Petrophysics
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Preface xi
1 Introduction 1
1.1 The basics 4
1.1.1 Units and abbreviations 6
1.1.2 Cores and logs 6
1.1.3 Lithology identification 8
1.1.4 Rock properties 10
1.1.5 Physics of a reservoir 10
1.1.6 Porosity 10
1.1.7 Water saturation 11
1.1.8 Permeability 13
1.1.9 Capillary pressure 14
1.1.10 Wettability 14
1.2 The results 15
1.2.1 Hydrocarbon pay 15
1.2.2 Simple volumetrics 15
1.2.3 3D static models 17
1.2.4 Value of information 17
1.3 Summary 18
2 Data Acquisition 19
2.1 Drilling data 19
2.2 Coring and core analysis 21
2.3 Wireline logging 24
2.4 Well test data 28
2.5 Borehole environment 29
2.6 Summary 33
3 Rock and Fluid Properties 35
3.1 Controls on rock properties 35
3.2 Lithology 39
3.2.1 Spontaneous potential 39
3.2.2 Gamma ray 42
3.3 Porosity 46
3.3.1 Core porosity 46
3.3.2 Log porosity 48
3.4 Water saturation 55
3.4.1 Core?]derived water saturation 57
3.4.2 Wettability 59
3.4.3 Electrical measurements 60
3.4.4 Log?]derived water saturation 61
3.5 Permeability 64
3.5.1 Core permeability 66
3.5.2 Log permeability 67
3.5.3 Porosity-permeability relationship 68
3.5.4 Overburden correction and Klinkenberg effect 70
3.6 Summary 71
4 Quality Control of Raw Data 73
4.1 Validation of log data 73
4.1.1 Labelling 74
4.1.2 Parametric ranges 76
4.1.3 Repeatability 76
4.1.4 Tension 76
4.1.5 Borehole conditions 76
4.1.6 Noise spikes and cycle skipping 77
4.1.7 Editing log data 77
4.1.8 Creation of pseudologs 77
4.2 Depth merging 78
4.3 Tool corrections 78
4.3.1 Environmental corrections 79
4.3.2 Invasion corrections 79
4.4 Core analysis data 80
4.5 Merging core and log data 80
4.6 Converting measured depth to true vertical depth 82
4.7 Summary 82
5 Characteristic Log Responses 83
5.1 Characteristic shale response 83
5.2 Matrix characteristics 89
5.3 Fluid characteristics 90
5.4 Hydrocarbon corrections 90
5.5 Shale corrections 91
5.6 Summary 91
6 Evaluation of Lithology, Porosity and Water Saturation 93
6.1 Evaluation of lithology 93
6.1.1 Histograms 95
6.1.2 Scatter plots 95
6.1.3 Lithology interpretation 100
6.2 Evaluation of porosity 101
6.2.1 Sonic porosity 101
6.2.2 Density porosity 103
6.2.3 Neutron porosity 103
6.2.4 Selection of reservoir porosity 104
6.2.5 Total and effective systems 105
6.3 Evaluation of water resistivity 106
6.3.1 SP method 106
6.3.2 Resistivity cross?]plot method 107
6.3.3 Pickett plot 109
6.3.4 Apparent Rw method (Rwa) 110
6.4 Estimation of water saturation 110
6.4.1 Clean sands 111
6.4.2 Shaly sands 112
6.5 Summary 114
7 Petrophysical Workflows 115
7.1 Data management 115
7.2 Quick?]look interpretation 116
7.3 Full petrophysical interpretation 118
7.3.1 Permeability estimation 122
7.3.2 Evaluation of cut?]off parameters 123
7.3.3 Determination of zone averages 124
7.3.4 Reporting 126
8 Beyond Log Analysis 127
8.1 Pressure measurements, gradients and contacts 127
8.2 Saturation?]height functions 131
8.2.1 Single?]predictor algorithms 133
8.2.2 Multi?]predictor algorithms 134
8.2.3 Normalized functions 134
8.3 Electrofacies and facies analysis 135
8.4 Rock typing 138
8.5 Integration with seismic 141
8.5.1 Depth conversion 142
8.5.2 Fluid substitution 143
8.6 Production logging 145
8.6.1 Pulsed neutron logging 145
8.7 Geo?]steering 146
8.8 Petrophysics of unconventional reservoirs 147
8.8.1 Total organic content 148
8.8.2 Porosity estimation 150
8.8.3 Gas in place 150
9 Carbonate Reservoir Evaluation 151
9.1 Rock fabric classification 151
9.2 Petrophysical interpretation 153
9.2.1 Porosity 153
9.2.2 Water saturation 154
10 Petrophysics for Reservoir Modelling 157
10.1 Multi?]scale modelling 158
10.2 Petrophysical issues 158
10.3 Blocking logs 160
10.4 Geological issues 162
10.5 Engineering issues 164
10.6 Volumetrics 165
10.7 Uncertainty 165
10.8 Epilog 167
Appendix 1 Petrophysical Report 169
Appendix 2 Data Collection and Management 179
Appendix 3 Oilfield Glossary 189
References 197
