Modern Log Analysis
Authors: Tixier M.P. 
Published: December 2007

TIXIER, M.P., MEMBER AIME, SCHLUMBERGER WELL SURVEYING CORP., HOUSTON, TEX.

The logging services most favored by the oil and gas industry are reviewed, and their characteristics and uses are discussed. Log evaluation is especially efficient when all the zones are investigated in such a way that the interesting levels stand out. Simple techniques that allow visual detection of oil and gas saturation are of great value, and are featured in the text. The selection of logging services must be governed by hole conditions and type of formations encountered. For each group, examples are shown which illustrate the preferred tools and techniques. Comments are made on various philosophies that are relevant to the problems. Conceptions that have existed in the past, as well as in the present thinking, and future trends are discussed.

Introduction

Modern well logging uses tools and techniques that were unknown a few years ago. Changes have been brought about not only by the discovery of new tools, but also by the ever-changing needs of the industry. This paper presents a resume of modern logging methods and of the information and utility they afford. A good set of logs permits correlation, assures depth control, defines the thickness of possible pays, and identifies zones for testing and further evaluation. To evaluate a zone, several quantities must be obtained: the effective and total porosity, the fractional water saturation, and evidence of permeability. Porosity is derived in a more-or-less direct manner by measuring the formation transit time (with a Sonic, or acoustic velocity, log), the formation density (with a Formation Density log), or the amount of hydrogen in the formation (with a Neutron log). Porosity may also be derived, indirectly, from a knowledge of F, the formation resistivity factor. The value of F is obtained from the responses of various resistivity devices that are chosen according to hole and invasion conditions. The empirical relation by which the porosity ( ) is derived from F is of the form

.................(1)

where a and m are based upon experience. Conversely, the value of F can be obtained from a knowledge of the porosity. The determination of water saturation Sw uses Archie's relation,

..............(2)

The resistivity of the formation water, Rw, may be known from measured samples, or it can be found from the SP curve. Rw may also be found from a study of porosity and resistivity values in clean water-bearing formations. The true resistivity Rt is preferably obtained from a focused log, such as a deep-investigation Induction log or a Laterolog. A value of 2.0 is normally assigned to the n exponent in Eq. 2.Evidence of permeability is best shown by the Microlog, since it locates mud cakes deposited during filtration into permeable zones. A hole caliper survey can also detect the mud cake in many cases. Furthermore, conclusions as to permeability can often be made as a result of detailed log analysis. Hole conditions, as well as formation characteristics, have a great influence on the selection of logs. Thus, it is essential to group together the various conditions that dictate specific logs which are combined into a proper logging program. Most logs in wide usage today are made with essentially focused devices. Logging programs in water-base muds will be discussed under two main categories, according to the resistivity contrast between formations and mud. Some special logging techniques will also be examined, and future improvements will be discussed.

The most important part of log evaluation is to so interpret the different levels that the interesting ones clearly stand out. To that end, a simple analysis that gives a visual evidence of hydrocarbon saturation is greatly preferred by the majority of people using well logs.

Low Resistivity Contrast

Logging Programs

This section applies to the logging of most sands and sandstones and to some carbonate formations of relatively high porosity in relatively fresh muds. The resistivities of such formations generally are not very great, and the resistivity contrast between formations and mud (the ratio of Rt to Rm) generally will not exceed 200.

JPT

P. 1327^