic benefits of core analysis, core analysis
projects can be developed that are optimised both technically and economically. Such justification applies to all aspects of coring and core analysis and to data acquisition in general.
The economics of core analysis is driven largely by its role in reducing uncertainty in formation properties, particularly hydrocarbon volume and hydrocarbon saturation. While accurate hydrocarbon volume determination depends on a number of variables such as geological architecture and reservoir distribution, a critical initial step is the determination of hydrocarbon saturation. Hydrocarbon saturation is mostly obtained from wireline resistivity logs. However, the actual relationship between hydrocarbon saturation and log resistivity is extremely variable, making calibration with core measurements an essential step. One Opco has estimated that an accurate understanding of their resistivity index-saturation (I-Sw) relations is worth at least US
$12.5 million annually in effective economic benefit, which is far more than the expenditure on core analysis. Other core analysis parameters are critical to decisions in managing a hydrocarbon resource. For example, sizing waterflood or water handling facilities can depend critically on relative permeability parameters, such as water endpoint relative permeability. Again economic impact studies show the value of the core analysis project usually far exceeds project cost. To determine the economic impact of a core analysis programme, Value of Information (VOI) concepts will be used which are detailed in Appendix 1. A summary of value of information concepts is given in section 2.1 and examples of how value of information is applied are given for a number of cases in section 2.2. Further discussion as to applications of VOI calculations are provided there.
2.1
Value of Information (VOl)
In making a decision whether to gather information, it is possible to determine the economic value of the information itself. Value of Information (VOl) concepts rationalise the decision as an act of choosing between alternatives, each of which have an economic impact. One alternative involves the gathering of the required (core analysis) information and the economic impact is calculated based upon the information provided. The other alternative is the economic impact calculated in the absence of the information. The difference in economic impact between the two alternatives is the value of information. At this point the value of information does not include the cost of the information gathering itself. Economic impact is calculated as Net Present Value (NPV).
Once the value of information is calculated, the decision whether to proceed or not is then based on the value of the information versus the cost of the information gathering. If the value of information is larger (see section 2.3 on Justification) than the cost of information gathering, then it is economically justified to obtain the information. The difference between the value of
information and the cost of the information gathering is called the Value of Appraisal. For core analysis, the economic alternatives are simply whether to proceed or not with the core analysis project. The fundamental choice of whether to proceed with a core analysis project is addressed in two different ways:
• prospect screening where there is very large uncertainty typically associated with exploration appraisal;
• project optimisation, where quantifying rock and fluid properties can narrow design criteria in development planning.
The basic theory behind the VOl calculation is presented in Appendix 1 (Appendix 1.1 for prospect screening and Appendix 1.2 for optimisation).
In the next sections, the VOl nomenclature and a summary of the concepts is presented so that the example presented in section 2.2 can be better understood but the reader is referred to Appendix 1 for complete details.
2.1.1 VOl Nomenclature
The terms used in VOl calculations in this manual are defined here.
NPV Net Present Value - economic impact of a development is expressed in terms of net present value. (-NPV denotes negative value.)
NPVi Net Present Value of ith branch
P(high) Probability of having high reserves; taken to be 0.33 because P(high), P(medium) and P(low) are taken to have equal probability of occurring.
P(low) Probability of having low reserves; taken to be 0.33. P(medium) Probability of having medium reserves; taken to be 0.33.
POCM Probability of correct measurement in core analysis. Value is very high for Shell E & P laboratories and about 0.75 for core contractor laboratories. (0.75 is a very conservative number based on estimates from Shell core analysis experts, some of whom feel that it is much lower especially for many special core analysis services such as resistivity index, relative permeability and compressibility.) POS . Probability of success.
VOA Value of Appraisal- value of information (VOl) minus the cost of gathering the information.
VOl Value of Information - value of information itself without any regard to the cost of gathering the information.