Index 201
1
Introduction
What is petrophysics? Petrophysics, as understood in the oil and gas industry, is the characterization and interaction of the rock and fluid properties of reservoirs and non-reservoirs:
- determining the nature of an interconnected network of pore spaces - porosity;
- the distribution of oil, water and gas in the pore spaces - water saturation; and
- the potential for the fluids to flow through the network - permeability.
Petrophysical interpretation is fundamental to the much of the work on the subsurface carried out by geologists, geophysicists and reservoir engineers and drillers. To characterize the subsurface successfully requires physical samples, electrical, chemical, nuclear and magnetic measurements made through surface logging, coring and drilling and wireline tools (sondes). Terms such as 'formation evaluation' and 'log analysis' are often used to capture specific parts of the petrophysical workflow, but should not be seen as synonyms. 'Rock physics', which sounds as though it might be similar, is usually reserved for the study of the seismic properties of a reservoir; similar concepts apply but at larger scale.
The evaluation, analysis and interpretation of these petrophysical data is as much an art as a science, as it requires an understanding of geology, chemistry, physics, electronics, mechanics and drilling technology. At its simplest, petrophysics determines the porosity and water saturation of a reservoir, then estimates the permeability of the rock and the mobility of the fluids in place. The interpretation is dependent on the lithology of the rocks being evaluated, as sandstone, limestone, shale and any other potential hydrocarbon-bearing rocks all have differing characteristics. The acquisition and interpretation techniques applied in formation evaluation have been developed over the last century primarily by the oil and gas industry, but the principles are equally relevant in coal mining, hydrogeology and environmental science. The type of data acquired is generic and can be used in a number of different analytical ways; indeed, as computing power and microelectronics have developed over the last 30 years, more high-resolution data can be collected and used for ever more detailed interpretation. However, measurements can be influenced by a number of variables, including the borehole environment; borehole diameter, temperature, pressure and drilling fluid, all affect the quality and type of data acquired. The reservoir rocks and the fluids therein can further affect the data quality and interpretation - a virtuous or viscous circle depending on how you look at it.
This book can be divided into two sections: first data acquisition and second interpretations, applications and workflow. This introductory chapter reviews the basics of petrophysics, including the confusing topics of measurement units, reservoir lithology, basic measurements and how the results may be used and the value of information and data management.
- Chapter 2 reviews data acquisition in some detail, from drilling data to core analysis and wireline logs. I have not tried to give a detailed description of wireline tool technology, because I am not a physicist or electronics engineer; I refer you to the appropriate manufacturers' publications. In an appendix I have tried to collect basic tool information, but I would direct you to the third edition of The Geological Interpretation of Well Logs (Rider and Kennedy, 2011) for a full description and discussion of the range of logging tools available.
- Chapter 3 discusses rock and fluid properties and what controls porosity, water saturation and permeability in the reservoir. Each property is defined and described and how the measurements are made, with a discussion of uncertainty.
- Chapter 4 is focused on data quality control, especially the validation of log data and the integration with core data.
