Practical petrophysics

402 Pages·2015·49.589 MB·English

by  Kennedy, Martin

Developments in Petroleum Science
Volume 62
Practical Petrophysics
Series Editor
John Cubitt
Holt, Wales
Developments in Petroleum Science
Volume 62
Practical Petrophysics
Martin Kennedy
MSK Scientific Consulting, pty ltd. Perth, Australia
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ISBN: 978-0-444-63270-8
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Series Editor’s Preface
This is the third book in the Developments in Petroleum Science series since it
incorporated The Handbook of Petroleum Exploration and Production in 2013.
After books on geophysics and stratigraphic reservoir characterization, we now
look at the equally important field of petrophysics.
Petrophysics is described as the study of the physical properties of rocks,
their pore systems and the fluids they contain. As such it plays a key role in
the geosciences and reservoir engineering and is a cornerstone to petroleum
exploration and production. In J.H. Schon’s 2011 workbook in the Handbook
of Petroleum Exploration and Production entitled Physical Properties of
Rocks, he discusses and defines the fundamental parameters we can measure by
petrophysics including fluid types and volume, porosity, and lithological rock
types as well as some of the properties needed for reservoir characterization and
simulation.
Martin Kennedy builds on these fundamentals in this book and in his words
‘show how to achieve a balance between the rigorous principles that ultimately
determine the petrophysical properties and how our measuring instruments
respond to them on the one hand and what is realistically achievable with
limited time and resources on the other’. In other words he is moving away
from purely idealized or theoretical petrophysics to the more complicated and
multi-disciplinary nature of the real petrophysical world.
The idealized or theoretical vision of petrophysics has been established over
the last 90+ years, and is essential to make sense of our rock, fluid and log meas -
urements. However, as our application of petrophysics has grown ever more so-
phisticated so has the risk that we employ techniques or equipment that exceed
their theoretical limits. Here Martin Kennedy has emphasized how important it
is for all practicing geoscientists or petroleum engineers to understand how a
particular equation or technique is derived and what its limits are. He describes
these techniques, equipment and limitations in detail and provides a book that
will as a result be essential reading for those active in petroleum exploration
and production.
John Cubitt
Holt, Wales
xi
Preface
The number of specialist textbooks dealing with Petrophysics or Log Analysis
is relatively small and some of them are getting quite ‘long in the tooth’ now.
This book was written to fulfil three objectives:
1. To be up to date both in terms of general measurements and techniques.
2. To present the basic principles of petrophysics in a straightforward and –
I hope – readable form.
3. To show how these principles can be applied in a pragmatic way to estimate
petrophysical properties.
It is not intended to be a comprehensive description of every equation and algorithm
that is available to petrophysicists. Neither does it attempt to describe every logging
tool and core analysis measurement currently available. The latter is a hopelessly
ambitious task and to my certain knowledge half a dozen new logging tools have
appeared on the market between nfi ishing the text and writing this preface. More
importantly, it is not intended to be a ‘How to’ manual. There is undoubtedly a
place for such books but blindly following recipes without some understanding of
the underlying principles is asking for trouble in any technical discipline (including
cooking). It is particularly dangerous in petrophysics where most of the equations
are either purely empirical or are based on a number of approximations and
assumptions. This book is intended to provide that essential background.
In terms of column inches much of this book is devoted to log analysis. This
is quite deliberate and is a reflection of the relative amounts of log and core data
that are typically available in the field. No matter how highly an individual or
organization values core the fact of the matter is that there will inevitably be
large gaps in the core record that only logs can fill. The best interpretations use
the two types of data to complement each other and although not often explicitly
stated, I hope the book leaves the reader with that impression.
The emphasis on principles means that some of the day-to-day tasks of a
petrophysicist are given scant coverage in this book. Topics which I have at best
mentioned in passing and generally ignored include getting logging tools into
places where they do not want to go and the limitations created by high tem-
perature and pressure, corrosive fluids, general HSE and money. These are all
important but at the end of the day they have no influence on how well Archie's
equation works and so have no place in this book.
