Martin Kennedy

Chapter 14 – Geophysical Applications

This chapter looks at the way borehole measurements can be integrated with surface geophysical surveys. The first few sections look at how sonic and density logs can be used to enhance surface seismic through defining the time–depth curve and synthetic seismogram. This includes a brief survey of the Gassman equation and fluid substitution. The final sections look at two deep reading borehole measurements: borehole gravity and resistivity. Although these are run from a wireline logging unit the logs are recorded in stations and generally require post-processing to produce a useable log.

Chapter 12 - Complex Lithology

This chapter looks at reservoirs where a petrophysical model with just two solid components (shale and matrix) is not an adequate description. Generally the matrix consists of variable amounts of two or more minerals and this causes some of the log analysis parameters to vary significantly throughout the reservoir. The chapter looks at various log-based tools such as photo-electric factor that can help build a more complicated model. These tools are illustrated with reference to a worked example of a limestone–dolomite system.

Chapter 10 - Fluid Distribution

This chapter discusses what controls the distribution of water and hydrocarbons in a trap. It ultimately deals with saturation-height functions: fundamentals, development and applications. The emphasis of the chapter is on the opposing effects of gravity trying to pull water as deep into a structure as possible and the intermolecular forces, which cause water to adhere to the mineral grains. It starts with sections on buoyancy and so-called capillary forces. The former are introduced using conventional hydrostatics and the latter are discussed at the microscopic level. The concepts of wettability and interfacial tension are defined and then all the separate concepts are integrated in a section on capillary pressure curves. Some practical issues with the measurement of these are discussed before setting out the steps to convert from capillary pressure in the laboratory to column height in the reservoir. The construction of a saturation-height function is presented as a curve fitting exercise and many of the commonly used functions are described (including their advantages and disadvantages). The chapter concludes by looking at how pressure–depth plots are obtained from formation testers and how they are used to find the free water level.

Chapter 2 - Petrophysical Properties

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.

Chapter 9 - Hydrocarbon Corrections

The final chapter on log analysis looks at how hydrocarbon effects on the input logs can be accounted for. Much of the chapter looks at how iterative methods are used to integrate the porosity and saturation calculations to produce a consistent interpretation. The process is introduced by considering a specific model integrating density porosity and Archie. The way this can be generalised to more complicated porosity and saturation methods is discussed and some of the pitfalls that can occur are introduced. The chapter concludes by looking at some of the specific hydrocarbon effects on density, neutron and sonic logs.

Chapter 4 - Logs Part I: General Characteristics and Passive Measurements

Abstract This is the first of two chapters devoted to logging measurements. The intention is to look at the general features of borehole measurements and not to give a detailed description of different tool types and their operation. The two ways of getting instruments into a well are compared and contrasted: wireline and logging while drilling (LWD). The volumes investigated by logging tools are discussed in some depth. In particular the concepts of depth of investigation, vertical resolution and azimuthal distribution are introduced. The second half of the chapter applies these general concepts to specific ‘passive’ measurements, that is logs that do not put any energy into the formation. The measurements that are looked at are gamma ray, spontaneous potential, calliper and temperature.

Introduction to Log Analysis: Shale Volume and Parameter Picking

Abstract This is the first of four chapters on log analysis considered as the conversion of physical properties, measured by logs, to the petrophysical properties that are needed to characterise a formation. The first half of the chapter looks at the general approach to calculating a property and breaks this down into two steps: choosing an equation that converts a log or logs to property and the selection of the specific parameters for the formation of interest. This part of the chapter concludes with a description of how the raw data is prepared for input to a computer log analysis package and some of the tools that can help choose interpretation parameters. These general ideas are then applied to the specific problem of calculating shale volume. The gamma ray and density–neutron cross-plot methods are discussed in detail. But other methods including the use of NMR and geochemical logs are also described.

Log Analysis Part II: Water Saturation

Abstract This is one of the longest chapters in the book. Water saturation is found from logs by estimating the volume fraction of water in the formation and the chapter starts by looking at the logs, which are particularly sensitive to water. NMR, neutron lifetime logs and dielectric measurements are briefly considered as ways to measure water volume before discussion moves to resistivity. The origin and implications of the Archie equation are discussed at length. The way the parameters can be estimated are described. How pore structure controls the Archie parameters are then discussed by reference to simple models and empirical data. Uncertainty is explicitly discussed with reference to variation in the saturation parameters and the play types with inherently high uncertainty are highlighted. The chapter then moves on to look at the problem of excess conductivity. Excess conductivity is introduced by looking at rocks that contain pyrite before moving on to look at clays and shaly-sand equations. The chapter concludes with a lengthy discussion of the derivation and application of the Waxman–Smits equation.

Log Analysis Part I: Porosity

Abstract This chapter shows how log analysis is used to estimate total and/or effective porosity from logs. It is implicitly assumed that the formation is water bearing so that hydrocarbon effects can be ignored at this stage. The general approach introduced in Chapter 6 is applied to the specific case of finding porosity, including shale corrections, from density. The discussion is then extended to look at other single log porosities. The density–neutron cross-plot as a porosity tool is used to introduce the more general topic of using more than one curve as an input to porosity. The use of NMR as a porosity tool, including the ways it can help calculate effective porosity is discussed. The chapter concludes by showing how core data can enhance porosity calculation.

Core and Other Real Rock Measurements

Abstract This chapter looks at the direct measurement of petrophysical properties on rock samples. After a brief description of cores and coring it looks at the ways in which porosity, grain density and permeability can be measured on these samples. The more complicated measurements of special core analysis are briefly introduced although at this stage more emphasis is placed on sample selection. The chapter concludes by describing how techniques that were developed for rocks with relatively high permeabilities are extended to oil and gas shales.