2.1.2 Value of prospect screening - Summary
In prospect screening discussed in Appendix 1.1, the VOl equation shows that information has a high value and that information gathering is economically justified. The justification stems mainly from the fact that expenditure for data acquisition is done to eliminate the possibility of making an unprofitable investment. Data acquisition is done to save on the loss of NPV if the field turns out to have insufficient hydrocarbon reserves. The VOl calculation covers all aspects of data acquisition from seismic data acquisition, wireline logging, production testing and core analysis. The VOl does not distinguish the value of the core analysis by itself and some method must by realised to assign benefit to the core analysis. In fact, the VOl of acquiring data is so high that it is easy to justify core analysis in exploration appraisal situations. Factors that must be known to properly evaluate the VOl for prospect screening are:
• probability of success, POS;
• the economic impact of the development if reserves are not present, -NPV3, which denotes a loss;
• a method of assigning the benefit of the VOl calculation to core analysis as part of overall data acquisition.
Using these factors in equation (A1.4) determines VOl for prospect screening.
2.1.3 Value of project optimisation - Summary
In optimisation discussed in Appendix 1.2, the VOI is calculated to assess the economic impact of reducing uncertainty by performing the core analysis project. The VOI calculation is based on reservoir simulation results which are aimed at determining the NPV impact when the uncertainty in a critical parameter is examined. The sensitivity analysis for this critical parameter is done by using the maximum and minimum parameter values. In optimisation calculations, the total benefit of VOI calculation is due to the core analysis project. A VOI optimisation calculation requires the economic impact of 4 scenarios to be performed usually by reservoir simulation. For a given rock parameter, such as relative permeability, 4 cases are considered as follows:
• high reserves, optimised high with economic impact, NPV1, in this case, the most optimistic scenarios are used which translates into using a high oil relative permeability curve and a low water relative permeability curve and optimistic endpoint saturations;
• low reserves, optimised low with economic impact, NPV3, in this case, the most pessimistic scenario is used e.g. a low oil relative permeability curve and a high water relative
permeability curve and pessimistic endpoint saturations;
• high reserves, base case with economic impact, NPV 4, is the case where an average case is used which might be using average relative permeability curves with optimistic endpoint saturations;
• low reserves, base case with economic impact, NPV6, is the case where an average case is used which might be using average relative permeability curves with pessimistic endpoint saturations.
The results of these cases are used in equation (A1.8) to determine VOI for project optimisation. As described in Appendix 1.2, NPV2 and NPV5 do not enter in the calculation of VOI.
2.1.4 Value of correct core analysis data - Summary
VOI methods can be used to evaluate any economic decision. Section 2.3 describes other aspects of VOI calculations pertinent to core analysis but an important example is the value of correctly measured data. Basing decisions on incorrectly measured data carries risk and this is discussed in Appendix 1.3. It turns out that using incorrect data puts at risk the primary benefit of the development and it is in fact, worse to use incorrect data than to have no data. These rough calculations can assess the risk of inaccurately measured data. Such a calculation requires: • probability of success, POS;
• the economic impact of the development case, NPV1;
• the economic impact of the development if reserves are not proven, NPV3. Equation (A1.12) is used to determine VOI of correctly measured data.
It is not always possible to use KSEPL or another Shell E&P laboratory due to the demands of accessibility, regional preferences or the availability of Shell E&P laboratories. It is nevertheless true that improperly measured data carries a significant and generally underestimated risk.
2.2
Value of information examples as applied to core analysis projects
In this section, a number of VOI examples will be shown to demonstrate how VOI techniques can be applied to justify core analysis projects. Both VOI and VOA are calculated. One of theexamples (Example 4 - Section 2.2.4) is an Opco VOI example used recently to justify a core analysis programme.
NOTE: For figures in this section, a red shaded rectangle denotes a human decision while a yellow shaded ellipse represents the consequences of measurement, which are various
outcomes each with a given probability of occurring. Positive economic impact is given as NPVi. A negative impact is shown as -NPVi.
2.2.1 Example 1 - Prospect Screening (Unconsolidated Sandstone)
The details of prospect screening are given in Appendix 1.1. Economic options are shown in Figure 2.1. The option of performing core analysis is clearly indicated by "Yes". However, the results of core analysis are to indicate whether or not to proceed with development given with a weighting by the probability of success. Net present value figures for each option are shown on the right hand side which are used to quantify the value of core analysis. Without core analysis, the "No" option, proceeding with development carries the risk of attempting to develop insufficient reserves. Avoiding the loss in an unprofitable development is the value of the core analysis.