Martin Kennedy
xiii
Chapter 1
Introduction
Chapter Outline
1.1 What is Petrophysics? 1 1.5.1 The Archie Equation:
1.2 Early History 2 A Case Study 8
1.3 Petrophysical Data 3 1.6 The Petrophysical Model 10
1.4 Quantitative Description 1.7 Physical Properties of Rocks 12
of Mixtures 4 1.8 Fundamentals of Log Analysis 16
1.5 The Practice of Petrophysics 1.9 A Word on Nomenclature 18
and Petrophysics in Practice 7 1.10 The Future of the Profession 18
1.1 WHAT IS PETROPHYSICS?
Petrophysics is the study of the physical properties of rocks. As a pure science
its objective would probably be to explain why rocks have the properties they
do. In particular how the relative amounts and arrangements of the minerals that
comprise them determine their physical properties. In practice, most of the time
we are concerned with the reverse problem of using physical properties to try
and find out what the rock is made of. This is valuable information for anyone
who works with rocks whether as a resource, a substrate or a storage medium.
But, as will be seen below, petrophysics has its origins in the oil industry and
is still most widely used for describing the rocks that make up hydrocarbon
traps. For this reason most of the tools and techniques that are described in this
book were originally developed to deal with porous, sedimentary rocks in the
sub-surface. In particular the problem of determining what the rock is made of
often reduces to finding how much of the rock is fluid, how much of that fluid is
water and how that fluid is distributed (as that will give some indication of how
easily it can be extracted).
More succinctly petrophysics in the oil industry is used to find the following:
1. Porosity – How much fluid can the rock store?
2. Saturation – How much of it is water?
3. Permeability – How quickly can it be extracted?
These are often referred to as ‘petrophysical properties’ or even just ‘proper-
ties’. The tools and techniques that were developed to estimate them can often
be used to find other information of practical importance, for example identify -
ing special minerals or modelling the seismic response of a sand/shale interface
Developments in Petroleum Science, Vol. 62. http://dx.doi.org/10.1016/B978-0-444-63270-8.00001-3
Copyright © 2015 Elsevier B.V. All rights reserved. 1
2 Practical Petrophysics
(we will look at this later in the book). Moreover in order to estimate these three
properties we often have to go through intermediate steps so that a full petro-
physical analysis may well end up producing a lot more information.
Since this book is ultimately concerned with the properties of rocks we
should explain what we mean by a ‘rock’ and also how big it is. For our pur-
poses rocks are physical mixtures of minerals. Minerals are for the most part
chemically pure substances that may be solid, liquid or gas (so in this book
at least water, oil and gas are considered minerals). For convenience we will
also include mixtures of similar compounds as minerals. An obvious example
is crude oil, which is invariably a mixture of hydrocarbon molecules as well
as some more complicated organic compounds. Examples of solid mixtures are
some of the clay minerals, which can have a range of compositions and a single
grain may show a variation in composition from one side to the other.
The size of the rocks we are interested in is largely determined by our meas-
urements. In the laboratory, samples may be minute, in fact some techniques
can be applied to single mineral grains. But in this book we will frequently
deal with borehole logging measurements, which typically cover volumes from
tens of cubic centimetres to several cubic metres. Even small core plugs have
volumes of several cubic centimetres. So to put it simply, the volumes we deal
with vary in size from hand specimens to boulders.
1.2 EARLY HISTORY
No doubt scientists have been measuring and exploiting certain physical proper-
ties of rocks for centuries but most petrophysicists would date their profession to
the 1940s. Fittingly the noun ‘Petrophysics’ was coined by G.E. (‘Gus’) Archie
in the late 1940s to satisfy what he felt was the need for a word to describe the
study of the physics of rocks. Even if someone else had invented the name, Ar-
chie would almost certainly still be regarded as the founder of the profession. In
1941 he developed the empirical equation, that bears his name, which relates the
electrical resistivity of a porous rock (R ) to its porosity (Ø) and the resistivity
0
of the fluid – invariably salt water – contained within its pores (R ). In general
w
R =f(Rw, Ø) R =f(R ,Ø) (1.1)
0 0 w
This is a classic case of petrophysics in action. The equation describes how
the resistivity of the rock depends on the relative amount of one of the minerals
in the rock (water); and as we will see later, how that water is distributed within
the rock. Historically, it is considered to be the first attempt to explain why a
physical property has the value it does.
Of course being able to predict how resistivity depends on porosity or vice
versa, might be interesting but if it was limited to laboratory measurements on
core plugs it would have few practical applications. Fortunately, resistivity had
been measured in boreholes since 1929, when the Schlumberger brothers ran an
experimental tool in a well in Alsace. This was the rfi st wireline log (in this book
Introduction Chapter | 1 3
we will henceforth simply refer to wireline logs as ‘logs’). The technique rapidly
caught-on and by the time Archie published his results, resistivity logs were rou-
tinely run in many parts of the world. Their principle application was however,
correlation and qualitative interpretation such as identifying sands and sometimes
distinguishing water and oil in the pore space. Archie’s work allowed the logs to
be used to estimate porosity along the well bore. Log analysis is now the standard
way to determine the petrophysical properties in the sub-surface.
