Practical Petrophysics (eBook)

Martin Kennedy (Autor)

eBook Download: PDF | EPUB

2015 | 1. Auflage
420 Seiten
Elsevier Science (Verlag)
978-0-444-63271-5 (ISBN)

Practical Petrophysics looks at both the principles and practice of petrophysics in understanding petroleum reservoirs. It concentrates on the tools and techniques in everyday use, and addresses all types of reservoirs, including unconventionals.

The book provides useful explanations on how to perform fit for purpose interpretations of petrophysical data, with emphasis on what the interpreter needs and what is practically possible with real data. Readers are not limited to static reservoir properties for input to volumetrics, as the book also includes applications such as reservoir performance, seismic attribute, geo-mechanics, source rock characterization, and more.

Principles and practice are given equal emphasis
Simple models and concepts explain the underlying principles
Extensive use of contemporary, real-life examples

Practical Petrophysics looks at both the principles and practice of petrophysics in understanding petroleum reservoirs. It concentrates on the tools and techniques in everyday use, and addresses all types of reservoirs, including unconventionals. The book provides useful explanations on how to perform fit for purpose interpretations of petrophysical data, with emphasis on what the interpreter needs and what is practically possible with real data. Readers are not limited to static reservoir properties for input to volumetrics, as the book also includes applications such as reservoir performance, seismic attribute, geo-mechanics, source rock characterization, and more. Principles and practice are given equal emphasis Simple models and concepts explain the underlying principles Extensive use of contemporary, real-life examples

Chapter 2

Petrophysical Properties

Martin Kennedy

Abstract

This chapter defines the quantitative properties that make up the petrophysical model and discusses what controls them. Firstly porosity, saturation and permeability are defined. The alternative ways of defining these are described and in particular the differences between total and effective porosity are carefully explained. Next, shale and clay volume are discussed and it is explained why clays are singled out for special attention in the petrophysical model. The concepts of heterogeneity and anisotropy are introduced and some of the ways of quantifying them are described. Finally the contentious issue of Net, Pay and reservoir average properties are discussed. The chapter also includes a section on relationships between properties and the use of regression to determine these.

Keywords

porosity

saturation

permeability

total and effective porosity

effective permeability

relative permeability

shale and clay volume

anisotropy

heterogeneity

Net

Pay

correlation coefficients

regression

2.1. Introduction

In the previous chapter, it was noted that much of practical petrophysics boils down to finding porosity, water saturation and permeability. For sure, some or all of these petrophysical properties may require some intermediate property to be determined first (e.g. shale volume). Furthermore, the so-called unconventional reservoirs require several additional properties for an adequate description, but even here the ‘Big Three’ are still key inputs to their evaluation. In this chapter, we will look at the petrophysical properties that are the end product of an interpretation. This includes apparently easily defined properties such as porosity, as well as the more subjective measures that are used to describe reservoirs (‘Net’ for example). Many of these properties are easy to define on paper but when they are applied to real rocks with water occupying at least some of the pore space, the definitions can become quite ambiguous.

2.2. Porosity

Porosity is the ratio of pore volume to bulk volume or, if you prefer, the volume fraction of fluids in the rock. That seems simple enough and for many artificial systems such as a porous solid made from plastic spheres it really is that simple. Unfortunately, for real rocks with water in the pore space it is not so obvious what constitutes the pore volume. As we shall see this is the result of water interacting strongly with many of the commonly occurring minerals. This ultimately leads to two rival ways of describing porosity: total and effective. Both are equally valid descriptions but one needs to be consistent and clear which is being used. We will return to this issue later but to begin with we will assume there is no ambiguity about what is solid and fluid.

As a ratio, porosity can have a value between zero and one (the latter is pure fluid of course). It is also commonly expressed as a percentage or in porosity units (pu), which are actually the same thing. Porosities for real rocks can vary from 0 to at least 50% although porosities in excess of 35% are unusual. Where porosities in excess of 35% do occur, they typically – but not exclusively – apply to carbonates. Figure 2.1 shows some porosity ranges for some real examples of clastic and carbonate reservoir rocks.

Figure 2.1 Porosity ranges for clastics (a) and carbonates (b).