An unconsolidated sandstone play has an estimated probability of success of 0.5. A suite of rock measurements for the evaluation of this play is given in section 3.2.1. The core and core
evaluation cost is DFL 5 mIn. The entire data acquisition programme including seismic and well testing is US $15 mIn. Proposed development costs include a platform and facilities at a cost of US $400 mIn. Economic impact if the reserves are inadequate is a loss of US $200 mIn. Here in example 1, POS = 0.5
NPV3 = - US $200 mIn. Equation (A1.4) yields VOl = US $100 mln
The value of information is US $100 mln and the value of appraisal is obtained by subtracting a cost of US $15 mln to obtain:
VOA = VOI - Cost
= US $85 mln
Note that the value of information here reduces the uncertainty to zero i.e. the probability of success is now unity, which is the combined effect of all the data gathered and not just core analysis. The benefit assigned to core analysis, is the amount by which the core analysis increases the probability of success. In prospect screening, core analysis is easily justified because the economic benefits are so large. The reader is urged to consider section 2.3 on other aspects of VOI calculations for core analysis.
2.2.2 Example 2 - Ekofisk
Ekofisk has experienced surface subsidence which has cost a lot of money for remediation. This problem might have been avoided or its impact reduced had sufficient special core analysis, particularly compaction measurements, been performed to obtain critical data necessary for field development. Hindsight is able to provide the economic impact of the various decisions made in field development. Economic choices are shown in Figure 2.2. The probability of success is estimated at 0.75. Net present value is presented on the right hand side of the figure. Remediation is estimated conservatively at US $500 million.
A coring and core analysis programme would have cost about US $2 million. But the cost of repairing the Ekofisk structure is about US $500 million. Figure 2.2 summarises the economic choices. POS is estimated as 0.75.
Here, in example 2, POS = 0.75
NPV3 = - US $500 mIn. Equation (A1.4) yields VOl = US $125 mIn.
The value of information is US $125 mln and the value of appraisal is obtained by subtracting a cost of US $2 mln to obtain:
VOA = US $123 mIn.
The value of core analysis is far greater than the cost of the core analysis project. With hindsight, it is reasonable to assign most of the value of acquisition entirely to core analysis. However, in a prospect screening situation, it is not reasonable to do this.
Even if the probability of success were 0.90, then the value of information becomes US $50 million and the value of appraisal becomes US $48 million, which indicate significant value in core analysis.
2.2.3 Example 3 - Project optimisation
Much special core analysis is performed for field development where core data allows project optimisation. The details of optimisation economics is given in Appendix 1.2. The economic choices are shown in Figure 2.3. Net present value figures are shown on the right hand side and should be obtained from reservoir engineering computer simulation as well as incorporating costs estimated from production engineering. The value in core analysis lies in being able to ensure that the project is properly sized. Project optimisation occurs because of reduction in uncertainty of a key parameter determined from simulation of the process under study.
A water-flood development project depends on detailed knowledge of relative permeability and capillary pressure. Reservoir simulation shows that the economic impact for the optimised high reserves case is US $20 mln, for the high reserves but unoptimised is US $10 mln and for the low reserves case optimised is US $8 mln and that for the unoptimised low reserves case is US $3 mIn. Core is to be taken with special precautions for preserving wettability and is to cost about US $200,000 including collection of appropriate fluids, well-site core handling and transportation. The total cost for taking core and performing core analysis is US $0.5 mIn.
Equation (A1.8) yields: VOl = 0.33 * { 20 + 8 - 10 - 3 }
= US $5 mIn.
The value of information calculation here results in a value of information of US $5 mIn. The cost of the core and core analysis programme is US $1 mIn. Therefore, the value of appraisal, VOA, is
VOA = VOI - cost = US $4.5 mIn.
Here the entire value of information benefit can be assigned entirely to the core analysis project.