Almost from the start, logs were an oil industry tool and it is hardly surpris-
ing that Archie too came from that industry (specifically Shell Oil). To this day
the major developments in petrophysics hardware and interpretation tend to be
driven by the needs of the hydrocarbon industry. Nevertheless, it can, and is ap-
plied to all industries that deal with rocks.
We will look at Archie’s equation in a bit more detail in subsequent sections
and a lot more detail in a later chapter. Before doing that it is only fair to point
out that Archie himself had much wider interests than electrical resistivity. He
studied almost any rock property that could be expressed numerically and in
the 1950 AAPG paper in which he introduced the word ‘petrophysics’ he was
already describing applications of porosity–permeability cross-plots, capillary
pressure curves, the SP log, neutron logging and of course resistivity. Signifi -
cantly he also showed how these properties depend on the geometry of the pore
system. In short he did not leave much for his successors to work on.
1.3 PETROPHYSICAL DATA
Almost all the petrophysical data discussed in this book comes from wells, this
imposes some important constraints on the accuracy of our estimates. There are
two, fundamentally different, sources of data:
1. Instrumental methods that measure physical properties.
2. Actual samples of rocks, which can be analysed in a laboratory.
(For completeness we should add the various types of well test to this short
list but we will defer any further discussion of these until much later in the book.)
The former obviously refers to the various types of geophysical log (which
we will simply call ‘logs’). These provide a continuous record of one or more
physical properties along the path of the well. Log analysis converts physical
properties to petrophysical properties and much of this book concerns it. This is
entirely appropriate: log analysis is not a synonym for petrophysics but it is an
indispensable part of it.
‘Samples’ include cuttings and various types of core. Cores are normally
quite localised, either in relatively short intervals, in the case of a whole core,
or widely spaced depth points, in the case of sidewall cores. Cuttings do give a
continuous record, but the drilling process always results in a certain amount of
mixing, possible loss of some minerals and sometimes they are so finely ground
that it is impossible to tell their original lithology. Even so, all these different
4 Practical Petrophysics
types of information should complement each other, and if properly integrated
their individual shortcomings can be overcome to an extent.
Well-bores are difficult places to make measurements and so there are rela -
tively few instrumental techniques we can adapt for that environment. Of these,
few can read more than a few centimetres into the formation. Unfortunately,
drilling inevitably alters the formation near to the well bore so even when the
logging tool is working perfectly, it will make an accurate measurement of rock
that has been changed in some way.
On the other hand, core material can be studied ‘at leisure’ using almost any
technique one desires. Unfortunately, whole cores are expensive and sometimes
nearly impossible to acquire. Sidewall cores are a cheaper alternative but there
is a limit to how many can be taken from one well and so they are generally
quite widely spaced. Also, depending on the type of tool that was used to obtain
them, they may not be suitable for all types of analysis. In any case, regardless
of what type of core was taken, the rock goes through some drastic changes be-
tween coring, being brought to surface and then being cleaned and prepared for
analysis. Cuttings give the greatest coverage for any sample type and they are
always present (although for the top-holes of some offshore wells they never get
beyond the sea bed). On the other hand they also suffer the greatest alteration on
their journey to the surface.
Even if we can obtain a complete set of well logs, cores and cuttings we
can only really be certain that we have characterised the reservoir in the near
well bore region. Because most logs can only read at most a few metres we
have no direct knowledge of what happens beyond. The net effect of this is
that petrophysics can often provide a very accurate and precise description of
the sub-surface but only at a few points across the reservoir (i.e. the wells). The
greatest uncertainty is often associated with what is going on between the wells.
1.4 QUANTITATIVE DESCRIPTION OF MIXTURES
As noted earlier a lot of applied petrophysics involves finding the relative pro -
portions of the minerals – including water and hydrocarbons – that make up
a rock. When we describe a mixture we have a choice in how to express the
relative amounts of each of the components. For describing rocks the simplest
choices are:
1. by volume fraction.
2. by mass fraction.
By convention, but also for convenience, in petrophysics and log analy-
sis the proportions are invariably expressed as volume fractions. Porosity,
for example is the volume fraction of fluids in the rock and ‘shale volume’ is
self-explanatory. Many of the laboratory techniques that are applied to cores
and cuttings, however give the results as mass fractions. It is obviously impor-
tant to know which system is being used and since we often wish to integrate