- Chapter 5 looks at the characteristic response of different logs to reservoir rocks and fluids and how the data may be used in log analysis. The response to shales and matrix and fluid properties are fundamental.
- Chapter 6 is about the evaluation of porosity and formation water resistivity and estimation of water saturation.
- Chapter 7 looks at different petrophysical workflows, starting with data management and then quick-look single-well analyses, followed by multi-well studies. This part of the process is supported by worked examples.
- Chapter 8 is called 'beyond log analysis' and looks at permeability estimation, cut-offs and zone averages, saturation height relationships, pressure measurements and fluid contacts. There is also a discussion of lithology prediction, facies analysis and rock typing and also integration with seismic data.
- Chapter 9 looks at carbonate reservoir characterization.
- Chapter 10 describes the role of petrophysics in reservoir modelling, with a particular emphasis on property modelling in three dimensions.
One outcome of a petrophysical analysis forms the basis of the estimation of fluids in place, upon which, together with the gross rock volume of a reservoir, major investment decisions are made by oil and gas companies: the quality of the interpretation will change with time as new wells and new data are collected, so there is a need for consistency in approach at all times. One aspect that should never be forgotten is that most of the measurements that are made are a proxy for the real property that we are trying evaluate: porosity is never actually measured but interpreted from a density or neutron log; water saturation is interpreted from a resistivity measurement, dependent on the analyst knowing some fundamental properties of the formation fluid. A petrophysicist therefore has to be a general scientist with a strong numerical bias to be able to cut through the complex analytical methods and uncertainties inherent in the process of evaluating a reservoir; above all, a petrophysicist must be imaginative and thorough in their analysis and be flexible in their attitude to an interpretation that will change over time through either additional data or greater insight.
Beyond volumetric estimation, petrophysics is at the core of many other subsurface disciplines: the geophysicist relies on correctly edited and calibrated logs for depth conversion and rock property analysis, likewise the geologist for well correlation, reservoir modelling and fluid contact estimation, and the engineers for well completions and pressure prediction and as input for dynamic simulation. How you approach a petrophysical data set will often depend on the objective of the study: a single-well log analysis without core data requires a very different workflow to that adopted for a full-field petrophysical review.
Petrophysics is not just log analysis - it is log analysis within a geological context or framework, supported by adequate calibration data, including sedimentology, core analysis and dynamic data from pressure measurements and well tests (Figure 1.1). Logs do not measure porosity, permeability or water saturation; they make measurements of acoustic velocity, electrical conductivity and various nuclear relationships between the rock and the fluids to allow computer programs to process and interpret the results. The petrophysicist role is to validate and organize the input data and to understand and calibrate the results. A little harsh, you may say, but how many petrophysicists do the job without using log analysis software and how many integrate the analysis with the geological interpretation?
Figure 1.1 Petrophysical evaluation: schematic showing the primary data sources, products and deliverables of an integrated petrophysical evaluation.
1.1 The basics
It is worthwhile looking at the context in which the rest of the book lies before diving into the detail. Although not attempting to be a primer in geology, physics or chemistry, we will touch on these disciplines as we progress, so I will try to set the scene and leave the reader to dig deeper into interesting subject matter from the references. However, it is worth considering that both of our primary sources of data, wireline/LWD (logging while drilling) and core data, present challenges in terms of sampling, data quality and integration. Log measurements, although made in situ, are invariably indirect; we seldom measure an actual property of the rock, only one inferred from its response to physical input: core measurements are broadly speaking direct but they are ex situ. It is not my intention to describe in any detail the tool physics behind logging measurements, as there are many other books that cover this vital part of the technology; rather, this handbook is designed for the user of these data to evaluate the potential commercial value of a hydrocarbon reservoir.
All the log measurements that are made come from one or more penetrations of a reservoir made by a drill bit usually between 6 and 12½ inches in diameter, attached to a drill-string often several thousands of feet or metres long; we use this penetration to infer reservoir properties tens to thousands of metres away from the borehole (Figure 1.2). The borehole environment at depth is hostile;...