2.2.4 Example 4 - An Opco VOl Example
In this example, a well is being drilled. A core analysis, project can quantify the permeability development below the gas-water contact and reduce the uncertainty in predicting aquifer behaviour before the field is put on production. With proper characterisation of the aquifer, the possibility exists that aquifer influx may be limited which would avoid the drilling of an extra well. Without core analysis the drilling of an extra well is necessarily included in the development plan. Net present value figures were supplied from the Opco. Economic choices (simplified) are shown in Figure 2.4.
The cost of the core analysis project including coring is estimated at DFL 500,000 including DFL 130,000 for core analysis. The core analysis project called for the acquisition of 4 coring runs below the GWC for a total of 72 m. In fact, the original Opco analysis included the option of continuous coring for 150 m which was found to have a slightly lower value of information and is not included here to maintain simplicity.
The values of the top branch, Vyes' and bottom branch, Vno' are given by:
Vyes = 0.5 * 0.4 * -12.36 + 0.5 * 0.6 * 12.36
= - 6.18 mln Vno = DFL -12.36 mln
VOl = Vyes - Vno
= DFL 6.18 mln
and thus the value of appraisal is obtained by subtracting cost: VOA = VOI - cost = DFL 5.68 mIn.
A value of appraisal of DFL 5.68 mln is calculated because of the probability that the drilling of an extra well can be avoided.
2.3
Core analysis aspects of VOl
This section deals with issues that arise in using VOI calculations for core analysis justification. In some ways these issues can be considered consequences or corollaries to the examples shown in the previous section.
• Justification - VOI calculations should show a value of information, VOI, that is many times programme cost, i.e. at least three times. Thus, the appraisal value, VOA, should be at least twice the cost of the project. VOI calculations which show less value than this are not justified economically. This is recommended as a reasonable guide to using VOI calculations for core analysis projects.
• Need for additional core - examples in section 2.2.1 and 2.2.2 discuss the situation of justifying initial core in an appraisal situation. VOl methods can be used equally well to justify the need for additional core even if there exists core from wells. Perhaps the previous cores were also justified by value of information techniques which indicated large positive value in core analysis. However, if the need for additional core is present then it must be due to some "failure" of previous cores. By "failure" we include the following:
- previous cores missed an important target zone;
- poor recovery in target zone (perhaps due to poor planning which allowed poor coring techniques and overlooked poor core handling. If the target zone is inherently difficult to core, then you must demonstrate that new insights into the coring process have increased the probability of success);
- incomplete planning as to core analysis needs (there now exists previously unanticipated needs for core material such as to measure a critical parameter like compressibility); - insufficient material (not enough material available from previous cores);
- poorly preserved core which indicates that the remaining core is unsuitable for measurement.
The justification for additional core carries with it the need to evaluate previous VOI calculations.
• What if coring risks the well? If the coring programme carries with it an identifiable risk of failure such as damaging an expensive well, such contingencies can be incorporated into the value of information calculation which is done by adding more branches allowing the
possibility of failure. This is similar to the calculation in the Appendix 1.3 which shows the problem of incorrectly measuring core analysis parameters. There, the extra risk is quantified by the parameter POCM, probability of correct measurement. In the same manner, the risk to a well is carried out by quantifying the risk that coring has on the well. Once that risk is quantified, then the VOl calculation will show a diminished value because the risk of losing the well is incorporated. If the VOl calculation is value neutral indicating that coring may be too risky, then additional scenarios can be evaluated such as by coring on by-pass. In coring on bypass, after a target zone has been drilled and logged and identified, the well is sidetracked above the target zone and then cored through the target zone. This technique has been employed in drilling the deepwater Gulf of Mexico turbidites and has the advantage that the coring point is clearly known and the amount of core required is clearly identified. • VOl Lookback - it is a good idea to maintain the examples of VOl calculations to ensure that
the assumptions made for the VOl calculation were reasonable. VOl calculations assume that the information will be successfully gathered. However, core analysis programmes can fail such as through bad coring practices, bad core handling techniques, or even poor core measurement techniques. In the case of such failures, it is likely that the full VOl was not obtained and this should be reviewed for future VOl calculations.
