Standard Methods of Geophysical Formation Evaluation

James K. Hallenburg

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000724134
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Environmental Science
• Index: Biological Sciences

These three works cover the entire field of formation evaluation, from basic concepts and theories, through standard methods used by the petroleum industry, on to new and exciting applications in environmental science and engineering, hydrogeology, and other fields. Designed to be used individually or as a set, these volumes represent the first comprehensive assessment of all exploration methodologies. No other books offer the breadth of information and range of applications available in this set.

1

Resistivity Methods

1.1 Introduction

Most of the discussion in this book will involve downhole methods. However, the techniques and instrumentation descriptions will generally apply to laboratory, oceanographic, and surface applications, also. We find three general classes of downhole, geophysical, resistivity measuring instruments: (1) standard or unfocused systems, (2) focused systems, and (3) electromagnetic systems. The unfocused methods include the single point, normal, lateral, micro-resistivities, older dipmeters, and many of the surface methods (except for the electromagnetic methods). The focused methods include the guard logs, micro-guard logs, high-resolution dipmeters, and the spherically focused logs. Focused methods are also used occasionally on the surface. These methods all use electrodes in the mud column (occasionally against the wall of the borehole) to introduce current into the formation material and to read the resulting voltage drops as signals.

The electromagnetic methods (EM) are the induction logs, the electromagnetic propagation time logs (EPT), the EM logs, and the EM surface methods. The electromagnetic methods generally use a coil or an antenna to induce an electromagnetic field into the formation and usually to read it, also.

Array and scanning methods are new categories. They will be covered in a separate chapter. Focused and unfocused resistivity methods and the electromagnetic methods are widely used in oceanographic studies and laboratory methods.

Electrodes may be combined if the circuit is properly designed. That is, a single electrode may have multiple uses, even simultaneous uses. Thus, the A electrode for the normals may be the Β electrode for the laterals. It is common practice to use the M₁₆ electrode for the SP measurement. Since measured borehole resistivity values usually need correction, the uncorrected value is called the apparent resistivity, R_a. For these discussions, the current required by a voltage measuring system is assumed to be zero. That is, the input impedance of the voltmeter is assumed to be infinite. This, of course, is not true. With modern circuitry, however, the input impedance is usually very high (10⁸ to 10¹⁵ ohms) and is effectively infinite. This is something which should be checked, especially when working with older equipment and older data.

All unfocused resistivity arrays have at least four electrodes. Some may be at an apparent infinite distance away or may be combined with other electrodes. But, when considering downhole methods, only the downhole electrodes involved with a type of device are usually discussed. The other electrodes are considered to be an infinite electrical distance away. At this distance, they would not affect the performance characteristics of the downhole system. Thus, a normal system may be called a two electrode system, because it must have an A electrode and an M electrode downhole. Of course, it also has an N and a B electrodes. But, they may be so far away that they are at an apparent infinite distance. In this case, the apparent infinitely distant electrodes are so far away they may be moved without noticeably affecting the measurement.

The media in which we will be working, the formation materials, will contain electrically charged bodies. Metallic conductors have free electrons with single negative charges. Semi-metallic conductors have electrons and holes (which have apparent positive charges). Solutions have positive ions (cations) and negative ions (anions) which will have one or more charges. The charged bodies can be made to flow in response to either (or both) an electrical field or a magnetic field. When these charged particles flow under the influence of an electrical or a magnetic field, they constitute an electrical current.

Resistivity methods constitute the most important single group of logging methods in petroleum formation evaluation. These methods are among the few which allow us to directly detect the presence of hydrocarbon. In addition, they tell us something about the clay/shale content, formation waters, stratigraphy, era and period of the zones, and many other things. They are important in all types of geophysical investigation. They are most important in downhole geophysical investigations, especially in the search for and the evaluation of hydrocarbon deposits. Electrical resistivity methods (these will simply be calle...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Preface
• The Author
• 1. Resistivity Methods
• 2. Focused Resistivity Methods
• 3. Induction Methods
• 4. Spontaneous Potentials
• 5. Resistivity Log Interpretation
• 6. Natural Gamma Radiation
• 7. Gamma Ray Spectroscopy
• 8. Scattered Gamma Ray Methods
• 9. Neutron Porosity Logging
• 10. Neutron Activation Methods
• 11. Acoustical Methods
• 12. Formation Evaluation — Standard Methods
• Glossary
• Bibliography
• Index