To make progress in petrophysics it is helpful to start with simple models (either physical or theoretical). Suitable models to understand porosity are porous solids made from spherical grains. The simplest examples use uniform spheres arranged in an array. In these cases the porosity can be calculated exactly. Some examples of small parts of such arrays are shown in Fig. 2.2a, it shows how four spheres pack together to occupy the smallest volume. If this is extended in all directions we get a ‘close packed’ array so-called because it is the most efficient way of packing uniform spheres. It has a porosity of 25.9% and this is therefore the lowest porosity possible with uniform spherical grains (the calculation of the porosity is simple but tedious). Any other arrangement, including one in which the spheres are not uniformly arranged will have a higher porosity.

Figure 2.2 Simple models of porous solids. (a) Close packed array of identical spherical grains. (b) Simple cubic array of cubic grains (it is particularly easy to calculate the porosity of this). (c) A porous solid made from elongated grains. (d) The simple cubic array with smaller spherical grains occupying some of the original pore space.

The porosity of the appropriately named simple cubic array is much easier to calculate and is left as an exercise (Fig. 2.2b). Using these simple models we can qualitatively predict what happens as we move to something more complicated and more realistic. In Fig. 2.2d smaller spheres are introduced into the spaces between the larger spheres. This simple model is therefore saying that porosity falls as sorting gets poorer. Similarly if we start to elongate the grains we expect the porosity to be reduced.

The simple models using spheres are a good way to understand clastic rocks but a similar approach could be applied to fracture porosity. In that case we would probably start by putting plane parallel-sided cracks with a well-defined aperture into a solid block. This approach illustrates a point made in Chapter 1 that practical petrophysics relies on integrating experimental results from real rocks with highly idealised models such as those in Fig. 2.2. The latter helps us to explain the former and in the process to give us confidence in the results.

It is now time to consider real rocks in contact with water because this is what leads to the two alternative descriptions of porosity. Water has the ability to form weak chemical bonds to the surfaces of some mineral grains, this causes the grains to be surrounded by a strongly adhering film of water. Substances with free oxygen atoms at their surfaces, such as the silicates, are particularly prone to this. Clays have a particularly strong affinity for water and furthermore their morphology results in a high surface area so, they can bind a relatively large volume. Because this water is not going to move under natural conditions it could be considered part of the matrix. Nearly all the silicates will bind to water and other examples that immobilise relatively large volumes of water include the micas – including glauconite – and chlorite. To keep the word count down, for the remainder of this chapter we will refer to ‘clays’ when we really mean any silicate that binds significant volumes of water. Actually, log analysts often include glauconite and chlorite as clays precisely because they can fix large amounts of water.

The way bound water is accounted for determines which of the two different ways of defining porosity to use. In the effective porosity system, bound water is no longer counted as part of the pore space. It’s proponents argue that, as it cannot move, it might just as well be solid rock. To supporters of the total porosity model, however, it is still water – i.e. fluid – and therefore it contributes to the pore volume. If no clay is present the two descriptions give the same or almost the same answer. Conversely, if all the water in the system is associated with clay, as will occur in a claystone or shale, the effective porosity will be zero but the total porosity will be finite. The relationship between total and effective porosity and the volume of clay is shown graphically in Fig. 2.3. The intelligent reader will see that there is merit to both arguments and will keep an open mind.

Figure 2.3 Relationship of total and effective porosity to the volume of clay in the system. In the effective porosity model water associated with the clay is excluded from porosity, so that at 100% clay the porosity is zero. In the total porosity model the clay water is still counted as water and in this particular case the porosity as at its highest at 100% clay. When there is no clay both systems agree.

As already noted, clay is not the only type of mineral capable of binding and immobilising water so that strictly speaking the above descriptions should have been made a bit more general. In fact, it is not even just silicates that have an affinity for water. Nevertheless in practice the difference between the two descriptions tends to be most significant in ‘shaly sands’ in other words sands with a high shale or clay content. We will look at these in more detail later.

Total porosity is often justified as the better description because that is what is measured in core analysis. This is because most companies thoroughly dry the core plugs before measuring porosity and even the water bound to the clays is driven off. So, if a porosity calculated from logs is compared to measurements on core plugs we are implicitly comparing to a total porosity. Furthermore, to find the total porosity from logs we only need to find the total volume of water in the system (although with conventional logs that may be easier said than done).

To find...

Erscheint lt. Verlag	27.5.2015
Sprache	englisch
Themenwelt	Naturwissenschaften ► Geowissenschaften ► Geologie
	Technik ► Bergbau
	Technik ► Elektrotechnik / Energietechnik
	Wirtschaft
ISBN-10	0-444-63271-9 / 0444632719
ISBN-13	978-0-444-63271-5 / 9780444632715

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