• Value and planning - after VOl has shown that the core analysis programme is of significant value, the task at hand is to ensure proper planning to achieve the programme objectives. We hope that recognising the value of a core analysis programme provides inspiration for carrying out the critical aspect of core analysis, namely planning the analysis programme in an integrated PE environment. Planning is the subject of the next two chapters.
2.4 Literature
Demirmen, F.,Subsurface Appraisal Justification: The Value of Information
June, 1994 EP 94-0585
E&P Economic Guidelines
Report EP 93-2150, October, 1993.
3.
Planning a core analysis programme
Group experience indicates that coring and/or core analysis is usually poorly planned, is left to a late stage, and results in under utilisation and poor application of expensive core material and core data. SIPM recommends core analysis project planning in order that the proper multi-disciplinary attention is paid to core analysis. Petrophysics, geology, reservoir engineering and other disciplines have significant input into the development of an effective core analysis
programme. The core analysis programme allows for consensus building in the multi-disciplinary environment and assists in the finalisation of programme goals. With proper planning core analysis projects are better able to deliver required data in a timely fashion.
Core analysis planning is shown in a flow diagram in Figure 3.1. Within the guidelines established for each Opco, coring and core requirements are defined for the prospect/field with input from each discipline. Once the design of the core analysis programme has reached consensus, the implementation of the programme can then take place.
The recommendation for core analysis planning is to follow the outline given in section 3.2. The programme outline covers all items that impact the core analysis programme including the core acquisition itself. The list given in section 3.2 is extensive but is done in such a way as to maximise the information that can be obtained from core analysis and to assist in data interpretation. Experience has shown that all aspects of the core analysis programme can be critically important for subsequent data application. Of course, some items may not be necessary for any given application but it is nevertheless worthwhile to consider the impact each item can have on programme implementation.
Each item addressed in the outline represents a separate section in chapter 4, where appropriate detail is found. Chapter 4 aims to present easy options for core analysis planners.
Many of the coring and core handling considerations are covered in the "Core Handling Manual", EP 93-2200, but are included here in summary form for completeness.
Core analysis project planning provides the following benefits:
• maximisation of quality in core analysis measurement through planning and selection of the appropriate measurement suite;
• obtaining input and consensus from a multidisciplinary EP team so that the majority of the core analysis needs can be anticipated and incorporated and reduce the need for later remeasurement;
• definition of clear timing constraints in order to properly impact appraisal or field development decisions.
• optimisation of core analysis programme potentially eliminating the need for future supplementary measurements by clearly defining expectations of the core analysis measurements. This is especially valuable if additional coring can be avoided;
• roles and responsibilities of each team member within a core analysis project schedule are clearly defined. Timely advice from each team member assists in on-time data delivery. • careful core selection and proper core screening increase the likelihood of measurement on
the most representative samples;
• an overall core analysis programme allows the core analysts to appreciate the entire scope of the project and to meet mutually agreed deadlines. Better planning and scheduling usually result;
• core analysis projects can be more easily justified by using value of information concepts and proper consideration of economic benefits;
• greater confidence and better utilisation of core analysis data is achieved. Improved application core analysis data in field development and resource management leads to improved field development planning;
• better project management which includes better continuity during staff changes because expectations and scheduling are clearly noted;
• easier presentation of core analysis plans and results to partners and other stakeholders in the hydrocarbon resource.
3.1
Planning in an integrated PE team
The multi-disciplinary planning of core analysis produces a better core analysis program with input from each PE discipline. In order to encourage such involvement this section addresses the roles and responsibilities of each PE discipline involved: namely petrophysics, geology and
reservoir engineering. Other disciplines such as production technology and exploration functions can also have needs that can be addressed through core analysis.
The value of the integrated team approach to core analysis is the synergy that can be obtained when each PE discipline contributes to the planning of the core analysis programme. More details of the requirements of the integrated PE team are given in section 4.2.
It has been customary for the petrophysicist and geologist to arrange coring and core analysis programmes. In the integrated PE environment, the petrophysicist remains likely to be the focal point for core handling and core analysis. However, over time it is expected that any member of the multi-disciplinary PE team can be focal point for a core analysis programme with the assistance of this manual.
